[Antennas] tune a wire antenna.

2003年8月29日 09:53:37 -0400

Hi, OM:
With some care and preparation, you can use a variable capacitor to tune a
wire antenna to a given frequency.
The basics go something like this. On 40M, a quarter-wave wire is around
34 feet long, and its feed-point impedance will be around 40 ohms if it is
fed against a "good" ground.
To get a "good enough" ground, connect a couple of 34-foot wires to the
ground-post of your rig, and run them in different directions around the
baseboards in the shack, and other rooms, as necessary. These wires form
what could be called a counterpoise. These may need length-adjustment later.
They will radiate to "some" degree.
To make the primary radiator, connect another wire, an odd-integer-multiple
(X1-X3-X5 &c) of 34 feet (34-102-170 feet &c) length, to the coax-connector
center on your rig, thru a 500-pF variable capacitor. Run it out the window
and do what you have to do to keep it off the soil and away from "metal
masses". Turn r-f on, and adjust the cap for max power out. If you want to
see the effect of changing the length of the primary radiator or the "good
enough" ground, double the wire back on itself a foot at a time and observe
results.
Scale for other bands, eg, on 20M, 34 feet becomes 17 feet, &c.
This is all I've done for field day the last few years. It's quick and
easy.
What all this does is put your rig at one of the current-loop(s) of the
antenna, which is a low-impedance point. Your rig will probably work okay
here.
I've left out the 100 years of lore in this quickie how-to, such as how a
38-40 ft wire raises the feedpoint impedance to about 50 ohms, but you still
need the cap cuz the antenna now "looks" inductive to the rig, and so forth.
You get that stuff on your own ;o)
You may have interference problems as well. I don't, so will defer to any
experts out there on that subject.
Go try this and let us know what happens.
GL and kind regards.
Dave, N3HE
Btw, for your sanity, on h-f wire projects, graduations smaller than 6
inches on your measuring-stick mostly just add to confusion.
----- Original Message ----- 
From: "Nicholas Janzen" <[email protected]>
To: <[email protected]>
Sent: Thursday, August 28, 2003 7:58 PM
Subject: [Antennas] tune a wire antenna.
> I am a fairly new ham, and am really enjoying the hobby so far, I do
> however have a question.
>> I would like to know how to tune a wire antenna to match up with a
> specific band/frequency.
SNIP