shared these priceless pearls of wisdom:

-The jpole is long for a straight installation on a tail on 121.5 - that
-is correct. However, the 1/4 whip without a ground plane IS NOT 50
-ohms. Maybe an an 'L' match?

What in the hell are you talking about? A 1/4 wave whip WITH a flat ground
plane isn't 50 ohms, either. More like 32 ohms. As you bend the ground plane
down, it matches 50 ohms at about 30° off horizontal. What do you intend to
L-match to? Certainly not a quarter-wave piece of wire without a counterpoise
of some sort.


-
-I did not limit the dipole to 90 degrees!. I said it could be installed
-with up to 90 degrees offset in the center. Actually more but there is
-some signal cancellation. At less than 90 degrees there is no signal
-concellation. A dipole that is bent in the middle is called a Vee
-Dipole and exhibits a more omni directional pattern.

You really need to take a few classes on this stuff. Signal cancellation is not
the correct term. Pattern modification perhaps.


-
-The J-Pole can be installed with a bend but that would also change the
-impedance, but not excessively. I am only putting out some ideas.
-EZ-NEC will model the antenna before you build it.

You can put out all the ideas you wish. However, it would be best if you had
some actual experience with aircraft antennas before coming in here and spouting
stuff of which you have no clue. Eznec is a fairly good antenna modeler; why
don't you foreshorten and/or bend that jpole to something that will fit inside
of a 3' high structure and then tell us how to match it over a 17% bandwidth.
You **do** realize that the jpole won't match decently over a 5% bandwidth,
don't you? Then the match reduces the bandwidth still further.



-
-On a metal plane I would opt for the 1/4 wave whip.
-On a composite plane the ground plane is gone so basically the feed
-coax becomes the ground. This will work but a matching "L' network may
-be required to get the antenna to feed at 50 ohms or so. 1.5 to 1 SWR
-or less. The radiation pattern will be distorted.

Oh, Lord. The old sleeve dipole trick. Please, sir, take your L-match over to
rec.radio.amateur and rachetjaw with the fuddies over there about HF antennas.
I have no idea what you mean about the radiation pattern being distorted.
Distorted relative to WHAT?



-
-A dipole can be made that is much smaller than the full length by
-adding inductance on each side. It will still perform well and be
-around 50 ohms. A dipole that is only 30% the normal half wave will
-radiate efficiently.

Efficiently relative to a strand of wet spaghetti in a copper septic tank,
perhaps. You ever mess around with Reg Edwards calculations over in the UK?



So on 121.5 that would be approximately
-(3.7 * .30) total length in feet. The inductance may be added at the
-feed point. Each side would be (3.7 * .30)/2.
-
-Short dipoles are quite popular in the ham realm especially at lower
-frequencies. Several commercial vendors market shortened dipoles.

Don'cha just love ham radio "experts" in HF coming in here telling us how to do
stuff at VHF in aircraft?

Jim


-Rob NV7F




Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com