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M0TCF
24th Jun 2015, 22:40
Most of my time, I tend only to use my VHF gear mobile/portable, mainly due to the constraints of an HF antenna on the roof of my van i.e. they don't like it up there and get bent, broken or come off! Anyway, ages ago I found an interesting 20m band antenna project and purchased the relevant items to put it together, then like all good things it got stuffed on a shelf when something more important got thrown at me! So it was only over the last week or so that having caught up on all the 'other' more important things SHMBO thought I should be doing I finally got down to making it all. In real terms it should've took me about an hour, but not being massively experienced in these things I decided to take it easy, one stage at a time and enjoy it. Over a couple of afternoons I managed to get it all together and having stuck it on the MFJ analyser was more than please when it checked at with an SW of 1:1.5, with a bit more tweaking I was able to get this down to 1:1.3 in the middle of the band, good enough for me so I sealed it all down and closed the lid on the box :)

Tonight, I decided it was a nice enough evening to give my gear a go - so off I set for the sea front car park at Exmouth on the east cost of Devon. Setting up my 10m fishing pole with the 'new' antenna took only a few minutes and then I was in the cab with the matcher box by the window and the rig next to it on the dashboard. On with the power, and a tune around, the rig boomed into life and lo, there was Europe and his mates coming in 5 by 9+10, go so the band was open but would anybody hear me?? After the usual checks I tried my luck and got a response from UA6NT Gennady, from Krasnaya Polayna, Rostovskaya obl, Russia! A great first hit, for a first time portable/QRP attempt! I continued to trawl the band and called a few times, knowing my 5 watts was making it out there, but also wondering if anybody was listening??? A while later I got a reply back from IK3RIY, working from Lido Island near Venice. initially he had a hard time copying me but all of a sudden the conditions changed and my signal report rose from a 3/3 to 5/5! I explained my working conditions and Marco shared his opinion of my home made antenna with me - despite my best efforts I don't think he understood it was designed to be vertical and didn't need radials........ Having promised my wife I would be back at a reasonable hour I decided to call it a day having proved the antenna worked - I now have some plans to see if I can develop a multi-band version, however I am not going to buy a 20m long fishing pole, the 10m one I used this evening drew a fair bit of attention!

All in all, a great little dxpedition for a very first attempt from my point of view; I can't think of a better way to spend an hour or so on a summers evening in the sun :)

M0LMK
25th Jun 2015, 07:44
Great results with 5W for an antenna with 1/2 missing ;)

Are the plans on line to share? I'm interested in the design.

Don't forget that you can use your activation towards claiming an award. :cool:

M0TCF
25th Jun 2015, 09:15
Matt,

Thanks for the reply. Hmm which award to go for??? Escaped the Shack or Lifes a Beach?? :)

With regards to the antenna design have a look here (http://www.mds975.co.uk/Content/G0KYA_EFHW_construction.pdf). Further reading indicates that work has been done on a multi band design using traps, I want to explore multiple switched winding's on a single core, but using different lengths of wire as matched radiators on the same pole.

A few construction notes. Instead of an expensive box from Maplin I went to Tool Station and purchased a 12 way plastic 'knock out box' for about £1.20. Leave a reasonable length of wire spare at the ends of the primary and secondary winding's, esp if you use fairly thick wire like I did. Make the winding's reasonably tight, but not so much as you can't slide them around the toroid ring. The fella who wrote the instructions was obviously very clever - however he wasn't that good at actually writing instructions for people to follow! To avoid any confusion, I broke the paragraphs down into single lines, editing as necessary! At one point the instructions seem contradictory - (whilst it might be obvious to some, it wasn't to me) but the reference to the coax 'tuning stub' center and outer braid means both from the same end and not opposites i.e. you connect the center to one point, having stripped back enough insulation to allow you to feather & twist the outer braid wires together to connect to the other specified point. T200-2 toroids are available over the net, and are, I think, colour coded to some extent, anyway the one I used was coated with a deep red, cost about £7.00 - I am pretty sure you can get them cheaper than that, but there really are only a very few suppliers here in the UK. Whilst tuning, we discovered that gentle winding of the antenna wire ( great stuff, cut from an old military 150 footer), with evenly spaced turns brought the SWR down even further!

G0DZB
25th Jun 2015, 10:19
Fixed it for you:
"Over a couple of afternoons I managed to get it all together and having stuck it on the MFJ analyser was more than please when it checked out with an SWR of 1.5:1, with a bit more tweaking I was able to get this down to 1.3:1 in the middle of the band, good enough for me so I sealed it all down and closed the lid on the box."

M0LMK
26th Jun 2015, 10:29
Ah, yes, the End Fed Half Wave.

One of our local club members built one and brought it along to a special event station. The only problem was the coax radiated more signal than the antenna and wiped out the other HF bands that the station was working on! Even the very tight bandpass filters we were running didn't stop it!

I've read elsewhere that the EFHW can be quite a potent antenna when set up correctly and most designs I've seen have used a 1:1 choke in the coax somewhere between the radio and the antenna (1/2 wave maybe?).

G0DZB
26th Jun 2015, 10:52
An other alternative to the tuned transformer for matching is a simple L-match. I have a T200-2 wound with thick copper wire, and a small variable C that I put together for feeding a high-Z antenna. I also put a ferrite choke between the radio and the L-match.

Here's a page someone has already written :-) http://www.hamuniverse.com/qrplmatch.html

M0TCF
26th Jun 2015, 15:49
Matt,

Thanks for the info regarding the issues around the end -fed dipole I'll have a look at the harmonics pattern, and as a precaution will put an rf choke in the system too - I'm going to put it to the test again soon on a club HF field day, so that may well be the proof in the pudding hi hi!!!

Peter, thanks for the link, some interesting reading there.