[MRCA] AS-2259/GR questions

[ersmar at comcast.net]

Doran:

     What I learned from my replies to my recent NVIS postings is that the
matching transformer base is NOT part of the basic AS-2259 antenna (as you
learned from the sewn parts list on the case.)  That base was to be used
with  a short (<2 ft) piece of coax going to a PRC-104 (20W) or other QRP
rig, NOT to a PRC-47 (100 W.)   Maybe the bases you have are shown here:
http://www.telex.com/Wireless/Files.nsf/Pages/1990brochure/$file/1990.pdf .

     I did some half-fast antenna modeling last week and found that the
resonant freqs of the stock -2259 were about 6.05 and 9.25 MHz.  My model
used dipole leg lengths of about 38 and 25 feet, respectively, the lengths
shown in http://www.vcars.org/tech/NVIS.html  which I am assuming (a BIG
mistake) are similar to the stock -2259's.  The feedpoint Z at 3.8 and 7.2
MHz were cal'd to be 12 -j 460 Ohms and 860 +j340 Ohms, respectively.
Neither is close to a good 50 Ohm match.

     I remodeled the antenna (no longer calling it an AS-2259) with dipole
leg lengths of about 59 and 30 feet.  Antennas showed resonances at 3.95 and
7.2 MHz, respectively, with SWR below 2:1 for both.  This assumes (!) that
the dipoles are fed directly at the top of the mast.  I don't know what the
impedance of the stock AS-2259 coaxial mast is supposed to be so I didn't
include it in the model.

     My conclusions:
1.  The stock AS-2259 and its derivatives are NOT resonant in any Ham band.
(But I think we all knew that.)
2.  The matching transformer is NOT for use with PRC-47's and similar QRO
(>20 W) rigs;
3.  The matching transformer's intended purpose is not clear (running EZNEC
with 12.5 Ohms as the feedpoint impedance rather than 50 Ohms shows no place
where SWR is below 2:1 from 3.5 to 10.0 MHz.);
4.  To get a crossed-dipole array similar to the AS-2259 to work as an NVIS
antenna on the Ham bands requires changing the lengths of the stock wires to
something approaching the lengths of conventional inverted-Vs for the 80 and
40M (or 60M) Ham bands.

     BTW:  To get a 60/40M array use 42 feet and 30 feet as the dipole leg
lengths; for 80/60M use 59 and 42 feet; and for a three-band version use leg
lengths of 58 feet, 42.5 feet and 32.5 feet, respectively.  In this
configuration the 40M wires are underneath the 80M wires; there is some
interaction or coupling between these two sets of wires, so the lengths are
different from the other versions, above.

73 de
Gene Smar  AD3F
Rockville, MD

P.S.  If you'd like you can send me the actual wire lengths from your -2259
(all four lengths!) and I'll plug them into the model and let us all know
the results.


----- Original Message -----
From: "doran platt" <jeepp at adelphia.net>
To: "Military Radio Collectors Association" <mrca at mailman.qth.net>
Sent: Monday, October 10, 2005 2:41 PM
Subject: [MRCA] AS-2259/GR questions


> Second try for this query.....
> I've have one of these antennas in, what appears to be, as new condition
> and complete?  In reading the various posts on the antenna and what I've
> been able to glean from a Marine Corps manual, I seem to be missing the
> coupler.  My antenna roll has all the parts on the sewn-in list "list"
> and there's no mention nor place for a coupler to be stored.  I do have
> two bottom adaptors.  One is to fit directly onto the PRC-47 antenna
> insulator, the other, a coaxial affair, is what I assume would plug into
> a coupler.  Given what's been said about the power handling capability
> of the coupler, it appears to be useless for all but genuine QRP usage.
> The Marine Corps book says the coupler is a 125ohm to 50ohm
> transformer.  Might I assume that something like a small MFJ coupler,
> installed in a suitable base mount that would accommodate the threaded
> PRC-47 bottom section would suffice the job?  I haven't laid out the
> wires yet to see how long they are but the Corps book says there's two
> symmetrical dipoles provided; one for a "high" frequency and one for a
> "lower" frequency.  I assume I could cut or add to bring it into 75 and
> 40 meters.  Any insights would be appreciated.  I took an HF course at
> GW University a while back and they covered NVIS rather well, to include
> one of the original NRL and Monmouth team members as a speaker.  The
> phenomena does work... to be sure.
> Regards  de K3HVG
>
>
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