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Old December 6th 05, 05:26 PM posted to rec.radio.amateur.antenna
Steve Nosko
 
Posts: n/a
Default dipole and balun question

At the risk of starting something... Owen, I didn't see this point in your
initial post. I've read a few of your posts and you seem to give pretty
good explanations.



I thought the answer would be that a wideband Balun (my assumption that the
OP is referring to this type) of the type commonly used (Ferrite core &
windings) is not recommended for a multi-band antenna where the impedances
vary widely. These baluns, I thought, are suitable only for impedances near
their design center. For example, a 1:1 balun works well in a 50-50 ohm,
50-75 ohm or 75-75 or 50-33 ohm system. A 4:1 (or perhaps more correctly
1:4) works ok with 50-200. 50-300, 50-133.

Using a 1:1 between tuner and antenna (at either end of the feed line) when
the antenna impedance may be 200 or 1000 ohms on some band, causes grief for
the balun in the form of poor efficiency and possibly a burned up balun -
because the balun is operating in a system impedance much different than
this design center. These baluns are not suitable for a wide range of
impedances.



Do I need to change my brand of drink, or have I got a reasonable
understanding?



Owen,

It is my understanding that (ignoring patterns and focusing initially on
the antenna) using a dipole which is not resonant on one of the harmonically
related ham bands ( I believe the original Windom-circa 1945 and G5RV are in
this category) avoids the problem of matching to the very high impedance at
the even overtones (a 40M dipole is two half waves on 20M and a high Z).
Yes, using ladder line vs. coax helps with this greatly, but then you have
this age old problem of the bal to unbal issue.

This, of course, somewhat ignores feed line loss, but this is a first
approximation focusing on the antenna initially - and I realize that
ignoring feed line loss is not what you do when looking at the WHOLE system.

.. I'll have to read your referenced article more fully (next). (:-)





73, Steve, K,9.D;C'I



P.S. For the beginners, A signal has a harmonic. A resonant object
(antenna, crystal, resonator, filter, etc) has an overtone - which is a
resonance, typically but not necessarily, harmonically related - frequently
slightly off the harmonic due to strays.



"Owen Duffy" wrote in message
...
On Mon, 5 Dec 2005 19:32:03 -0600, "RB"
wrote:


Was just wondering if coax feed with balun at centerpoint would be better
than ladderline feed with balun at tuner output (and no, my tuner doesn't
have an internal balun).


I can only assume that if you are still asking that question 6 hours
after I wrote you a response, that you didn't read / understand the
response and the referenced article!

If that was too hard to understand, the short answer is NO. If you
want to understand why, go back and read my earlier posting and the
article.

Owen
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