This is preliminary, but just for an experiment and to satisfy my curiosity, I
have made some basic measurements of the Bird Feeder's SWR and Resonant
Frequency Changes as the tides move in and out.I am certainly not in possession
of an Electrical Engineering degree. They ain't got none of them at Emory
University. I was just lucky enough to learn a few tooth things. Maybe someone
else has done this before. Bird Feeder=disguised Inverted "L" (35' Vertical,
95' Horizontal) on a seawall and the Feed point is at High Tide about 6" above
salt water, and within a few inches horizontally from water. Don't make fun,
it is all I got. We have 6-9 foot Tidal fluctuations here on the GA
coast.Approx 16 Radials of varying lengths, some lay on the Marsh floor at Low
tide, hooked into 8' Ground Rods 3' apart in the marsh floor, K9YC Balun at
feed, but measurements were taken without the Balun. Approx 90' of RG213 to
shack. Measurements w/ MFJ259B. At the FEED POINT of the Bird Feede
r, LOW TIDE:1800 1.6:1 R=29 X=71825 1.6:1 R=29 X=01850
1.8:1 R=30 X=171875 2.2:1 R=30 X=301900 2.2:1 R=29 X=44
High Tide1775 2.3:1 R=34 X=331800 1.9:1 R=33 X=231825
1.6:1 R=32 X=81850 1.4:1 R=33 X=01875 1.6:1 R=34
X=121900 1.9:1 R=34 X=27 Conclusion: Hi-Tides raise the frequency of
the minimum SWR point and raise the Radiation Resistance about 10%. In the
shack, the Resonant Freq (Low SWR anyway) was around 1800khz @ 1.6:1 at Low
Tide and was 1.2:1 at 1850 @ High tide. Just to solve my curiosity,( I always
noted some variance from time to time in using a Tuner to make the SWR Flat at
the Linear....) I now conclude the fluctuating water table changes Impedance
and Resonance of the Antenna. Why and How, I will leave to those who know the
basics and more. I just am sharing this observation, that is all. What to do
with this information, I don't know. Maybe I now need a r
emote tuning device which is Tide Controlled. See you in the pileups with the
lowest antenna on Top band this season.P.S. The Birdfeeder now comes in a New
and Improved version using an LC circuit and Vacuum Relay to make it switchable
1/2 wave on 80M. The Tide also lowers SWR on 80M as well at high tides.
ValN4RJ
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