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Homebrew Helix Questions


In the quest for some comparative analysis of my existing s-band helix
(commercial) I set about this weekend attempting to build my own. Regular
readers will be aware that my ability to build antennas which function
better than a dummy load with a large reactive component may realise that my
attention span has lapsed somewhat. I seem to have forgotten that I am
incapable of building anything more than a wire dipole.

However that aside I'd be interested on some comments on my technique
(constructive please!) and where I'm going wrong...

I built 2 helices (one LHCP, one RHCP) based on Jerry's [K5OE] 16 turn helix
dimensions, identical except:

(a) 21 turns;
(b) I used 0.25" conducting tape which is usually used for sticking to
windows for security;
(c) I used an 8" x 4" piece of PCB as the reflector.

The basic dimensions are 1" turn spacing, helix diameter 1.4", frequency

My comparative antennas were a 20 turn commercial RHCP helix and a 10cm
piece of wire ('random wire') stuffed into an N socket.

My signal source was a modified (TX opened up) IC-T81 transmitting on low
power (~0.2 W) on 480MHz, fed into a V/U/L triplexer with the V & U ports
going to dummy loads and the L port going to a scanner antenna. I was using
the 5th harmonic.

The receiver is an AR-5000, with an uncalibrated S meter.

Maximum achievable S points with scanner antenna on TX:
S7 20 turn commercial RHCP
S5 21 turn homebrew RHCP & LHCP
S3 random wire

Maximum achievable S points with homebrew RHCP on TX:
S9 20 turn commercial RHCP
S7 21 turn homebrew LHCP
S5 random wire

I found the homebrew antennas had a very well defined forward lobe, in
exactly the direction you'd expect. The commercial antenna was nowhere near
as well defined.

On the plus side I get pretty consistent 2 S point (12dB) gain over the
random wire, but unfortunately I also get a 2 S point loss over the
commercial antenna.

Also there doesn't seem to be much loss in using the homebrew RHCP pointing
to the homebrew LHCP.

So any comments on where I'm going wrong would be appreciated. Some ideas:

o Maybe the conductive strip isn't conductive enough (DC resistance seems
OK, but maybe skin effect could be a problem).

o Although the first 180 degrees is very close to the reflector, the
conductive surface area against the reflector ('shim') is very low: should I
add some 'shim'?

o Is the self adhesive strip conductor too wide and too thin?

73 Howard G6LVB

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