Carl Pattinson wrote:
> Its a while now but our VOR is a copper tape dipole mounted
> horizontally in the bottom of the rear fuselage - about 18" behind
> the rear baggage bulkhead. Not sure of the dimensions but would guess
> a 1/4 or 1/2 wavelength.
It should be a 1/2 wavelength.
For what it is worth: I have in the bottom of the rear fuselage also a
1/2 wave dipole. Not straight, but in a V. It is flat in the vertical
plane, but a V in the horizontal plane. The middle of the V points to
the rear, both tips to the front.
The V serves a couple of purposes: A straight dipole is not sensitive in
the direction of the tips if it would be straight. So, if you would put
it parallel to the length axis of the ship, it would be sensitive to the
sides, but not to the front and rear of the ship. If you would turn it
90 degrees, it wouldn't fit as the fuselage is not wide enough for a
straight 1/2 wave dipole. By shaping it in a V, it becomes sensitive for
all directions (with a slight bias to the front of the ship) and it will
fit nicely in the rear fuselage.
Keep in mind though that if you do this, the dimensions of the dipole
will change. I used an antenna analyzer to tune it properly for the
VOR-band.
Sure I know that the front of the ship generates some noise, but this
noise will get into the radio anyway. I do not expect humans and the
engine to be serious obstacles for the radio waves. Both are quite small
compared to the wave length.
An antenna in the wing tip could be a nice idea, but maintenance of the
coax cable (they do age) would be troublesome, and wing tip strobes (as
I have) will spoil the advantage, as strobes produces lots of noise. A
V-shaped antenna in the middle of the ship will null out noise from the
strobes as they are symmetrical to the antenna from that position.
Have not been flying with it, but if I reach that stage, I will let you
know whether it works or not.
Frans
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