Whilst thumbing through the fuselage manual I glanced at the fin mounted
antenna details. I don't know how these are performing in actuality but
the upper arm might be a bit sensitive to noise currents from any static
discharge from the upper fin edge.
It struck me also that if the antenna elements were bonded to the closeout
face then they would be close against the lossy polystyrene core which would
certainly damp the antenna. Bonded to the thin glass hinge mounting area would
be better in this respect.
Adding a second full length element in parallel to and joined to the extreme
ends of the dipole will turn it into a folded dipole which is inherently less
sensitive to noise and beneficially increases the bandwidth slightly.
Bandwidth is determined by the length/thickness ratio, so wide tapes work
better than thin wire.
The terminal impedance will rise to around 300 ohms but that can be dealt with
simply with a half wave coax balun instead of the ferrite balun. Tuning is
easily accomplished by moving the shorting links inwards or outwards equally
until the aerial is resonant at the geometric band centre.
Alternatively the feed point could be moved downwards so it becomes
a match to 50 ohm coax, I have a magic formula somewhere to work out the
precise numbers. This would have the benefit that the coax can be led off
perpendicularly straight up the fuselage and be further removed from the
stabiliser torque tube and balancer. Because the half wave dipole centre
point is some way up the fin the coax feed will have to run close to the lower
element which is detuning. Ideally it should leave perpendicular to the antenna
for least disturbance of the far field pattern and least excitation of surface
wave currents on the outer braid.
Finally it is unlikely that tuning the antenna with a Standing Wave meter will
be that meaningful. Aircraft band transmitters have wide bandwidth tuned output
stages which often means that they don't look much like a pure 50 ohm source
even at centre frequency. Also if they are that badly mismatched then their
internal protection circuit will shut down the power level again shifting the
output impedance. This is why a low VSWR reading can be obtained and the aerial
can remain off resonance. The net result is the transmit power output remains
low and the receive signal level is attenuated. You can dead short coax and by
choosing the right point to measure VSWR still obtain 1:1, move it a half wave
along and it will should show infinity.
Tuning is better accomplished by measuring the field strength with a separate
test antenna and diode detector. Alternatively insert a 50 ohm non reactive
resistor between the fin antenna terminals and measure the received field
strength with a diode detector excited by an second external generator
transmitting into an antenna located a couple of wavelengths away. (An Icom
handheld for instance) By moving the second antenna up or down with respect to
the fin antenna centreline a fair measure of directivity can be estimated.
Ideally the fin antenna should show a slight downward tilt in it's radiation
pattern somethimg which can be achieved by off centre feeding or with an
inverted 1/4 wave monopole and groundplane.
Either way is a acceptable because all passive antennas are reciprocal.
--
Graham Edward Laucht
Birmingham UK
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