MARTIN - G8JNJ

ECLECTIC AETHER - Adventures with Amateur Radio

Multi Band Wire HF Antennas

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.

 

Antenna reviews, especially for the various compact designs, usually state 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.

 

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

 

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

 

When I originally started building a multiband HF antenna I had several criteria in mind:-

 

  • Operation on as many bands as possible 80-10m Inc WARC bands.
  • As efficient as possible
  • Minimise interference to, and from, other equipment and neighbouring property
  • Low visual impact
  • 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 it didn’t require a radial system, could be orientated to minimise RF levels towards the house and could 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.

 

At this stage I wasn’t keen on the Windom as I considered the off-set feed and balun arrangement would present both mechanical and interference 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.

 

After further investigation I built the ZS6BKW/G0GSF version of the G5RV. This is an improved design, which has been optimised for true multiband operation with a 50 ohm coax 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 originally started building the G5RV, I went to buy some 450 ohm ladder line, 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 tried 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.

 

Many people don’t believe that a G5RV ‘type’ antenna can present a good 50 ohm match on most of the HF amateur bands, but the following table shows the VSWR and impedance at the end of the 450 ohm feeder measured with an Autek VA1 Vector RX Analyst

 

Frequency

VSWR

R

X

Z

1.9MHz

H

25

-372

374

3.6MHz

6.4 : 1

12

36

37

7.1MHz

1.28 : 1

61

-7

62

10.1MHz

H

H

H

H

14.1MHz

1.55 : 1

54

-23

58

18.1MHz

2.0 : 1

89

-30

97

21.1MHz

15.2 : 1

573

-329

668

24.9MHz

1.68 : 1

68

25

71

28.5MHz

3.76 : 1

138

-83

158

29.0MHz

2.02 : 1

101

9

101

29.5MHz

3.96 : 1

137

87

160

 

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 here.

 

A handy 50’ high pine tree at the bottom of the neighbour’s garden made a convenient anchor point, the house formed the other. 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.

 

I also improved performance on 80m slightly 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 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 to 20 turns of coax on a 3" diameter plastic waste pipe provided the correct value of inductance.

 

 

 

 

Multiband Verticals

 

In addition to the ZS6BKW/G0GSF antenna, I also occasionally use a 9m long SOTA pole (top 1m has been removed) 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 auto tuner. A good elevated counterpoise mounted on the fence below the antenna seems to perform about as well as a few radials lying on the ground. 

 

I also use an LDG-Z11pro auto-tuner remotely sited away from the antenna. This is not designed for use with random lengths of wire, but can be used with them providing you choose the length 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

 

For more ideas and an improved design see my next set of notes describing Wire antennas for the LF bands

 

 

 

Martin – G8JNJ – 12/08/2008 – V2.0