Thanks for that Fred. I intend to give a try, if only for the fun of it, but
who knows, the answer may lie therein.
Please have a look at the attached sketch to see if I have interpreted
properly. If so, can you answer a couple of questions for me?
1. Can we use aluminium rod/tube instead of the copper wire. Using a
mechanical joint instead of a soldered one?
2. Would the various metal items in the rear of the fuselage degrade the
performance of the antenna if it is mounted behind the 'D' panel as
illustrated?
3. Would there be any mutual effect with the Transponder ariel quite close
by?
Best regards. Bryan
----- Original Message -----
From: "Fred Fillinger" <n3eu@comcast.net>
Subject: *** SPAM *** Europa-List: Monopole Antenna for Troubleshooting
>
> 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.
>
>
>
|