Matching less important
John Doty (firstname.lastname@example.org)
Date: August, 1995
Original source: Usenet's rec.radio.amateur.antenna
You can live with large losses in a medium wave receiving antenna
because signal and noise levels are very high...
this mean that it in everyday, normal life is quite okay to connect
a 50 ohm coax to that long stretch of copper wire - and simply
forget all about those baluns?
forget the term "balun" and substitute "matching transformer".
A balun is a special type of transformer that performs better
than a conventional transformer in certain specialized circumstances.
Conventional transformers work fine for receiving below a few
hundred MHz. Conventional transformers are simpler to construct
(just a primary and secondary winding) and offer a wider variety
of transformation ratios.
Baluns are an amateur radio affectation that's misplaced in shortwave
(or longer) listening. The transmission of power at HF is
one of the specialized applications where baluns are appropriate,
but reception isn't.
antenna is running right under a local power line (low power)
in approx. 90 degrees. This works fairly well when the weather
is dry, with slight interference. Things tend to change when rain,
mist and snow sets in. Somehow the wire is electrified, and occasionally
(mostly in foggy and snowy weather conditons) even sends out pretty
uncomfortable sparks. I suppose a coax from my QTH to well beyond
the other side of the power line (approx. 100 meter) could solve
an excellent application for coax. Be sure you ground the shield
both at the base of the antenna itself and near your house. Burying
the coax will help.
The sparks are probably not due to the power line, but rather
normal "precipitation static". For a large antenna it is highly
desirable to bleed this static off. A matching transformer with
DC continuity to ground will do this, or you can use a resistor
to ground: a few thousand ohms is a good value.
I have to correct any mismatch when using a coax, or would it
be better to ignore the balun totally (in spite of any mismatch)?
with matching is another amateur radio affectation that's misapplied
to reception. Matching is always important if you're transmitting,
but it's less important for reception. The longer the wire, the
less you need to worry about matching. The lower the frequency,
the less you have to worry about matching. At microwave frequencies
matching is almost always important. A coax-fed 6.5 meter wire
antenna will be seriously deaf in the 13 meter band without some
sort of matching device, but in other shortwave bands the mismatch
won't be a serious problem with an adequately sensitive receiver.
A Beverage antenna at mediumwave should be extremely tolerant
of mismatch unless your receiver is unusually insensitive.
It's easy to tell when you have an adequate match: if the noise
level in your receiver increases when you attach the antenna then
the match is good enough. In this case, any improvement in signal
transfer from antenna to receiver will increase signal and noise
by equal factors, so no improvement in signal to noise ratio will
be achieved by using a better match.