----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
To: "Scott W3TX" <superberthaguy@adelphia.net>; "Roger (K8RI)"
<K8RI-on-TowerTalk@tm.net>; "joe johnson" <joe.k3rr@gmail.com>;
<towertalk@contesting.com>
Sent: Thursday, July 05, 2007 8:25 AM
Subject: Re: [TowerTalk] Gamma-matched 4-square
> At 06:56 AM 7/5/2007, Scott W3TX wrote:
>>John ON4UN addresses this question in "Low Band DXing".
>>
>>His advice was to avoid shunt fed elements in a driven vertical array.
>
> But what was the basis for his recommendation?
>
> Is there some practical detail that creates a huge problem?
>
> or is it just the simplicity of getting the right element current
> when there's no matching network at the feedpoint, and the "current
> at the end of the coax == current in the element"
>
> or it allows you to leverage the decades of experience with AM
> directional arrays
>
Fundamentally there is no reason why you can't phase shunt-fed
towers and get performance equal to series fed elements. I think
the problem is generally the matching/phasing network
design. With insulated elements, it's fairly easy to measure
element self-impedance and also to verify element current in order
to verify that the matching/phasing networks are setup properly.
With shunt fed elements you have an extra matching/phasing
network to contend with (i.e. the gamma match arm), so AFAIK
self-impedance has to be modeled (perhaps there is a straighforward
way to measure it, but I am not aware of how you would do that).
I think you can measure the element current in a shunt fed
design, but your current transformer needs to enclose the tower
and shunt feed in order to get a measurement of the net current
(if your transformers were phase matched, you might be able to
make multiple measurements and add them vectorially).
It would be interesting if someone developed the technique as
there are a lot of contest stations out there with multiple towers
that could take advantage of it (it would make a good QEX
article).
FWIW, back in 80's KS8S and AD8P tried phasing Dean's 120ft
Rohn 45 tower with a 130ft self-supporting pyrod tower (both of
them shunt-fed) on 160 meters. I was never impressed that this
array worked any better than the Rohn 45 tower by itself, however.
K6SE (SK) up in Palmdale may have done it successfully (as I
recall he phased his two towers together on 160 meters), but I
don't know for sure. I think Goose W8AV who is a broadcast
engineer with lots of experience setting up phased arrays even
came out and tweaked on it a while, but it's not clear to me that
even Goose's magic touch did much good.
73, Mike W4EF............................................
>
> Sometimes, it pays to revisit the underlying assumptions behind the
> traditional wisdom.
>
>
> Jim, W6RMK
>
>
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