Making a start on the 630m band
John Adams G3ZSE explains transmitting on 630m – an easy approach.
In Part 1 we began to appreciate that a passable station on 630m is possible for many amateurs with standard size properties and modest equipment. Moving on to transmit, rather than just receive, is a little more involved. Here I will outline my approach, which may then act as a framework that you may need to modify, depending on your equipment, circumstances and ambitions for the band.
TheTransmitter
There are a number of possible options for a transmitter. I use my old Icom IC-706 MkII, which I wide-banded some years ago. A number of Icom rigs, when widebanded, will give an output at 472kHz. The waveform has a lot of undesirable harmonic content, but the ATU cleans this up. Operating outside the manufacturer’s parameters has to be at your own risk, of course, but the IC-706 is used by a number of people on the band and performs adequately, provided the power is set to the lowest level. Additionally, I cool the IC706 with an outboard fan, regardless of the band, mode or power, as I find it never runs too warm then. The IC-706 low power setting on USB is 4W, but at 472kHz it produces 3W. Don’t increase the power – it may dissipate too much heat internally. I regularly leave my IC-706 on a transmit/ receive cycle on WSPR overnight, and all has been well. Other options are to use a transverter (see links on 472khz.org), or to use a modified audio amplifier. I recommend starting out with transmitter power in the 3 to 10W range, as it keeps things simple and voltages lower, and will provide a working station on 630m. The transmit power allowed at the antenna is 5W EIRP (effective isotropic radiated power). To achieve that you may need a 1kW amplifier though.
Losses in theTransmit System
The diagram, Fig. 1, shows the equivalent circuit of the transmit system for a vertical antenna with a physical earth arrangement. In an ideal world we would only concern ourselves with the radiation resistance. At VHF you will often have a simple antenna of around 50Ω radiation resistance, and the unwanted loss resistances (shown in red in Fig. 1) would be either absent or negligible. RL is the effective AC resistance of the main ATU coil, and may be about 6Ω. (The inductance calculator on the Hamwaves site is useful to show this element.) RRAD will be far less than the 50Ω we would like – it may typically be 0.15Ω only. RWIRE will be fairly low normally (but usually greater than RRAD). RETH is generally the big one – weighing in at 20 to 40Ω. We need to maximise the iANT, the current in the antenna, to maximise the radiated power in a short vertical antenna. This is because the power will be equal to the current squared multiplied by the radiation resistance. The best chance of maximising iANT is to reduce the losses, particularly RETH. Inevitably we still end up putting most of the power into heating the earth, and we can only achieve a very low overall efficiency.
The Antenna and Earth
In Part 1 we looked at antennas and earth systems. The key thing is to do the absolute best possible to achieve a high vertical section with a horizontal top. I use a doublet antenna with the feeder wires strapped together at the shack. An inverted-L wire is also a good option. I know it is possible to use a VHF beam with the feeder strapped at the shack. Do take care not to use systems with components such as inductors, capacitors or ferrites in them. You might destroy these as well as impairing transmission. You will need to have at least one very good earth stake, and preferably more. I live in a suburban area where the soil is heavy clay, and not particularly good as an RF path, but three earth stakes gives me a reasonable setup. I would strongly advise against using loop antennas, either big or small, as the losses will often be worse than a simple antenna. One last safety point − do not link the transmit earth and the house mains safety earth. This is acknowledged bad practice and can lead to dangerous situations.
The ATU
In essence the ATU (Antenna Tuning Unit) shown in Fig. 2 is very simple, but is the crucial part of this project. Be prepared to spend some time experimenting to get it right for your particular antenna/ earth system. I recommend looking at the HB9DUL link on 472khz.org. This will give an extra insight to this topic and outline various approaches to the allimportant issue of a coil to provide the high inductance required in the ATU.
At the base connecting-point all short vertical antennas ‘look’ like a small radiation resistance in series with a large capacitor. The goal is to tune out the capacitance with a large inductor, and then match the remaining resistance (including the losses) to the transmitter. Various approaches can be taken, including mechanically complex variometers, to vary inductance and coupling.
After some experimentation I decided to take the approach I use on 160m, and this design has been in use for generations on the low HF bands. L1 is bigger than the inductance required to tune the antenna, so requires capacitor C as well. Because C is variable, this gives a simple way to resonate the system. The capacitor adds additional loss, but not significantly at low power and low current. Fig. 3 shows the implementation, which remains in its
experimental state. The main coil former is a 130mm length of standard 68mm rainwater downpipe, wound with 114 turns of 0.8mm enamelled copper wire. This takes about 25m of wire. At the outset I added five taps at points between 92 and 108 turns. These may be fine, but for your setup you may have to add others. It can be a bit tricky partially unwinding and adding another tap, but time spent getting your ATU right is well worth it.
The capacitor I used is an old valve receiver type, with three sections of 550pF paralleled up to give 1,650pF maximum. My system comes to tune with this capacitor about two-thirds meshed. The plate spacing on such capacitors is small but should suffice for low power operation. To couple the tuned antenna/earth to the transmitter a second coil, L2, is used. This is nine turns of the 0.8mm wire wound on to a former made by wrapping some card into a cylinder, such that it is a snug sliding fit over the earth end of the main coil. L2 is held together with masking tape. I have found it works best with the lower end of each coil aligned, but moving it a short way along L1 in either direction may aid tuning.
I have found it useful to keep the ATU in a semi-experimental state to facilitate trying out variations in the system. You will find it easier to try things such as different capacitors if your initial ATU is configured using croc clips and choc blocs, before putting it all together. Mine was laid out on the bench prior to assembling it on to a board. If you have software such as MMANA-GAL, you could try modelling your actual antenna. It predicted 0.15Ω for the radiation resistance of mine. Some modern antenna analysers may give you a measurement at 472kHz. Both these methods would throw more light on your particular antenna arrangement. You might want to try a different coil former, or some thicker wire. The Hamwaves website gives lots of help in coil design if you want to do that. My L1 measures a total of 430μH, and the tap I use is at 380μH.
Assessing Antenna Current
As discussed earlier, we aim for the maximum antenna current iANT. With my setup I have found that tuning the ATU results in the highest antenna current occurring at the minimum SWR. I use the built in SWR meter in the IC-706 to indicate this. Be wary if you use a separate SWR meter – some have high loss at low frequency. You can test this by tuning for minimum SWR and then removing the meter and checking if iANT increases. When starting out you have two options – a simple indication of iANT or making a more accurate measurement of it. You can make a pick-up coil of, say, ten turns of thin insulated wire bunched up, and fit this over the feed to the antenna and feed the small voltage induced across it to an oscilloscope. This suffices to show if tuning is resulting in current increasing or not.
My RF ammeter is built in two parts, the transformer sensor, located in line with the antenna feed, and the display meter linked by a short screened cable. The circuit is shown in Fig. 4 and is one which has cropped up in various forms in various literature for a long time. The photos, Figs. 5, 6 and 7, show the arrangement and boxes I used, although this is non-critical. A plastic box is probably better for the main unit. You could use 4mm post terminals for input and output. I used SO239 sockets and generally use 4mm plugs in the centre contact for 630m. I connected an ‘AC’ socket to the unit for using the oscilloscope for some monitoring. Initially you will probably be dealing with iANT in the range 150 to 500mA. R is a 100Ω resistor of 2W rating and this should allow up to 2A to be measured. I used 4 x 390Ω/0.5W in parallel. You may want to build it all in one plastic box – that’s fine.
The critical item is the transformer, where the primary ‘turn’ is the antenna cable going straight through the middle of the ferrite toroid, as happens in many SWR meter circuits. You need a toroid of around a one-inch centre hole, that is not lossy at 472kHz. Mine is an unmarked grey natural finish of 37mm OD, 23mm ID and 12mm deep. It’s useful if the toroid performs the same at 1.8MHz as it does at 472kHz because it is then possible to calibrate the meter by using a dummy load at 1.8MHz and computing the current. You will need to relate the microammeter readings to real current. Thermal ammeters are good for antenna current, but they usually don’t go below 1A. If you go for a simple indication of current with a pick-up coil, then with a few watts from a transmitter and a simple antenna as outlined, you could safely assume an EIRP of between 5 and 15mW. Certainly, you know you are well within the licence limit.
Cautionary Note
Large inductances at RF generally lead to very large voltages. A few watts of input power won’t get you into any real trouble, but there will rarely be less than 100V RMS at the antenna connection. At the other end of the scale with 5W EIRP you would need to cope with many kV at the antenna, and many high power stations have ended up setting fire to wooden supports such as fences and fascias!
The complete Transmit/Receive Setup
The diagram, Fig. 8, shows the completed setup for transmit and receive. As mentioned, you will need to have a means of measuring SWR, often from the transceiver being used. I often also run another receiver with a very short wire as
an antenna to monitor the signal going out. In Part 1, Fig. 1 shows that I feed the antenna/earth at 7ft above ground level. This is where the wires enter the shack through an air brick. Ground level would be better, but a slightly elevated feed is fine. Feeding the antenna from upstairs though, would probably be less effective.
Going on the Air
You will need to ensure that the ‘Radio’ tab in the settings for WSJT-X is appropriately set for your transmitter and is working correctly. It might help to consult the WSJT-X User Guide for this. Check that your computer clock is spot on – no more than a second out. Set your transmitter to 474.200kHz USB (or USB data), and then go into the WSPR mode and choose an audio tone frequency in the range 1,400 to 1,600 using the ‘Tx (Hz)’ box. You can use the WSPRnet website to check nobody else is using that frequency. Ensure the band is set at 630m in WSJT-X and set your power – probably around 10mW. Next set your transmit percentage, starting with something like 25%. The software will transmit for 25% of the time overall, using a random pattern that helps to stop stations all transmitting at the same time and thereby not hearing anyone else.
Use the ‘Tune’ button on the WSJT-X screen to transmit a signal so you can check SWR and iANT. Aim for maximum iANT and minimum SWR. (SWR should be 2:1 or less.) Then ensure your transmit ALC indication is within the zone for sideband transmission. You may need to set the level with a control on your interface unit. When finished, switch off tune, select ‘Enable Tx’ and ‘Tx Next’ and wait for the first transmit sequence to commence. After you have transmitted, go to WSPRnet and set up the database query to see who is spotting you. When conditions are good you can move on to trying the FT8 mode.
What Next?
Options for further experimenting at my station include improving the earth system, trying a bigger coil diameter with thicker wire, looking at higher power and ways to improve the antenna. You may want to try some of these things too.
UsefulWeb Resources
For everything about 630m, including an excellent ‘Useful Links’ page:
For coil calculations and much more:
WSJT-X Installation Package and User Guide:
WSPRnet: