[MRCA] AS-2259 antenna matching

[ersmar at comcast.net]

Gents:

     A couple of things.

     This past weekend I did some work with the Smith Chart (remember 
those?) to design a matching network for the mil version AS-2259 (leg 
lengths of appr 26 feet x2 and 38 feet x2.)  On 3.9 MHz it's pretty 
straightforward:  To match the 12 + j440 Ohms feedpoint impedance, insert an 
inductor of about 18 uH in series with the feedpoint of the Vs to cancel the 
capacitive reactance.  Then you need a 1:4 impedance balun (1:2 turns ratio) 
to transform up to 50 Ohms.

     But that's if you conncect directly to the four wires at the top of the 
mast.  In the real AS-2259, you have to connect at the BOTTOM of the 50 Ohm 
mast; and I'm not certain that the mast really IS 50 Ohms.

     That's where my first point is:  Where can I borrow , or buy cheaply, a 
real AS-2259 antenna system to experiment with?  I thought one of the 
visitors to the MRCA last year lived near me in Alexandria, VA and had 
access to several, possibly for a loan.  I want to measure the dimensions of 
the coax mast inner and outer conductors and calculate its actual impedance, 
then run that number into my Smith Chart work to come up with a matching 
network that can sit on the ground near the feedpoint.

     I know that two full-size V's on 80 and 40 M will match 50 Ohms coax 
quite nicely; I've done that already.  My design objective here is to 
develop an outboard matching box that would sit on the ground near the 
feedpoint of a real AS-2259 with short elements, possibly deployed during 
Field Day or actual emcomm, and permit long coax feed lines to be run to Ham 
rigs, either commercial or milrads.  It's easy to minimize SWR-based losses 
if you sit the xcvr right at the bottom of the mast.  But unless you're in 
Spec OPs in some third-world deployment, you will probably want to, or have 
to, set up the rig farther away from the base of this antenna through a coax 
cable.

Second point.

     If I construct a matching network with one series variable inductor and 
a 1:4 impedance transformer that works on 3.885 MHz, the AS-2259 system 
could be matched quite easily on 80 and 60M.  The Resistive part of the 
feedpoint Z is about 12-24 Ohms from 2.6 to 5.5 MHz or so.  The capacitive 
reactance varies from around -j500 Ohms at 3.6 MHz to about -j150 Ohms at 
5.4 MHz.  If your matching network cancelled the capacitive reactance (which 
the series inductor does), then you'd have a resistive component that would 
vary from 50 Ohms to about 100 Ohms on the balun output (coax cable side.) 
Most commercial and milrads can  match this quite easily with low loss.

     Forty meters is a bit trickier.  The AS-2259 exhibits a feedpoint 
impedance of about 860 +j340 Ohms at the feedpoint.  Adding the same 15 feet 
of coaxial mast changes the impedance even more.  I haven't figured out a 
matching system for this arrangement yet, but we won't be operating on 
anything but 80M during the Gilbert fieldex.  I think if I developed a 
matching unit for 80M I will be satisfied for this summer.

     So, can anyone direct me to a source for AS-2259's to borrow?  Possibly 
with the "hubcap" base that I could cannibalize to install a matching 
network?  Thanks for any help.

73 de
Gene Smar  AD3F
Rockville, MD