2 ELEMENT VERTICAL BEAM
After the tests earlier on in the year with the 2 element quad and the 2 element which used the antenna on the car as the driven element, it was time to continue with the vertical beam trials.
After finding the folding bases (see previous entries) and getting a couple more fishing rods (from Decathlon, the 7 metre long ones), and then doing all the calculations, it was time to see if it would all work.
The theory for today was a 2 element half wave for the 10m (28 MHz), using a driven element and reflector. The reason I chose 10m was because the 7m long fishing rods might have been too short for 15m, the band I was more interested in! The driven element was designed for resonance on 28.500 MHz and the reflector 0.96 x frequency of the driver, in this case 27.385 MHz. Lengths on paper should have been 2.58 metres for each leg of the driven element and 2.70 metres for each leg of the reflector.
I also drew up 2.49m for a first director and 2.46m for a second director, for another test another day.
Element spacing is 2.07 metres
Thanks to Doctor Dave HK1A as usual for all his input.
Setup and tuning
Prior to setup I had prepared about 11m of RG58U with a PL259 on on one end and bare tinned wire at the other. This went into a block connector.
I cut the wire for making the antenna as mentioned above and stripped back a bit of insulation at one end, for fitting in the block connector.
I set up a stand for one antenna and then attached the top reflector leg to the top part of one of the fishing rods. I then attached connected this to the centre core of the coax, and then connected the braid to the other half of the antenna, which continued down to the bottom.
The rod was not fully extended and the bottom of the antenna was about 75cm from the concrete dock floor.
I connected the PL259 and checked the curve on the Rigexpert AA-30 Antenna analyser. It was low in frequency, as was to be expected, so I tuned it by cutting a few centimetres off either end.
I took the rod down, disconnected the coax, and put the antenna away horizontal, far away in order to not affect the driver, which I set up next.
I repeated the operation and set up the driver. Once tuned, I made a quick test QSO with Hamdi 7Z1TT in Jeddah and he gave me a strong 59 report.
I then took a look round the band and heard some south american stations, so took out my compass to check where 240º was, and set up the 2 element antenna looking that way. The centre wires on the reflector were connected together.
The final SWR curve for the 2 element antenna looked like this:
On the air testing
I installed my usual 10 metre quarter wave whip to use as a comparison antenna, using a coax switch for quick change from one to the other. All contacts were made with 100w from the Kenwood TS850S.
First QSO was with a station in North Chile who gave me 55 on the beam and was unable to copy me on the 1/4 wave.
Second QSO with OA4Q, 800 km south of Lima, was 57 on the beam and 53 on the 1/4 wave.
Subsequent contacts in Argentina and Brasil gave average differences of 2 "s" units (12 dB) in favour of the beam.
I then started hearing stations in the USA and moved the beam heading to 300 degrees approx, by picking up the reflector element and moving it. To my surprise, all stations contacted agreed there to be no difference in the signal, or if there was, it was slighty in the 1/4 waves favour.
Finally I contaced a polish station who confirmed a 12 dB difference in the beams favour.
In general the conclusion is that as expected the beam antenna works better than the 1/4 vertical by an average of 2 "s" units. In comparison, this is the average difference between running barefoot and running 400w, and for another time this will be an interesting test to try out.
A possible reason for the USA signals not being different might be that they were not arriving on a direct path and may have been coming in on a skewed path over south america. I did not think of it at the time and so did not try changing the beam heading.
In general it should be taken into consideration that:
- the angle at which the feedline comes away from the centre of the vertical dipole affects quite considerably the resonant frequency.
- the distance between the reference 1/4 wave and the beam should have been further to ensure no interacion however I had to make do with the 11m coax I had prepared. Results could be affected by this.
- Tests should be conducted over a much longer period of time to be conclusive.
- These antennas will work much better over salt water (as is this case) than normal ground.
For more background information and a lot of detail on the subject, visit team vertical at www.k2kw.com