Even though the general availability of the 630 meter (472-479 kHz) and 2200 meter (135.7-137.8 kHz) bands to U.S. amateurs is a recent phenomenon, I've had interest in these frequency ranges for about as long as I can remember. Back in the "old" days (the 1980s, for me) I would listen in these low-frequency ranges (10kHz to 530 kHz) using my modified Drake TR-7 which has an "LF input" on the back panel.
From the very beginning, I discovered a few things that did not work well for receiving these frequencies:
The "discoveries":
Even as a teenager with limited experience and knowledge I realized that at such long wavelengths even a rather long piece of wire as a receive antenna would be akin to putting a paper clip in the antenna connection of an HF rig and expect to "hear the world" - but I also knew that it was possible to hear low frequencies quite well on a very short antenna: The short whip on my car could hear the entire AM broadcast band really well - so it was possible if done correctly.
These realizations told me several things:
The next "breakthrough" was to wind a simple 1:1 transformer on a chunk of ferrite - probably the flyback transformer core of an old TV - that allowed only magnetic coupling between the radio and its chassis, and the antenna and a connection that went directly to my kludgy system of buried ground rods. By doing this, the "noisy" ground of my receiver - which was connected throughout the house with its noisy devices - was no longer referenced to the antenna. Because the antenna must have a "ground" of some sort to "push" against I knew that if that "ground" was the radio itself, which was connected to the noisy house wiring, that this noise would, in effect, appear on the wire antenna. This transformer effectively decoupled the two, using, instead, the comparatively "pristine" ground rod for the antenna to "push" against.
Between the above two tricks an entirely new world opened up as I could now hear the (now defunct) Omega transmitters between about 10 and 14 kHz and a myriad of "NDBs"(non-directional beacons) and similar signals in the range from 190 through just below 530 kHz. To be sure, I had to do most of my listening at night when TVs and lights were turned off, but that's when most of these frequencies propagated best, anyway!
The "LowFER" band:
Somewhere around this time I learned of the so-called 1750 meter "LowFER" band - a spectral slice from 160 through 190 kHz where legal, unlicensed operation (according to FCC §15.217 - read more here) could occur with some very strict limitations (e.g. and antenna that was, at most, 15 meters "long" and a maximum of 1 watt of input power.) but the challenge of both transmitting a usable signal with these limitations and receiving it via conventional techniques (e.g. CW) had its appeal.
It was at about this time - in the late 1980s - that I purchased an LF Engineering LF-400B - a commercially-available active E-field whip antenna that seemed to have decent reviews in the various longwave-related newsletters to which I then subscribed. This antenna, with a built-in amplifier and a strong low-pass filter to remove signals above 500 kHz, was much more convenient than trying to string a long piece of wire and matching it as it was rated from "3 kHz to 500 kHz". One slight disadvantage of this - or any - active antenna is that it needs power, supplied in this case by a "power inserter" that ran from an external power supply or a pair of contained 9 volt batteries.
Being a E-field whip antenna it was still sensitive to the direct radiation of interference from the household and neighborhood wiring and appliances, but provided that I located it away from the house and "decoupled" its cable by winding as many turns as could fit on the core of a flyback transformer from a scrapped TV and grounding the shield at the antenna, it seemed to hear the background static very well.
It was during this time period that I actively listened on the LowFER band, managing to hear a number of stations that were 200-700 miles (about 300-1100km) away and, on one winter evening, hearing a station halfway across the continent - about 2000 miles (3200km) away. I also set up my own LowFER beacon that, although very modest, was occasionally heard, on CW, up to 700 miles (1100km) away.
Comment:
Fast forward to the 21st century:
As it happens, I still have the L-400B and it has been outside, on a roof, for most of the time since the late 1980s. Other than having to be repaired a time or two (usually due to condensation and related corrosion) it still works as well as it ever did. While I have not been as active on LF as I once was, I've been maintaining that receive antenna and with the recent availability of the 630 and 2200 meter bands, interest has been rekindled.
To this end, I decided to document my receive antenna installation showing what "works for me."
The antenna on the roof:
I will admit to a luxury that most others will not have: My house has a metal roof.
The metal roof not only acts as an excellent ground plane, but it is also an effective barrier between what is "inside" my house and the "outside world". This means that at VLF and LF frequencies, things in my house that generate noise (light dimmers, switching power supplies, TVs) are fairly effectively isolated from this antenna on the roof - at least in terms of direct radiation of energy from these devices.
If you are not "blessed" with a metal roof on your house - but you are willing to go through a bit of hassle - you could lay down a suitable ground plane: Many people have been known to put down a layer of chicken wire on the roof or an interconnected grid of wires to act as an effective shield. Practically speaking, it need not cover the entire roof, but if the radius of this ground plan is 1-2 times the height of the antenna over the roof, it will probably have reasonable effectiveness.
Somewhere, this ground plane must be grounded and it is best that this is done via its very own ground system - which could be as simple as a ground rod - which is preferred over tying into the house's "noisy" electrical ground.
As can be seen from the picture in Figure 1 the whip antenna is mounted to a vent pipe at a height of approximately 5 feet (1.5 meters) above the roof - which happened to be the length of the piece of aluminum that I'd found to mount the antenna. When experimenting with mounting this antenna I found that if I placed it just above the metal roof, it was very quiet and relatively insensitive - but much of that was due to the fact that the very E fields to which the antenna is sensitive decrease significantly with proximity to "ground". By raising the antenna above the roof the signals increase very dramatically, but still seemed to be within the "cone of silence" afforded by the metal roof.
Decoupling the coaxial cable at the receive antenna:
When I bought the LF-400B antenna it was offered only with a permanently attached RG-174 feedline, but after about a year of use, often hauling it into the wild to listen, away from the city, the coaxial cable fatigued and broke, so I carefully disassembled it and installed a BNC connector (later versions of this antenna have a choice of connectors as an option.) This modification allowed me to connect a ground directly to the bottom of the antenna.
As mentioned earlier, one of the "tricks" to a quiet E-field antenna is to prevent electrical noise from being conducted from the receiver and "lighting up the ground" of the antenna itself - a problem that is arguably worse than the antenna itself picking up noise, directly. One of the better ways to to do this is to "decouple" the coaxial cable between the antenna and receiver using a large amount of inductance on the feedline - and I chose to do this several ways.
As can be seen from figure 1 there is a (red) wire connected directly to the antenna's connector that, in turn, connects to the "local ground" - that is, the metal roof itself. By doing this, the "ground" of the antenna and the roof are at the same RF potential and the detection of "local" interference is reduced.
Also visible in figure 1 - and in more detail in figure 2 - is an inductor in the form of a portion of the connecting coaxial cable being wound around a large ferrite rod from a discarded AM radio. The location of this inductor places it between the antenna and the receiver and its inductance adds common mode impedance to signals that would be conducted along the coaxial cable, but will not affect the desired signals within the cable itself. A better choke for this location would be like that depicted in Figure 4 (and described below) as it has higher effective resistance at the frequencies of interest, but since I'd already installed this one, I left it in place.
Figure 4 shows the other end of the cable just after enters the house. Just as it enters through the window there is another BNC connector, and connected to the shield at that point is a wire that goes directly to a grounding system that is located immediately outside the window. Between this grounding point and the inside of the house where the connection to the radio is made the coaxial cable is wound around a much more substantial choke - this one consisting of as many turns of the RG-58 coaxial cable as will fit on a TV flyback transformer ferrite core that was scavanged from a discarded TV. The details of the locations of these chokes and the grounding points is detailed in Figure 3.
It is this second "inside" choke that does most of the work: Consisting of about 20 turns, it has a measured inductance of about 15 millihenries. In running the math we can see that this large amount of inductance is what is required to effectively isolate the coax at LF and VLF frequencies, as in:
Where inductive reactance is calculated using the equation:
At 500 kHz:
500 kHz * 15 milliHenries * 6.28 = 47100 ohms
Because this is a linear equation, we can then re-run the numbers which tells us that at 50 kHz, the reactance is 4710 ohms and that at 5 kHz it would be 471 ohms.
What this shows us is that even at very low (VLF) frequencies, our rather substantial inductance is still effective, so it will work nicely at both 630 and 2200 meters - and everything in between!
Obtaining the inductance:
While "current-mode" 1:1 baluns that isolate the feedline in the manner we desire are readily available, unless they were specifically designed for LF and VLF use they do not have enough reactance to operate effectively at these low frequencies! What this means is that unless a suitable product is offered by one of these companies that is has been designed for LF and VLF use, they will not work well! That this means is that you will probably need to construct your own coaxial choke.
Many years ago it was pretty easy to scavange flyback transformer cores from old CRT-based TVs or computer monitors, but these are getting harder to find - but it is a good thing since these transformers were part of the very circuit that caused a lot of interference at VLF and LF frequencies! Once one manages to get the core out of an old flyback transformer in the first place (sometimes a trick in and of itself!) the fact that these cores are in two pieces makes it easy to wind the coaxial cable over one half and then assemble it. When I come across a flyback transformer, I often resort to putting it in a toaster oven and heating it so that the glue softens. Often, the core breaks - but ferrite typically breaks very cleanly and the two pieces can be rejoined using a drop of cyanoacrylate (e.g. "super") glue with little change in performance.
If a flyback transformer core is not available, what can be used, instead?
While not as convenient as a flyback transformer core - which can be disassembled during winding - a ferrite toroidal core can be used, instead. To maximize the number of turns, smaller coax such as RG-174 would be used and the connectors would be installed/connected after winding was complete.
Take, for example, a common ferrite material designed for low frequencies - "Mix 75"(sometimes called "Mix J") with a typical permeability of about 5000. A reasonably large toroidal core would be the FT-240 (the complete part number would be either "FT-240-75" or "F240-75"). Note that the ferrite mixes that one would normally use for things like HF baluns aren't ideal for this purpose as they have lower permeability.
Extrapolating from a data sheet and rewriting the equation we can see that if we can manage to wind 30 turns on this particular toroidal core, we can expect:
Where:
So,
6850 * (30/1000)2 = 6.165mH
Clearly, this is a bit less than half as much as I'd measured on my discarded TV flyback, but if we use the equation above we still get 194 ohms at 5 kHz and over 5 kohms at 2200 meters - a respectable amount of reactance! Using this size of core (an inside diameter of 1.4 inches/3.5cm) it is likely that more than 30 turns could be wound on it - and if you make this type of core, by all means, put as many turns on at as you can!
Unfortunately, the "Mix 75" toroids are not as easy to find as the typical toroids designed for higher (HF) frequencies and if we use a more common mix such as Mix 31 the result will be between a quarter and a fifth of the inductance for the same number of turns whereas "Mix 77" will, for the same number of turns, yield about 1/3 of the inductance as Mix 75, but this would still imply between 1 and 2 kohms at 2200 meters - still quite good.
Where does one get this sort of toroid? Toroids can be found at a number of places, including:
Another possibility - Common-mode chokes:
While a coaxial-based choke is preferred, there are other devices - possibly in your junk box - that may be suitable: A common-mode choke used for power supply filtering. The best place to find these is from scrapped switching power supplies - such as those used in computers.
Figure 5 shows an assortment of typical devices - but not all of them are suitable. As noted in the caption, the self-contained power line filter (upper-left) blocks all RF and wouldn't work, but the other devices allow differential currents to flow while blocking common-mode currents - which is what we want.
In order for these devices to be suitable for our purpose, they need to have:
In short, the suitability these devices for our purpose is best determined experimentally.
How it is would be connected:
Figure 6 shows how such a device would be connected to coaxial connectors. Note that the winding for the shield on one side of the choke connects to the same shield on the other side. In theory, this wouldn't matter at RF, but because we may need to conduct DC to power the active antenna, we would also need to preserve the polarity.
Not also that both sides of the input and output coaxes connect to the same "side" of the dual winding choke as indicated by the dots - in other words, the two windings are in phase with each other: Were either one of the windings (ground or center conductor)"flipped", this choke would do exactly opposite that which we desire - that is, the signal on the coax would be blocked, leaving only noise!
For an inductor such as that depicted in Figures 5 and 6 that is not wound with coax, it doesn't matter which side is the shield and which is the center - just as long as the windings are "shield-to-shield" and "center-to-center".
More information about interference reduction:
While the above techniques will go a long way to reduce the amount of noise picked up by an E-field antenna - and, to a degree, any antenna - it is too-often the case that there will be some device that simply radiates a lot of noise. While at HF frequencies and higher it is possible to reduce this noise with the application of large ferrite devices on cables, power cords, etc. this tactic simply does not work well at VLF/LF/MF frequencies because it takes so much reactance (inductance) to introduce enough effective resistance in the wire conveying this noise and a "snap-on" choke simply cannot do this. Even if a device contains "good" noise suppressing components (not all do!) they simply may not be very effective at VLF/LF/MF frequencies.
If you are interested in listening on the LF and MF amateur bands, the necessary first steps are outlined above: Do what is necessary to prevent noise from being conducted out, onto the antenna in the first place.
Once that is done, you may need to "seek and destroy" devices that are particularly egregious when it comes to generation RF "grunge" - and the typical suspects are switching power supplies, light dimmers and some brands of LED lights. Plasma TVs are notoriously bad interference generators, but since they are no longer being made, their contribution to the miasma of QRM is slowly decreasing as they die off.
The best way to find noise that you can do something about is to power the receiver from a battery (NOT including an inverter!) and turn off all of the power to the house - including shutting down any UPSs that you might have: If the noise decreases or goes away, turn on a circuit at a time until it returns and upon finding the circuit, isolate the specific device that causes the problem. If the noise is just the same with your power off as it is on, there may be a noisy power line nearby and/or a neighbor may have a noisy device - and how you deal with those two entities is up to you!
If you find a device (or devices) that generate lots of interference, they might either be replaced with "quieter" ones or modified to be quiet. Unfortunately, the latter can be a challenge the links below include techniques for doing this. If your goal is interference reduction at VLF/LF/MF - and you are constructing better filtering - remember that the higher-inductance chokes will be best!
How well does my receive antenna system work?
In the late evening and overnight, I can easily hear the "band noise" - that is, the sounds of the ionosphere and propagated storm static. During the day time the noise level is typically lower as it seems as though propagated noise from a wide area is suppressed somewhat - possibly by the formation of the ionospheric E-layer. During the "busy" hours - particularly from, say, 5 to 11 PM, there can be a bit of interference from other peoples' TVs, appliances and whatnot, but it is usually not severe enough to completely quash reception.
In my ham shack I have some track lighting over the workbench that is equipped with LED floodlights and is controlled by a light dimmer. While I do not "hear" the LED's switching power supplies, I do get a significant "buzz" on 2200 meters from the dimmer - but I don't hear it on 630 meters. The work-around for this is to use a smaller work light near the workbench - both of them being fluorescent - one having an iron ballast and the other electronic - but neither of them causing detectable interference on either 630 or 2200 meters.
One device in my arsenal is a "Line Synchronous Noise Blanker" - that is, a device that will mute the antenna signal when an interfering pulse - which is usually in sync with the power mains - comes in. This devices is adjusted manually and can go a long way to knocking out this type of noise. This device is described on this page: A Line-Synchronous Noise Blanker for VLF/LF/MF use - link.
Links to other articles about power supply noise reduction:
Final comment:
The L-400B still seems to be available - at about twice the price as it was when I bought mine in 1987. The page with information on this and similar products may be found here - link.
[End]
This page stolen from ka7oei.blogspot.com
From the very beginning, I discovered a few things that did not work well for receiving these frequencies:
- Simply connecting an end-fed random wire to the "Low Frequency" input.
- Using my 40 meter dipole.
- Anything that was indoors.
The "discoveries":
 |
| Figure 1: The LF-400B active e-field whip on my roof. The antenna is about 5 feet (1.5 meters) above the roof, mounted to a vent pipe. The red ground wire connected to the coaxial cable's shield at the bottom of the antenna can be seen along with choke. Click on the image for a larger version. |
These realizations told me several things:
- I would probably have to match the "short" antenna to the receiver input to be able to hear anything. I determined that this could be done with a series inductor or some sort of high-impedance amplifier - or a combination of both.
- When in a car, I could be well-away from interference sources - such as power lines and noisy appliances - and could hear weak AM stations. Somehow I had to keep the interference from things in the house from finding their way into my receiver.
The next "breakthrough" was to wind a simple 1:1 transformer on a chunk of ferrite - probably the flyback transformer core of an old TV - that allowed only magnetic coupling between the radio and its chassis, and the antenna and a connection that went directly to my kludgy system of buried ground rods. By doing this, the "noisy" ground of my receiver - which was connected throughout the house with its noisy devices - was no longer referenced to the antenna. Because the antenna must have a "ground" of some sort to "push" against I knew that if that "ground" was the radio itself, which was connected to the noisy house wiring, that this noise would, in effect, appear on the wire antenna. This transformer effectively decoupled the two, using, instead, the comparatively "pristine" ground rod for the antenna to "push" against.
Between the above two tricks an entirely new world opened up as I could now hear the (now defunct) Omega transmitters between about 10 and 14 kHz and a myriad of "NDBs"(non-directional beacons) and similar signals in the range from 190 through just below 530 kHz. To be sure, I had to do most of my listening at night when TVs and lights were turned off, but that's when most of these frequencies propagated best, anyway!
The "LowFER" band:
Somewhere around this time I learned of the so-called 1750 meter "LowFER" band - a spectral slice from 160 through 190 kHz where legal, unlicensed operation (according to FCC §15.217 - read more here) could occur with some very strict limitations (e.g. and antenna that was, at most, 15 meters "long" and a maximum of 1 watt of input power.) but the challenge of both transmitting a usable signal with these limitations and receiving it via conventional techniques (e.g. CW) had its appeal.
It was at about this time - in the late 1980s - that I purchased an LF Engineering LF-400B - a commercially-available active E-field whip antenna that seemed to have decent reviews in the various longwave-related newsletters to which I then subscribed. This antenna, with a built-in amplifier and a strong low-pass filter to remove signals above 500 kHz, was much more convenient than trying to string a long piece of wire and matching it as it was rated from "3 kHz to 500 kHz". One slight disadvantage of this - or any - active antenna is that it needs power, supplied in this case by a "power inserter" that ran from an external power supply or a pair of contained 9 volt batteries.
Being a E-field whip antenna it was still sensitive to the direct radiation of interference from the household and neighborhood wiring and appliances, but provided that I located it away from the house and "decoupled" its cable by winding as many turns as could fit on the core of a flyback transformer from a scrapped TV and grounding the shield at the antenna, it seemed to hear the background static very well.
It was during this time period that I actively listened on the LowFER band, managing to hear a number of stations that were 200-700 miles (about 300-1100km) away and, on one winter evening, hearing a station halfway across the continent - about 2000 miles (3200km) away. I also set up my own LowFER beacon that, although very modest, was occasionally heard, on CW, up to 700 miles (1100km) away.
Comment:
Another antenna to consider for MF/LF/VLF reception is a shielded H-field loop. By its nature, it is less-sensitive to nearby E-field energy - often that which emanates from electrical devices' interference radiating from wiring.
Another advantage of a loop is that it has a "figure-8" pattern with two nulls, allowing the possibility of rotating it such that one of these nulls is oriented toward an interference source. The obvious disadvantage is that a loop should have provisions for rotation to steer it into the null for the worst interference - or take care of those instances where the desired station happens to be in the direction of the null.
Shielded loops are available and they can be constructed fairly easily, typically using a piece of coaxial cable. Unless they are rather large and/or actively tuned to the receive frequency, they - like a short E-field whip - must have an amplifier that is powered from somewhere.
Fast forward to the 21st century:
As it happens, I still have the L-400B and it has been outside, on a roof, for most of the time since the late 1980s. Other than having to be repaired a time or two (usually due to condensation and related corrosion) it still works as well as it ever did. While I have not been as active on LF as I once was, I've been maintaining that receive antenna and with the recent availability of the 630 and 2200 meter bands, interest has been rekindled.
To this end, I decided to document my receive antenna installation showing what "works for me."
The antenna on the roof:
I will admit to a luxury that most others will not have: My house has a metal roof.
 |
| Figure 2: A close-up of the coax choke at the antenna. This choke consists of 10 turns wound on a large ferrite bar. The coax used is solid-dielectric RG-58. The use of a solid dielectric rather than a foam dielectric - such as that found in RG-6 - allowed a very tight radius winding without worrying much about the center conductor "migrating" and shorting to the shield. A cable like RG-174 would have also been usable, allowing a tight radius and more turns. At these frequencies, the loss of the coaxial cable is insignificant. Click on the image for a larger version. |
If you are not "blessed" with a metal roof on your house - but you are willing to go through a bit of hassle - you could lay down a suitable ground plane: Many people have been known to put down a layer of chicken wire on the roof or an interconnected grid of wires to act as an effective shield. Practically speaking, it need not cover the entire roof, but if the radius of this ground plan is 1-2 times the height of the antenna over the roof, it will probably have reasonable effectiveness.
Somewhere, this ground plane must be grounded and it is best that this is done via its very own ground system - which could be as simple as a ground rod - which is preferred over tying into the house's "noisy" electrical ground.
As can be seen from the picture in Figure 1 the whip antenna is mounted to a vent pipe at a height of approximately 5 feet (1.5 meters) above the roof - which happened to be the length of the piece of aluminum that I'd found to mount the antenna. When experimenting with mounting this antenna I found that if I placed it just above the metal roof, it was very quiet and relatively insensitive - but much of that was due to the fact that the very E fields to which the antenna is sensitive decrease significantly with proximity to "ground". By raising the antenna above the roof the signals increase very dramatically, but still seemed to be within the "cone of silence" afforded by the metal roof.
Decoupling the coaxial cable at the receive antenna:
When I bought the LF-400B antenna it was offered only with a permanently attached RG-174 feedline, but after about a year of use, often hauling it into the wild to listen, away from the city, the coaxial cable fatigued and broke, so I carefully disassembled it and installed a BNC connector (later versions of this antenna have a choice of connectors as an option.) This modification allowed me to connect a ground directly to the bottom of the antenna.
 |
| Figure 3: A block diagram of the antenna and receive system showing the grounding and coax chokes. Note that the "roof ground" - which is, in my case, the metal roof itself, but it could be a grid of wire or fencing material laid on the roof and is grounded elsewhere - is connected directly at the shield of the e-field whip itself, "before" the coaxial choke. At the ground level, in close proximity to the building entry is another connection to a "clean" local ground such as several ground rods and/or some buried ground radials. The "Inside coax choke" has the most inductance and does most of the isolating of common-mode noise currents that could otherwise "light up" the antenna with electrical noise that would be conducted from the radio system's ground connection to the power mains. The DC power inserter puts DC on the coaxial cable to the antenna - but not on the coax to the receiver - to provide power. Click on the image for a larger version. |
As mentioned earlier, one of the "tricks" to a quiet E-field antenna is to prevent electrical noise from being conducted from the receiver and "lighting up the ground" of the antenna itself - a problem that is arguably worse than the antenna itself picking up noise, directly. One of the better ways to to do this is to "decouple" the coaxial cable between the antenna and receiver using a large amount of inductance on the feedline - and I chose to do this several ways.
As can be seen from figure 1 there is a (red) wire connected directly to the antenna's connector that, in turn, connects to the "local ground" - that is, the metal roof itself. By doing this, the "ground" of the antenna and the roof are at the same RF potential and the detection of "local" interference is reduced.
Also visible in figure 1 - and in more detail in figure 2 - is an inductor in the form of a portion of the connecting coaxial cable being wound around a large ferrite rod from a discarded AM radio. The location of this inductor places it between the antenna and the receiver and its inductance adds common mode impedance to signals that would be conducted along the coaxial cable, but will not affect the desired signals within the cable itself. A better choke for this location would be like that depicted in Figure 4 (and described below) as it has higher effective resistance at the frequencies of interest, but since I'd already installed this one, I left it in place.
Figure 4 shows the other end of the cable just after enters the house. Just as it enters through the window there is another BNC connector, and connected to the shield at that point is a wire that goes directly to a grounding system that is located immediately outside the window. Between this grounding point and the inside of the house where the connection to the radio is made the coaxial cable is wound around a much more substantial choke - this one consisting of as many turns of the RG-58 coaxial cable as will fit on a TV flyback transformer ferrite core that was scavanged from a discarded TV. The details of the locations of these chokes and the grounding points is detailed in Figure 3.
It is this second "inside" choke that does most of the work: Consisting of about 20 turns, it has a measured inductance of about 15 millihenries. In running the math we can see that this large amount of inductance is what is required to effectively isolate the coax at LF and VLF frequencies, as in:
Where inductive reactance is calculated using the equation:
Z = 2 * Pi * F * LWhere:
Z = Reactance in ohmsBecause we are dealing with milliHenries and kHz, the "10s" parts cancel out, so:
F = Frequency in Hz
L = Inductance in Henries
At 500 kHz:
500 kHz * 15 milliHenries * 6.28 = 47100 ohms
Because this is a linear equation, we can then re-run the numbers which tells us that at 50 kHz, the reactance is 4710 ohms and that at 5 kHz it would be 471 ohms.
What this shows us is that even at very low (VLF) frequencies, our rather substantial inductance is still effective, so it will work nicely at both 630 and 2200 meters - and everything in between!
 |
| Figure 4: The indoor coax choke consists of 20 turns of RG-58 wound on a TV flyback transformer core. This choke, with a measured inductance of about 15 milliHenries, provides excellent isolation even down below 10 kHz. If a flyback transformer core cannot be found, a suitable choke can be wound on a high-permeability ferrite core using smaller (e.g. RG-174) coaxial cable. Note that to be effective at these frequencies this choke really does need to have at least several milliHenries of inductance! Click on the image for a larger version. |
While "current-mode" 1:1 baluns that isolate the feedline in the manner we desire are readily available, unless they were specifically designed for LF and VLF use they do not have enough reactance to operate effectively at these low frequencies! What this means is that unless a suitable product is offered by one of these companies that is has been designed for LF and VLF use, they will not work well! That this means is that you will probably need to construct your own coaxial choke.
Many years ago it was pretty easy to scavange flyback transformer cores from old CRT-based TVs or computer monitors, but these are getting harder to find - but it is a good thing since these transformers were part of the very circuit that caused a lot of interference at VLF and LF frequencies! Once one manages to get the core out of an old flyback transformer in the first place (sometimes a trick in and of itself!) the fact that these cores are in two pieces makes it easy to wind the coaxial cable over one half and then assemble it. When I come across a flyback transformer, I often resort to putting it in a toaster oven and heating it so that the glue softens. Often, the core breaks - but ferrite typically breaks very cleanly and the two pieces can be rejoined using a drop of cyanoacrylate (e.g. "super") glue with little change in performance.
If a flyback transformer core is not available, what can be used, instead?
While not as convenient as a flyback transformer core - which can be disassembled during winding - a ferrite toroidal core can be used, instead. To maximize the number of turns, smaller coax such as RG-174 would be used and the connectors would be installed/connected after winding was complete.
Take, for example, a common ferrite material designed for low frequencies - "Mix 75"(sometimes called "Mix J") with a typical permeability of about 5000. A reasonably large toroidal core would be the FT-240 (the complete part number would be either "FT-240-75" or "F240-75"). Note that the ferrite mixes that one would normally use for things like HF baluns aren't ideal for this purpose as they have lower permeability.
Extrapolating from a data sheet and rewriting the equation we can see that if we can manage to wind 30 turns on this particular toroidal core, we can expect:
L = Al * (T/1000)2
Where:
L = Inductance in mH
T = Turns
Al = mH per 1000 turns from the spec. sheet - 6850 for an FT-240-75
So,
6850 * (30/1000)2 = 6.165mH
Clearly, this is a bit less than half as much as I'd measured on my discarded TV flyback, but if we use the equation above we still get 194 ohms at 5 kHz and over 5 kohms at 2200 meters - a respectable amount of reactance! Using this size of core (an inside diameter of 1.4 inches/3.5cm) it is likely that more than 30 turns could be wound on it - and if you make this type of core, by all means, put as many turns on at as you can!
Unfortunately, the "Mix 75" toroids are not as easy to find as the typical toroids designed for higher (HF) frequencies and if we use a more common mix such as Mix 31 the result will be between a quarter and a fifth of the inductance for the same number of turns whereas "Mix 77" will, for the same number of turns, yield about 1/3 of the inductance as Mix 75, but this would still imply between 1 and 2 kohms at 2200 meters - still quite good.
Where does one get this sort of toroid? Toroids can be found at a number of places, including:
- Palomar Engineers (link)
- Amidon Associates (link).
- Another distributor of some of these devices is the web site kf7p.com -link.
Another possibility - Common-mode chokes:
While a coaxial-based choke is preferred, there are other devices - possibly in your junk box - that may be suitable: A common-mode choke used for power supply filtering. The best place to find these is from scrapped switching power supplies - such as those used in computers.
 |
| Figure 5: An assortment of power line filtering chokes and devices. In the upper-left is a self-contained AC line filter, but it is not suitable for this purpose as it is designed to block all RF - both differential and common-mode. All of the other devices are dual-winding common-mode chokes that allow differential currents to pass, but will block common-mode currents - but not all of these devices are suitable for our purpose - see text. Click on the image for a larger version. |
In order for these devices to be suitable for our purpose, they need to have:
- Adequate inductance. As we noted above, we need milliHenries of inductance to effectively choke out interference at LF and VLF frequencies. The smaller toroidal chokes shown have hundreds of microHenries of inductance - which may be suitable at 630 meters, but could be marginal at 2200 meters. For example, a choke with 100 microHenries per winding will offer about 295 ohms of reactance at 630 meters, but only 86 ohms at 2200 meters. Because we want as much reactance as possible - at least in the many hundreds of ohms - we would hope to do better!
- Good balance. All of these chokes consists of two identical windings and the idea is that if a common mode signal appears across both windings, they will be suppressed. If, however, the two windings are not identical, this suppression will be incomplete. It is likely that the "transformer-looking" chokes will have reasonable suppression at 2200 meters - and maybe even 630 meters - but as one goes up in frequency even more the imbalance will grow.
- Low loss to differential signals. The reason that we can pass a signal through a coaxial cable wound on a large piece of ferrite without affecting the signal being carried by that cable is that the coaxial cable, by its very nature, is fairly low loss to the signals carried within where the signal on the inside conductor of the coax is precisely equal and opposite to that carried on the shield. If one has separate windings, each carrying an equal and opposite signals, imperfections in these two windings - sometimes the same as those that cause imbalance - can cause degradation of those signals. As one goes up in frequency these ferrite cores - which are designed to block low frequencies - can start to get lossy - and this doesn't include the self-capacitance of the windings which can cause other things to happen, such as strange resonances or coupling. In other words, they may work find at low frequencies, but "fall apart" at higher frequencies such as 160 meters and up.
 |
| Figure 6: An example of how a bifilar (or similar) choke would be connected to a coaxial cable. The diagram above depicts how the two windings would be connected, keeping straight which is the "center", and that which is the shield of the coaxial cable. The dots indicate "phasing" - that is, same ends of the two windings connect to the antenna side and the other ends connect to the receiver side. Click on the image for a larger version. |
How it is would be connected:
Figure 6 shows how such a device would be connected to coaxial connectors. Note that the winding for the shield on one side of the choke connects to the same shield on the other side. In theory, this wouldn't matter at RF, but because we may need to conduct DC to power the active antenna, we would also need to preserve the polarity.
Not also that both sides of the input and output coaxes connect to the same "side" of the dual winding choke as indicated by the dots - in other words, the two windings are in phase with each other: Were either one of the windings (ground or center conductor)"flipped", this choke would do exactly opposite that which we desire - that is, the signal on the coax would be blocked, leaving only noise!
For an inductor such as that depicted in Figures 5 and 6 that is not wound with coax, it doesn't matter which side is the shield and which is the center - just as long as the windings are "shield-to-shield" and "center-to-center".
More information about interference reduction:
While the above techniques will go a long way to reduce the amount of noise picked up by an E-field antenna - and, to a degree, any antenna - it is too-often the case that there will be some device that simply radiates a lot of noise. While at HF frequencies and higher it is possible to reduce this noise with the application of large ferrite devices on cables, power cords, etc. this tactic simply does not work well at VLF/LF/MF frequencies because it takes so much reactance (inductance) to introduce enough effective resistance in the wire conveying this noise and a "snap-on" choke simply cannot do this. Even if a device contains "good" noise suppressing components (not all do!) they simply may not be very effective at VLF/LF/MF frequencies.
If you are interested in listening on the LF and MF amateur bands, the necessary first steps are outlined above: Do what is necessary to prevent noise from being conducted out, onto the antenna in the first place.
Once that is done, you may need to "seek and destroy" devices that are particularly egregious when it comes to generation RF "grunge" - and the typical suspects are switching power supplies, light dimmers and some brands of LED lights. Plasma TVs are notoriously bad interference generators, but since they are no longer being made, their contribution to the miasma of QRM is slowly decreasing as they die off.
The best way to find noise that you can do something about is to power the receiver from a battery (NOT including an inverter!) and turn off all of the power to the house - including shutting down any UPSs that you might have: If the noise decreases or goes away, turn on a circuit at a time until it returns and upon finding the circuit, isolate the specific device that causes the problem. If the noise is just the same with your power off as it is on, there may be a noisy power line nearby and/or a neighbor may have a noisy device - and how you deal with those two entities is up to you!
If you find a device (or devices) that generate lots of interference, they might either be replaced with "quieter" ones or modified to be quiet. Unfortunately, the latter can be a challenge the links below include techniques for doing this. If your goal is interference reduction at VLF/LF/MF - and you are constructing better filtering - remember that the higher-inductance chokes will be best!
How well does my receive antenna system work?
In the late evening and overnight, I can easily hear the "band noise" - that is, the sounds of the ionosphere and propagated storm static. During the day time the noise level is typically lower as it seems as though propagated noise from a wide area is suppressed somewhat - possibly by the formation of the ionospheric E-layer. During the "busy" hours - particularly from, say, 5 to 11 PM, there can be a bit of interference from other peoples' TVs, appliances and whatnot, but it is usually not severe enough to completely quash reception.
In my ham shack I have some track lighting over the workbench that is equipped with LED floodlights and is controlled by a light dimmer. While I do not "hear" the LED's switching power supplies, I do get a significant "buzz" on 2200 meters from the dimmer - but I don't hear it on 630 meters. The work-around for this is to use a smaller work light near the workbench - both of them being fluorescent - one having an iron ballast and the other electronic - but neither of them causing detectable interference on either 630 or 2200 meters.
One device in my arsenal is a "Line Synchronous Noise Blanker" - that is, a device that will mute the antenna signal when an interfering pulse - which is usually in sync with the power mains - comes in. This devices is adjusted manually and can go a long way to knocking out this type of noise. This device is described on this page: A Line-Synchronous Noise Blanker for VLF/LF/MF use - link.
Links to other articles about power supply noise reduction:
- Completely containing switching power supply RFI - link. Sometimes it can be difficult to quiet a switching power supply, so it may be necessary to put it in a box with strong filtering on all of the conductors that enter/leave.
- Minimizing VHF (and HF) RFI from electronic ballasts and fluorescent tubes - link. Electronic light ballasts, like many switching power supplies, operate in the LF frequency range so "cleaning them up" at VLF/LF/MF frequencies can be a challenge.
- Quieting high current switching power supplies used in the shack - link. This page describes techniques that can be used to reduce the amount of RF energy produced by switching power supplies that you may be using to power your radios. Again, higher-inductance chokes may be required at VLF/LF/MF frequencies.
- Reducing switching supply racket - link. This describes general techniques that can be used to beef up the filtering for switching supplies in general.
Final comment:
The L-400B still seems to be available - at about twice the price as it was when I bought mine in 1987. The page with information on this and similar products may be found here - link.
[End]
This page stolen from ka7oei.blogspot.com