To: lancair.list@olsusa.com
Date: Fri, 12 Feb 1999 10:21:52 -0600
Subject: re: antenna efficiency
Message-ID: <19990212.102451.9726.0.rcusbob@juno.com>
From: Robert E Grossmann <rcusbob@juno.com>
Mime-Version: 1.0

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Dear Stu and Fellow Lancair Builders,

Undoubtedly the best place for the COM antenna in a 320 (and kin) is the
vertical stab. Only if you have a carbon fiber tail as in the IV would
this not be true. Nevertheless, there are other locations that will also
work well, such as against one side of the fuselage behind the baggage
area, where a lot of ELT antennas are placed.

In my thirty years of HAM radio experience, much of which has involved
antenna work, I have found that when there is a problem such as you
describe there is almost always a fault in the antenna itself, rather
than in the location. The first question is, what kind of antenna are you
using and where is it placed. One of the most important things about
antennas is that they are TWO-POLE devices. That is why in composite
aircraft you must use a dipole and both poles must be the right length to
tune to the band we use, namely the aircraft band. You could use a
vertical as we do on aluminum planes where the plane itself serves as the
second pole, but then you would have to place a large mat of screen or
metal in or on the plane to serve as this second pole. Such a metal plate
is the "ground plane." I have used ground planes seventy feet across for
vertical Ham antennas with great results. The ground plane for a COM
antenna needs to be about 30" in radius. Thus while we see transponder
antennas with a ground plane placed in composite aircraft because at
their high frequency the antenna poles need to be much shorter, in the
range of 6", this is not a useful plan for a COM antenna. Now, do you
have a dipole, or what are you trying to use?

Second question, have you checked that all the connections are absolutely
solid with no corrosion. The only realy way to make sure of that is to
solder all wire connections and use BNC connectors at the breaks. Even
then, one should start trouble-shooting by checking continuity from the
radio to the antenna poles. We hook up an ohm-meter to each end of the
line, for example the center conductor at the radio and the end of the
antenna pole connected to it. It should read zero ohms. (The radio itself
must be disconnected from the line for these tests because it will have
some connection between the two poles it is feeding. If there is a Balun
in the antenna end of the connection, that too will have a connection
between the two poles. In the Balun case the antenna and the line to it
must be tested separately. A simple way to do this is remove the radio
from its tray and connect the ohm-meter probe into the proper terminal at
the back of the tray. This is easily determined since the antenna fitting
is the round recepticle with a male center probe that fits into a female
fitting on the radio. I make this connection to the ohm-meter with a
miniature aligator clip available at Radio Shack.)  If there is
resistance, it must be eliminated by checking at places between the two
ends to find exactly where the resistance is. At the same time that the
center conductor and the shield must both have a perfect connection
between the radio and their particular pole of the antenna, there must be
no connection whatsoever between the center conductor and its pole of the
antenna and the shield and its pole. Are you sitll with me? You must have
a good connection between each pole of the antenna and its connection to
the radio.

Third question: is the antenna itself properly designed and installed. I
know this is a bear for those who are not radio nuts. Indeed, thinking of
this makes me think that you might be well to contact a local HAM Radio
organization to see if they have someone who can help you. Nevertheless,
it still can be checked by anyone who is willing to work at it. During
the CB craze all kinds of SWR or Standing Wave Ratio meters were sold and
used by truck drivers and anyone else. So there may even be one lying
around. The SWR is a fairly accurate measure of the efficiency of an
antenna. It is the measure of voltage on the antenna, both going to the
antenna, and being reflected back by the antenna. Radio Shack still sells
SWR meters and they are useful for all radio work. The best place to
measure SWR is at the antenna. Never the less it is usually easier to
measure it at the feed-point, that is the point at which the radio
connects to the feed line to the antenna.  One simple connects the SWR
meter between the line and the radio, or between the line and the
antenna, turns on the transmitter and switches the SWR meter switch
between "forward" and "reflected" power to see it the antenna is working
properly. If it is, there wll be a large difference between the two, if
not, they will be pretty much the same. It is pretty dramatic to see a
meter go full scale on forward power (there is a knob to set the meter a
full scale on forward setting), and then see it drop to zero on
reflected. When it does, you know the antenna is working perfectly, AND
that any radio reciever worth a hoot will hear you loud and clear.

Well, that's the beginning. I think it would be good to get a friendly
Ham involved. They are complete nuts and like nothing better than helping
solve someone's radio problem (as you can see, I wrote all this gibberish
knowing that you might not understand a word, but I would like to help).
Actually, we are not that far apart. I am near Mason City, IA, and fly a
Mooney out of Forest City (FCW) while I am building a Lancair. I have
tested my COM antenna with the vertical stab on a stepladder and can
communicate well from there. So, if you wish to fly in, we have a 4000'
runway, I'd be happy to take a look.

Bob Grossmann