Part 15 AM Coverage
(Re-posted from another list. Author of the comments is R. Fry, the submitter of this message to the Part15.us Forum)
LH wrote:
>Next time a non-D AM tower falls over, let's clean up the mess,
>then install a Rangemaster or other high quality Part 15 transmitter
>on the ground at the center of the radial system and see how far
>it gets out, measure the field strength, etc. The official 3 meter
>antenna to be used.
_____________
An interesting concept.
A 3-meter vertical radiator is ~3.6 electrical degrees at 1 MHz. Using
Figure 32 in George Brown's paper I referred to in an earlier post, the FCC
efficiency of such a vertical with a ground system consisting of 113 radials
of 0.27 wavelengths each is about 40 mV/m at 1 mile for 1 kW of radiated
power.
Using that efficiency with the FCC's AM curves for 1 MHz, and assuming that
the Part 15 AM tx could supply 80 mW to the radiator,* here are the
parameters for a ground conductivity of 8 mS/m:
2.00 mV/m 0.1751 miles
0.500 mV/m 0.6695 miles
0.100 mV/m 2.9125 miles
NEC-2 calculates a base impedance of 0.04 -j6500 ohms for this set of
conditions (4mm constant OD radiator). No practical transmitter could
deliver its rated power into such an impedance, but we have ignored that for
these calculations. (The reactance term can be cancelled by using a loading
coil, but the required coil would absorb a large amount of the available
power due to I^2R loss).
Considering that Part 15 AM stations using intentional radiators neither
have the ground system described above, nor can cause the RF current
equivalent to 80 mW to flow in the radiating portion of a 3-meter antenna,
it is clear that any Part 15 "coverage" claimed that approaches or exceeds
what is shown in the calculations above must be related to the use of an
illegal antenna system, and/or illegally high tx power.
* AM Part 15 limits the tx to 100 mW of input power.
Output power will be less -- 80 mW was assumed
for this calculation.
Richard Fry, Broadcast Applications Engineer (retired)
[email protected]
I love theory.
Now - for practical application and real life:
A free radiating Part15 AM TX running just under 100mW and using the tesla antenna (the Manteca Magnum) was getting about a 6 mile radius. Completely legally. This system was built by a Stanford Physics Professor who likes practical application more than paper shuffling. It stood the FCC Field Agent test, as there were 2 of them living in listening range to the station. I spoke with one of those agents regularly.
Another station in NJ, using a single Rangemaster was getting very good coverage of several miles. Again - withstanding the FCC test, as there were visits by them per the station managers request - because *he* wanted to make sure he was in line with the rules.
Several stations run by the gentleman who runs the lpam.net site. You guessed it: more legal Rangemasters that stood up to the test.
You can put your calculations in one hand...and the practical app in the other... the practical app weighs in.
Now - if you continue to needlessly and mindlessly troll this board, you won't be greeted with a positive response. We've kept things pretty much open, and while we may not agree with one another even on a personal basis, we have enough respect for each other not to try and dissuade one another from our chosen hobby.
We've been over this here and on other boards a hundred times. And when it comes down to it, we have done, and will continue to do this - its what we do and we like it. We take a very simple to read, straight forward, no nonsense Rule, and we make it work for us.
Now, you seem like you know something that could be valuable to the group, so stop trying to impress us with numbers that, while they jive on paper, obviously don’t stand up to real life, and join in the fun. We could use someone who can perform more than basic math functions...