> 1. Can we use aluminium rod/tube instead of the copper wire. Using a
> mechanical joint instead of a soldered one?
Mechanical joints eventually corrode, unless attention is paid to
metal fasteners. Else, it will not alter performance.
> 3. Would there be any mutual effect with the Transponder
> ariel quite close by?
The VHF antenna sees the xpondr antenna as a tiny thingy of no
consequence. However, the coax to the xpondr antenna can certainly
annoy a nearby VHF antenna. Further, is your coax from the VHF
antenna running parallel to one of the dipole legs? That's worst case
for trashing a radiating pattern. Must run perpendicular from the
center >= 2 feet, then make any sharp turn.
> 2. Would the various metal items in the rear of the fuselage
degrade...
Malicious devils. The texts tend to show the energy radiating from
the base of the element (the center of a dipole, if such). If not
merely for illustration, I think I can dig that. In the point in an
RF cycle where voltage is highest, it can't xmit squat at the tip, and
therefore does not radiate from there. Voltage and current -- the
heavy lifter -- are 90-deg out of phase, and 90-deg is 1/4 wavelength.
What tuning is all about. Anything near the tip is bad, because that
I guess is where the local "e-field" is at (voltage). Or the near
field, of no use to us, but don't upset it. Besides whacking a bunch
of dB by detuning, we're also creating a crude directional antenna,
meaning the radiation field may not be where ATC is at. Note that
it's easier to visualize xmitting; reception is the precise
reciprocal.
> ...the performance of the antenna if it is mounted
> behind the 'D' panel as illustrated?
Attachments must not come thru the list, but in that location there
can be long perpendicular metal nearby. If on the left side, your
body is in the way for communicating forward. Carbon-based life forms
absorb; trees thoroughly trash GPS. Better might be further back.
The longitudinal stuff will be more in the dipole's center, and put up
with a few dB loss on the curvature of the elements.
I don't see anywhere in the fuselage that's good, though others come
out OK (really just huge, dynamic range of our comm boxes). Mine was
thus "micro'd" in the trailing edge of the rudder. I might suffer
some static buildup, though.
I went upstairs yesterday in the AA-5 to check worst case conditions.
Comm #2 was bought off eBay for $200, connected to a bent whip
antenna; its attachment screws are too rusty to remove; I personally
aligned the box w/o a service manual. :-) At 2,500 AGL, put that
big air carrier field behind me and just keep going, and going. I
don't mean listening to tower; he's not goin' away until well into
Pennsylvania. It's the guys on the ground. Still strong
transmissions must be top mounted antennas. Weak ones, or no response
heard is maybe bottom-mount, or that huge Convention Center bldg, for
the ones at the end of the landing roll on 24L. Or ask Tower for
permission to talk to JetLine 236. A sexy little $50 million RJ with
a foil dipole; that's just so wrong. :-) At 55 miles, it was losing
educational value, and I didn't have all that much fuel aboard!
Something has to be radically wrong, if no joy at like 25 miles.
Reg,
Fred F.
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