Topband: Delta Loop Match

[D Rodman MD]

Looking for some help matching my 160 loop.  I made three changes to the 
system this year and the VSWR has increased to about 2:1 at resonance 
which is about the same frequency.  Here is what I have.

The antenna is fed at a convenient point up from a bottom corner and is 
hung apex high at about 140 feet.  I have a series match with a 75 ohm 
line that is about 20' or so from the feed point.  The horizontal 
portion is pulled about 20' away from the lower portion of the tower. I 
designed the impedance of the antenna for 100 ohms pure resistance using 
MININEC and the system has been in place for about 15 years.  I don't 
know of any damage anywhere on the antenna.

Here is what I changed this year.  I built the 80 4-square nearby.  The 
two closest elements are about 40 ft away.  The loop was shorted to the 
tower at the apex due to proximity to one of the guy positions (all 
Philystran).  It was just moved a few weeks ago.  I found bad series 
coax filled with water and it was replaced during the summer.  I used 
the antenna for the first time over the weekend for the contest.

Basically, I would like some guidance on how to MEASURE feedpoint R and 
X.  I have never been able to do so with a standard antenna meter.  I 
presume this is either because the induced voltages on the antenna from 
the BC band are higher than the signal transmitted by the box or it is 
not happy with the closed loop configuration.  If I knew the feedpoint R 
and X, I would be able to calculate a better match and construct it.

It looks to me like a lower Z antenna needs a shorter series match moved 
farther from the feed point and a higher Z is a longer line moved closer 
to the feedpoint.  I haven't put a quarter wave 75 ohm at the feed.  On 
site I have lots of test equipment, including a service monitor, 
spectrum analyzer and a network analyzer.  I can put my portable 
transceiver close to the antenna if I need to.

Thanks for any help.

-- 
David J Rodman MD
Assistant Clinical Professor
Department of Ophthalmology
SUNY/Buffalo

Office 716-857-8654