A 45 ft vertical for 160 is a little problematic. Looking at bottom
loading, you need about 80 uH. Now lets assume some reasonable numbers,
wire = #12, coil Q = 400 which gives about 2.3 ohms loss, and 64 quarter
wave radials which gives about 7 ohms ground loss. This resonates at
1.85 MHz, feedpoint impedance is 13 ohms, 2:1 bandwidth, if matched, is
17 KHz. So you need a 50 to 13 ohm matching network at the base, but
since you only have 17 KHz bandwidth, you may want a variable matching
To build an 80 uH coil you need #10 wire, 6 inches long, 4 inches
diameter, and 38 turns. It sould be built similar to the way air dux
coils are made to reduce losses. In other words, don't use a PVC form.
Try a couple pieces of plastic at right angles. If you use a form that
adds significantly more capacitnace you will need to increase the number
Radiation resistance of this antenna (no R losses) is about 3.2 ohms.
There is a 3 dB loss due to resistance. There is 2.3 ohms in the coil,
7 ohms in the ground, and 0.83 ohms in the 45 ft wire. A matching
network at the base to match 50 ohms to 13 ohms is likely to have
another 3 dB loss since this is a particularly difficult task at 160 meters.
Best way to improve the antenna is to raise the radiation resistance.
That means making it longer. A little improvement can be made by using
center loading. More improvement can be made by using top loading.
Converting it to an inverted L would be much better, provided that the
end of the horizontal wire is at a significant height. Running it back
to a lower height will lower the radiation resistance again.
Jim Miller wrote:
>This reflector - Only antennas that go on towers?
>I've been thinking about putting up a 160 loaded vertical of 12 gauge wire
>but did not intend to "buy" the coil. And my 60 ft tree just died and I had
>to cut it down. Now I'm at about 40-45 feet, maybe. Would someone do the
>calcs for a coil of 12 gauge copper wire at the feedpoint to make a 160
>antenna of a 45 ft vertical? Just a starting point is fine. Capable of
>full legal limit. Sorry, phone band (for now).
>Any mods to the request welcome with buildable examples.
>Use chain link fence top rail 2x21=42 ft.(probably galvanized steel) for the
>vertical instead of 12 ga copper?
>Bite the bullet and buy copper or aluminum pipe for the vertical? How big?
>Coil at the midpoint?
>Top hat (reasonable)?
>Copper or aluminum tubing instead of wire for coil?
>Would a center loaded 1" aluminum vertical of 45 ft with a top hat (5 or 6
>ft or less in diameter) work? Rope over the tree to hold it in place.
>I just want something relatively inexpensive I can build.
>Thanks, de Jim KG0KP
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