Saturday, 31 January 2009
About two years ago I built a pair of EH antennas. I couldn’t really get either to work properly and they gathered dust in the back of the garage. In retrospect I should have used a choke balun near to the antenna. I moved house and couldn’t bring myself to throw them away, especially as I had bought a three-metre length of copper tube to build them that a cost of about £50.
Anyway, fast-forward two years and I thought I would have another go. I lent the small one to a friend to play with and set to on the bigger one.
I now have a 20m EH with two 20inch x 1.75 inch copper cylinders and a 10-turn tuning coil. It has a 10-turn choke 1 metre down from the antenna.
It is vertically mounted off a rafter in the loft and my other antennas are horizontal to cut down on cross talk/re-radiation effects. Does it work? Well, the 2.5:1 SWR bandwidth is just about 14-14.350MHz, the minimum SWR is about 1:1.4 and it hears almost as well as a dipole and a Windom, sometimes minus 1-2 S-points.
The noise level is lower too, which suggests little coax pick-up.
I have worked many stations, including Kazakstan and the recent Tunisian DXpedition with 59 reports - the latter on first try.
You can definitely see that fading is due to polarisation changes as one or other antenna improves as the other goes down. Does it work by Poynting Vector Synthesis? Don't know and may never know. Does it radiate from the coax? Don't think so (it has big choke near the feed point) and the fact that the field strength near the coax is less than my Windom and dipoles says something.
I have to admit that I am quite surprised. Earlier tests about two years ago were inconclusive, but I have learned that:
1. You need to earth the MFJ analyser to get reasonable readings.
2. You need the choke.
3. Make the tuning coil slightly longer than you think you need as you can always space out the turns to increase the frequency.
4. Spacing out the turns at the top of the tuning coil changes the resonant frequency, spacing out the bottom changes the SWR.
5. I did have it so that the maximum radiation according to the FS meter was the same as the minimum SWR, but this has changed slightly now that the antenna is on a different length of coax.
Does it work by Poynting Vector Synthesis? Probably not, but it does appear to work and the aim of this post was to encourage people toat least play with the design.
The noise level is lower too, which suggests little coax pick-up. I am only reporting what I am seeing. Build one according to the instructions and try for yourself. I was very sceptical and now I'm amazed.
For more details see http://www.eh-antenna.com/. You can get construction details at the EH Yahoo group (eh-antenna.yahoogroups.com)
Turns out that if you use a long multiband antenna as an inverted V it really changes the radiation pattern. The lobes contract quite badly and the radiation angle goes up dramatically. I lost about 7db east-west and the low-angle DX capability went through the floor.
An inverted V half-wave dipole doesn’t do this, so if you must have an inverted V for 20m, use a half-wave dipole, not a G5RV or OCF (Windom).
Save your inverted V G5RV or Windom for 80m where they seem to work quite well with high-angle radiation.
Above right: The red trace is a G5RV inverted V with the apex at 12m and the ends at three metres. The blue trace is a flat-top G5RV at 12m. These were produced with MMANA-GAL, an excellent program.
Thursday, 29 January 2009
Feel free to download them.
You can also look at the latest trans-terminator results on 3210kHz (October 2005).
Radcom published the results of the first round of tests in May 2005 - you can download it in PDF format (500kb)
This is the original presentation on Greyline and the studies given to the RSGB Convention in 2005 (20Mb download - PDF.
Download the "3Y0X Propagation Predictions to G v Reality" presentation given at the RSGB's HF convention in October 2006 in either Adobe Acrobat PDF or Powerpoint formats (both 5mb). While not strictly about greyline it does show the effect of morning/sunrise enhancements on 80m very well. (October 2006)
The Maldol gives you access to all of the HF bands, plus 6m, and apparently all without the need for an ATU. I wasn't terribly impressed, but any antenna is better than no antenna I guess.
Over the years I have experimented with magnetic loops, crossed field antennas and E-H verticals and have come to the conclusion that, for now at least, the magnetic loop seems to offer the best alternative to beams and dipoles if you have no space.
Unfortunately, some say the evidence is stacked against him and people have argued about the success or otherwise of his achievement for years.
Read Steve's feature from the December 2007 issue of QST on the work of the GB3SSS experiment (3Mb PDF).
IMD is usually held on the Saturday closest to Marconi's birthday, when amateur radio stations are established and operated from original historic sites, or nearby. These stations are known as the 'Award Stations'. This year's event is on Saturday 25th April 2009.
My wife called it a “geek’s weekend”. I can’t see the local tourist board adopting the title, but if you’ve ever been interested in technology and wondered how it developed, Cornwall is a great place to visit and find out.
Not only is this south-west corner of England the home of radio and satellite communications, but it’s also where the original world wide web was developed – back in the late 1800s.
Download the full feature in PDF format