NEC-LIST:Current-sensing loops on AM broadcast towers

Chuck, a rule of thumb for towers shorter than 90 degrees is to place the
sample loop perhaps 20 feet up the tower. However the current maximum in
the case of a taller tower is about 80 degrees from the top, so if you want
to sample the maximum current in a tall tower, you have to place the sample
loop higher up. This is based on the theory that the current maximum bears
the best relationship to radiated field. But in the case of
self-supporting tapered towers, a correction factor needs to be considered.
 Also with a little MoM modeling you will find that the best relationship
between current sample and field is not necessarily at the current maximum
on a tall tower. I would recommend that you make a quick study of this
using a typical phased array of monopoles.

If you attach the sample loop directly to the tower, you will need some
form of isolation at the base insulator, unless your tower is grounded and
shunt fed. Otherwise standoff insulators will have to be used for the loop
and its co-ax, unless you mount the loop on a separate non-conducting pole
near the tower base.

It is general practice to make all the sample lines from the towers to the
antenna monitor equal length, phase stabilized at the factory. This
accomodates electrical line length changes with temperature. I prefer type
N connectors.

2. In the FCC licensing data published on the web, do the tower-excitation
amplitude and phase data refer to the positions of these loops?

these refer to the "field ratios," which the FCC assumes the consulting
engineer has obtained by adjusting and then reading the antenna monitor
connected to a valid sampling system. The final antenna monitor readings
may differ considerably from the initially proposed field ratios and
angles. The important thing is that the station operator has a means to
indirectly determine that his radiated pattern is suitable and legal. An
extensive series of field intensity measurements and analysis are part of
the antenna proof that is submitted to the FCC. See part 73 of the rules
and regs.

Grant Bingeman
www.qsl.net/km5kg

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