I've done this many times, for freqs other than VHF comm. Real
slap-dash affairs with cardboard, wire, and duck tape. A few dB of
antenna abuse is no problem if the avionics sitting on the right seat
hot-wired to the cig lighter can still be adequately tested. Below is
a more careful effort, and it should isolate antenna vs. receiver
problems. It will be useful also to see if a 1/4-wave bent whip,
pointed up or down, will work somewhere back in the fuselage. If it
does, don't buy an actual aircraft antenna. Take foam blocks and gobs
of RTV back there and permanently install!
Go to a home improvement place or a good hdwe and buy solid copper
wire; if insulated we can remove for soldering in a few places. #6
wire works real well; #8 OK; even #10, but #6 is more wideband,
roughly doubly better. The stiffness will help keep it seated in the
right-seat pax area.
Cut 5 lengths of the stuff to 21.5 inches each. A soldering pencil on
the vertical element will show what a heat sink does, so hit the end
of that good with a blue-flame torch and then melt a blob of solder
onto it. The center conductor of RG-58 coax will go there. A blob
you can solder to, not just a coating. Check it by glomping with
pliers to see if it pops off easily. That means a cold joint.
Take 1/2" or 3/4" wood stock, and cut a 1.5"-2" block. Drill a
perpendicular hole in the center. At each corner, drill (by eye is
OK) 45-deg holes for the ground plane legs. Angled inward toward the
center hole. What we want is a ground plane of 4 equally spaced legs,
angled 45-deg downward. 5-minute epoxy the legs in the block.
Take a few feet of RG-58 terminated in a male BNC. "Pre-tin" the
center conductor and solder it to the blob of solder of the prepared
element. Feed this element into the center hole of the block and
epoxy in place. To electrically connect the ground plane legs, strip
insulation off about 12" of solid wire. Or strip common zip cord. On
the bottom of the block, scrape the base of the 4 fence posts of
copper for good electrical bond. Wind this wire tightly several times
on each leg, and continue similarly around the fence posts. This wire
is where the shield of the coax is soldered. Note, if to be
permanently installed as mentioned, these junctions must be soldered.
Blob silicone RTV to secure the center conductor, but not the solder
joint, so usage of the contraption doesn't break the connection.
Straighten all elements and adjust for about 45-deg on the ground
plane legs. At a halfway point in the vertical element, bent it back
only as needed so it fits in the airplane. It affects radiation
pattern only, probably in a beneficial way.
Incidentally, the 45-deg droop of the legs takes an antenna with a
natural impedance of 37 ohms and makes it 50, which we want. However,
it doesn't do that across the full band of interest, 118-137 MHz. So
where a ground
plane on a monopole antenna is constructed as a flat ground plane,
90-deg to the vertical element, it doesn't matter much. This is true
also of a dipole, but there's no easy fix either.
However, if #6 wire, this antenna could show "zero reactance" from
about 120-130MHz, rather preferable in testing. When the receiver
looks down the coax, it wants to see 50 ohms of any kind of resistor.
A 50-ohm resistor which won't transmit squat, but merely get hot, will
do. It has no clue as to an actual resonant antenna, or a combination
of resistances. When reactance = 0, and SWR is minimal, virtually
all electrons are sent heading for the hills.
The remaining issue is interference with the antenna. An analyzer will
report almost nothing by introducing any metal in the vicinity of the
legs in the horizontal plane. There will be metal in the central
tunnel, an you'll be sitting next to it, to unknown effect on
radiation pattern, but hopefully not as bad as an installed antenna
allegedly not working so good. You can also freely transmit on it, as
there will be more radiation received by talking on a 3W handheld, due
to distance, or on a cell phone for an hour for that matter.
So all we need is to bring the comm receiver antenna coax out to a BNC
in the cockpit. Re-route the existing antenna to cockpit. BNC/RG58
patch cables are available cheap from allelectronics.com. Buy a
female-female BNC coupler (Radio Shack in the States) to mate either
your anorexic passenger or your recalcitrant dipole to the comm, and
go test fly. Doesn't take long to build, and I'd be curious what
results come back if someone so ventures. Can even be fun!
Reg,
Fred F.
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