|Assembled in G3VJM Box|
|Assembled in G3VJM Box|
The following question was posted on the QRPL list a few days ago:
I plan on some portable operations(camping, hiking, etc…) and I was
wondering what are the optimum length of radials for verticals, or
counterpoise for End-fed half waves wires? Frequencies are 40 through 10
meters (including 30,17, and 12 meters).
I seem to remember something about at least 2 or 3 ¼ wavelengths laid on
the ground. I just want to get a consensus of the group here about what
seems to work best.
My response may be of interest to some members of CARC:
I am not sure whether the attachment will make it through the reflector (it’s only 21KB), but it’s worth a look if it does.
It is Fig 30 from Ground System as a Factor in Antenna Efficiency, which is the seminal paper by Brown, Lewis and Epstein published in Proc IRE vol 25, #6 June 1937.
This is the paper where the 200 radial comes from, although they ran out of wire at 113 radials as you can see from the graph.
This shows that with two radials at 1/8 lambda, received field intensity is slightly less 50% down on theoretical maximum. Received power goes as the square of intensity, so two radials are slightly less than 6db down on theoretical maximum, i.e. less than 1 S-point.
If you go up to 15 radial at 1/4 lambda to get to something like 148/196 intensity, i.e. 2.5dB down.
Given that 15 @ 1/4 lambda represents a practical maximum for temporary operation, you have to work pretty hard to win the 3.5 dB relative to the two 1/8 lambda wires.
Two other points of note are that you get within 1dB of maximum with
60 radial at 1/8 lambda and if you go up to 113 radials 1/10 lambda seems to be about 1dB down. However I do worry how accurate the top left corner of the graph is.
I would of course be interested in knowing whether there is later (definitive) evidence that contradicts George Brown’s paper, but the conclusion that very few radial are all this is needed seem startling compare to the advice of the seasoned DX community, which I am sure disheartens operators of ordinary means.
The following was a report to HARC on our recent talk on GB3MH and GB7MH. This was written by Alister, G3ZBU and we thank him for permission to reproduce it here.
Your hon sec went along to our friends in Crawley for their monthly talk which was on the Ashdown Forest Group’s repeater GB3MH, which is located at a church in Turner’s Hill. This 2m repeater is co-located with a 70cm D-star repeater and they share the same aerial;
the aerial is a 2m/70cm co-linear cunningly disguised inside a flagpole. Power is about 5W ERP, and the aerial height is about 600ft above sea level, so it covers from the North Downs to the South Downs. Coverage is not so good to the West.
What is interesting about the 2m repeater is that it has an Internet link to the rest of the world. It can be used as a normal repeater for local contacts on 145.625MHz/145.025MHz, and it gereates a Morse ‘K’ between overs. However, if you have a DTMF keypad, you can make contacts all round the world using either Echolink or IRLP modes. To access an Echolink node, press star/asterisk/* before the node ID, e.g. to access an EchoTest node, press *9999.
Echolink nodes include mobile phones, so it is possible to access someone’s mobile phone. We had a demo where a phone runs an ‘app’ that has a soft PTT button on the touchscreen, so it makes the phone work in simplex mode. To use a phone, or computer for that matter, you have to send off a copy of your licence and become registered.
IRLP is a mode where the two ends of the link must be done by amateur RF. One doesn’t need special registration because using RF sort of infers that one is licensed, or maybe a pirate…
When linked, the ‘K’ signal is replaced with a ‘courtesy tone’ ( 3 tones rising or falling in frequency ). If you hear the ‘K’ you know the linking has been undone! To unlink, key in 73.
Dave 2E0DDG accessed repeaters around the world but no-one replied! Possibly it was very early in the morning in ZL-land! He impressed upon us the operating procedures to be used: always listen first, then ask if the repeater can be linked. Of most importance is to leave a large pause between transmitting to allow all other repeaters to change from tx to rx. This can take up to 3 seconds if the end point is on the other side of the planet. Without a large pause, the repaters can end up in silly states e.g. locked in transmit!
The reader may like to read an article originally published in the Feb 2010 issue of RadCom by G4ULF.
The aerial had to be professionally erected, and considerable expense went into all the legalities of planning permission. It cost something like £2,000 to get the kit on the air. Most of the electronics was kindly donated. Annual running expenses are mainly for internet access and electricity, as the site rent is a ‘peppercorn’ rate.
The tone levels are not quite correct as the ‘K’ is far louder than the received audio. If enough people complain then this may get sorted out.
We meet on Wednesday evenings 20:00 to 22:00 approx. and Sunday mornings 11:00 to 13:30 approx. at our dedicated radio shack Hut 18 in Tilgate Forest Recreational Area. near K2
Tap here for a map.