Jim Tucker (Jim_Tucker@mindlink.bc.ca) asked:
What is better to add to a simple longwire antenna -- a balun or an antenna tuner?
Neither.
Start with a well-grounded good-quality coax feedline. Move the
base of your longwire antenna as far as possible from noise sources such a
houses, powerlines, etc. Connect the center of the coax to the loingwire.
Ground the coax shield at the base of the antenna and near your house.
Bury the coax a few inches under ground. Use RG-6 or RG-213 or "RG-8" coax
- avoid Radio Shack RG-58, as it's shielding is not good enough. You want
something with at least a 95% shield coverage.
Later, you can add a matching transformer or antenna tuner, as you prefer.
Use the antenna tuner if you like to twiddle knobs, otherwise use a
matching transformer (sometimes miscalled a "balun").
From: Steve Byan (steve@hi.com)
Date: April 27, 1995
Original source: Usenet's rec.radio.shortwave
Suggestion:
A balun is also of primary value for transmitting, again trying to move as much power as possible from the feedline to the antenna. It won't hurt for receiving, but frankly, it won't do too much for you, either.
Given a resonant dipole antenna (i.e. one that is cut to the right length
for the frequency where you are listening) with a coax feed (as opposed
to a balanced feedline like TV twinlead), a balun will decouple the
outside of the coax shield from the antenna.
Without the balun, RF noise
from your house will be picked up by the outside of the coax shield,
propagated along the coax to the antenna, and coupled back into the inside
of the coax, and back to your receiver.
So for resonant dipole antennas,
a balun can be a big help.
Unfortunately, when the antenna is not resonant, its impedance quickly
rises to the point where the balun does not have sufficient inductance to
decouple the outside of the coax from the antenna. In this situation,
which is common for SWLs, it is better to use either:
1) an unbalanced
antenna, such as an inverted-L or vertical monopole, or
2) with a balanced
antenna such as a dipole, use a balanced line like TV twinlead.
One is still left with the problem of connecting the balanced feedline to
the unbalanced antenna input of the typical shortwave receiver.
The best
method is to connect the twinlead to an antenna tuner, and then connect
the antenna tuner to the receiver through a balun. You use the antenna
tuner to transform the impedance at the end of the feedline down to the
impedance that the balun is designed to handle.
Or just ignore the
problem, and hope that the imbalance in the line doesn't result in too
much noise pickup.