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Hopperdhh(at)aol.com
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 Posted: Tue Jun 06, 2006 3:47 am Post subject: -- Baluns |
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Bob and others on the list,
It is my understanding that the purpose of the balun is mainly to prevent
the feed line from distorting the pattern of the antenna. That is, to keep the
antenna purely horizontal so that it will be less receptive to reflections
from mountains and large buildings for example. When waves reflect off of
things they tend to lose the original polarity (horizontal or vertical). The
balun isolates the antenna from the feed line and makes it more like a pure
horizontal dipole.
Having said all of that, I have a dipole attached to the canopy of my RV-7A
with NO balun (because I didn't have one handy) and it appears to work just
fine. Perhaps it would work even better with the balun!
By the way, I experimented with the length using my MFJ-259B Antenna
Analyzer looking for the lowest SWR across the 108 to 118 MHz nav band. Start with
the antenna long, say cut for 100 MHz and trim to move the SWR higher in
frequency. The reason it is necessary to experiment here is that the antenna is
not in the clear, and also it is not running straight.
The length of an ideal dipole antenna (in feet) is approximately
468/Freq(MHz) from any of the ARRL Handbooks.
Hope this helps,
Dan Hopper K9WEK
In a message dated 6/4/2006 3:48:34 P.M. Eastern Daylight Time,
nuckollsr(at)cox.net writes:
http://n-lemma.com/calcs/dipole/balun.htm
http://en.wikipedia.org/wiki/Dipole_antenna
We used to wire our VOR cat-whiskers on the
single engined Cessnas with a balun back in the
60' but gave it up after a series of experiments
showed no perceivable difference between antennas
with baluns and antennas that simply attached
the center conductor to one leg of the dipole
and coax braid to the other leg.
Some folks believe in slipping ferrite toroidal
cores over the coax like this figure from the
AeroElectric Connection:
http://www.aeroelectric.com/Pictures/Antenna/Dipole_Antenna.pdf
However, subsequent to publishing that figure I've
done some testing in the lab and found that the
ferrite cores offer no observable improvement in
VOR receiver or GS receiver performance by their
use.
Therefore, it's my recommendation that you forgo
the use of either balun or ferrite cores and simply
attach the legs of your dipole to the center conductor
and shield of the coax.
Bob . . .
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noelloveys(at)yahoo.ca
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| Posted: Wed Jun 07, 2006 7:16 pm Post subject: -- Baluns |
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Balun.... Balanced to unbalanced.. Coax is unbalanced line the dipole
antennas are in fact balanced antennas. The balun is a device which allows
you to use an unbalanced line to feed a balanced antenna.
Will your dipole work better with a balun ???? The only thing I could say
is try it. You won't need a very big or heavy balun because the transmit
power on an aircraft is usually pretty low.... On the receive side of things
I doubt you will see any difference at all.
Don't forget that a horizontal dipole antenna has a directional factor to
it. This is the reason that most com radios have vertical antennas.
We used 300/F(mhz) to give an answer in metric.... A little easier to
remember.
Noel
[quote] --
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