Paul,

The SWR should be as low as possible. Ideally 1:1 over the entire
aero-band. Which is pretty impossible for a 19 MHz span around 125
MHz.
If the SWR is high, an increasing part of the RF power sent to the
antenna is reflected back to the transmitter, and back again.
Depending on the quality and the length of the coaxial cable, more or
less power is lost in it each time the RF energy runs through the
cable, with less RF energy radiated by the antenna as a result.
Practically, considering the relative good quality of RG-58 coax at
these frequencies, and the short length of the cable between the
transceiver and the antenna, an SWR of, say 2:1 at some frequencies is
far from dramatic.

You should first plot the curve of SWR versus frequency for the
antenna as is. Put the antenna on some insulated pole, with no
metallic parts near the antenna. Connect the SWR bridge between the
antenna and the RF generator (an aero-handheld XCVR will do the job).
Measure the SWR for each Mhz. You will get some kind of a 'U'-shaped
curve, with the SWR increasing below and above the antenna's resonant
frequency.
If the lowest SWR-frequency is higher than 123.4, your antenna is too
short.
If the dip is on a lower frequency, you antenna must be clipped.
Cut no more than 10 mm at once, to start.
If you cut too much, getting a resonant frequency, say at 120 MHZ,
your antenna can still be used over the entire band, and nobody will
notice...

I used to do some curveplotting and there is a difference between a
glider on the ground and in flight, obviously due to ground proximity.
SWR stands for 'Standing Wave Ratio'. Broadly speaking, if the
caracteristic impedance of the output stage of the transmitter, the
coaxial cable, the connectors and the antenna are all the same (52 ohm
in most cases), you get a 'travelling wave' from the transmitter to
the antenna, and no part of the wave is reflected.
If on the other hand there is a impedance mismatch, the vectorial sum
of the incoming and reflected travelling waves results in a 'Standing
Wave'. The ratio between them is what your SWR-bridge measures and
calculates.

Does this answer your question?

Willy VINKEN - Cirrus OO-ZNY - Ham callsign: ON5WV
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On Wed, 24 Sep 2008 19:07:15 GMT, "Paul Remde" wrote:

Hi,

I'm hoping that someone on r.a.s. that is an expert in radio antennas and
SWR meters can help me out. I just brought in a new line of 5/8 wave
antennas, but the manufacturer can't tell me what length to make them. I
want to cut them for 123.4 MHz so they will work well at glider frequencies.
However, theoretically calculated antenna lengths don't usually agree with
ideal lengths because of things like "end effects" and coils.

I have an SWR meter, but I have not used it yet. The instructions are
non-intuitive.

My guess is that I will need to connect a radio - transmitting on 123.4 MHz
and measure and record the SWR value of the antenna. Then I'll need to cut
a bit of the antenna off and see if the SWR value improves. I am completely
ignorant so I don't know if low or high SWR value is better. At some point
in the process the SWR value should level off. I'd hate to continue on
until the SWR value goes the other way - because that would be a waste of a
perfectly good antenna.

Any suggestions? Is there an easier way to do this?

I appreciate any help you can offer.

Soon I will offer some very nice antennas with precisely optimized lengths.

Thanks,

Paul Remde
Cumulus Soaring, Inc.
http://www.cumulus-soaring.com