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Field Day Techniques

by VK1DA

May 2009

These notes reflect the approach I have developed over dozens of field days. There is nothing really unusual here, but if you are a newcomer to field day operation you may find some useful ideas here.

Antenna and equipment setup

For VHF/UHF antennas, rotary beam antennas are fairly important. For VHF DX work you need gain, you need horiontal polarisation and if you are on a decent hill, you need to get the antennas above the ground - but not too high, as noted below.

The masts I use are either a two-section Hills telomast or a 4m section of aluminium tubing. The Hills masts are predrilled and shaped to match their guying rings and slip rings but a plain tubing mast requires the equivalent fittings, to be rotatable as easily as the Hills is.

The guys are permanently attached to the guy rings. This makes for quicker assembly of the mast. The Hills masts have a guying point every 3m (10 ft) so with a 120 degree angle between the guys the stakes should be spaced the same distance out from the mast. It is amazing how difficult it is to get those angles right without something to judge it by, especially on sloping ground, so I place something at the planned mast base and estimate the guying points from that.

I assemble the mast on the rotator with just the guys attached, hammer in the three stakes in a 120 degree pattern and walk the bare mast up in that position to check I have the guy lengths roughly right. The desired outcome is that two of the guys then have the correct lengths and are attached to their stakes firmly. With the mast supported by the saw horse I then attach the antennas, feedlines and 432 preamp and walk it up again. The base of the mast is moved into the correct position to tension the two fixed guys. The mast is vertical and the third guy is then walked out to its stake and attached. A final check for "verticality" which can be tricky on sloping ground, then all guys are retensioned for security.

With a large 2m antenna at the 6m (20 ft) point of the mast and a long 70cm antenna down at say 4 to 4.5m, plus the weight of the coax, you can readily see that the top of the mast weighs a lot more than the lower section. Even with a rotator attached to the mast base, it is common that by the time I have walked the antenna up to about the 45 degree point, the mast base is being lifted off the ground by the leverage of the antennas. To avoid having the base lift off the ground and make it impossible to raise the mast further, I attach an additional heavy object to the rotator. My gas bottle comes in handy for this purpose. A better solution would be a correctly designed hinged base that was anchored to the ground in the correct position. Each field day sees an incremental improvement in arrangements like this so I estimate that by about 2020 I will have a much better system than my current lashed up solution.

As the mast is rotating within the guy ropes, the feedline cables must be placed so that they allow for 360 degree rotation of the antennas without fouling the guy ropes at either extremity of the rotation. I find a 360 degree loop in the feedlines with part of the loop above the guy ropes and part of it below the guys achieves this and does not risk putting strain on the feedlines or the guys themselves.

What about FM on 2m and 70cm?

If you want to use the FM channels, you will find rotary antennas are helpful, but because most FM work is designed to be compatible with mobile stations, vertical polarisation is the standard. So you'd want vertical beams designed for the 146.5 MHz area of the 2m band in Australia. Unless you have a high gain beam, you may well find that one of the longer omnidirectional verticals would work as well as a small beam, and would not require rotating. This is a very simple station, ideal for working local stations or depending on your location, stations within say 100km. It should be possible to work other portables within 150km if both stations have say 50w output power and an 8 dB omni antenna.

A big drawback with FM is that most activity tends to be on the national simplex frequency of 146.5 in Australia. A tiny minority knows that 146.55 is a quieter frequency and will operate there. Almost no-one appears to use 146.6 or 146.45, let alone the 25 kHz channels between all those. What chance will you have of working into a nearby city if all the handheld, mobile and home stations in that city are already using that frequency for local contacts, and few of them will even know you are on the frequency? Well, none. For this reason, I find FM channels quite frustrating and of little value in field day events and contests.

For longer distances you need SSB and horizontal polarisation.

Feedlines for VHF and UHF

Until 2009 I used a mixture of surplus RG213 and similar cables selected for their length, on all bands. These were about 10m long which was sufficient to reach the radios in a field setup. With these antenna cables I worked distances of up to 750 km on 2m, depending on conditions. From the Canberra area to Melbourne is around 450 km.

In 2008 I decided to improve the quality of my feedlines for the higher bands, took the plunge and purchased new CNT400 cables. This had some unexpected results. One of my antennas turned out to be very poorly matched and with the newer low loss cable this mismatch was evident for the first time. Once the antenna is fixed I am sure it will be working much better than it had been.

The feedline improvement did make a difference on 432 MHz and 1296 MHz.

Evolution of field day setup

When I first operated my own field station I had 6m SSB, 2m FM and HF. I had some tropo contacts into the Sydney area on 6m and my 2m contacts were all with locals, possibly extending out to 80km or so. Adding 2m ssb gear and better antennas is something that evolved over many years.

Much later I graduated to multimode radios for 2m and 70cm with increased power, and better antennas. This made the world of difference to the distances worked on those bands. In the 90s I had an IC271H and an IC471H for 2m and 70cm. These workhorses have good receivers and produce plenty of output power for field work.

Like other radios they are voltage sensitive, so you have to make sure you feed them with nothing less than about 12.5 volts. A dying car battery drops below 12v and then the internal voltage regulators on many radios no longer work correctly, allowing the oscillators to change frequency as you transmit.

This gear was used for several portable operations in the Canberra area during the 90s and also in 2001 on a trip to Tasmania, where I operated from some sites in the Burnie area, making easy contacts across Bass Strait into VK3 on 2m, 70cm and 6m. On 6m I used a dipole which was fed with RG58 cable and was lying across a bush, elevation 1m at the time of a contact with Norm 3DUT.

Until 2007 I had no equipment for 23cm so was limited to the most popular bands 2m, 70cm and 6m. Most of the points I earned in field day contests were on 2m and 70cm.

In 2007 I purchased an IC910H with the 23cm band module and that expanded the contest for me into a new band with different performance and antenna requirements.

In 2008 I added a transverter for 2.4 GHz and while I haven't made any longer distance contacts with it, it nevertheless adds another dimension to the field day.

Future plans include improving my power output on 1296 with a converted surplus amplifier producing 60 to 70 watts, adding a masthead preamp as well to negate the feedline losses. And more bands...

What bands are really needed for VHF field days?

I encourage newcomers to field operations on the VHF/UHF bands to start with modest expectations and just try to enjoy what you do have, then improve it one step at a time when you can.

The feedline change mentioned above was something I changed after operating for years successfully using RG213 type cable on all frequencies up to and including 432 MHz. The point is that while it isn't as good as it could be, if that's what your resources permit, that's what you use. Don't let idealism get in the way of having some fun from a field day location.

The multiband radios such as the IC706 series, IC7000, FT100, FT817/857/897 and the TS2000 now owned by many operators can be the basis of a one-radio, 3 band vhf station that only needs antennas to be a self contained portable station. An antenna for 6m can be as simple as a quarter wave whip and if you don't mind a slight compromise, you can use a whip intended as a 5/8 vertical for 2m. These are very close to resonant on 6m as a quarter wave and work fine.

For 2m and 70cm, remember that the first 3 elements contribute the first 8 dB or so of the gain. Additional elements do increase the gain but by a relatively lower margin. It is tempting to build very long yagis for the higher frequencies to get every last dB of gain, but in practice this is not a good approach to field operation. Reliability and physical strength are almost more important.

A two band yagi can be made simply for 2m and 70cm on a common boom. See Kent Britain's cheap yagis for an example of how effective antennas can be built without breaking the bank and without using difficult construction techniques. If you can drill holes, solder coax onto copper or brass elements, attach a coax connector to a cable and mount an antenna on a mast, you are in business with these antennas. Simple antennas could be the basis of a simple field day setup combined with your IC706, FT817/857/897/100 multiband radio.

Operate from any hill and you'll be surprised by the signals you hear and the distances you can work with a simple setup.

But a word of caution: don't expect signal levels to be as high as your local FM repeater. And don't expect signal quality to be excellent, with noise free reception. There will be background noises, QRM from nearby stations, the dx will probably be weak and at times impossible to copy. But the trick is to persist. If the signal fades down, you can almost always be sure that it will fade back up. Will it take 10 seconds, 50 seconds or 200 seconds to return? The only way to find out is to keep listening and calling in short bursts.

My approach to antennas for VHF portable operation

A lower antenna means a short feedline which cuts losses whereas a higher antennna requires a longer feedline, which increases feedline losses. This means there is a compromise between the biggest possible signal at the antenna and the biggest possible signal into your radio (ie. after taking line losses into account). Unless you believe you will get really improved signal levels with a higher antenna, don't put your antenna way up in the air. If you are on a hill you already have a big advantage over home stations in the valleys, so don't waste it with long runs of lossy cables. 3 or 4 metres is high enough on 2m and 70cm and above.

On 6m where feedline losses are lower, placing a yagi at a better height (say 6m) can help you work longer distances. A 6m beam should not be any lower than 3m.

There is some advantage in having antennas that allow you to hear and be heard in a wide angle of directions from you field site. This reduces anteanna rotation, cutting down on rotator wear, worrying about getting the antenna pointed in precisely the right direction and helps you avoid missing out on a contact that a very sharp antenna will not let you hear. Having a wider beamwidth means less gain, but on a field day you want contacts and you are not there to try to set records. Records require full legal limit power and the best of conditions, including low QRM, none of which can be guaranteed to be in force on a field day. The simplest way to obtain a wider beamwidth is to use a smaller yagi, such as 4 to 6 elements instead of 8 to 12.

Some US field operations are now using stacked 5 element antennas for 2m. This arrangement gives the wide horizontal beamwidth but would narrow the vertical beamwidth, which translates into increased gain. This should provide the same contacts as the longer beam but with simplified beam pointing requirements. On top of that, stations in a wider arc will hear your transmissions, making it less likely you miss out on contacts with stations just off the beam from a sharper antenna.

My best 2m dx from Mt Ginini, other than sporadic E contacts up to the north of Queensland (VK4) at almost 2000 km, were some VK5 stations worked in November 08, at about 1100 km. Yet at the time I was using the same old 8 element yagi I have used on many previous field days. I'm sure this antenna does not have as much gain as others I have used. The difference would be only several dB. The VK5 contacts were made possible by conditions, ie. the right combination of air pressures in the right parts of the path. No antenna can give the signal lift those conditions create. Using a shorter, robust antenna is a better field day option. It isn't long enough to get damaged in transit and is light enough to be put at the 20ft mark on my mast and lifted with ease.

I have used a longer antenna, a DL6WU-type 12 element antenna for 2m. On the occasions I used that antenna I did not work any additional stations or longer distances.

So my advice is not to try to take the biggest, highest gain antenna. Instead, choose something reliable and predictable, something you have tested, that you know will survive the trip up the hill and will not fall apart when you drop it on the ground at some stage, as you will.

I hope these notes are helpful to others planning a field operation.

Andrew

VK1DA