Antenna and Coax Length

John Scott wrote:
My glider has a dipole antenna built into the rear edge of the rudder. How
do I tune the VSWR for this antenna. Cutting a chunk off is not possible.

John

Rear edge? Odd, most gliders have them in the leading edge of the
vertical stabilizer.

Anyway, the bottom line is that you cannot change the length of the
antenna. We are all [retty much in the same boat. It would be great
if we could retune the antenna (change the length) for 123.3 (or
whatever frequency you use). This would optimize our power out.

However, this is not practical so we live with antennas that are
probably optimized only for the middle of the 118.000 - 136.975Mhz
range. What is the middle? 127.4875Mhz. This is at least pretty
close to where we usually transmit.

John Scott wrote:
My glider has a dipole antenna built into the rear edge of the rudder. How
do I tune the VSWR for this antenna. Cutting a chunk off is not possible.

What glider? Some of the old ones did have a tuning adjustment. First,
measure the VSWR at the output from the radio: less than 2 - you're
golden; over 3, it's worth some effort to discover the problem.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly
* "Transponders in Sailplanes" http://tinyurl.com/y739x4
* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org

Generally, close is good enough. As Eric says, under 2:1 is probably fine.
A number of ham operators have impedance bridges or you can use a reflected
power meter. However, before cussing at your antenna, be sure the meter is
designed for the frequency range and is of good quality. The cb meters will
not give you reliable readings here.

If your swr is bad, or you are having communication problems, the connector
is more likely the problem than the antenna. I have helped out on a number
of installations and in almost every case the antenna failure is in one of
the connectors.

Remember, also, that if you place a metal object or wire in the close
proximity of the antenna, it may affect the resonance.

Colin

COLIN LAMB wrote:
Generally, close is good enough. As Eric says, under 2:1 is probably fine.
A number of ham operators have impedance bridges or you can use a reflected
power meter. However, before cussing at your antenna, be sure the meter is
designed for the frequency range and is of good quality. The cb meters will
not give you reliable readings here.

It's measuring a ratio, not an absolute quantity, so they work just
fine. Sure, the response is lower, but you calibrate the meter each time
you use it, and that compensates for the response difference as well as
the output power of the transmitter. That was the conclusion of a long
thread on this a year or two ago. Radio Shack has them cheap enough, as
do other places, so every pilot can own one, or know a pilot that does.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly
* "Transponders in Sailplanes" http://tinyurl.com/y739x4
* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org

Although inexpensive bridges calibrated for one frequency or band can be
useful at a different frequency for comparative standing wave ratio
measurement, I have found enough unreliable instruments to not trust them.
If you compare them in the aviation band with a known meter, that is one
thing - but trusting them off the shelf may give you a false reading. Many
of the meters must zero the reading into a 50 ohm load at the frequency in
question. That zero can vary with frequency.

You can generally check the usefulness of the meter by doing two things.
First, test the meter with a known 50 ohm dummy load at the frequency in
question. It should read zero reflected power. Note the forward power or
ratio. Then, reverse the leads. The same readings should be observed with
the meter readings reversed. If not, the meter is not going to tell you the
truth.

Some excellent meters, like the Bird wattmeter, will work fine as an swr
indicator far outside the design frequency, even though the power indication
is off.

Colin

It is an APIS. The antenna is in the trailing edge of the rudder due to the
fuselage, vertical stab, and most of the rudder being carbon fiber.

I do have an inexpensive VSWR meter. I also have a 50 ohm dummy load (left
over from thin wire ethernet days....). I'll check my meter with that.
I haven't installed my Microair radio in the glider yet. I did a check
after installing the wiring and connector using my handheld ICOM and the
VSWR was a little less than 2 then. Looks like I should be okay.

John

Hi John:

Remember that your dummy load may not have the power rating of your
Microair. I think your Microair is rated at 3 watts carrier and your dummy
load may only be 1 watt. That will still work - but keep the test
transmissions short - enough to get a useful reading. 10 second or so
transmissions should be fine.

Colin