After 2-hours going through threads here on the site, pouring over the Part 15 Rules and searching my many documents, I still am not sure. Is it possible to do FM leaky-cable over a given area?
After 2-hours going through threads here on the site, pouring over the Part 15 Rules and searching my many documents, I still am not sure. Is it possible to do FM leaky-cable over a given area?
Leaky FM Cable
Carl,
Part 15.239 which covers operation in the FM band has no restriction on the type of antenna so if the field strength and other technical requirements are met leaky cable operation is OK.
I recall that there is a company which produces units for retirement apartments, nursing homes, etc. which operate this way.
This is only my opinion and not legal advice.
Neil
Yes I See
Yes, Radio 8Z, I see what you referenced and have noticed that the Rules allow up to 50-Watts to drive a cable system, although that amount of power would only apply in the case of a very large system, whereas covering the length of an ordinary lot would probably be under 10-Watts, since the radiation from the leaky cable must not exceed those same limits that would apply if an antenna were used.
That paragraph seems to fit the rules, unless someone has a correction, so I move on to another idea that seems economical, since the actual cable wiring would be fairly expensive: double use of the cable to serve both AM and FM.
If an AM and FM transmitter were combined at the outputs with proper matching, both signals could likely drive the one cable. Comments?
AM and FM Cable System
Here are some thoughts on this. AM and FM signals could be superimposed on a cable but the “leakage factor” will most likely be very different for the two signals. I suppose this could be compensated for by properly adjusting the respective powers.
Regarding expense, the cable itself need not be anything special, in fact it takes effort and expensive materials to make a coax cable “RF tight”. I know from experience how difficult this can be. The foil shielded cables do a good job but there still can be problems at the connectors.
The problem with AM will be that operation will have to be under part 15.209 which is a field strength limit rather than a physical limit on dimension. The allowed field strength is much less than that produced by a system compliant with the 3 meter rules. This may not matter if the intended audience is close to the cable such as in a high density residence or similar facility.
It would be simple to try this using some basic high pass and low pass filters. The difficulty for an experimenter would be properly measuring the field strengths.
Neil
Response to Neil
Radio 8Z, you have made points that match what I have presumed in my abstract view of this idea of mixing AM and FM on a single leaky cable. It would be good if I described the objective.
At present no antenna arrangement for either AM or FM covers the full length of my long but narrow yard, where I often travel about doing landscaping. FM is, as you would probably expect, worse than AM. As of now I have not built outdoor antenna sites, which might also be somewhat beneficial. What I am thinking for the moment is that the cable would uniformly cover the entire yard with an equal signal.
To meet the radiation requirements it seems to me that minimizing power so that even 100′ beyond the yard the signal would be almost gone, sacrificing the more common rule-of-thumb of 200,’ which I think has been quoted as an informal measure for both AM and FM. In that way perhaps I would fall well below the limits of a true RF measurement.
What I am beginning to envision if having several systems, each one refined to solve a different problem. The leaky cable would solve the problem of yard coverage, and for AM could probably use a very low frequency, such as 570kHz, which probably wouldn’t tend to escape the boundaries by very much.
Hi Carl:
I had an idea that is sort of like the solar powered
repeater I am working on, but a little bit different.
It is expensive, but it’s just another idea.
Imagine the shape of your yard. In my case, it is
a rectangle. Now I know this isn’t easy, but it’s
just theoretical.
Run an audio and supply voltage cable around your
yard. In my case – it would be just on the outer
edge of the rectangle. Put an FM transmitter on
each corner of the rectangle – or just on each leg.
In other words, one on the front center, one on the
back center, one on the left side center, and one on
the right side center.
I thought of trying this with 4 Maxell P-13s, which
only cost about $20 each. (Or course the cable will
cost money too.) Put all 4 transmitters on the same
frequency and walk around the whole yard and see
what it all sounds like on a radio. My guess is: (and
it is only a guess) that the transmitters are so low
in power that a good FM radio will just chose the
strongest one and you will have coverage of the
entire yard, not too mention whatever coverage is
left beyond the yard.
I really don’t know if this would work but it’s another
idea. Also, I know there is some kind of FCC limit
on how many Part 15 transmitters you can own.
I don’t remember the actual rule.
I can tell you it might work in my yard. The lot is
75 feet wide and 200 feet long – a big rectangle.
Also one more piece of trivia – I have had two Part
15 FM transmitters on in my house on the same
frequency by mistake. Again, it seemed as if the
radio chose the dominant transmitter depending on
where the radio was located. The 250 uV/m@3 m
field strength is not that strong. And that is the key.
There did not seem
to be any place where there was no quality signal and just
a mess. Well, I suppose this is something to experiment
with.
I will definitely try this with 2 Part 15 FM transmitters spaced
about 100 feet apart. I will report my findings.
For those of you who have DXed FM radio, you will probably
remember E-skip openings where multiple FM stations would
fight for dominance on one frequency. Sometimes multiple stations
would just go back and forth with no messy interference sounds.
This would be as if you were flicking a switch between the stations.
My idea is based on that observation of weak FM signals.
Best Wishes
Bruce, MICRO1690/1700
Hi Carl: One Question For You
Hi Carl:
I suddenly have a feeling that your yard is MUCH
bigger than mine is.
Is that true? If it’s A LOT bigger than my yard then
the multiple FM transmitter idea would not work.
Best Wishes,
Bruce, MICRO1690/1700
Worth Trying
Bruce MICRO 1700 I find your idea well worth adding to the tool kit as a possible solution to FM coverage. I will also do a two-transmitter-on-same-frequency trial just indoors.
Our yard dimensions are almost identical, except I think my width is closer to 60-feet. I should go read blue-prints.
My biggest problem on AM is the uphill side, where my AM signals refuse to climb the hill. Down front at the bottom of the hill coverage is perfect.
For FM these little certified transmitters are silly toys, because their signals can’t make it across one room and rarely into another, but of course the rule of height is absolutely the way to cover more area… just get it way up in the air, above the ceiling, above the roof.
Here’s Another Question For You Carl:
How far does your C Crane transmitter go and
what kind of radio are you using to receive it?
Same question with your Whole House Version 2.0.
I just thought of something to support the theory
of weak FM signals just sort of switching back and
forth. There are three local/semi local FM stations
heard here in West Hartford on 107.7 MHz. One
is a weak college station running low power which
is about 4 miles away but is blocked by terrain. The
other two are higher power stations that are farther
away. As I rotate my radio, the stations just flick
back and forth.
For what it’s worth.
Bruce, MICRO1690/1700
Not Quite Laboratory
My testing methods so far may not have been as “laboratory” as necessary. But here is some description.
The Crane sends from my computer to a Technics FM tuner one room away where the audio feeds SStran AM 1550. At first the Crane was constantly interupted by my walking in the signal path, which happens to be a doorway. So I added a 1-and-a-half foot clip lead wire horizontal to the Crane’s vertical antenna and that solidified the FM signal. The tuner has one of those T-shaped antennas made from 300-ohm flat cable, but it’s at a funny angle because of a wall.
I hooked the Whole House right with the Crane, but its antenna is only vertical and does not have the horizontal add-on. At the tuner, it comes in at half-scale, whereas the Crane is full-scale. The two frequencies are 101.9 and 107.1
Earlier studies found the Crane and Whole House to be equal.
My CCRadio Plus has a very sensitive FM section, but is right close to the two FM transmitters and gets good reception. Over there in another room at a desk is a Sangeon ATS 505 which has a great FM section and picks up both signals.
The real test is the walk-about Grundig FR-200 which is tops with AM but perhaps the FM is too sensitive, because it is swamped by high power signals. Getting either of my weak signals is very poor in the house and almost non-existent outside the brick walls.
The transmitter that really plows through the walls and delivers a solid signal is the Scosche FMT4R at 87.7, across 1-1/2 rooms
Most distances referred here are around 50-feet.
The heights of the FM transmitters are about shoulder level, and I’m making an indoor tower out of bamboo poles to get the antennas up by the ceiling.
Another thing is that my transmit antennas are flat against the wall, until the tower allows getting them out into the room a few feet.