[BC] AM loads

[RichardBJohnson at comcast.net]

Sanity, sanity! The power going to a single-stick antenna is
measured as the base current squared times the measured antenna
resistance. Regardless of the mismatch as seen by the transmitter,
if the current was correct and the resistance was correctly measured,
then the antenna is receiving the correct power.

If there is more current flowing in the transmission line than is
normal for the power because of a poor match, then there may be a
small increase in the transmission line losses. For all practical
purposes, you won't be able to detect these increased losses with
a load from about 30 ohms to 70 ohms from a nominal 50 ohm transmission
line. However, modern solid-state transmitters won't make power or
perform properly unless the load impedance is very nearly 50 ohms.

Vacuum tube transmitters were able to feed practically anything
because you had both tuning (phase angle) and loading (magnitude)
control. An improperly matched transmission line could be accommodated
quite well and, in fact, its impedance transformation became part of
the total network solution. The only problem being if the current was
so high that the losses become excessive or the voltage so high that
arc-over occurs.

To get an idea of the losses, suppose we had some “awful” RG-8/U for
transmission line at 1 kW. The current will be sqrt(1000/50) = 4.47A.
RG-8 has an effective resistance of 1.7 ohms per 1000 ft at 1 MHz.
This introduces a loss of 4.47^2 * 1.7 = 34 watts. Suppose that because
of a poor match, the current doubled to 8.94 A. The losses would be
8.94^2 * 1.7 = 236 watts. If the transmitter could make this power
output, you wouldn't measure or hear the difference. Most 1kW transmitters
are rated at 1.2 kW to be able to compensate for directional antenna
phasing equipment losses. Now a 236 watt loss is a very expensive
loss because that RF power was generated with an overall efficiency
of about 60 percent so it's going to increase the electric bill as
though it was 236/0.60 = 394 watts --every hour you are on the air.
Nevertheless, you can't hear the difference.

Even if the power going to the antenna was only 1/2 what it should be,
the coverage, the volts-per-meter, would only go down by sqrt(0.5) = .707.
It is unlikely that any customer would be able to detect a 0.7 reduction
in signal strength if the signal was ordinarily strong enough to operate
the normal receiver AGC circuits, i.e., within the 1 mV/m contour.

This does NOT mean that I am an advocate of throwing hard-earned RF power
away because of transmission-line losses, but when deciding what to
fix or upgrade it is essential to put things in their proper perspective.

Cheers,
Richard B. Johnson
Book: http://www.AbominableFirebug.com/

----- Original Message -----
From: "Scott Cason" <scott at lagrange-com.com>

>> The solution is real simple give your transmitter a good load and you
have nothing to worry about.

Something else to consider with the bad load is, how much of your power is
really making it to the antenna?  Four years ago, I replaced a Bauer 707
with a Nautel J1000.  The Nautel would not come up.  We stuck a OIB on the
line and the load was severely mis matched and had been running that way for
DECADES!  Once we had retapped the ATU and got the Nautel running, suddenly
we could hear the station in areas it had been unlistenable.  The owner
could hear it at his home.  They were going through a set of tube a year in
the Bauer.