[evlatests] P-band dipole performance

[Paul Harden]

With most of the low band receivers now working, a little attention has 
been given to the performance of the old P-band dipoles.  I tested the 
dipoles from EA10 about two years ago, and the one from EA22 last week, 
with fairly consistent behavior between the two.

Summary:
Though the P-band dipoles are old (1980s), *they still work quite well.*

The optical alignment plates mounted to the dipoles, to define the 
subreflector center, detunes the dipoles to broaden out the bandwidth 
quite a bit - advantageous for the wider bandwidth of the new system.

The old dipoles offer a good response over the expanded 230-470 MHz 
P-band, with some return loss concerns at the very bottom end, about 
230-250 MHz.  This does not seem to cause a significant change in 
sensitivity at the very low end.

When the dipoles are tuned in the lab for best performance, they 
maintain the tuning once reinstalled in the antenna.  Reflected power 
remains virtually unchanged and the resonant frequency drops downward 
about 20 MHz due to the apex and quadrupod legs (presummably) - not 
significant for the overall bandwidth.

The only real problem is the alignment plate is mounted with nylon 
hardware for electrical isolation from the dipole elements.  The New 
Mexico sun fries the nylon making them brittle or even to break, causing 
misalignment of the plates and degrades dipole performance. 
Historically, John McClendon always rebuilds this portion of the dipoles 
when an antenna enters the barn for maintenance to keep this problem at 
a minimum.

A .pdf file of the dipole test results is attached.

Paul
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