Whilst building G-BXTD I checked out the dipole installation using a
hand-held radio and the VSWR readings were good. When I came to fly the
bird, using the same radio, ground stations using the middle range of
frequencies had difficulty receiving my transmissions. I checked the aerial
with a VSWR meter again and found that the output was indeed dreadful in the
mid range. After discussions with Ferg (of this forum) I put this down to
interference from the wires running to a fin mounted strobe, which I had
fitted subsequently to my original VSWR test. I disconnected the dipole and
constructed an aerial/ground-plane within the rear fuselage, which performs
adequately, but the readings are not as good as the original ones from the
dipole.
Conclusion - the dipole is great but highly susceptible to degradation.
----- Original Message -----
From: "Brian and Pat Tarmar" <freefolk@screaming.net>
Subject: Europa-List: Europa VHF Antenna Problems
<freefolk@screaming.net>
>
> During the test flying programme of G-OBJT it became apparent that
although the radio worked perfectly at the lower end of the VHF band, when
we selected frequencies above 125.0 ground stations were having difficulty
in receiving us. I borrowed a Bird watt meter today and came up with some
rather bizarre results. The radio (Walter Dittel) delivered 5 1/2 watts
throughout the range into a dummy load (50 ohms), but as soon as the Europa
half wave dipole aerial was used very high readings were obtained at either
end of the spectrum (11 watts at 118.0 and 9 watts at 136.0) but down to 3
watts between 126.0 and 130.0. Can anyone explain this and suggest a cure.
I used a quality VSWR meter during production of the antenna and obtained
reasonable gain figures.
>
> Brian Tarmar - G-OBJT Trigear - test flying complete and awaiting a full
permit( subject to a working radio!)
>
>
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