ku8e@bellsouth.net wrote:
Adam,
You might want to try to short the center and braid of you dipole
together and use
it as a "T" antenna on 160 meters. It will work even better if you can
lay out some radials. A "T" will act as it is a top loaded vertical...
73, jeff
------------------------------
One thing I think Jeff just forgot. If you do the above, you have to
use a balun or a tuner which will take a single line feed, or you'll
burn up your Tx. If you simply short the center and braid, you have
exactly that. A short.
On the subject of T antennas, Jeff is quite correct in his assertions.
They will work much better than an "inverted L" because the top of the
T effectively cancels radiation from the entire wire which represents
the top of the "T".
An even better method, which I use here, is to make the top of the "T"
long enough to raise the input impedance. For example, mine is 89' up,
and 100' across the top. I tie a line from the top of the mast on my
89' crankup ( and the mast extends some 15 feet out of the
top---moly-be-damned ) and then run the line to a tree about 75' tall
about 150' feet away. This raises the input impedance to about 4300
ohms, and since efficiency of radiation is a function of ground loss
and input impedance, will almost eliminate the use of radials. In this
case, the antenna efficiency is about 99.996%, versus the usual
efficiency of a 1/4 wave vertical ( 36.? ohms + typically 13-15 ohms
of ground loss )
of around 65% with no radials.
On the feed part, it's a simple matter to build a parallel LC network
into one of the XYL's plastic bowls. Use some RG8 ( about 30 pf per
foot and 4000 volts ) for the parallel capacitor,wind a coil of about
14 mh, and then tap the coil up from the bottom about 3 or 4 turns for
your coax feed. Adjust for 1:1 by varying the tap and the length of
the coax capacitor.
I can't lay my hands on the exact formula now, but it's a very old
one.
LC=~22500/F^2 ( where F^2 is the frequency squared ). So, for let's
say 3.550, we have
LC=22500/12.25 and a LC constant equal to ~ 1836. Now, since we know
our coil is about 14 mH, we can rewrite the formula as (12)C=1836, and
from there C=1836/12.25 =146.88pf.
And dividing that by 30 means we need a piece of coax about 4.9' long.
( and start out with a 5'+ piece and then trim it down. Be careful
when you get close to a 1:1, as it is a good idea to remove about
1/4-1/2" of the shield on the open end of the coax to prevent RF flash
over. Tape it, wind it any way you please, or just leave it on the
ground and your circuit is complete.
Now, the center of the coax, the top of the coil and the bottom of the
antenna are all connected together, and of course, the bottom of the
coil and shield of the coax all go to your ground rod.
One last point. It is obvious that if you make C greater, less coil is
required. But if you take this too far, you will narrow the bandwidth
because the Q of the circuit is too high. Same thing goes if you
reduce L of course.
And of course, building it for 40 is merely a matter of dividing the
coil C and L by 2.
Hope this gives you a idea or two.
73
Ed
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