Producing signal augmentation requires at least two conductors that due to their current and voltage ratios, plus combined phase angle and spacing, cause the additive and subtractive radiated waves to augment the transmitted wave in predicted directions. This arrangement produces signal lobes (more signal) or signal nulls (less signal) providing more signal strength or less signal strength in certain directions.
The phase angle is a product of the percentage of a full wavelength measured in degrees (360 degrees is a full wave).
90 degree spacing for example at 1610 KHz would be approximately 150 feet. Even with a passive reflector (not fed by an active transmitter), it would be longer than 150 feet and about 75-80 feet away from the broadcast antenna. Just the virtual size would cause a violation.
Because Part 15 certified AM transmitting systems are only allowed 3 meters in total length, using traditional directional antenna theory becomes more than problematic. And if we were to substitute, the antenna elements with lump inductors and capacitors the efficiency of the system would be abysmal.
Using 2 transmitters, on the same frequency, presents several problems. One large problem is that AM signals on the same frequency, with both reaching the same receiver simultaneously must be in phase or the audio will be seriously degraded because the audio is carried on the amplitude or outside of the radio signal sine wave. And some where between the two transmitters signal loss would be very noticeable.
Keith Hamilton has studied this challenge for many years and has designed and constructed several systems that work to expand the service area for a signal audio/program source. I would suggest contacting him for his expertise and experience.
This is not an exhaustive treatise of AM BCB antenna system signal augmentation. I have been personally involved, over the past 40 years, in designing directional arrays for broadcast stations as a broadcast engineer, as well as, designing and constructing several directional arrays for shortwave and ham radio stations.
Hi Guys:
Two brief comments because you guys have
really covered it.
I noticed nulls on my signal at some street
intersections. Also, even with 16 radials
around the antenna, my coverage area
was more like a figure 8, instead of a
circle. I am rebuilding the AM system in
the spring - it isn't on now.
I thought it would be a fun experiment
to feed two transmitters with the same
crystal oscillator. The oscillator would be
in the house. The transmitters would be
outside. The length of feed line to each
transmitter would be the same. This is
a simple idea that I'm sure is really
very complicated.
Best Wishes
Bruce, MICRO1690/1700
Bruce MICRO1700
You may be able to feed your transmission lines with more than a mere oscillator. There are 2 very fascinating documents on the site which describe what is essentially a cable system feeding remote final amplifiers where the main requirement is 100mW to the finals. All the schematics and everything are included. The transmitter can actually be 1-watt or more as long as the finals are not over-driven.
Low Power Broadcasting by James R. Cunningham
and
Carrier-Current Techniques by Ernest G. Wilson
I also imagine building one of these systems some day.
As always, Carl, I appreciate your
comments. I will check those sources
out.
Things are real busy here, and I didn't
expect that. I might not be on here as much.
The 13 MHz rig on the other thread is
still proceeding, just very slowly.
Thanks again,
Bruce, MICRO1690/1700