Topband: Inverted L questions
Telegrapher9 at aol.com
Telegrapher9 at aol.com
Mon Nov 20 13:46:56 EST 2006
Peter,
a slightly short Inverted-L with a shunt coil to GND might be the way to go
since you don't have a variable capacitor. The 65' tall Inverted-L has a
radiation resistance of 18 ohms. Depending on your ground system resistance the
feedpoint resistance will be 18 ohms on up. To match 18 ohms to 50 ohms using a
shunt inductor the antenna impedance needs to be about 18 -j24 ohms. The shunt
inductor is 3.2 uH. The total antenna length is about 131', or 4' short of
resonance.
A nice 3.2 uH inductor can be built on 2" PVC pipe using #14 THHN wire. Close
wind 7 turns and you have 3.4 uH. Here are inductances for antenna feepoint
resistances:
18 ohms 3.2 uH 7 turns
25 ohms 4.3 uH 8 turns
30 ohms 5.3 uH 9 turns
35 ohms 6.4 uH 10 turns
The 4.3 uH coil will allow the VSWR to be adjusted (by antenna length) to
1.4:1 or less for a system resistance of 18 to 35 ohms. The procedure can be to
tune the antenna to resonance. Measure the VSWR or use an impedance analyzer.
Calculate the resistance from the VSWR or read the resistive number on the
impedance analzyer. Add the correct shunt inductor. Trim the antenna for minimum
VSWR.
The shunt inductor can provide a 1:1 VSWR at one frequency. At large
frequency excursions the VSWR is the same as with no shunt inductor. Because of this,
and my narrow band antennas, I simply run the 2:1 VSWR at resonance antenna
into a good antenna tuner and let it do the work.
The Inverted-L radiation pattern is essentially omni-directional. You can run
the horizontal portion in any direction.
Dave WX7G
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