This subject has come up a
little bit here and there.
What I have gotten from this,
is that Part 15 FM is legal
for about 200 feet on an
average radio, such as an FM portable -
that is: a radio that is not made
to pick up DX.
And - for a car radio, which is much much
more sensitive, 600 or so feet from the transmitter shows that all is well.
Maybe even a little more range if the radio
is a real DX machine.
That is what I "think" is correct.
However, others have said that anything
more than 200 feet away from the
FM transmitter with a very very sensitive
radio is about the limit. Anything more
than that range under those conditions
would indicate that the transmitter is
over the Part 15 limit.
I tend to go with the first idea, just
knowing what 250 uV/meter @ 3 meters from
the transmitter can do with a very sensitive
receiver in a car.
I guess in a way, if you are playing by
the rules on FM, the signal is going
to be weak - so nobody will hear it anyway,
unless they live in a very nearby house with
a really good radio. I suppose I'm just
splitting hairs here. Right now, I don't
have any FM transmitters here - certified
or otherwise. So I can't really make a test.
(Some of my radio equipment has been put into
storage for the time being.)
Just wondering. This comes up once in a
while. The result of this is that Part
15 AM is the way to go anyway.
Now, our friends in Canada and New Zealand
are more fortunate with this range question -
especially New Zealand. Maybe I'll move there
and get a fresh start.
Bruce, The Dog Radio Group
Some of the better car radios have sensitivity specs of 1.5uv with quieting (generally about 30-40 db). But it takes a greater field strength - on the order of 2.5 times that - to induce that kind of signal into the radio (that's an estimate from previous calculations here). That translates into a required field strength of 3.75uv/m.
If you do the math, starting with a field strength of 250uv/m at 3 meters, you'll get that approximate level of field strength at 200 meters, or about 650 feet. And that's line of sight to the antenna, no obstructions.
However, you could expect to get a little more range with less quieting with that kind of radio - the signal will be noisier, but still listenable, at least for a while. And a mono signal will eke out a bit more distance. That's where in a previous post I came up with the 600-800 feet estimate.
However, since most consumer radios have sensitivities probably on the order of 25 to 50uv, you'll get much less range on them (and that's ignoring their lack of adjacent signal rejection, which can reduce range for your puny Part 15 signal even more). If we're charitable, and assume 25uv, then you need a field strength on the order of 62.5uv/m. Doing the math, you'll get 125uv/m at 6 meters, and 62.5 at 12 meters or under 50 feet. That 200 foot estimate is pretty generous for ordinary radios.
I think you're right.
That makes more sense,
and you gave the numbers
to show it.
I appreciate that.
Best Wishes,
Bruce, The Dog Radio Group
Just a further comment. Canada IS better than the U.S. for unlicensed FM broadcasting, but even 100uv/m at 30 meters (or 1000uv/m at 3 meters) is pretty pathetic. Line of sight I got up to 3200 feet range. My transmitter was located on a rooftop on the side of a hill overlooking my desired coverage area, so it was effectively line of sight for a lot of listeners.
However, that was to the front. At the back, there was a cliff, and behind the cliff a little ways, blocked from line of sight (and only a few hundred feet away) was a road, fronted by some houses. You could not receive the signal back there, even in a car on the road.
And the range to the front decreased dramatically where there were obstructions (mostly trees) to 1/4 - 1/2 km, with some entirely dead areas.
Hi again, Artisan!
I have read the specs on your
permitted FM field strength
many times, but I always keep
forgetting what they are. Maybe
now I will remember.
It's always good to hear from
you. There are many people here
who really know what they are
doing, and you are one of them.
This is a subject for another thread,
but, did you order one of those 13.560
rigs? If I'm correct and you did,
it will be fun to hear about it when
you are ready to do that.
Bruce, The Dog Radio Group
Yep, I actually ordered 2 (one for backup, or in the event that I screw up the first one changing the crystal controlled frequency to 13.560).
I'm still working on how to reduce the output to conform to RSS210 specs (our Part 15) but the builder has provided me with several modification suggestions (yet another reason to have a backup).
I'll certainly report what happens in here.
I should remember these things.
By the way, if you chose to
lower the output of the 13 MHz
transmitter with an attenuator,
you shouldn't have any problem.
I built a 10 dB attenuator for
my carrier current transmitter.
It reduced 6 watts down to
600 mW and only cost a few dollars.
There are lots of places on the web
to find schematics and parts values
for RF attenuators, but I'm sure
you know that. I guess the big thing
IS to get that HF transmitter to
operate correctly on the new 13.560
frequency.
Also, I believe RSS-210 permits
another unlicensed HF allocation
in Canada. Isn't it in the 7 or
8 MHz range?
Best Wishes,
Bruce, The Dog Radio Group
Just a side comment, the FCC seems more interested in enforcement of the rules for FM than AM as evidenced by reviewing the NOUOs issued. This could also be due to the relative numbers of FM units in use vs. the number of AM units in use but the difference in the number of cites is remarkable.
It is also much easier for a user to comply with the power and antenna limits for AM (15.219) as opposed to the field strength limits for FM (15.239) primarily due to the difficulty of making appropriate measurements.
My AM system, which I believe is compliant, gives a "car radio" range of 1 mile but given the typical low receiver sensitivity and high noise environment which are factors for AM reception the usable range for fixed receiver locations is most likely much less than a mile.
I have heard an AM "Talking House" transmission which originated about two miles from here but this was done using a good communication grade receiver and an outdoor dipole antenna.
Neil
we had 100uV/m @ 30m prior to 1989 when it was changed to 250uV/m @ 3m
i'd like to have the old FM standard back.