I'll take that as a yes (to my question, not the OP's).
73, Steve, K9DCI

You've got a lot of interesting stuff on your site. Too bad I don't have
the time to read it all.



"Owen Duffy" wrote in message
...
On Tue, 6 Dec 2005 11:26:40 -0600, "Steve Nosko"
wrote:

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.

The answer was in the graphs in the referenced article. If a reader
would be satisfied by a simple answer, the answer is no, it isn't
practical. But if they wanted to know why it wasn't practical, the
information and a better solution were both in that article which was
generally about the issue of whether the traditional wisdom of a
dipole, one half wave long at the lowest operating frequency, fed with
coax and "loaded up" with an ATU would be a good multiband antenna.

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.

Steve, it is certainly a challenge to implement a balun that works
over the range of impedances that would be encountered in a 140'
dipole from 1.8 to 30 MHz.

To my mind, the very high coax loss dismisses the solution before you
visit the balun problem.

Without detailed analysis, my guess is that the 140' dipole will be
far enough off resonance to cause high coax loss on 80m, there is the
slimmest chance that it would fall into 30m as a 3 x 1/2 waves, and it
is unlikely to have low coax loss on any other band. Hence my
statement that it is unlikely to have acceptable coax loss on any
band, save possibly one.


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.

A design and implementation challenge, indeed.




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). (:-)

Ok, that is where the answer to the question about coax feed to a
multi band dipole lays. The two graphs show (1, 1a) show in an instant
that the problem is in the feedline, and other graphs show feedline
solutions that are better.

Nevertheless, the balun problem remains if you have an unbalanced ATU.

I think the balun design can't be adequately addressed by considering
just passive component between an unbalanced generator and balanced /
floating load. A more complete answer is found by modelling the entire
system (inc real balun and feedline) as part of the antenna system in
NEC. The unbalanced shunt L and the value of the series R and L
introduced by real baluns will have different effects depending on the
length of the feeder, where it is earthed, how it is coupled to the
nominal radiator etc. This approach is interesting in considering
whether the balun should be on the tx or ant side of the ATU.

I ran a range of models following my G5RV article at
http://www.vk1od.net/G5RV/index.htm exploring baluns in that context,
and finding a solution that reduces common mode feedline current
significantly is challenging. The closest I came was a W2DU style
choke balun that was four times (or more) the traditional lengths, and
would reduce common mode feeline current to less than 10% of the
nominal radiator current. It is a work in progress!

I lament the unavailability of quality commercial ATUs that deliver
uncompromised balanced output. (Yes, I know MFJ have recently released
a product, but I did say "quality").

Owen
--