[Antennas] balun and dipole question

Thu Dec 8 10:34:02 EST 2005

Dear RB,
[quote]
Some questions about a 140' dipole fed with ladderline from an unbalanced
tuner to work 160-10m:
1. Does it matter if the balun is placed on the tuner output, or at the
dipole center feedpoint?
2. Which type of balun (voltage/current) would be best for
centerfeedpoint
use?
4. What ratio balun would be optimal for either application?
[/quote]
#1 - Yes - it matters, but your selection of possible answers omits the one 
with the most promise, in my opinion. A balun placed at the center feed 
point on an all-band antenna is challenged by a range of impedances with 
which it cannot cope.
Therefore, that is not a good solution. However, it becomes more tractable 
if you limit operation to those bands where you have a current loop at the 
feedpoint, because then you could use a 1:1 current balun.
A balun placed at the tuner output feeding an antenna and transmission line 
that is essentially a random impedance over a wide range of frequencies is 
severely challenged. At some frequencies it would be better as a 4:1, at 
other frequencies a 1:4 would be better. There would be some frequencies at 
which 1:1 might be better. I won't even venture an opinion as to whether it 
would be better as a voltage balun or a current balun, because I doubt that 
it matters with so many concerns over the impedance mis-match.
A balun placed at the tuner output is of no advantage, so far as I can see. 
The balun should probably be on the other side of the tuner where it sees a 
better match. See the Dick Measures tuner design in Feb 1990 QST.
#2 For those circumstances where the balun sees a low to nominal impedance, 
I would say the current balance is more effective.
#3. You forgot #3.
#4 The ratios of commercially available baluns are generally limited to 1:1 
and 4:1, although there are other designs. It has been my experience that 
unless the antenna is used on limited range of frequencies and is looking 
into its nominal impedance, its performance is a serious compromise. In 
other words, baluns are no longer very effective for their intended purposes 
when confronted with a wide range of load impedances.
In addition to the dismal situation indicated above, most balun applications 
in ham radio are for the purpose of impeding the flow of RF currents on the 
outer shield of coax - impedance transformation was not the reason for their 
use.
The only baluns that I know of that could be effective in your situation 
might be:
a) the baluns by W2DU of the type consisting of ferrite beads with the coax 
running through them. They are available from most suppliers of amateur 
radio antenna components.
b) baluns made by coiling several turns of coax into coil of several inches 
in diameter and tying it into that shape with cable ties.
73 de W9DMK - Bob Lay, Dahlgren, VA
http://www.qsl.net/w9dmk
w9dmk at crosslink.net 