In-shack termination control has worked well for me, both on standard
size antennas and on the car-roof micro-SuperLoop used at beach
DXpeditions.
I should mention that the longest feedline I've used is 150 ft. / 45m.
That in-shack pot termination still works with 600 ft. of 100-ohm line
is a bit of a revelation since I think I had read earlier reports of
ineffectiveness with that long a run. It's good that you don't have to
go with the more-cumbersome Vactrol approach.
I use ordinary speaker wire but CAT-5 is probably better. 9:1
transformers at each end of the antenna are each 12 turns : 4 turns on
Fair-Rite 2873000202 / Amidon BN-73-202 binocular core. Toroid
equivalent would be 45 turns : 15 turns (overlapped, occupying ~80% of
core) on Amidon FT-114-J or Steward 35T0501-10H.
With transformers at each end, the two 100-ohm lines are routed to an
in-shack box with a 4-pole 2-throw switch that sends the two conductors
of one feedline to the 2:1 transformer going to the receiver coaxial
cable and the other feedline to the termination pot. I would use a 500
ohm pot rather than 100 ohms since sometimes you might be in the 120
ohm range (= 1080 ohms at antenna) for a given cardioid null. Maybe
having a series resistor of about 47 ohms in series with the 100 ohm
pot makes sense.
The switch box allows instant pattern reversal, something quite useful
in evaluating antenna performance and determining if an unidentified
signal is "pest" or DX.
In-shack termination is well suited to double loop designs (DKAZ,
Bowtie, etc.) as these tend to be more finicky about termination
setting than ordinary Kaz Deltas, Flags, and SuperLoops. Most of those
can be "set-and-forget" terminated with something between 800 ohms and
1K.
Mark Connelly, WA1ION
South Yarmouth, MA
<<
Bill: NICE work on your report! That's a lot of info; easy to
understand.
Interesting what you're finding with 'over-sized' D-Kaz antennas. I
had
settled on a 120 ft/20ft and found it suffered a bit on the low end of
the
MW band...but it turned out to be hot a long way up into SW. What
height
are you using with the 160-foot array?
Here's a thought that may or may not make sense. I've often wondered
if
extending the null-pot to the shack might disrupt the linearity of the
D-Kaz
null.
When the weather's nice, I work from the null-pot location, extending
the
receiver feed instead.
9:1 at the receive end is almost a perfect match to Cat5. I run the
Cat5
around to the null-pot end (staying away from the antenna). The Cat5
feeds
a 2:1 that perfectly matches the Perseus input.
(Still found some RF amplification was a good idea for the MW low-end,
but
the stability was rock-solid and the null was fairly uniform across a
couple
of octaves.)
Regards,
Mark Durenberger
-----Original Message-----
From: Bill Whitacre
Sent: Saturday, March 08, 2014 10:05 AM
To: Mailing list for the International Radio Club of America ;
a...@nrcdxas.org
Subject: [NRC-AM] Grayland - Feb. 19-22, 2014
I visited Grayland, WA February 19-22, 2014 and did some TiVo SDR DXing
using a 180' DKAZ antenna at ~280 degrees.
I'm still reviewing wav files but results thus far are here:
<http://realmonitor.com/am_logs_grayland9.php>
Nothing earth-shattering though hearing Indonesia [on 999kHz] at any
time is
welcome. A few observations:
- 180' DKAZ is a GREAT low-band antenna. I've experimented with 88,
112,
128 and 140' versions before and this is by far the best for the low
end.
Unfortunately, high end nulls suffer as a result and it may be that
160' is
a 'sweet spot' that covers nearly the WHOLE band.
- Feb. 21 and 22 were like two different seasons as the 21st had nice
sunrise enhancement to the Far East while the 22nd was more DU thanks
to an
A-index of over 40.
- I used remote termination - 9:1 transformer at the null end of the
antenna, ~600' of CAT5 to Room 15 at the Grayland Motel where a 100 Ohm
pot
'controlled' the null. Worked very well and I was able to put a decent
null
on stations off-the-back as well as Portland area by virtue of being a
little too close to the neighbors cyclone fence on the south side of
the
property which I believe skewed things a bit.
- I also ran a ~800' BOG directly west from just outside Room 15. Not
very
impressive results on 'live' listening but I've yet to review Perseus
wav
files.
Check out the 'Summary stats', 'Map' and 'AzBar' links on the grayland9
page
as well. They're fun and maybe even useful.
Oh, and if you haven't visited one of my previous DXpedition webpages
there's no 'traditional' logs -- just links to sounds along the far
right
side. Click on the 'little x' and you'll hear what I heard -- dates &
times
as column heads.
Enjoy!
Bill Whitacre
Alexandria, VA
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