Hi John,

Good points. Thanks for the note. Thank you to everyone who sent the many
helpful notes.

The antennas I'm trying to pre-tune for my customers are magnet base
antennas (for use on a car) and a base station antenna for mounting to an RV
or truck. The base station antenna has 4 small antennas that spread out at
the base of the main antenna which probably act as the ground plane - so I
won't worry about a ground plane for that one. They will all ship with
coaxial cable and a BNC connector for connection to a handheld radio or
Dittel base station radio. I'm selling a quarter wave antenna and a 5/8
wave antenna.

I agree that 122.5 is directly in the middle of the aircraft radio frequency
band, but I think that most customers would prefer that the antenna be tuned
for 123.4 (between 123.3 and 123.5) - which would still give excellent
response at the nearby 122.8, 122.9 and 123.0 common airport frequencies -
and all other aircraft band frequencies. I would think that most car to
glider communication would be at 123.3 or 123.5.

The testing is necessary because the antennas are nowhere near the correct
length. The manufacturer of the 1/4 antenna gives a very nice length table
that I will use. But for the 5/8 wave antenna they don't have any
recommendations. I have talked to the manufacturer directly.

There have been many very helpful responses to my query. I appreciate them
all very much. A common note was that a coil is required on the 5/8 wave
antenna. I have that under control. I have all the right parts.

I think that if I can make the magnet base antennas work well on my car they
will work fine on any car. I understand that every car will be a little
different, but I believe they will be close enough.

I think I am now armed with enough knowledge to tune the antennas.

This newsgroup is a fantastic connection to a community of very
knowledgeable and helpful glider pilots!

Thank you,

Paul Remde


"ContestID67" wrote in message
...
I think that what Paul is asking is..."I have a bunch of antennas
ready to sell. I want to pre-tune them to 123.4 so that my customers
can install them right away for optimal glider performance."
Hopefully this is correct and it is an admirable goal.

Paul - You may not be able to get there from here. Remember that
tuning for VSWR is from the connector at the radio end to the antenna
which includes the coax. I looked at your catalog and see that most
of your antennas (antennae?) are sold without coax. Thus if you tune/
trim the antenna with a piece of coax you had lying around, odds are
that it won't be right for your customer.

For those antenna that come with coax, you would assume that the
manufacturer cut the coax for the "right" length for the middle of the
aircraft band (you'd need to ask them). The middle appears to be
122.5Mhz (108-137mhz) which seems pretty darn close to what you want
to tune for. Then you have to worry that your customers will cut the
factory length of coax to make a nice neat installation.

So if you want to experiment, using a transmission source (your radio
presumably), a VSWR meter, and the manufactured antenna/coax combo,
you should be able to trim the antenna for maximum performance (as
close to 1:1 as possible). Let's say you were able to go from a VSWR
of 1.7:1 to 1.6:1. How much further can you now transmit? I have no
clue. I'm the type of EE called a "bit pusher", not an RF jock. Your
goal is making your tranmission go further, but It is nearly
impossible to predict how much further you will get by retuning short
of renting some very expensive facilities).

So you have to ask yourself, is your re-tuning in your office really
worth it to your customers? My answer would be no. My $0.02.

- John DeRosa