On Tuesday, 28 June 2011 11:21:50 UTC+10, Wayne wrote:
I have seen two commercial antennas advertised that are each 43 feet tall,
and have no traps/stubs, etc.
There appears to be a 4:1 balun at the feedpoint. Mentioned, is the
requirement to have a tuner at the driving end of the feedline, and max
VSWRs in the 4:1 range.

Several manufacturers supplied and recommended a 4:1 voltage balun at the feed point of their 43' unloaded vertical. It would seem driven by one seller, and a mob of sheep following.

If you look on eHam at the evaluations of the configuration, you will find that most reviewers thought it a great antenna system.

I commented on that in some web articles and many postings in online forums over years. My thoughts in seeking an understanding of how it "works" are in the article at http://www.vk1od.net/balun/application/Z5.html .

I should note that after a particularly vigorous discussion in one forum a year of two back, one of the sellers changed his recommendation to an unun and advised existing owners how to modify their balun to an unun. The method was quite the same as I previously documented at http://vk1od.net/antenna/multibandunloadedvertical/ .

The whole saga reminds us that anything "works", and flexible sellers remain poised to supply to the current fashion.

Owen

On 02/07/2011 2:40, Wayne wrote:
....
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Thanks for the interesting links, Owen. The 4:1 balun did look unusual
to me, and EZNEC didn't help reveal the magic involved. As I mentioned

It is that it is a voltage balun that is the main issue. The
transformation ratio is questionable, but not nearly as clear an issue
as being a voltage balun between a coaxial feedline and ground mounted
vertical where one terminal of each should be close to the same
(~ground) potential.

There is often good argument for a current choke or current balun
feeding such an antenna. If there is good argument for a 4:1
transformation ratio, then there is good argument for a 4:1 current
balun. I suspect that the changed recommendation to a 4:1 unun is
because that can be achieved with a simple reconfiguration of the
existing component, a low cost customer support solution.

Seller's are caught between supplying product for which there is demand
by the ham buyers, and one that can be explained rationally. One balun
seller explained that ham buyers want 4:1 voltage baluns for multiband
dipoles because when they replace them with 1:1 current baluns, they
have problems with flashovers, and they can't match up on some bands. It
is true that voltage baluns can facilitate matching on some extreme
loads, at the expense of system efficiency, similar effect to some
extent to the subject case.

NEC2 does not model conductors passing into the ground well, so you are
unlikely to build a good model.

I toyed with creating a model in NEC4, but figured that those who
wouldn't see the folly of the AS/Z5 balun idea would not be informed by
NEC models anyway.

before, I have challenging antenna restrictions (not restrictive
covenants), and will look some more at a shorter version for higher
frequencies.

Just because the 4:1 voltage balun seems irrational, that doesn't
dismiss the unloaded vertical, nor benefit of a current balun.

But, if you are primarily interested in higher bands, a shorter vertical
with elevated radials will likely be more efficient and less a cloud
warmer pattern wise.

Owen