That's great, Rich. Thanks a lot.
I wonder if it would be possible, for the scenario with the greatest field strengths, to rerun the simulations and get the results for 16, 32 and 64 radials. It not only would show the effects of the number of radials, but give us what might be the absolute best we could ever possibly hope to do with a legal system.
The reference to greater fields being possible with radiating ground leads responded to ArtisanRadio's comment in reply 27 about coverage of unlicensed systems to a radius of six miles. That, along with higher powered transmitters could be the reaon for that range.
ArtisanRadio wrote: ... I wonder if it would be possible, for the scenario with the greatest field strengths, to rerun the simulations and get the results for 16, 32 and 64 radials. It not only would show the effects of the number of radials, but give us what might be the absolute best we could ever possibly hope to do with a legal system.
Actually, the fields for 30 mS/m earth conductivity in my table in Reply 28 would change very little for those higher numbers of radials. So in effect those fields are about the best one could hope for, for the legal system described in that graphic.
Rich,
A while ago when i was working on my flagpole antenna design, you were fairly neutral about a capacitance hat being legal, possibly due to the fact that the hat itself does not radiate, but enhances the vertical antenna by increasing current higher up the vertical radiator.
Do you think it would be acceptable for part 15 am (15.219)?
Paul.
Paul, this has a legal answer (or opinion) and a technical answer, and the two may not agree.
An FCC field inspector would have to decide if a flagpole using a horizontal "hat" to top load it complied with FCC §15.219(b). If the inspector found that the vertical flagpole itself + the wire path from the transmitter chassis to r-f ground was 3 meters or less, and understood that the top hat was not contributing useful radiation to the far-field signal, then that system might be acceptable to the inspector as far as §15.219(b) is concerned.
If the length(s) of all of the conductors in the antenna system including the top hat were 3 meters or less, total, that would be compliant with 15.219(b), but would not be a very efficient antenna system for the medium-wave broadcast band.
There is nothing in §15.219 limiting the field strength a compliant system can produce, or concerning the r-f current distribution along the conductors of its antenna system.
Thanks Rich.
So, basically, with either good ground conductivity, or a sufficient number of radials (and not taking into account obstructions, radiating power lines, etc.), a signal would be just above the typical noise level at 3 miles.
So, basically, with either good ground conductivity, or a sufficient number of radials (and not taking into account obstructions, radiating power lines, etc.), a signal would be just above the typical noise level at 3 miles.
That essentially is true for 30 mS/m earth conductivity, assuming the r-f noise level at 3 miles is just above 37 µV/m.
But that is less true for poor conductivities. Using more and longer buried radials with those poorer earth conductivities may keep ground losses in the antenna system (and therefore its ERP) reasonably constant over the range of conductivities in my table.
But groundwave propagation loss accumulates as the radiated signal travels along the surface of that poor earth for miles. The longer the propagation path, the poorer the earth conductivity, and the higher the transmit frequency, the more the loss to the received signal.
The biggest killer I have found to our signals happens to be any obstruction of any kind, mainly buildings. Power lines have amazingly not been a problem for my signal, there is only one line in town the signal "ducks" under. There is a set of power lines in the next town over about 6 miles from my transmitter that roar the station back above the noise floor.
The charts are pretty much on par for my rural setup.
Since I live in an extremely rural area in North Texas I have fantastic ground conductivity with nothing to stop my signals. As an experiement when I first got my rangemaster I mounted it on a 10 foot metal pole. It had a long ground lead (8ft) going to an 8ft ground rod. Around the ground rod were 8 radials nearing 15 feet long each. With the rangemaster set to 100mW the signal went 4 miles. Upping the power to 980 mW provided a signal exceeding 7 miles.
However my coverage to the southweat was severely limited by the small town in that direction. The farther you went the signal would start getting some very intersting propation characteristics. In the neighboring town about 5 miles out you would hear the signal shoot back up in strength as you went through intersections. (I assume this had to be due to the long roads that spanned the entire town, making for an unblocked path)
The most intersting interference was experiencing phase issues due to what I assume was skip, though it sounded like when you are on the edge of a cancellation zone where the station interferes with itself.
In Reply 37 I wrote: " ... That essentially is true for 30 mS/m earth conductivity, assuming the r-f noise level at 3 miles is just above 37 µV/m. ..."
On reviewing my post in Reply 37 of this thread I see that I should have written just below 37 µV/m.
My mistake.
Perhaps some of that increased signal strength driving through town may have been due to the overhead power lines acting as passive repeaters or directors re-radiating your signal.
I originally thought that too Mram, until it was still doing it on areas with buried power lines. Some lines in neighboring towns do enhance the signal though.
It is commonly experienced that overhead power lines conduct or reflect RF signals causing a noticible boost to automobile or portable radios, but what else might be caused by man-made infra-structure?
How about overhead cable and telephone lines?
Pipes underground perhaps?
Underground water ?
Rock formations or cavities underground?
Metalic structures along the way - trucks, cars, metal fences, metalwork in buildings?