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>From n5kb Tue May 18 12:19:57 1999
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From: Eric Gustafson <n7cl@mmsi.com>
To: towertalk@contesting.com
CC: wb1dog@hotmail.com
In-reply-to: <19990518000052.65329.qmail@hotmail.com> (wb1dog@hotmail.com)
Subject: Re: [TowerTalk] Feeding a multi band loop (cloud burner) with balanced
line
Reply-to: n7cl@mmsi.com
References: <19990518000052.65329.qmail@hotmail.com>
Hi Jim,
I think you are right to suspect the balun. If it is a "true"
W2DU, it can be shorted at the output (balanced) end. I have
found several in this state. The hardware in the top cap (screw
eyes, etc.) is designed to be a spark gap to the ground strap
that goes from the top hanger screw eye down to the coax shield
at the bottom. Sometimes their tolerances permit one or the
other of the output terminals to be shorted to the ground strap.
A DC conductivity check will always show a short to the coax
shield regardless of the state of the top hardware. I have also
found this type of balun with an internal "open" connection. But
this is much more rare.
I don't think you can see an actual SWR as high as 20:1 on bands
above 80 meters regardless of the feed method without a fault in
the system somewhere (shorted balun, shorted or open coax
connector, open connection in the balun or elsewhere, etc.). So
you should carefully check all of the system components and
connections.
The 1:1 bead sleeve balun is the right type to use for this
application. Another type that could be used is to roll up 8
turns or so of the coax on a piece of 5 inch PVC tubing long
enough to get a turn to turn spacing of 0.5 to 1 inch. Unless
you are getting a pathological impedance transformation due to an
unfortunate length of open line, I don't think you should see an
SWR much higher than 8:1 worst case. I have run 80 meter loops
and dipoles on all higher bands (up to 30 MHz?) through a 1:1
balun with no problems.
You should probably eliminate all system components except coax,
balun, and open line until you get the system sorted out. Then
add the protection devices, knife switch, loc (whatever that is)
, etc. back into the system one at a time.
I have a few questions about your system.
1. What kind of tuner are you using? What does LDG stand for?
Does it have continuously variable inductor and capacitors?
2. Why bother with the twinlead for such a short run?
3. What balancing device were you using when the loop was coax
fed? This antenna requires feedline decoupling regardless of
the apparent feed impedance. If you weren't using a balun of
some sort before, you may have radically changed the antenna
by disconnecting a major fraction of the radiator from the
loop when using balanced line. This could show up as a
radical change in feedpoint impedance.
4. What do you mean by grounding one side of the balun? What
are you actually grounding (coax shield, one of the balanced
output terminals)? What ae you actually grounding it to?
5. What are the actual dimensions of your loop? Is it fed on a
corner or in the middle of a side? How high above ground is
it? Do you know what its resonant frequency is? Is it
square, rectangular, triangular, circular, etc.?
73, Eric N7CL
>From: "James Apple" <wb1dog@hotmail.com>
To: <towertalk@contesting.com>
>Date: Mon, 17 May 1999 17:00:51 PDT
>
>I just upgraded my 80 meter full wave horizontal loop to use
>balanced line (450 ohm) for the first 50 feet then coax
>(bury-flex) for the last 20. At the transition I'm using a w2du
>1:1 balun. I expected this to be a upgrade for the system, less
>loss etc, but my tuners now only find a match only on 15 meters.
>When the antenna was feed with all coax I could get a good match
>on all bands. Even on 15 I seem to only get to about 3:1. I
>have tried both a LDG tuner and the internal tuner in a TS-570,
>they both just give up right away. I put a Autek RF-1 on it and
>it just reads "H" which I think is > 20:1. With coax I never
>saw > 10:1.
>
>I'm suspicious of the balun, I went with a 1:1 because I figured
>at some freq's the impedance would already be very low. But now
>I'm wondering if anyone else have tried this configuration or is
>a 4:1 balun the answer. Or maybe the balun is bad, if while the
>RF-1 is connected I ground one side of the balun, the swr drops
>? I don't know why I even tried this test, but I've tried about
>everything.
>
>Any help would be great, over all with a ICE balanced arrestor,
>ladder line, loc, knife switch and balun this was about 200
>dollar "upgrade".
>
>Thanks in Advance
>
>Jim Apple (WB1DOG)
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