"Carl J. Hixon" wrote in message news:jo_gd.85932$hj.69553@fed1read07...
I saw the following antenna being made by some ultralight guys in San Diego.
I was wondering what to think of this. I am new to antennas and just
starting down the path to hamdom. Anything I need to know before starting
to experiment with making one? I don't want to fry my only radio, a Vertex
Standard VX-700.

Below are the plans for an antenna which does not require a ground plane.

This antenna is a 1/2 wave vertical dipole made out of RG-58/U coax and a
Radio shack FM radio antenna. The beauty of this design is that it is cheap,
simple, and easy to make with readily available materials. The radiator
should be 3', 9 1/2 inches long for 123.450 operation, but only the top foot
or two needs to be the telescoping FM radio antenna. Or, you can make the
entire radiator length out of regular antenna wire if you wish. The only
critical thing is the total length.

Start out with enough coax so that you will have enough to run up to the
radio from your antenna mounting location.

First, solder the telescoping antenna to the middle conductor on the coax.
Make sure to strip back the shield far enough so that it does not short to
the "radiator" or telescoping part of the antenna. Remember, you can make
the "radiator" entirely out of the center conductor on the coax, so if you
are going to do that, you must make sure the dimensions are correct. Next,
on the other end of the coax, I cut off a piece of coax that is going to be
the shorting stub. In all cases, I always make the lengths to the long side.
This way the antenna can be trimmed in to the desired frequency with a SWR
bridge.

To connect the shorting stub to the feedline, carefully remove a small
section of insulation from the center conductor (no more than 1/4 of an
inch). Solder and tape the inner conductor, then join and solder the
shields. To insure proper shielding, split a spare piece of braid lengthwise
on top of the shield connection, and carefully solder the shield together.
(Rather than disecting the coax cable and soldering, we used a BNC
connector for the shorting stub. We then trimed back for tuning and ended
up with almost no wire needed on the shorting stub. VERY SHORT.)

Don't forget to put a BNC connector on the radio end.

Finally, hang up your antenna in an open space, and tune it in with a SWR
meter. I have found that adjusting the length of the shorting stub has the
most affect in adjusting for minimum SWR but be careful. If you cut too much
off you will have to re-do it.

Radiator Length: 3' 9.5"
Radiator to shorting stub: 12.9"
Shorting Stub: 1.9" (imagine no BNC connector when
measuring)

QUESTIONS:

(1) So how do I calculate lengths for other frequencies? ie 121.5MHz,
146.73MHZ, 123.0 MHz

For a half wave it's 5616 ÷ frequency in Mhz = Radiator length in
inches.

For a quarter wave it's 2808 ÷ freq.

(2) Any chance of damaging my Vertex Standard VX-700 if I start playing
around with this?

No. It protects itself, don't worry about it.

(3) Where can I finde a descent but cheap SWR meter?

There is great beauty in the KISS theory. Don't bother with an swr
bridge for no-brainers like this with an HT, it's overkill for any
purposes, close enough is close enough. Cobble together two antennas,
one for the air band and one for the 2M ham band, swap antennas
depending on the band in use and be done with it which is easy with
BNC connectors.

The simplest cheapest type of VHF antenna is the ¼ wave coaxial "no
radials vertical" and is made from RG-58. Calculate the length of the
¼ wave radiator minus one inch to roughly correct for the velocity
factor and strip off the plastic jacket to that length plus ½ inch.
Don't strip the insulation off the center conductor. Then horse the
braided shield back down over the jacket of the coax toward the radio
end. Lay a length of nylon cord alongside the radiator which extends
some number of feet beyond the upper end of the radiator. Seal the
whole assembly with heat shrink tubing, plug the top end of it with
some Goop and hang it up somewhere handy. This is not a particularly
portable "shack on the belt" sort of solution but it's essentially
bulletproof.

I strung one of these up in the airframe of my Quicksilver MX in my
ultralight flying days and it didn't just work, it drove me batty, at
altitude I heard every weekender UNICOM chatterbox for a couple
hundred miles around and I quickly retreated back to the rubber duck
to get some "attenuation".

Thanks,
Carl

w3rv