I would like to share my experiences with 160m operations during the SP
contest last December.
My rental house in St George Isl FL had a 140 ft pier over the bay. Also a
power line to the end of the pier raising questions about RFI.
I set inv L at the end of the pier with 40ft Spiderpole, 70ft horizontal
and 70ft radial on the pier. Horizontal with 24 gauge enameled wire for
low weight, and the rest with 18 ga THHN. Matching was with a transformer
on FT140-61 and coil on T200-2. Minimal SWR at 1.84.
One hr before the sunset the EU was booming. RFI was pretty high but the
signals even stronger. I tried to use KX3 and Expert 1.3k combination, but
SWR become very unstable causing the Expert to turn off. I was looking for
culprits but could not find anything. Frustrated, I was forced to use
FT-891 at 100W.
The setup was still pretty good as several EU answered my CQ.
The next day I noticed that the radial on the pier that was touching
underneath (salt water sprayed) had melted insulation. Apparently this
caused sparcing and then variations in SWR.
I relocated the radial away from the pier, over the bay. The resonance
moved from 1.84 to 1.96, and the impedance at resonance reduced from 40 to
25 ohms. So the pier (or power line) was loading the antenna, surely
generating losses. Operating with a KW was no longer a problem.
Here are my lessons:
1. 40ft Spiderpole is enough for highly efficient inv L over salt water.
2. Beware of insulated wire touching anything and especially wet and salty
at high power.
3. Even with problems, the performance of an antenna over salt water is
terrific.
4. Matching units do not have to be big.
5. With good antenna, $550 FT-891 is OK.
Ignacy, NO9E
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