MARTIN - G8JNJ

Now don’t get me started………

Almost anything will work as an antenna – it just depends how well.

You can work DX on any HF band above 7MHz, using almost any bit of wire, if the conditions are in your favour.

I read claims made for various compact antennas such as ‘I worked XXX using this’ and I think, wouldn’t it have been a lot easier if you had used something which worked properly.

I only start to believe claims for designs if they can work reasonably efficiently on the lower frequency bands.

All antennas are a compromise between size, efficiency, frequency of operation and useable bandwidth.

When originally planning my HF antenna I had several criteria in mind.

1. Operation on as many bands as possible 80-10m Inc WARC bands.

2. As efficient as possible

2. Minimise interference to, and from, other equipment and neighbouring property

3. Low visual impact

4. Ease of construction and erection using available materials and supports

I initially considered a vertical, but finally decided upon a balanced horizontal antenna. This was because they don’t require a radial system, can be orientated to minimise RF levels towards housing and can provide good NVIS coverage on 80 and 40m for inter-UK working.  So, like many others before me, the options got narrowed down to G5RV or Windom variants.

I wasn’t keen on the Windom as I considered the off-set feed and balun arrangement would present mechanical problems and more RFI problems than with a true balanced feed. I believe that performance comparisons of simple wire antennas are mainly limited by the overall length of wire and their height above ground. If you place 30m of wire in the air at 10m height, then no matter what configuration you use, you will get very similar overall results. Perhaps the peaks and troughs in the polar plot will move around a bit, but the gains and angle of radiation will be substantially the same.

For general HF operation I use the ZS6BKW/G0GSF version of the G5RV. This is an improved version of the original design, which has been optimised for a 50 ohm feed. Although it may not the best single-band antenna in the world, I can’t think of any other balanced design which offers multiband operation without a tuner, reasonable performance and ease of construction.

It’s a very good starting point and offers a good baseline which you can measure other antennas against.

When I started operating using a G5RV, I went to buy some 300ohm open wire feeder to make my own, but the shop was out of stock. So I ended up buying a ready made version using 300ohm feeder in order to quickly get me on the air.

I had this up for a few weeks and it worked reasonably well, but didn’t give a particularly good match on all the bands it was supposed to. I then came across the ZS6BKW/G0GSF design.

I obtained some 450 ohm open feeder and cut it to the correct length using a sweep generator. This gave me a surprise, as the tuned length was about a metre out when compared to the calculated velocity factor. When I got the antenna up I immediately noticed a difference, the match was much better on all bands and signal reports were also improved.

The following table shows the VSWR and impedance at the end of the 450 ohm feeder measured with an Autek VA1 Vector RX Analyst

As a bonus the antenna also works on 50MHz quite well with a VSWR of less than 2:1 over most of the band.

When I got around to measuring the commercial G5RV I had bought, I discovered that the 300ohm feeder had been cut to the wrong length. I believe the manufacturer had used the calculated velocity factor rather than actually measuring it. They corrected this error by adjusting the length of the wire elements to compensate. As a result the performance was only good on one band.

This made me wonder, how many other amateurs are using non-optimised antennas such as this, and not realising that they have a problem.

Details of acceptable dimensions can be found at:-

http://www.reflections.co.nz/wordpress/my-uploads/zs6bkw-antenna-from-the-horses-mouth-by-g0gsf-ex-zs6bkw-sprat-_130-spring-2007.pdf

A handy 50’ high pine tree at the bottom of the neighbour’s garden made a convenient feed point, the house and a 9m fibre-glass pole provided supports for the ends. Connection to the radio is via 200’ of Heliax which snakes along the base of the fencing, up the side of the house, through the loft and into the radio room. Although it may seem pointless using such low loss cable at HF frequencies it makes a big difference on the non-resonant bands when using a remote tuner. This is because the mismatch loss becomes very significant when the VSWR increases above 10:1.

This works well as the nearest point of the antenna is still about 60’ away from the house, so interference and noise problems are minimised. I was initially disappointed with the receive noise levels on 80 (S6) and 160m (S8), but replacing the ‘few turns of coax’ balun at the base of the antenna with a good lump of ferrite, brought these levels down to S zero. In fact apart from occasional static busts both bands now have very low man-made noise levels.

Performance on 80m has been enhanced by adding multiple lengths of wire along the fencing running below the antenna at a height of about 2m. This forms a reflector about 10m below the antenna wire, which improves the NVIS performance for inter-UK operation.

For operation on 160m and 80m I have built a remote switch box which allows me to strap the feeders together so that the antenna can function as a T. Which is described here The ability to quickly switch between different antennas is very useful and allows you to find the best performer for a given distance and operating frequency. In order to improve performance on 160m I have coiled a short length of 50 ohm coax at the base of the 450ohm line, before it enters the switch box. This acts as a 1:1 balun on the HF bands when the antenna is used in its normal mode, but works as  a base loading coil when the feeders are strapped for use on 160m. About 10 turns of coax on a 3" diameter plastic waste pipe provided the correct value of inductance.

In addition to the ZS6BKW/G0GSF antenna, I also use the 9m long SOTA pole (top 1m has been removed) http://www.sotabeams.co.uk/maxi.htm which supports one end of the ZS6BKW/G0GSF as a vertical.

A 7m length of wire, resonant on 10.4MHz gives reasonable multi-band performance when connected to a 4:1 balun at the base with a suitable autotuner. A good elevated counterpoise mounted on the fence below the antenna seems to perform better than radials lying on the ground. As the end of the ZS6BKW/G0GSF antenna is about 0.5m away from the top of the vertical section, so there is a small amount of interaction. The worst case isolation between the two antennas is about 20dB, so in practice this shouldn’t cause too much of a problem.

I also use an LDG-Z11pro auto tuner remotely sited away from the antennas. This is not designed for use with random lengths of wire, but can be used with them providing you choose your length of wire carefully. If lengths of approx 7.2m (resonant on 10.4MHz) or 9.0m (resonant on 8.3MHz) are chosen the impedance doesn’t rise much above 1K ohms on the majority of amateur bands, so the tuner can be used directly connected to the wire. If you wish to use the tuner remotely (even with a short length of coax) then the best compromise is to use an external 4:1 balun at the base of the antenna.

More information about baluns can be found here

The SOTA pole is very useful as a reference antenna, or as a support for use when testing other designs.

When I first started experimenting with antennas, I realised that I needed to be able to make a reasonably accurate measurement of radiated field strength. So that I could compare different configurations, or fine tune individual designs. In order to do this I have used several methods.

1. A home constructed RF field strength / level meter using an Analog devices  Log Amp / Detector
2. An upconvertor and TV / Satellite level meter or calibrated spectrum analyser
3. A remote controlled receiver and active antenna located a few miles away
4. Off-air signal comparisons and reports

Once you have the capability to make repeatable measurements it’s surprising what you can discover about antennas, especially their actual performance and gain, when compared against other antennas.

This chart shows the relative field strength of various antennas at a distance of about 2mile from the TX site. The signal levels are measured in dBm (dB relative to 1mW) and the receiver is using a short vertical active antenna, which is reasonable frequency independent. The closer to 0 dBm the stronger the signal. e.g. -70dBm is stronger than -76dBm.

Note that it’s actually quite difficult to compare the performance of different antennas, as factors such as the radiation angle and receive noise performance, also need to be taken into consideration when deciding which antenna provides the best results for a given operating frequency and distance.

As an example the ZS6BKW antenna seems to have poor performance on 7MHz, but this is due to a null in the lobe in the direction of the RX site. Also note that the difference in signal levels on the 28 and 50MHz bands are due to the lobes changing position as the frequency varies. This is not apparent with the omni-directional vertical antenna.

Even though a particular antenna may seem to produce the strongest field strength locally, it may not produce the best results. For example low horizontal antennas produce the best NVIS performance for medium distance communication on the LF bands, but a vertical antenna may be better for long distance HF communication due to its low angle of radiation.

However this measuring technique is useful when testing new antenna designs or simply optimising existing ones. For example removing an earth connection to the elevated radial system increased the field strength by an average of 2dB. It is difficult to quantify small improvements such as this unless you are able to make repeatable measurements.

I frequently read antenna reviews, especially the various new compact designs, how well they perform, what DX has been worked (usually with CW on 12, 15 or 17m) etc. But most of this is just subjective, as I have said before almost anything will work on frequencies higher than 7MHz if the conditions are good. Don’t believe me ? Then take a listen to some of the worldwide beacons which transmit at different power levels. If you can hear one when it’s transmitting at 100w, you can usually still hear it when it’s only using 100mW. A difference in output power of 30dB and you can still hear it !

Many people are happy to operate with (or purchase) an antenna which has a gain of perhaps 10 or 20dB less than ¼  wave vertical or  a half wave dipole, but because they have a few good contacts and have not been able to compare it against a anything better, they believe it is working well. A common misconception being that ‘the VSWR is good. If you have a good match, across a wide bandwidth when using an electrically small antenna, beware, something is wrong.

Martin – G8JNJ – 05/02/2008 – V1.4