In message , dave
writes
JimK wrote:
- Terminate the random wire away from the
- electrical noise near the receiver by using
- co-ax and the balun.

Generally a Balun works best along with a
good earthen Ground {connection}.
.
Dave and RHF, thanks. I'll read up on the balun. What is the purpose
of impedence (or RF?) matching between random wire and coax?
Thanks,
Jim


Think of "impedance" as hose diameter. You can dump water from a
skinny hose (low Z) into a fat hose (Hi Z) but not the other way around
(most of the water is spilled).

That's a rather unfortunate analogy. To the flow of water, a skinny hose
would have a high impedance, and a fat hose would have a low impedance.
But the principle is 'sort of' correct.

The matching "Bal Un" is usually an autotransformer which converts the
450 Ohms (max Z of the random wire) to 50 Ohms (good match for RG-58,
RG-59, RG-6, RG-8X, etc.) Note that at resonance, the random wire is
already at 50 Ohms, so the transformer lowers it to ca. 6 Ohms, but it
works fine for receiving).

Although it's certainly high, I don't think that the maximum impedance
of a random wire is limited to 450 ohms. Nevertheless, the transformer
still provides a better match than you would get with a direct
connection between the coax and the wire, and you don't lose as much
signal.

At frequencies where the wire presents a relatively low impedance, the
transformer may transform the impedance of the antenna to a value a lot
less than the coax. On those frequencies, you could actually get less
signal than you would if there had been a direct connection.

However, it's a case of swings and roundabouts. For most of the spectrum
between (say) 1 and 30MHz, you will get more signal (and, on some
frequencies, quite a lot more). On a few relatively narrow parts of the
spectrum, you will get somewhat less.

A second benefit is that the "BalUn" is DC grounded everywhere
(provided you ground the cable sheath where it enters your house per
NEC). This will keep static charges out of your radio.

You don't really need an un-un to do this. You can add an RF choke (or
high-ish value resistor) at some point across the coax. This won't do
much for lightning protection, but it will prevent the build-up of a
static charge on the wire. However, an un-un will probably give you more
against the effects of nearby lightning strikes.

You should at least provide a good ground for the coax screen at the
antenna feedpoint. Hopefully, this will be remote from the house. For
good luck, you can also ground it where it enters the house (and
anywhere else you care to along its length), but it's the ground at the
antenna end which is the most important.

Your brother is right. Whips are usually "active" antennas, with an
extra transistor to convert the Hi Z antenna to Low-Z, to match the EXT
ANT input, which you should use, if you have a proper coaxial deed.
--
Ian