In a message dated 7/30/01 8:38:34 AM Pacific Daylight Time,
n4zr@contesting.com writes:
<<
This is my ignorant question for today -- with only an antenna analyzer,
how can I determine the resonant frequency of my tower? I plan to attach a
first-attempt at shunt feed at about the 30-foot level, with 30-inch
spacing from the tower. Can I tell from the analyzer reading at the bottom
of that shunt wire where the tower is resonant? Do I need to put the
series capacitance in before attempting the measurement? Or is it all
fruitless?
Clearly my education was neglected or I'd know the answer, so thanks for
your help and patience.
73, Pete N4ZR >>
Pete: There is a simple way with a reverse connection to the radial to coax.
Assume a tower 50' high with a 20M beam. The beam adds a good top load to
it--how much?
Connect the coax shield to the tower starting at the bottom and add say a 80M
radial to the center lead and run it horizontal at first. Check the resonant
frequency with a MFJ SWR Analyzer. Keep moving the shield connection up the
tower with a vice grip until the lowest SWR is obtained on 80M. The tower
length above the shield connection and loading affect of the beam will be
about 1/4 wave electrically and to the "Resonant Eye". You can droop the
radial down to about 10' off the ground as you raise the shield connection
and also alter the lowest SWR. No series capacitor is needed either other
than if the lengths aren't right. I did this for 6 different bands with no
apparent affect on each band as long as the radials weren't too close to each
other. Sometimes grounding the tower has no affect below the shield
connection. The primary "Phantom Ground" was at the shield connection
insofar as the RF and resonance was concerned. It might be necessary to cool
other cables coming into the shack but I never had a problem--others may.
Works like a charm.
As the shield connection is raised to obtain resonance and the band is
higher, It starts to look like a Sloper. When a low SWR point is found it
means the length of the tower above and below the shield connection looks
like a 1/4 wave. Short towers seldom work well for 160-80M top guy wire
slopers as the length above and the often grounded tower below just aren't
resonant. A low shield connection often works great for 80 and even 160M.
If not on 160M add a wire extension like a guy wire to resonant it on 160M.
K7GCO
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