RFI (Radio Frequency Interference) sleuthing with the TinySA - Part 1: The gear

Although focused on the use of the TinySA, the techniques and equipment described in this article may be applied to using any other type of Spectrum analyzer - or even a simple receiver - for the detection of interference/noise sources or general-purpose direction-finding.

Figure 1: The Tiny SA, connected to an outdoor HF antenna during daylight hours.  The displayed span is 2-22 MHz. Click on the image for a larger version

About the TinySA:

The "TinySA" is a small spectrum analyzer with a 2.4" touch screen, costing approximately U.S. $50.  Capable of operating from below 100 kHz to as high as 950 MHz (the optimal range being between 100 kHz and 350 MHz) its minimum resolution bandwidth (RBW) is about 2.6 kHz.  This resolution bandwidth is too wide for precise analysis of the components of an SSB or NBFM signal, but it is useful for general RF surveying - including the measurement of harmonic and off-frequency spurious components of a transmitter in addition to the detection of low-level signal sources.

While it may seem silly to use a spectrum analyzer for the detection of signals, the TinySA, at about U.S. $50,  is only about 2-3 times the cost of the least expensive, battery-powered shortwave receiver that you'll find online - and it has the advantage of "seeing" a large swath of spectrum in a single view, allowing detection of signal sources that may otherwise go unnoticed.

The usable sensitivity of the TinySA in the range of 1 MHz to 350 MHz (e.g. the "Low" input) is approximately -152dB/Hz meaning that its sensitivity is slightly poorer - but roughly comparable to - "real" spectrum analyzers with expensive-sounding names:  This value is quite a bit poorer than a typical communications-grade receiver which will likely have a sensitivity better than -160dB/Hz.

The TinySA comes with a telescoping antenna, but being only about 12"(25cm) long it's usefulness doesn't extend below VHF frequencies (e.g. 50-100 MHz):  At HF, the combination of the 50 ohm input impedance of the analyzer and the phenomenally poor mismatch of the small telescoping whip results in uselessly-poor sensitivity, meaning that one must be nearly atop a signal source before it may even be seen on the analyzer.  Clearly, more help is needed here!

The TinySA is NOT the NanoVNA! The TinySA is NOT electrically similar to a NanoVNA, despite physical similarities:  It has completely different circuitry and works like a "real" spectrum analyzer in that it's a proper, swept frequency, narrowband logarithmic receiver-detector.   While a NanoVNA does have a detector that tracks its built-in signal generator, it makes a terriblespectrum analyzer!

Where to get a TinySA?

The TinySA is widely available - but there are apparently many inferior clones out there (e.g. lack of shielding, poorer quality or omitted components, etc.)  

For the U.S. readers, I suggest R&L Electronics as a source (see the link HERE).  I have no connection to R&L and suggest them only because they are supplying the genuine article and they are an established equipment dealer (e.g. more likely to help if you have any problems with the unit), and they are offering it for a decent price.

* * * * *

Active antennas to the rescue:

Adhering to the theme of portability, a useful companion to the TinySA would be some sort of small, active antenna - and two of the most common types are the active (E-field) whip and the electrically-small loop.

The Active Whip:

A popular active whip is of the "PA0RDT" design - the so-called "Mini-Whip".  It is the intent that this antenna be mounted outside and in the clear with a quiet "local" ground - that is, well away from noise sources, grounded to a mass of metal (or a ground system) that is isolated from the (noisy!) shack ground with a common-mode choke or two - so using it as a "sense" antenna for sniffing RF sources isn't exactly what the designer had in mind.

Figure 2: A homebrew E-field whip using the PA0RDT design. This unit is built into a piece of 1/2" PVC irrigation pipe. A BNC connector was chosen over an SMA for durability. Click on the image for a larger version

The reason for this specification of a "clean" ground and being placed outside, in the clear is that this antenna - being a tiny fraction of a wavelength on the highest frequency for which it is to be used - is essentially a capacitor that couples into the "æther"(e.g. free space).  To accomplish this, the whip is just a Hi-Z probe that has no gain of its own, but rather a circuit that matches the impedance of the "antenna" portion (usually a piece of wire or a plate of copper on a circuit board) to 50 ohms typically using JFET and bipolar follower circuits, avoiding what would otherwise be tremendous losses due to mismatch - but this is only half of the antenna:  The feedline - and the gear to which it is connected - is the other half - and this is the basis of the recommendation of a "local" ground with the feedline being decoupled via a common mode choke to minimize the conduction of RF energy from a "noisy" shack ground.

As a "sense" antenna in this application, its purpose is largely to determine the presence of RF energy:  Sources of this energy may be located by noting the amplitude increase as one moves the antenna nearer to the potentially-noisy device - or near a conductor (e.g. power cord, cable, etc.) that is conducting or radiating this RF energy.  Because the antenna will be carried by the user, the "ground" portion of the antenna will be nothing more than the hand capacitance of the user holding the equipment and the connecting cables - and it is imperative that these not be a source of interference in their own right or connected to something that could be a source of such signals.

Figure 3: Inside the E-field whip of Figure 2. The "antenna" is the copper foil "plate" with the electronics having been built on an SMD prototype board.  This version uses the BF862 JFET as the source-follower and a BFU589GX as  the emitter-follower. Click on the image for a larger version.

Because of the vagaries of coupling energy into this "whip" - which include the inconsistencies mentioned above - it's difficult to make absolute signal level measurements, but relative amplitude measurements can be made by noting signal levels as it's moved about.  Because it lacks directionality and is relatively insensitive to polarization, proximity is indicated mostly by relative amplitude:  If one happens to be close to the suspected device or interference source, this is easy to discern as even small changes of distance to the source can markedly change amplitude, but if you are somewhat distant from it this will not be the case and the lack of a directional response can be a challenge as one may not know in which direction one should move to get closer.

A few more articles about the small whip - its construction and its operation:

dl1dbc.net/SAQ/Mwhip/Article_pa0rdt-Mini-Whip_English.pdf   - The diagram of this ubiquitous circuit is that in Figure 7.

472khz.org/media/pa0rdt-Mini-Whip-Manual.pdf

pa3fwm.nl/technotes/tn07.html

And an "improved" version:

pa3fwm.nl/projects/miniwhip/

 

Where to get a mini-whip?

If you choose not to build one, the so-called "Mini-Whip" and its clones may be found on EvilBay and Amazon - to mention but two places - typically for $25 U.S. or less.  All of these devices work reasonably well - the ones from Eastern Europe often performing slightly better and being more consistent in quality than those from Asia - and they come with a "power inserter"(a.k.a. "Bias Tee") - a device that injects DC power for the amplified antenna onto its coaxial cable.

These antennas are also easy to construct, the details being found - and the theory of operation - in the above links.

* * * * *

The Small Loop:

This name refers to an "electrically small" loop - that is, one with a circumference that is a small fraction of the wavelength of the highest frequency for which it to be used.  For LF, MF and HF purposes, we are referring to a receive-only loop that is typically 1-3 feet (25-75cm) in diameter.

Figure 4: The completed shielded loop.  The gap in the shield is on the top with the transformer and amplifier in the box at the bottom.  The loop itself is 3/8""Heliax" supported by a frame made of 3/4" PVC irrigation pipe, using a "cross" in the center and "tees" on the sides. Click on the image for a larger version

Unlike the active whip, this antenna can be entirely "self contained" in the sense that the amount of energy impinging on the loop that is being conveyed to the receiving device and the cable itself has little effect on this level.  Additionally, this type of antenna exhibits a degree of directionality with sharp, deep nulls being present normal to the plane of the loop - which is another way of saying that if you were to hold the loop flat against your chest, the nulls would be directly in front of and behind you with very broad peaks to your left and right.

This loop, being electrically small for the intended wavelengths, has negative gain compared to a full-sized antenna, but even a chunk of coaxial cable connected directly to the loop with no attempt at matching will work "less badly" than a piece of wire or telescoping antenna - with no amplification or matching - of similar size.  Having said this, including a modest amount of gain in the loop's signal path is extremely helpful.

Having consistent "gain" lobes and nulls means that the amplitude readings from this type of antenna are more predictable and it is possible to divine the apparent bearing of the noise source by exclusion.  Because this type of antenna has two nulls (and two broad peaks) one must move about to resolve this ambiguity, noting the geometry of the direction of the these null and your surroundings to determine if it is very close or distant - and whether it is in front of or behind you!

Where do I get a loop like this?

Unfortunately, these loop antennas are nowhere near as ubiquitous as the "Mini Whip" - and those that are available tend to cost about an order of magnitude more than a mini-whip.  Fortunately, these types of antennas are not particularly difficult to construct - and such a loop is described below.

While this type of antenna may be constructed using just  loop of wire - and the techniques shown in Figure 7, below, will work fine with just a piece of wire, the preferred version is the "shielded loop" as it is somewhat less sensitive to very nearby E-field interference and its pattern of broad peaks and nulls can be more symmetrical and of better quality - as discussed here:

w8ji.com/magnetic_receiving_loops.htm

and

owenduffy.net/antenna/shieldedloop/index.htm

 * * * * *

Which type of antenna should I use?

If I had to choose just ONE of these types of antennas, I would pick the loop antenna due to its directional properties using the nulls.

Having said that, I would prefer to have both on hand:  The whip is much smaller and can easily be held near suspect devices and conductors to aid in detection/exclusion.

* * * * *

Constructing a shielded, symmetrical, untuned loop:

This portion of the article will described the "untuned, balanced, shielded" loop, and adding a simple amplifier to extend the noise floor of the TinySA.  There are many ways to construct this type of loop, but what follows is a rather simple and effective version that will tolerate component variations quite well.

In addition to locating sources of noise and interference, it provides reasonable performance for general listening on frequencies from below the AM broadcast band to the top of HF, and its ability to cast a null toward a noise source may prove to be useful.  While lacking the apparent "gain" response of the resonance peak of a tuned loop at a specific frequency, an untuned loop is easier to build and more convenient to use in a broadband application.

What to use for the loop itself?

It common to use "Heliax" (tm) coaxial cable (50 or 75 ohm is fine) to construct the loop itself - typically of the 1/2 inch or 3/8 inch variety as it is fairly rigid and can support itself fairly well, mechanically.  Aluminum-jacketed CATV (cable TV)"hardline" will work as well - although unlike the Heliax - which has a copper jacket - making connection to the outside shield can be a challenge.  

"I don't have any hardline/Heliax - can I use 'normal' coax?"

Barring the availability of "scraps" of Heliax cable, ordinary coaxial cable will also work quite well, but it must be mechanically supported as it may not be able to hold any shape -  The frame depicted in Figure 4, above, can do this.  If you are using "ordinary" coaxial cable, practically any coax will work, from RG-58 to RG-11 (the impedance doesn't really matter):  Even inexpensive RG-6 cable will work if one uses standard "F" connectors  - both on the cable itself and on the box containing the electronics - to make reliable connections to the aluminum shield found on most cables of this type.

Figure 5: Details of the gap in the middle of the shield - the dielectric foam being retained for additional support.  This gap should be covered with heat-shrinkable tubing or electrical tape to protect it from moisture.  Sliding a piece of rigid tubing over the gap is recommended to immobilize the cable at the gap in the shield.  A small piece of metal tubing may be used for this as long as it's insulated from the shields. Click on the image for a larger version.

For the loop described, I used a 66"(167cm) long scrap of 3/8" Heliax (FSJ2 type) which would correlate with a loop diameter of approximately 21"(53cm) - but the precise length and diameter is unimportant in this case:  While a larger loop (with a greater area inside the loop) will intercept more signal, a loop that is between 12" and 24"(25-50cm) diameter will work.

In the center of the loop, as depicted in Figure 5, there must be a small gap where the shield is opened, symmetrical about the loop's circumference:  Were a gap not present, this would simply be a loop of coax and a signal could not be excited on its center conductor with respect to the shield portion.  This type of loop has the advantage that an electrostatic field cannot easily excite the inner conductor, but the lack of the "shorted turn" provided by the gap in the shield means that the magnetic field portion of the signal is unimpeded.    (Hint:  Determine the center of the length of coaxial cable that you plan to use for your loop before you form it into a loop!)

Feeding the loop:

Figure 6: Inside the box - the transformer and loop amplifier. The transformer (T101 in Figure 7, below) may be seen in the middle.  The circuitry is built "dead bug" on a piece of glass- epoxy circuit board material bolted to the bottom of the box. The shields of the halves of the loop are connected with heavy wire to the circuit board - both for electrical bonding and mechanical support - and wire (a.k.a. "zip") ties are used to prevent the cable from being pulled out through the sides of the box.  The RF out/DC in is via a BNC connector mounted to the side of the box - this being preferable to the bottom of the box to help protect it from damage in handling. Click on the image for a larger version.

The two open ends of the loop are brought together opposite the shield gap and it is there that the shields are connected to each other at a common signal "ground" - and this is typically done in a small enclosure:  While a metal box is preferred, it's not absolutely necessary.  The box shown in Figure 6 is a die-cast aluminum enclosure approximately 5-3/4" x 2-1/2" and 1-1/2" deep (about 122 x 65 x 40mm).

While one may simply connect a 50 ohm coax to either side of the center conductor of the loop - and connecting the shield of that cable to the shield of the loop itself - this will cause a bit of asymmetry.  Practically speaking, this isn't going to cause much of an issue in terms of directionality, but it is preferred that balance be maintained to provide the best symmetry in its response.  While there are many designs out there that include differential amplifiers, the use of a simple, ferrite transformer will be just fine for this - and most - applications.  Remember:  We are looking for noise sources!

For this, a small transformer must be constructed.  A good choice for this is a toroid - either an FT37-75 or FT50-75.  Experimentally I determined that either 2 or 3 turns on the primary and 3 or 4 turns on the secondary will provide good response across the HF spectrum (I used 3 turns on each, for a 1:1 turns ratio).  The gauge of wire is not critical and something in the range of 18-26 is suggested with a preference on the larger size.

Amplification:

Because of its small size, the gain of the loop will be very low compared to a full-sized antenna and to bring the signal level up to where weaker signals may be "seen" by the TinySA, a simple amplifier is used, depicted schematically in Figure 7, below.  This amplifier could be considered to be a general-purpose "gain block" as it is quite well-behaved in terms of stability and input and output impedance - and it is very forgiving in terms of component variation.

Circuit description:

Figure 7: Schematic of the loop/amplifier and power inserter (a.k.a. "Bias Tee"). Click on the image for a larger version

 

The shielded loop is schematically depicted in Figure 7, above, along with the transformer T101, which I found to work reasonably well with a 1:1 turns ratio.  Using a VNA and with a bit of empirical testing, this configuration was optimized for low-mid HF (e.g. 3-15 MHz or so) but it seemed to work quite well through the AM broadcast band and below.  Like many similar loops - and the Mini-Whip - its performance will start to drop off at higher frequencies (above 20 MHz) and a bit of circuit redesign would be required to optimize for these higher frequencies.  As noted, the circumference of the loop must be a small fraction of the wavelength at the highest frequency at which it will be used in order to maintain the directionality of the nulls, so a loop of approximately 18" diameter will work well throughout HF, but not offer the desired properties on, say, 2 meters where its circumference would be approximately a quarter wave!

T101 does a reasonable job of maintaining the symmetry of the loop itself - although purists would insist on a somewhat different topology to eke out every bit.  As described, the symmetry is quite good and it is possible to completely null local 50 kW AM broadcasts stations.

Amplifier portion:

Q101 and associated components form a simple feedback-type RF amplifier.  This basic circuit is well-behaved and has reasonable input and output matching to 50 ohms.  Shown is the use of the common 2N3904 transistor which is perfectly acceptable for this sort of use - and similar devices, such as the 2N2222 and 2N4401 - work pretty well.  A "better" device would be the 2N5109, 2N3866 or similar RF amplifier which will offer a bit better performance in terms of intermodulation distortion and gain at higher frequencies, but they are more expensive and harder to find - and are likely overkill for a "sniffing" device.  This amplifier is not the penultimate in performance (e.g. IP3, P1dB, etc.) but it has very good performance and fairly low noise (6-8 dB noise figure) considering its simplicity.

To a degree, the gain of this stage may be adjusted by varying the value of feedback resistor R101 - resistances over the range of 330 to 680 ohms being useful, with higher gain (roughly 18 dB) being associated with higher resistance.  The purists will note changing devices or feedback resistance will alter the properties of the amplifier (e.g. input/output impedance, etc.) but one can generally ignore this in all but the most critical applications - such as matching to an impedance-sensitive filter network.

Although a battery could be placed within the loop's enclosure - in which case L101 could be eliminated and the V+ lead connected to the junction of C104/R104 - it is common to use a power inserter (a.k.a. "Bias Tee") that couples DC onto same coaxial cable that conducts receive RF from the loop, making one bias tee useful for multiple antennas!

How much RF noise does the TinySA produce? Because it's a computer with a display, one might wonder how much RF noise the TinySA itself produces.   The designers of the TinySA appear to have been very careful about this - for example, critical components are shielded and they to have chosen to use linear voltage regulators instead of more-efficient switching-type regulators.  (I can't speak to the construction of the many clones out there!)   In testing, the TinySA was held up to the E-field whip and placed inside (and moved around) inside the circumference of the loop:  Through the range of 2-22 MHz, there are no obvious "spikes" or lumps of noise that appear on the display - at least above the ambient RF noise floor in my ham shack.   Clearly, one isn't likely to walk around with the TinySA held against the antenna - but the initial glance shows that you probably could get away with it!

The power for the amplifier is picked off the coaxial cable by decoupling choke L101.  The value of this choke is not critical - and anything above about 100uH will work fine to a bit below the AM broadcast band.  

Molded chokes may work, but they tend to have quite high internal resistance and the current consumption of the amplifier itself (50-80 milliamps) can cause a significant voltage drop.  An alternative is to use the same core as that used for T101 (you did get several FT50-75 toroids, didn't you?)

Power inserter (a.k.a. "Bias Tee")

The "power inserter"(a.k.a. "Bias Tee") is also included on the diagram and its job is to combine the RF and DC onto the same cable, using the same type of choke (L201) as on the amplifier portion.  Optional enhancements to this device would be a diode (D201) to protect against accidental reverse-polarity application to the antenna and an LED to show that power is turned on (LED201) with its current intentionally set low (e.g. a 10k limiting resistor for about 1mA) to minimize battery drain.  The addition of a fuse - preferably of the self-resetting thermal type (F201, with a current rating of 100-300 mA) is a good idea as well to prevent damage to the blocking choke, L201 should the output be accidentally shorted and to limit current into protection diode D201 should reverse polarity be applied.

If you happen to get a "Mini Whip" from one of the online sources, it will likely come with a power inserter/bias tee that is electrically very similar to that depicted above - and that device will work just fine with the loop/amplifier depicted in Figure 7.

* * * * * 

Using a shortwave receiver for RFI sensing and source identification:

Up to now we haven't mentioned one RFI-locating tool that may be already in your possession  - a portable shortwave receiver.

Figure 8: A small selection of inexpensive portable shortwave receivers.  Upper-right:  A DAK MR-101 - an inexpensive receiver from the early-mid 90s.  Lower-right:  A Grundig YB-400PE - the only one shown that is capable of CW/ SSB reception.  On the left:  A $15 receiver (labeled LCJ 310) - one of many nearly-identical units available from. Amazon capable of tuning from 5.9-21.85 MHz on SW and on the AM and FM broadcast bands.  Of the receivers pictured, only the Grundig and DAK have external antenna connections (a 3.5mm "phono" connector) which would be required to use the directionality of the loop antenna. This connector would be needed to be added to the "LCJ" receiver to take advantage of this antenna property. Click on the image for a larger version.

While one can only "hear" a small bit of spectrum with a receiver rather than "see" a larger slice with an analyzer and increase the likelihood of detecting an otherwise-unnoticed interference source, being able to tune into an interference source and listen to it can give a clue as to the source of this signal and type of device.  If your portable shortwave receiver has an external antenna jack - especially one that automatically disconnects the built-in antenna when a connection is made - you can use the same antennas (whip, loop) described above and take advantage of any directional/nulling properties.

For example, a mains-frequency "hum" in the noise implies a switching power supply while a sharp "buzz" might indicate a triac light dimmer.  If your receiver has a BFO, one may be able to hear the harmonics of a switching supply and be able to specifically identify it by it's unique "sound" as it's powered on and off, being able to distinguish it from other devices.

A caveat with the use of a receiver:  Other than "missing" interference sources on frequencies other than that to which the receiver is tuned, a receiver will have an AGC which, by its nature, will adjust the internal gain to keep the volume constant.  When trying to locate a signal or noise source, this can work against you as it may be difficult to determine if the signal is moderately weak, strong, or very strong unless the AGC can be defeated and a manual RF gain control be operated - something that cannot usually be done with very inexpensive receivers.  Even if this is the case, the "sound" of the interference can still be useful in providing a clue as to the type of device that may be causing the interference.

* * * * *

Again, there are many possible ways to do this, but the gear described above has been proven to be useful.  The next installmentof this two-part series will include details on how it has been used and what to expect when doing so.

This page stolen from ka7oei.blogspot.com

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