Michael Lawson wrote:
"starman" wrote in message
...
Pierre Vachon wrote:
Hi there, I am trying to build a longwire antenna for use in the
20 meter
band to 70 meter band area. I was looking for advice on what
materials to
use for the antenna and the lead in lines. I will hook it up to a
Drake r8B
radio. What is the minimum height it has to be? Where to ground
it?.
Actually, does it have to be grounded if the radio is grounded at
the
outlet?
I am a little limited in space as my yard is only 75 feet long and
there are
power lines at the front. I assume that they are the source of an
irritating
hum on the receiver on certain frequencies.
Thanks for the help in advance.
Pierre
A real longwire antenna is much longer than what you are thinking of
building. The antenna you describe is called a 'random wire' or
inverted-L. This kind of antenna is not tuned for a specific band or
range of frequencies. In fact, it performs well throughout the
shortwave
spectrum. See the following website for instructions on building a
good
low noise inverted-L antenna. I use this kind with my R8B.
http://www.anarc.org/naswa/badx/ante...e_antenna.html
Hmm. That brings up a question that I'd thought of earlier, but
when I was writing my posts, I forgot to put it in.
How do you figure out the impedance of various random
wires of different gauges?? John Doty's article mentioned
an 18 gauge wire hung more than a few feet above the
ground, but I'm just curious what sort of differences there
would be using, say, 14, 16 or 22 gauge wire in a similar
scenario. I can't imagine trying to test it without a load of
some sort.
--Mike L.
The size (gauge) of the wire is not important for a *reception* antenna.
It does matter for a transmitting antenna. The typical inverted-L has an
impedance of several hundred ohms for much of it's frequency range.
That's why a 9:1 (impedance) balun is often used. If the receiver has an
antenna input impedance of about 50-ohms, a 9:1 balun will match it to a
450-ohm antenna (9x50=450). The wire turns ratio for a 9:1 balun is
actually 3:1. My balun has a primary winding with 10-turns and a
secondary with 30-turns. I used #22 enameled wire on a 1/2" dia. type-43
ferrite core. I plan to make a new balun using a 'binocular' core. This
core shape makes the balun's impedance more consistent throughout it's
frequency range.
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