So perhaps it's the 140-foot size, rather than the use of Cat-5 extenders,
that created the 15 db difference in F/B nulling. Previously, I was doing
better than 40 db at times with the 120-footer (though that WAS with the
null-pot right at the antenna).
Earl Higgins asked for some loggings; they're on the way.
Regards,
Mark Durenberger
-----Original Message----- From: neilkaz
Sent: Monday, May 19, 2014 1:45 PM
To: am@xxxxxxxxxxx
Cc: irca@xxxxxxxxxxxxxxxx
Subject: Re: [IRCA] [NRC-AM] Reversible Double-Delta
Sounds like it was working well and good to hear that there basically was
little re-nulling needed. This is a very important aspect of this design
since SDR users want to record the entire band. On mine at home using
fixed
resistors I can get 30+ to 40 dB nulls on locals. This means most back
nulled regionals go away at night as they are overwhelmed by DX from the
desired direction.
You can always phase it vs something else if a single freq very deep nulls
is needed. When I got KCEG on 780 14 miles NNW of WBBM, I phased one of
these vs a random piece of wire about 30 ft long on the ground.
Non SDR users would do very well phasing one end of a Double Delta (Double
KAZ or whatever you want to call it) vs the other. Nulls clearly exceed 40
dB, but are very narrow banded..ie basically just the frequency nulled +/-
not much more than 10 KHz. The phased configuration is also bi-directional
since you switch the phasing to null the opposite direction.
140 ft length is likely the max for good high band F/B as it seems to
worsen if you make the antenna much larger.
73 KAZ
-----Original Message-----
From: Mark Durenberger <Mark4@xxxxxxxxxxxxxxx>
Sent: May 18, 2014 10:13 PM
To: MNDXC <MDXC@xxxxxxxxxxxxxxx>, "DX @NRC" <am@xxxxxxxxxxx>, DX-IRCA <
irca@xxxxxxxxxxxxxxxx>, Shafer Mike <mikegjco@xxxxxxxxx>, Baumgartner
Fred-ARRL <K0FMB@xxxxxxxx>
Subject: [NRC-AM] Reversible Double-Delta
This evening, 3 Phools in the Phield set up a reversible 140-ft.
Double-Delta antenna at a quiet place in Burnsville MN. The goal was to
see
what sort of nulling we could do off the "backside" of the antenna
(whether
the "backside" was the North end or the South end). Think of it as a
super-hot loop antenna, but unidirectional in pickup rather than
bi-directional.
It works.
On a number of frequencies we were doing armchair copy of one station;
flip
the antenna and the first disappeared while another came up just as
well...from the opposite direction.
We were oriented North-South since most of our local pests were North of
us.
Antenna reversal is as simple as flipping a switch.
Interestingly...the null held up across the band; little or no re-nulling
was needed.
Surprisingly, we could do no better than about 27 db nulls of local
stations. Our conclusion was that the Double-Delta seems to like low-
and
medium-angle skywave and doesn't discriminate as well with strong
ground-wave signals.
We will also investigate whether the extension of the null pot and RF
takeoff VIA UNEQUAL LENGTHS OF CAT-5 degrades the null. (I've seen
better
than 40 db null on these guys when the null pot is right at the leg of
the
antenna; in this case we had run 150 ft. of Cat-5 to get to the
receiver.)
Mike Bates and Jim Dale may chime in with their own observations. It was
an
evening well-spent.
Regards,
Mark Durenberger, CPBE
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