The half square antenna
During the last couple of days I've been making a half square antenna for 30 metres. I had a listen on it today and it does seem to work as well as they say they should. I was listening on 40 metres and some Russian stations did seem to be quite loud and clear, and on 17 metres, even though the antenna was miles away from resonance a station in the US was the best and strongest I've heard for years, maybe be was blowing smoke with a killowatt, I don't know.
Now my question before I go out and buy some more wire. Would it be possible to make a smaller version for 20 metres, have it suspended within the 30 metre antenna and use the same feedpoint and feeder and would it work properly ?
Can someone remind me, well, it's a bit late now really because I've made the antenna, but when cutting the wire using the formula's 492/F Mhz and 240/ F Mhz should the lengths be increased or reduced by 5% if the wire is plastic covered.? My excuse for forgetting is old age.
I'm going to have a listen on 30 metres in a few minutes I hope the Chinese radar's gone away.
Originally Posted by G0GQK
W4NRL (now SK ) did quite a bit of work on the half square antenna. Rather than reinventing the wheel, it's probably best to get signed up with his site and read his excellent articles.
However.....based on the TCC* factor for any radio station's performance, I'd go out and buy as much wire as you can, anyway.
*total copper content
"A minute of measurement trumps a decade of debate."
Mel, when a wire is covered with insulation, it reduces the velocity factor thereby making the wire electrically longer. PVC insulation results in a velocity factor of 0.96, therefore a PVC insulated wire needs to be 96% of the length of a bare wire to give the same resonant frequency. So you need to shorten it a bit - by 4%. But, it's safer to start close to the required bare wire length and trim unless you know exactly what the velocity factor of your wire is.
Now my mistakes travel at the speed of light!:cool:
Pete, that's too simplistic. It depends on the insulation thickness as well as the dielectric constant of the insulation. So not all PVC covered wires have the same velocity factor.
Originally Posted by M3KXZ
Thanks Pete and Steve,
I have all the required information on the half square, made it to the specifications, and its good. The only "fly in the ointment" if I might be so bold, is that the SWR is about 1:5 or 6 rather than the specified near to 50 ohms of 1:1 but put through the ATU I can pull it down. Apparently the higher the antenna is raised the lower the SWR but I'm at the maximum of 24 feet above earth.
Thanks for the reminder about the VF, I'm getting old and the "vast knowlege" I have is now leaking so I just couldn't remember, lengthen or shorten. What I will do is to shorten the verticals and the top by a little and see if that brings down the SWR, but its not really a problem.
I compared it to my R5 when "tuned" to 20 metres and of course its not as good as the vertical, but it works. Even though its made for 30 metres it works well on 40 metres, and if I had enough energy in a morning I'd get up early and try and catch a VK. Then I'd know it was good !
Kind regards, Mel G0GQK
Back again, what is the opinion of those who know of these things ?
If I made a 20 metre half square to nest inside the 30 metre half square, fed by the same coax, would the 30 metre horizontal wire cause a problem for the radiation ability of the 20 metre antenna.? Probably the quick answer is "Dunno, just try it.