A reason sometimes given for using elevated mounting of unlicensed Part 15 AM transmit systems is so that the radiated signal will clear nearby structures or other path obstructions, and produce a greater coverage area.
Probably this is a belief based on the performance of transmit systems operating in the VHF spectrum (and above), which depend on a direct, or space wave. Their relatively short wavelengths can be significantly affected (blocked and/or reflected) by typical path obstructions, so elevating the tx and rx antennas can improve system performance in those bands.
However MW propagation isn't much affected by such path obstructions, because those obstructions are relatively small compared to the wavelength.
Below is an illustration of this, based on measurements of the fields of a MW broadcast station for two path lengths of about 1 mile. One path is blocked and the other is essentially clear. Note that the received field is the same for both of those paths.
Elevating a Part 15 AM system might produce higher fields at a given distance than if it wasn't elevated (other things equal). But most likely that improvement is based on additional radiation from one or more radiating conductors leading away from the elevated transmitter -- such as conductors to a buried ground rod or water pipe, plus conductors carrying audio program and d-c power up to the transmitter.
So while it may seem a surprise, that coverage improvement will not be based on the height of the transmitter and its ~3-meter whip above the earth.
I don't believe it as my experience has shown otherwise.
I have a Talking Sign, certified. Currently it gets poor range, mounted inside at ground level, with 2 stories above it and many townhouses surrounding it. That same transmitter, installed in a rancher, with the wire antenna just reaching over the rooftop (about the same height, maybe a little higher), got over a mile range in a direction with open fields.
Almost identical installs, dramatically different ranges.
Results trump theory.
Absolutely correct, Rich, and I hope all those interested in transmission in the medium wave band finally get it straight after so long being sold the "higher is better" myth.
In my experience I have taken note that AM signals blocked by my house are not blocked by my house because those signals fold-around or flow-around the exterior of the building and appear on the other side the same as water might do in a large flood.
What I think can also be read into your presentation is that elevating MW antennas does not reduce effectiveness, which is important if the reason for elevation is equipment security, i.e., keeping transmitter out of easy reach.
... Almost identical installs, dramatically different ranges. Results trump theory.
However, such different results can be produced by factors that have not been observed, and recognized.
Such results can be predicted by theory, also, when all installation parameters are accounted for.
... What I think can also be read into your presentation is that elevating MW antennas does not reduce effectiveness, which is important if the reason for elevation is equipment security, i.e., keeping transmitter out of easy reach.
In fact, elevating Part 15 AM systems increases their effectiveness, when (other things equal) the length of their radiating conductors exceeds the 3-meter total length permitted by FCC §15.219(b).
Security of the installed system is important, for sure, but would that be a sufficient reason for non-compliance with Part 15?
Rather than tackle the security issue I'm turning attention to the installation examples given by manufacturers of certified part 15 AM transmitters, which at latest check happen to be elevated installations. I am confident they would not propose conditions that would exceed their own certification.
What I sense is that this subject is being used as a back-door to get us back into the now legendary "long ground lead issue." Yes?
... What I sense is that this subject is being used as a back-door to get us back into the now legendary "long ground lead issue." Yes?
I'm sure that such will be assumed, and I'll have to take the flak.
But when the reason for the increase in radiated fields from an elevated Part 15 AM system is attributed only (and falsely) to the height of the transmitter+whip above the earth, it becomes important to state the real reason for the increase in those radiated fields.
Obstructions DO affect MW signals in my experience. Exactly what factors explain my example. Two identical installs, wire antennas at the same height, one with no obstructions getting over a mile range, the other with range measured in several hundreds of feet.
There are 2 issues here. There may very well be an increase in field strength in an elevated install due to a lengthy ground wire (or something). Or there may not be if you don't have a ground as some have experimented with.
But there is no doubt in my mind from actual ly doing something that if the field strengths are equal, obstructions can and do attenuate your signal. Particularly in urban settings. So there is always an advantage in getting your antenna out in the open, no matter how you do it. As long as you don't have a long ground that violates the rules, of course.
Given that in these many decades of the existence of Part 15 and the FCC this subject is only now being raised, you speak alone. If it were a real matter the FCC would have long since issued a position or ruling.
Sticking to facts, the audio and power leads that are obviously necessary to attach to a part 15 transmitter are not ground leads as legally defined.
Do no harm.
... But there is no doubt in my mind from actually doing something that if the field strengths are equal, obstructions can and do attenuate your signal.
AR, et al: Note that I actually did something to investigate this subject matter, and that such effort was not made using computer simulations.
It was based on the measured fields received from a real-world, active transmit system, and shows that terrain obstructions can produce little effect on received ground wave fields in the MW band.
Multiple houses along even a "flat earth" path produce less of an effect (attenuation) along such path lengths than does total path blockage along that same path caused by terrain elevation(s).
... Do no harm.
Do not even mislead.
The more I think about this suddenly new claim that audio & power-leads take a transmitter out of compliance with part 15, as well as the demonstration of physical obstacles not mattering at MW frequencies doesn't resonate with years of entirely other points of view expressed here in the forums.
First, Part 15.219 says nothing about physical location of the 3-meter antenna/transmission-line/ground-lead... which means it can be placed indoors or outdoors, on the ground or up in the air.
Part 15.219 says nothing about power or audio lines! Repeat that because it's important.
MOST licensed AM stations place their antennas in open fields, often slightly higher than surrounding terrain, free of physical obstructions.
My previously announced plans to experiment with indoor AM antennas has been universally criticized for the very reason that the physical construction of the building attenuates the signal as it passes through walls, thus ushering in the entire STANDARD of placing an out door antenna IN THE OPEN for best results.
In his antenna observations Neil Radio8Z has noted that vegetation LOWERS the effective radiant field especially when wet!
Everyone knows that AM reception under bridges and overpasses is crushed because of the construction materials surrounding the car.
Something goofy this way comes.
The more I think about this suddenly new claim that audio & power-leads take a transmitter out of compliance with part 15, as well as the demonstration of physical obstacles not mattering at MW frequencies doesn't resonate with years of entirely other points of view expressed here in the forums.
First, Part 15.219 says nothing about physical location of the 3-meter antenna/transmission-line/ground-lead... which means it can be placed indoors or outdoors, on the ground or up in the air.
Part 15.219 says nothing about power or audio lines! Repeat that because it's important.
MOST licensed AM stations place their antennas in open fields, often slightly higher than surrounding terrain, free of physical obstructions.
My previously announced plans to experiment with indoor AM antennas has been universally criticized for the very reason that the physical construction of the building attenuates the signal as it passes through walls, thus ushering in the entire STANDARD of placing an out door antenna IN THE OPEN for best results.
In his antenna observations Neil Radio8Z has noted that vegetation LOWERS the effective radiant field especially when wet!
Everyone knows that AM reception under bridges and overpasses is crushed because of the construction materials surrounding the car.
Something goofy this way comes.
Carl wrote "In his antenna observations Neil Radio8Z has noted that vegetation LOWERS the effective radiant field especially when wet!"
While true even to this day with abundant foliage I attribute the measured loss in field strength to the detuning of the antenna due to nearby foliage and not necessarily to absorbed radiation. This is supported by a measured increase in the total antenna system resistance when returned to resonance vs. the resistance seen without foliage.
Neil