In article ,
Dave wrote:

On Fri, 11 Jul 2003 21:16:37 GMT, Telamon
wrote:

In article ,
Michalkun wrote:

How does one can determine the impendance of a wire to get the right balun
for it, so it can be hooked up to the coaxial cable?

The impedance of the wire will depend on:

1. The diameter of the wire. The larger the diameter (smaller AWG
number) the lower the impedance will be.

2. The height of the wire above ground. The higher the wire the higher
the impedance will be.

3. The ground conductivity. The more conductive the ground the lower
the impedance will be. Also note here that this is affected by how the
antenna is grounded. If you have just a ground stake or whether you have
radials will make a big difference on how well the wire will perform.
The poorer the ground conductivity the more how you provide grounding
will determine how well the wire will work.

Why grounding is so important is because the wire is just half the
antenna with the ground being the other half. You have to give the RF
some place to go to complete the circuit that is your antenna or it
will not work well.

The coax back to your radio can be that ground but that has the
disadvantage of mixing the antenna currents with the power line noise
at the radios location reducing the signal to noise. One reason why
people are advocates of Baluns is because the antenna can have its own
ground independent of the radio ground.

For a wire antenna one radial run directly under the antenna wire will
do the most good as a minimalist approach.

All that being said a typical wire will be something in the 400 to 600
hundreds of ohms range so the 9 to 1 type of transformer would be the
best type.

The diameter is usually not significantly related to the impedance, it
affects Q a lot more.

Two AWG wire sizes will change the impedance about 6%. I was trying to
give a sense of how all the parameters of the wire affect the impedance.
The Q of the wire is a complex thing and fairly advanced concept
compared to its impedance. Increasing the wire diameter will reduce the
DC resistance of the wire increasing the Q. Typically this also infers a
narrowing of a resonant peak but other factors conspire to broaden the
peak in this case. Are you concerned with this? I think this is a
non-issue for most receiving antennas.

Impedance is high except at resonance, where it lowers dramatically
(e.g. 500 Ohms to 50 Ohms).

You are confusing the wires intrinsic impedance to its reactance to some
specific frequency of signal energy. This is a common mistake.

You are asking for trouble with 2 grounds. Any difference in
potential can mean noise. I ground my co-ax on the roof (the mast,
grounded at the bottom) and use the outer conductor for the radio
ground, deep in the bowells of my house.

There are two possibilities he

1. You operate the radio on batteries and there is no power line noise
to contend with. From the signal to noise standpoint one or two grounds
are a non-issue.

2. You operate the radio from a AC supply. Here two grounds will reduce
the possibility of power line noise being conducted common mode to the
antenna and then into the radio input. With one ground signal to noise
will be worse if there is any noise on the power line and there always
is some there.

Technically, I should use a ground lift on the IEC cord, but I don't
unlesss there's a noticeable loop.

This is a quick and dirty way to solve a problem. It can be dangerous
and is not recommended. This can also make things worse instead of
better because power supplies in most devices generate some AC noise
currents on the device ground.

Ground loops can cause problems in measurements systems by creating
error voltages and should be avoided. If you don't use two grounds here
a ground loop is formed so noise from the power line, which powers the
radio is added to the measurement and connecting the measurement device
provides the other half of the antenna changing the measurement. Looking
at it this way the radio input is a voltage or power measurement device
that is not floating, which we use to measure the voltage or power from
the antenna. For a single random / long wire antenna the wire is just
half the antenna. The other half is its ground. You don't want your
measurement device ground to influence the measurement so a separate
antenna ground is required. The measurement is the potential difference
between the random wire and its ground terminated in its characteristic
impedance. You then measure the voltage or power across the termination.
The antenna output is some distance from the radio (measurement device)
use coax to convey the signal to it. Here the coax impedance should be
at the antenna output impedance and also the receivers input impedance.
If the antennas output impedance is different then use a transformation
device at the antenna output to change it.

In this way you will get a similar result of signal level whether the
radio is powered from batteries or the AC mains. You can see that if the
antenna does not have its own ground that how the radio is powered will
make a big difference on received signal strength and signal to noise.

--
Telamon
Ventura, California