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Magnetic-U/DMS observations (LONG)

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Subject: Magnetic-U/DMS observations (LONG)
From: (jim henderson)
Date: Fri, 07 Feb 1997 23:44:10 +0000
From: "Charlie Morrison" <>
Subject: K4VX-DMS=Double Magentic Slot Antenna - ARRL AntCompV4

     From ARRL Antenna Compendium Vol 4
     K4VX described a variation of N5RP/W0UN's work
     on large magnetic loops of wire.
     Gordan reports higher low angle gain as the advantage
     of this antenna even at low height (60' on 75M).
     Anyone try one yet ????
     Charlie -N1RR


Charlie, and Reflectees:

        If you want some empirical, unprecise operating input, here is some.

        Since 1993 I have used 2 Mag-U's, not the DMS. I built one each for
18mHz and 7mHz, and both were only a few feet (h < 1 degree) off the desert
sands of southern New Mexico. Both aimed at eastern Africa. These were to be
compared in practice to my LPDA on both bands, to my ground-mounted,
carefully tweaked HF6V butternut on 40m, and my 1/2 wave 40m sloper,
antennas that have proven themselves thru the trusty ol' "A/B" testing over
the years.

        I was totally unprepared for the sudden jump in practical
performance, especially on 40m. Previously, the LPDA (@75') and the HF6V
provided good DX results on the morning LP to EU, 3B8, HZ, and so on; the
peak was short, and with about 500-600 honest watts out, I had trouble in
getting thru both the Texans to the east, then the stations to the north and
northwest as the greyline advantage shifted. Also the RSTs were highly variable.

        After switching to the standard 30-degree Mag-U's, and comparing
over a couple of years, I found: 1) the apparent signal-to-noise was better
here (quieter RX), 2) the sigs at all the other ends went up a reported 1
S-unit or better, 3) the RSTs were very consistent from day to day, 4) the
average duration of the LP openings (from DX fade-in to fade-out) was up to
75 minutes longer, and 5) my ability to get thru the seething masses of RF
from the guys with better stations was dramatically improved. In fact, with
the Mag-U on 40m, I was still able to compete well even an hour after our
greyline had peaked. The "A/B" testing was continued, but the winner was
clearly and unmistakably the standard Mag-U. 

        Since reading up on the in-line version, the DMS, I have not had the
chance to erect one to compare with the Mag-U. I have, however, removed the
40m sloper and HF6V... And I am presently working on a 15-degree, dual
(4-loop) Mag-U for 160m with Russ Prack. Have received some info from Lew
(K4VX), but have not communicated with W0UN, yet. (Hi John!)

        I have no scientific study and only some simple computer models (by
NEC spin-offs) to give, only this anecdotal or empiricle testing for 2
years. I was so impressed with the performance of this original design (I
believe the DMS and others mostly result from the improved performance seen
in similar "A/B" testing by others), that I checked my other 40m antennas
several times to see if something was amiss.  The improvement in DXing on
the Short Path was also very sharp, but perhaps not as dramatic as I
experienced on LP. 

        As a low-cost, simple, and low-profile antenna, is really shines.
Even the 17m Mag-U was a good performer, earning nearly as good a report
from the DX on SP (no 17m LP heard during the test period) and giving a
quieter RX noise level at my end. This is especially desirable during the
New Mexico "Harmatan" dust storms.

        Finally, I wish I had experienced the Mag-U before operating on the
low bands from Uganda in 1993 (5X1XX). We had a full sized 80m loop at 100+
feet in the trees, on the shore of Lake Victoria. Had I known then what I
know now, I would have put up Mag-U's for 160/80/40, and there is no doubt I
would have doubled the 80/160m totals. If anyone in Reflectorland has used
these on a comparative basis on a DXpedition, say VS The Battle Creek
Special or phased verticals, I would love to hear from you. I have a bulging
notebook on wire antennas I have tried from various DX locations in the last
28 years, and there is just a wee bit of room left to cover Mag-Us. 

        The only drawback to this antenna now seems to be it is catching on,
and I'll have to come up with something better next year.

        BTW, has anyone got any real data to support the contention that, as
primarily a magnetic radiator in the near field, the Mag-U/DMS/variants have
less low angle, far field attenuation in a sandy/rocky/rotten RF ground
environment than does an equivalent electric-field radiator? Practical
experience supports it, but I've never measured it.

73 de Jim, KF7E
ex: 5X1XX, 7Q7JH, ZM7AH, ad nauseum

Jim Henderson

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