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| Figure 1: The LF/MF transmit station, configured for 630 meter operation. At the time this picture was taken I had not yet completed the gear and put the various pieces in their boxes, hence the mess of clip-leaded- together modules sitting on my workbench. Not readily visible is the low-pass filter in the power amplifier box. The pictured variometer has been supplanted with a "new" one wound with 660/42 Litz wire for lower loss. Click on the image for a larger version. |
It took only two evenings to put together the transmit converter and power amplifier as I had the parts on hand - and none of them were particularly exotic. While the transmit converter will be described in greater detail in a future post, the signal path for the transmitter is approximately thus:
(See the block diagram in Figure 2, below.)
- A 5 MHz IF is used, allowing a "broadbanded" FT-817 to serve as the exciter.
- The 5+ MHz signal (about 5137 kHz for 2200 meters, 5475 for 630 meters) is mixed (using a 74HC4066) with a 5 MHz local oscillator to yield the LF/MF signals.
- A low-power post-mixer amplifier boosts this signal to a level capable of driving the power amplifier.
- A MOSFET-based broadband power amplifier, running on 12-24 volts, provides between 10 and 30 watts of RF at either 630 or 2200 meters. Because the transmit converter is broadband, it is agnostic to the operating frequency meaning that one needs only use the appropriate low-pass filter to change to a different band. (The 630 meter low-pass is always in line - another filter is added for 2200 meter operation.) This power amplifier is designed to be driven by either the transmit converter or another device, such as a QRP Labs Ultimate 3S beacon transmitter equipped for these bands.
- The 50 ohm output of the power amplifier goes to a tapped autotransformer wound on an FT-240 ferrite core and is used to match the transmitter's output to input resistance of the loading coil.
- The loading coil, placed in series, cancels out the capacitive reactance of my antenna system.
- Not shown in the drawing is a relay the disconnects the loading coil from the autotransformer when I'm not transmitting. This was necessary to prevent the transmit antenna from "sucking out" some of the receive signal being intercepted by my E-field whip and also to prevent the transmit antenna from conducting "house noise" from the transmitter onto the transmit antenna and partially desensing the receive system.
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| Figure 2: Block diagram of the 630 and 2200 meter transmit chain. Click on the image for a larger version. |
When I made my first-ever transmission I had not yet constructed the variometer. Fishing around in my "box-o-inductors" and found several Litz-wound ferrite coils that were probably rescued from some scrapped TVs or computer monitors and wiring enough of these in series I was able to achieve resonance with about 750mA of antenna current and on the first WSPR transmission I managed to be "heard" by several stations (See Figure 3, below.)
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| Figure 3: A screen shot (from WSPRnet.org) of the very first 630 meter WSPR transmission that I made with the badly-kludged loading coil. Not too bad for a temporary lash-up! |
On the next night, after observing a few stations engaging in JT-9 QSOs I answered a CQ by VE7SL and he replied, giving me a signal report of -22dB while I gave him -19dB. This was quickly followed by two other QSOs as both W7IUV and NC0B noticed the "new guy" on the band!
Over the next several days I got around to constructing the "new" variometer depicted in Figure 4 and this boosted my antenna current to about 1.25 amps - a theoretical improvement of about 4.4dB with more QSOs to follow - including 2.5 (one "partial") CW contacts on the band. After operating for a while it became apparent that, for the most part, I could work anyone that I could "hear".
A few days later I constructed yet another variometer for 630 meters - this time using some 660/42 (e.g. 660 strands of 42 AWG) Litz wire which reduced the skin-effect losses by a significant amount and this, along with minor improvements of the ground system, decreased losses and resulted in a further increase of antenna current to over 2 amps - a theoretical ERP improvement of more than 8.5dB compared to my original configuration. The measured resistance at the input of the 630 meter Litz coil is about 13.5 ohms, implying an overall antenna system efficiency of less than 1% - but still enough to work quite a few stations with a few 10s of watts of RF.
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| Figure 4: The "Mark 1 version of the 630 meter variometer. This device is wound on "4 inch" ABS triple-wall sewer pipe using 22 AWG insulated hookup wire. Inside is "3/4-inch" ABS waste pipe (actual O.D. about 1-1/8") that forms the rotatable portion of the variometer. This unit has an adjustment range of approximately 175-235 uH. Click on the image for a larger version. |
Operation on 2200 meters:
I have since wound yet another variometer - also on 4" ABS pipe - for 2200 meters. This coil, adjustable from about 1.7-2.0mH, uses the same 22 AWG hook-up wire as my original 630 meter loading coil. As it turned out, this coil, by itself, doesn't have quite enough inductance to resonate my antenna at 137 kHz so I place the other two coils in series with it. As compared to the 630 meter loading coils, it is somewhat lossy, but I am able to obtain about 900mA of antenna current: Not surprisingly, this coil runs slightly warm in operation due to the losses - but these are, no doubt, minor in comparison with the ground losses. The measured resistance at the input of this loading coil is about 43 ohms, implying an overall antenna system efficiency of well under 0.25%.
Not surprisingly, operation on 2200 meters - even at this power level - can be a bit hazardous. With the rather low antenna capacitance the voltages on the feed are quite high - an estimated 5000-8000 volts! What this means is that the feed wire has to be kept well clear of other conductors or else corona will occur, sapping transmit power, filling the room with ozone and becoming a potential fire hazard. Fortunately, at this modest power level - and with the current-regulated power supply that I'm using - almost any sort of fault will detune the antenna system to the point that the high voltage will all but disappear and/or the power supply will go into current limiting and effectively shut down the transmitter.
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| Figure 5: Left to right: The original 630 meter variometer (seen in figure 4) wound with 22 AWG stranded wire , the new 630 meter wound with 660/42 Litz wire and the 2200 meter variometer, wound with the same 22 AWG stranded wire. In the lower right corner is the autotransformer wound on an FT-240 ferrite core. Click on the image for a larger version. |
Despite this simple arrangement I've managed to be "heard" by seven other stations in the western U.S. and Canada using WSPR to date, but I've not yet made any 2-way contacts. The relative scarcity of stations that listen or transmit on 2200 meters coupled with my rather weak signal means that a contact will probably have to be arranged and conducted using a weak signal mode like JT-9 or QRSS.
Improvements:
There are plenty of improvements to be made, most notably getting the feed of my antenna a bit higher, laying out a few additional ground wires to further-reduce losses and improving the variometer for 2200 meters - but there are only so many things that I can do on my relatively small city lot. This entire arrangement has so far been precariously sitting on my workbench, meaning that the high RF voltages are also also nearby, just waiting to leap out at me when I reach over to tweak a variometer.
At some point I'll "remote" the matching network just outside, but I need to get/build a few other items first, namely some stepper motors, control circuity, more vacuum relays and a means of remotely monitoring the antenna current.
Comment: Despite having the feedpoint in my shack, I've not had any problems at all with transmit RF getting into computer speakers or other devices in my house.
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My recent operation, as of the date of this post, I seem to be the only actively transmitting station on either 630 or 2200 meters in Utah and have been running WSPR on 2200 meters most of the time, occasionally switching to 630 meters in the local evenings when the activity level is highest.
If you are QRV on 2200 or 630 meters and would like to arrange a CW, JT-9 or QRSS contact with me - or if you are interested in just "hearing" my signal (via your ears or with a computer+sound card) drop me a line using my callsign at arrl dot net.
Other entries on related topics found at this site:
- A (semi)-typical suburban E-field whip receive system for the 630 and 2200 meter bands - link. This page describes various techniques to effectively use an E-field whip in a noisy environment for LF/MF reception.
- Low-pass filter for LF/MF reception - link. Many HF rigs, while capable of reception at 630 metes (and possibly 2200 meters) are overloaded by signals from local AM broadcast stations. This filter can help reduce this problem.
- Completely containing switching power supply RFI - link. Switching power supplies are the band of the LF/MF listener and this page - and the pages linked to it - contain information about how these devices may be quieted.
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This post stolen from ka7oei.blogspot.com