Hi John, and thanks ...
To be clear, this has nothing to do with Ham radio, only Part 15 as it applies to midband AM. I think some of the gov't radio towers may be slightly above the limit ... I'd have to do more serious checking as I just eye-balled them.
When you say "That being said, given the locale, you should be able to do jest fine with AM part 15." -- what do you mean as refers to an antenna system? Do you think it would work well on the ground as oppose to taking chances with ground for a 1.5' mount, 16' of mast, 1' of loading coil, and 8.5' whip antenna?
IOW, will it work well with just the 1.5' base mount, the 1' coil, and 8.5' whip? What if I use just one mast section (8')?
I knowm, but if had a ham license, you could build a tower, say 50 feet, and then mount ur Part15 on that. It sounds like your town is run by........
Hi John,
Easy enough to go 1000 yards and be outside town limits. If you have time, take a look at the location for the KSJU 91.9 LPFM antenna system, as spec'd in the FCC construction permit. It's a decent location all things considered, just outside town, on the highest ground in that area. I'm sure it will be as much as it can legally be. As you may know, one of the requirements for LPFM licensing is EAS capability, so that should quell any other complaints. We'll have to see.
Back on topic ... So the question remains:
Will adequate ground radials (how many, how long?) in a half-moon pattern (close to the back wall of the building my studio is in) be enough if it's mounted close to the ground, or should I go for height and a ground rod?
I think the best way to go is to experiment.
Wires to ground rods can radiate, and I suspect that that is the real reason you will get the 1-2 mile range that the Rangemaster and other similar transmitters claim. Plus you have to determine if that signal is listenable for extendable periods of time, and on what kind of radio.
If you can get your antenna up on a roof, and put radials on the roof, that could be a solution (the more and the longer, the better). Or, better yet, if you can mount the transmitter just inches above a metal roof (even on a metal shed), you will get a capacitive effect and the roof will act as a large ground plane (which is what radials simulate). I've personally seen a huge impact to a micro power FM signal with this situation, and I plan to try it for AM as well.
Otherwise, I'd go for the ground mount and radials, the more the better and the longer the better (up to 1/4 wavelength).
Be sure to check the Ground Conductivity Charts, here to see what your're working with.
Here's a good general grounding article, Grounding in RF Environments By William D. Chesney, N8SA
Using the Talking House and remote ATU antenna, I recently added 6 radials each about 10 feet long evenly spaced around the base connected to the antenna tuner ground tie point.
There is a definite range increase, not dramatic but still noticable.
The radials were made using 18 awg insulated solid copper wire.
Already know about general ground conductivity here ... not so good. The airport recently built a brand-spankin'-new NDB antenna, 284 kHz. It looks about 50 ft., capacity hat, and 8 ground radials, I think. They bulldozed and disked the area, turned over a lot of soil, removed lots of boulders. Not commissioned yet. Cost a lot of $$ ... we'll see how good it works.
As to the other article ... there is something I can't understand: If the transmitter ground-side is connected to the building ground through the audio and power cables, and the other ground wire from the transmitter is connected to a separate RF ground system, say a ground rod and some radials, all bonded at the base of the antenna, how are disparities between the building's ground grid and the transmitters RF ground system resolved?
32 to 64 radials should be installed as a minimum on any ground radial system. Beyond 64, the improvement is negligable. There has been a lot of discussion as to insulated and uninsulated.
Right now, I'm using 4 25 foot insultated radials, laid on top of the ground, screwed into a piece of copper tubing that I've got surrounding my mount post. Transmitter is the Rangemaster AM1000. Range is 1/4 to 1/2 mile of usable (listenable) signal (with some noise) in a fairly hilly and treed area (the longest is through a valley and across a small lake).
There is so much conflicting information out there about radials, regarding numbers and length. And it all sounds reasonable.
I'm going to experiment with using CAT-5 cable - trying 8 10 foot pieces (spreading out the ends as suggested) for a total of 64 radials, 16 10 foot pieces for a total of 128 radials, and then see what the effect is of 8 20-25 foot pieces. Luckily, I have a lot of this cable lying around not doing anything - I once used it to suppy power and audio to a roof-mounted transmitter. I think I'm even going to try 4 1/4 wave lengths (for a total of 32 1/4 wave radials).
I'll report back here when I get the results.
Ken,
Once again you brought up a good point and an excellent question which I will try to answer.
You wrote:
As to the other article ... there is something I can't understand: If the transmitter ground-side is connected to the building ground through the audio and power cables, and the other ground wire from the transmitter is connected to a separate RF ground system, say a ground rod and some radials, all bonded at the base of the antenna, how are disparities between the building's ground grid and the transmitters RF ground system resolved?
Since the building ground system designed for 60 Hz power applications is not necessarily useful as an RF ground, consider that the radial ground system is the principle focus for RF.
Indeed, having two grounds for low frequencies in a system can cause problems which, in this case, will result in power line hum in the audio signal. The simplest and an inexpensive way to resolve this is to use an audio coupling transformer in the audio circuit to "break" the ground loop. Others here have suggested using a "ground isolator" which is available from Radio Shack for $16 or so. I would wait to see if I had a problem first but this could be a solution.
Looking forward to you posting the results of your experiments.
Neil
How about wire gauge? I'd think that would have more impact than insulation. I'm a bit concerned about corrosion ... this is generally a marine environment, hard on electronics any ol' way. I do have about 100' of 12 AWG insulated multi.
I don't have enough wire lying around to make that many 20' radials, but then I'm up close to a building anyway (can't go more than yard or so to the building side of the mount). I do have about 100' of 12 AWG insulated multi.
Meanwhile, I need to get it approved, up and tuned by Friday.Whatever I can get out of it.
After that, I think I'll move ahead on Dick Rich's advice and use the formulas he gave me (out of a 1939 physics book!) to figure induction of a 10' diameter loop. If the values look good, we'll go ahead and build an RF network to to tune it.