To expand on the subject of an elevated Part 15 AM transmitter with attached ~3-meter whip and two horizontal radials, on the roof of an A-frame building...
To expand on the subject of an elevated Part 15 AM transmitter with attached ~3-meter whip and two horizontal radials, on the roof of an A-frame building...
WIRES: Dr George Brown of RCA Laboratories in Princeton, NJ is the inventor of the "ground plane" antenna, which usually consists of a vertical conductor driven against 4 horizontal conductors at the base of the vertical. This design is essentially what is being discussed here. In Brown's autobiography he wrote that only 2 horizontal conductors are required, but early customers thought that the antenna would be bi-directional, so RCA marketing people decided to use 4 horizontal wires, to avoid the perception problem.
Brown used conventional mathematics to determine the radiation pattern and gain of a ground plane. Nowadays NEC software makes it rather easy to show that there is no difference in the radiation pattern and gain of a ground plane when using 4 radials vs. 2 radials. This is true because the only useful radiation from this configuration is produced by the vertical conductor, alone.
Using more than 2 radials is very important if those radials are buried. But their performance is not the same when they are elevated above the earth. Two elevated 1/4-wave horizontal conductors used as a counterpoise for a vertical monopole can give essentially the same result as using 120 x 1/4-wave buried radial wires.
LIGHTNING: A 3-m whip driven against 2 elevated horizontal wires as a counterpoise can produce about the same field strengths and coverage area as when installed at the surface of the earth using a connection to the earth rather than the counterpoise. When using a typical transmitter rated for 100 mW input power these fields will be much greater than permitted by FCC 15.209.
But the elevated system using the counterpoise has no means of draining off static charges that could build up on the system, and is more susceptible to damage from nearby lightning activity.
The solution for this is to install a "lightning ground" conductor from the transmitter to an electrical ground such as a ground rod buried in the earth. But then we have the issue that using such a conductor length of more than a foot or so likely will be non-compliant with Part15.219(b).
Radiation from that ground lead can be made negligible, and still allow reasonably good lightning protection for the system by installing components that prevent r-f current in the MW band (only)from entering the ground conductor (and the power and audio conductors) where they attach to the transmitter.
Use of ferrite chokes along the lengths of those conductors also might be required, because they could have r-f currents induced on them by radiation from the driven conductors of the antenna system. Unfortunately that also may reduce the performance of that path as a deterrent to lightning damage.
But whether or not the FCC would accept such a system configuration as compliant with 15.219(b) is unknown.
This matter would be good topic to be investigated by the manufacturers of Part 15 AM transmitters, who could develop the necessary hardware, in collaboration with the FCC as to the compliance issues. Successful resolution of this could produce increased interest in Part 15 AM, and more equipment sales for the manufacturers.
...but is another solution to prevent lightning damage as simple as turning the transmitter off and unplugging it during an electrical storm? Then the only way the transmitter would get fried would be if you've lost the "lightning lottery" and your antenna gets a direct hit from a lightning strike, right?
I'm new to radio so please cut me some slack if this is the world's dumbest radio question.
...but is another solution to prevent lightning damage as simple as turning the transmitter off and unplugging it during an electrical storm? Then the only way the transmitter would get fried would be if you've lost the "lightning lottery" and your antenna gets a direct hit from a lightning strike, right?
I'm new to radio so please cut me some slack if this is the world's dumbest radio question.
...but is another solution to prevent lightning damage as simple as turning the transmitter off and unplugging it during an electrical storm? Then the only way the transmitter would get fried would be if you've lost the "lightning lottery" and your antenna gets a direct hit from a lightning strike, right?
I'm new to radio so please cut me some slack if this is the world's dumbest radio question.
My Cricket Wireless mobile broadband connection kept dropping out every time I hit save and somehow my comment was posted 3 times. I apologize for bombarding the forum with duplicates.
Channelx1610 wrote ...but is another solution to prevent lightning damage as simple as turning the transmitter off and unplugging it during an electrical storm? Then the only way the transmitter would get fried would be if you've lost the "lightning lottery" and your antenna gets a direct hit from a lightning strike, right?
Yes, that's a good start to protect the transmitter from lightning transients arriving through the a-c power line.
To carry on with that idea, during a lightning storm it would also be good to connect the base of the 3-meter antenna, and the chassis of the transmitter to a good, low-impedance earth ground.
A Part 15 AM transmitter and its attached ~3-meter antenna both installed indoors probably don't need more protection during a lightning storm than just disconnecting the a-c power supply -- or at least connecting the power supply via a plug strip with built-in transient protection for the a-c line.
Channelx1610,
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Neil
Does a counterpoise technically work the same way as a mobile CB radio grounded to an automotive frame? My dad has a Cobra CB radio and he couldn't transmit very far until he grounded the antenna to the frame. Is this technically the same concept except the transmitter and antenna are grounded to radials on the roof?
Another idea I thought of is possibly grounding the transmitter to the aluminum gutters and attaching a wire from the downspout to the ground. I don't know if this would be legal or not. You could use an 8 inch length of wire to connect from transmitter to gutter and another 8 inch section from downspout to grounding rod. The only downsides to this idea are 1. it wouldn't be a radial system, and 2. If your transmitter gets struck by lightning, your gutters would be severely scorched.
"To carry on with that idea, during a lightning storm it would also be good to connect the base of the 3-meter antenna, and the chassis of the transmitter to a good, low-impedance earth ground."
This is a good idea, though a hassle to deal with connecting and disconnecting whenever a storm rolls through. You could install a shorting shunt via a relay in off state so that when the relay has no power applied, it closes contacts to shunt the 3 meter antenna radiator directly to ground. A blade switch controlled by a servo like that used in the bumpers of pin ball machines. You could go even further and install a photo sensor that would control the blade switch so that at a certain light intensity flash point that servo engages the blade switch and grounds the antenna system while at the same time, either shutting off the TX, or taking it's output to an appropriate dummy load contained within the same enclosure as the TX is and matching coil etc.
A simpler approach would be to use a small engine spark plug and let that be your "spark balls". One side of the spark plug is at ground potential (case) and the anode connected to the 3 meter rod. Just gap the plug .25 or so and it will work fine.
You could combine these things into one and have a pretty darn well protected system and automate it too. It's all a matter of ideas and trying them out.
RFB