G7FEK Restricted Space antenna tested at Sidmouth Observatory
Posted by Mike, G7FEK on Sat, 02 May 2009 12:15:55 +0100

On Saturday night 28th February my limited space antenna was tested at the Norman Lockyer Observatory Sidmouth to answer specific questions of the antenna.

The antenna is a multi band antenna for small gardens, smaller than a half size G5RV, but unlike the 1/2 size G5RV, it works on 80m. Click here for antenna details (PDF).



Iain, M1OOO, was still a bit sceptical that my antenna works well for DX on 80m, so he wanted to see it in the flesh and test it for himself. For a useful experiment, Iain had asked me to answer four specific questions of the antenna:
 
  1. Could it be built quickly and cheaply out of easily available parts ?
  2. Could it really be made to work without an ATU on 80m?
  3. Can it be used portable on a SOTA pole (fishing pole) ?
  4. Does it really have the DX performance claimed on 80m, or is it relying on a big antenna the other end ?
 
To answer the above questions, I suggested we built the antenna from scratch, on site, using his small SOTA fishing pole as the main support, and that we used it in the observatory grounds as a portable station.

To answer Q4 I arranged for Scott, VE3SCP, in Canada to sked with us that night. He uses exactly the same 46 x24ft antenna with just 100W, so this test will be a small G7FEK antenna and 100W at *BOTH* ends !!. If this works, it is by far the best way to answer Q4.

80m DX

To make a transatlantic contact with any station on 80m is a hit and miss affair, even with big full size antennas or arrays and a linear, so to schedule a specific station for a specific frequency and time, and expect to make a useful contact, is asking a lot of 80m at sunspot minimum - and then to do it with two small antennas at just 100W on both ends might be madness!!. In addition Iain had asked for the test to be done *without* the ATU so I had to use just the two simple radials to keep VSWR below 1.5:1, rather than go for maximum efficiency with 4 or 6 elevated radials. He gave me a real challenge!!.

Construction

The antenna was built by me in about 1/2 an hour while Iain sorted out the support. It was made exactly as per the published pdf but using "choc block", scraps of coax and random pieces of scrap wire. The only item I provided was the 24ft length of twin ribbon feeder. The radials were simply 1.5mm insulated wire, laid on the ground and raised at the ends only, a technique which I have found to achieve low VSWR easily with this antenna while keeping performance acceptable. Because the pole was bending a lot in the wind, we also used the vertical section as a guy, pinned down with a screwdriver, and added an extra piece of string as a second guy on the other side.



The antenna on a fishing pole, before an additional guy was added


Ugly lash-up construction - Here's the feed-point made with Choc Block with two radials


View from underneath along the fishing pole, showing how the feeder and an
additional length of string were used to guy the pole and stop it bending in the wind.

Iain raised the pole to full height and did the first MFJ analyser tests. VSWR was 1.2:1 on 3.85 and 7.3 MHz - a little high in frequency but bandwidth looked about right. So down it came for a small length adjustment, adding about 1 ft at each end. This brought resonance to 3.74 and 7.11 MHz and was the only setting up needed. A quick 59 contact was made to USA on 18MHz to test it on that band just before it closed, then a quick listen around 160m to the many stations we could hear. Iain was impressed with the MF receive performance of the antenna. Then we went for fish and chips.

That evening, despite poor band conditions on 80m, several UK contacts were made to warm things up before we went to observe the comet "Lulin" and Saturn's moons. Then it was back to the radio for our Sked.
 
The Sked
 
At about 23:58 Iain called Scott via the Exeter City UHF Echolink Gateway to give him my working frequency of 3,744 KHz.

By the time Iain had walked back into the dome to let me know, I was already talking to Scott on his first call, with a signal of 58 fading from 33 to 59+10 at the peak. The signal was about the same in both directions, but Scott was suffering from splatter from high power US stations. Even this did not stop us from working each other on various frequencies with reasonable ease until 01:15 when we finished. I think this contact has answered Q4 without any shadow of doubt !!.
 
So the first known transatlantic contact on 80m with this antenna at both ends was a big success. The antenna has also proved itself to be both repeatable and portable.

Iain is now making up this antenna properly to give him more DX options and bands when working portable. I knew I'd never get my feeder back !!.

-

Many thanks to all those stations that gave us useful signal reports on the test day. We will provide special QSL cards direct to those that contact us at: M1OOO, PO Box 2965, Calne, SN11 8WQ, England.

For more information about the Norman Lockyer Observatory at Sidmouth, visit:
www.normanlockyer.org.





Comment by 'Mike, G7FEK' on 1 Jun 2010 - comment
“Hi Peter, I am in the process of moving house so I cannot test and publish a 10m modification right now. Take a look at Juko's 30m mod, shown in the current pdf document on my website. It is similar to the mod required for 30m, but for 10m make the shorted stub only appx 2.5 meters along from the vertical section, not 5.5meters as Juko says. This will make a tuned stub for 28MHz instead of 10MHz. This is quite complicated to get exactly right and you will need an analyser to find the exact length needed but it should be about 2.2 to 2.5m in length and open at the end near to the vertical feeder just like the picture. You need to use 450 ohm twin to make the stub. I hope this helps. All the best,   Mike, G7FEK.”

Comment by 'M6PAV - Peter' on 1 Jun 2010 - comment
“hi mike i built this antenna and was totally amazed at how it works. Ive had many a contact into the north states and canada in the past 2 weeks, and all on 80mts. i was woondering if you had the 10 metre`s mod available? if so could you send it to my private e-mail please.. many thanks Peter M6PAV”

Comment by 'Mike G7FEK.' on 23 Jul 2009 - comment
“David, I use a 9m fibreglass fishing pole with the top section removed. For this test we used a 7m pole inserted into a metal pole at the base giving an extra 50cm of height.”

Comment by 'David M6WOW' on 20 Jun 2009 - comment
“Do you use a Spider beam pole, Mike? I put my version up on my SOTABeam pole when I first built it but it wasn't tall enough. I'm buying a FT101ZD so will need to use it with your antenna at chimney height and partnered with a 12 metre pole. I shall build the VK5JST analyser kit first. M0XLE and I haven't managed to get the antenna tuned correctly yet due to no analyser to hand. I'll get there. ”

Comment by 'Mike G7FEK' on 14 Apr 2009 - comment
“Andre, let me know how you get on with your build. My antenna is down at the moment due to a fractured pole.”

Comment by 'Andre, 2E0SKI' on 30 Mar 2009 - comment
“I was thinking about adding a choke to the coax feed-line somewhere. Now I have the answer! Funnily, I was also wondering if the coax would function like a counterpoise :-)”

Comment by 'Mike G7FEK' on 15 Mar 2009 - comment
“I should add that a simple choke made from winding 22ft of the RG58 coax on a 4 inch PVC drainpipe was added near the radio end and about 50ft down the coax from the antenna. When using this antenna with such a simple counterpoise, this choke is essential to ensure that the RF return path is via the counterpoise and not via the radio!. Placing it about 50ft down the coax near the radio end allows the coax itself to contribute to the grounding effect by working like an additional counterpoise. This addition has also proved very effective with my own installation at home.”