Scott,
A couple of questions:
1) Where are you measuring the impedance - at the
feedpoint or through a length of coax?
2) If through a length of coax, how long and what type?
3) Did you cross over the conductors of the parallel metal
strip feedline on the 40 meter dual driven element to get
the 180 deg phase shift?
4) Are you sure the metal strips that form the parallel
feedline on the 40 meter dual driven element aren't
shorted together?
If you are measuring thru a feedline, my guess is
that you have a short somewhere on the parallel
strip transmission line. The periodic variation of the
impedance would be consistent with a short circuit
being rotated in phase around the smith chart by
a ~200' long feedline. The 5:1 (versus infinite)
VSWR would be due to the finite loss of the
transmission line (~1.7dB one-way loss). If the
short is far enough from the balun (~1/4 wave),
it will get reflected back to the balun as an
open. This would explain why you might get good
VSWR on 15 meters (the driven elements of the
40M4 are resonant here) where the distance
between the driven elements on the 40M4 is
probably getting close to a 1/4 wave on 15
meters. You can check for this by disconnecting
the metal strip feeders from the balun so you
are only using 1 driven element. If there is a short
on the other end of the metal strip line, you should
see a dip in VSWR at around 7 MHz when you
disconnect it from the feedpoint.
I doubt the 10 meter antenna is having much
affect.
73 de Mike, W4EF.....................
----- Original Message -----
From: "Scott R." <w4pa@yahoo.com>
To: <towertalk@contesting.com>
Sent: Monday, July 07, 2003 12:32 PM
Subject: [TowerTalk] Is my 5 el 10 detuning my 4 el 40? ?
> Put your heads together and help K4JNY and myself come up with a
> solution to this problem:
>
> KLM 4 el 40m beam at 120 feet, 5 el 10 meter beam on a 28' boom at 125
> feet on the same mast. 45G tower guyed with Phillystran.
>
> The KLM antenna is (was) new in box and was built with all WØUN mods
> and all DX Engineering hardware. It's in the air, and exhibiting some
> weird behavior.
>
> According to our MFJ 259B antenna analyzer, the SWR is about 5:1 from
> 1.8 to 20 MHz. At 20 MHz, SWR slowly drops to flat, remains flat to
> about 22 MHz, then rises back up to where it was.
>
Are you talking about the 40 meter antenna or the 15
>
Every 800 kHz to 1 MHz of spectrum, we can see the antenna impedance
> dip from about 400 ohms down to about 12 ohms and then rise back again.
> SWR does not vary when this happens - indicates the same value of
> about 5:1, until the dip at 20 to 22 MHz. We tried the same procedure
> with an HF radio and got the same results.
>
> The antenna does have some pattern, we can tell by rotating it and
> listening to stations on 40 in different locations.
>
> What we have tried so far: Swapped out the 4:1 balun with a different
> 4:1, then a 1:1 just for the heck of it. No change was noted. Nothing
> appears to be disconnected or shorted and we were very meticulous when
> assembling the antenna to not miss anything, use Penetrox on all
> connections and NyLok hardware, etc. Different feedlines were tried -
> all act the same.
>
> The only thing we can think of is that the 10 meter beam is severely
> detuning/interacting with the 4 el 40. We tried shorting the 10 meter
> coax out to see if it had any effect on the 40m - no effect (which
> makes us wonder if it is, in fact, this.)
>
> What we plan to try next:
> Rotating the 10m beam 90 degrees off the 40m beam.
> If that doesn't work, pull the 10m beam down off the tower.
>
> Are we crazy? Or is there something we haven't thought of? We've
> been puzzling this over for the past two days and have decided we just
> "don't get it". Any help?
>
> Scott W4PA
>
>
> =====
> Always been the radio's child
> Quick-to-smile precious baby
> Search the dial and electrify
>
> --Widespread Panic "Radio Child"
>
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