Monday, 30 November 2015

The triple (half wave) dipole for 4m.

Triple (half wave) wire dipole.
Before I moved into Drogheda (1995) I was living out in the country side where room for antennas was not a problem, I was away from neighbours so did not worry as to what I had up in the air. But in 1995 I moved back into Drogheda and was faced with a postage stamp back yard, it was tiny… I did manage to get a 40 foot tower up, much to the delight of my neighbours and the local council, but planning permission was granted.
However with the tower up I could only put up a few small antennas on it and could not use my 4 element 50 Mhz beam….
I decided to experiment around with wire dipoles, but these do not have any gain. I measured the distance between the back wall and the house and I realised it was about three times the length of a dipole for 6m.
I cut approximately the length of wire I needed and made the dipole, put it up and bingo!!! Yes of course the SWR was too high…
However while running up and down the frequency with an Analyzer I seen that the SWR was flat on 48 Mhz with an impedance of approx. 50 Ohms. So with a little careful snipping I got the dipole up to where I wanted it on 6m with a flat SWR.
So with that thought I made a 70 Mhz version recently to centre around 70.200 Mhz, the measurement I used was 468/frq(mhz)x12 which equals the length of a 4 dipole in inches, now all I had to do was multiply the answer by 3 to get the triple dipole length, add a few inches each side for fine tuning.

This is not an exact measurement but will put you close to where you want to be on the spectrum.
One thing I have noticed is the bandwidth is very narrow, only about 200 Khz or so, but on 4m that is not an issue, so on theory then… a half wave dipole for a frequency is 0dBd, (3dBi), so then the triple should be 6dBd (9dBi), well that’s what theory says… I cannot lay claim to that as I have no way of measuring the gain but it will be better than a half wave dipole and most importantly you will not have to buy a Balun…

This of course can be scaled up or down for any frequency.

I hope to use mine next year when I buy a transverter for 4m, but in the meantime I have it plugged into my SDR Dongle and ready to listen in on 4m SSB when the band opens in 2016… 

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