Is the RF ground in the C. Crane FM Transmitter-2 separate/filtered from the power or audio grounds?
I have a C. Crane FM Transmitter-2. I want to put it in a weather proof box up high outside. It would be easiest if I could run a single cable to it. I am thinking either using a coax cable with audio/DC combined or a balanced audio cable with an arrangement similar to phantom power. Either way, I end up tying the power ground and the audio ground together and they get tied to the shield of the cable.
I have thought about the RF ground a bit. If the short antenna on the FM Transmitter-2 wants something to push against, RF could end up on the shield of the combined audio/power cable. I could either suppress that with chokes as close to the FM Transmitter-2 as possible, or I could see if some length of coax between the board I used to split the audio from the power and the chokes gives me more range.
In the C. Crane manual for the FM Transmitter-2 they suggest putting the transmitter over a metal ground plane to make it work better. That makes me think they are counting on capacitive RF coupling between the FM Transmitter-2 and the metal surface. That got me thinking about how I could really work to get the shield of the coax to act as the other half of a dipole for the FM Transmitter-2.
I would make a circuit board the size of the FM Transmitter-2. One side would be a ground plane. The other side would have the capacitors and inductors to split audio from power on the single cable up, as well as a jack for the audio to the FM Transmitter-2 and a cable to run power to the DC jack on the FM Transmitter-2. The ground plane on the circuit board would be tied to the shield on the cable feeding the FM Transmitter-2, it would be tied to the DC ground, and the audio ground. The goal would be to get as much capacitive coupling between the board and the FM Transmitter-2. Then I would put chokes on the cable at the distance from the board that gives me the best signal. As a first guess I would think around a ¼ wavelength of the frequency I am transmitting on.
Then if I got complaints or an inspection, I could pull a unmodified FM Transmitter-2 from the box; all I would have to do is unplug the audio cable from the board, and the power cable from the FM Transmitter-2.
Part of figuring all that out depends on what the FM Transmitter-2 does with RF and ground.
I have looked for the same ground point in the C.Crane FM 1 and the Wholehouse 2.0, whose only external access to "ground" is the audio jack.
The schematic for Crane FM 1 is linked here
http://scwis.home.comcast.net/~scwis/files/ccsch.pdf
We see that power leads and audio ground have in-line inductors.
I agree that direct access to RF ground in these transmitters would be useful in antenna planning, and I hope someone has more information to share on the subject.
... In the C. Crane manual for the FM Transmitter-2 they suggest putting the transmitter over a metal ground plane to make it work better
One entry in the list of FAQs at http://www.ccrane.com/transmitter states that the C. Crane FM Transmitter 2 is certified by the FCC.
If that is true, then operating it in the configuration in which it was certified could not legally produce a field intensity greater than 250 µV/m 3 meters away in any direction from its fully-extended whip antenna.
Its maximum radiated field 3 meters away from that system using that FCC-certified physical/electrical configuration could legally be less than that, however.
The C. Crane website suggests for the FM Transmitter 2 that to increase range, use a clip-lead to connect its built-in whip antenna to a longer radiator.
The FAQ list there also suggests that installing it over a ground plane such as a filing cabinet or refrigerator might help.
All of that could be possible, and __might__ not result in a system that is non-compliant with FCC §15.239.
Unfortunately, accurate measurements of such radiated fields requires test equipment that almost no Part 15 operator has.
But if any installation configuration of an unlicensed FM broadcast band transmitter whether or not FCC-certified under Part 15 produces a radiated field in excess of 250 µV/m 3 meters away from that system in any direction, then it would not be compliant with FCC §15.239.
The importance of this is left to others to decide.
It is the FCC (and not Rich(F)) which, in my opinion, is talking down to the audience in the case of FM Part 15.239.
The "audience" in this case are the users of this equipment, certified or not.
While the rule based on field strength serves as a guideline for the manufacturer seeking certification it is of no benefit to the customer who hopes to reach the very limit of what's allowed without exceeding the micro-power-level.
There are excellent FM transmitter kits, but no part of the rules offer a reasonable guideline for determining their compliance.
The FCC rule committe must have taken a long lunch after writing 15.23(b)...
"It is recognized that the individual builder of (home built) equipment may not possess the means to perform the measurements for determining compliance with the regulations. In this case, the builder is expected to employ good engineering practices to meet the specified technical standards to the greatest extent practicable."
Although the FCC makes distinctions between certified, kit and self built, to me any and all equipment employed by a home-user is HOME BUILT by the time it's all put together.
It's easy to be cavalier and shrug-off small people lacking corporate wealth, but it is not easy to respect such a regime.
Ground is any point on the circuit board. Any area where there are no components(brown or green or whatever color the board is painted), if you scrape the coloring off and expose the copper underneath, that is ground. The components leads, ICs included, that are on the + side will have a separate insulating ring on the circuit board isolating that contact from ground.(hope I explained it right)
So you can find a suitable spot on the board to solder the shield of the coax.
Not that I am encouraging you to violate FCC rules but you can turn the POT. fully counterclockwise to get max signal.
You will see this when you open the case up and take the back off.
Mark
... to me any and all equipment employed by a home-user is HOME BUILT by the time it's all put together. ...
And no problem there, as long as said home-users can convince FCC field inspectors that the unlicensed system(s) the FCC is about to (or did) cite for non-compliance with Part 15 are entirely HOME BUILT, and comply with "good engineering practices to meet the specified technical standards to the greatest extent practicable," even though they use transmitters that were NOT home-built by those operators.
A small piece of electronic equipment such as a C.Crane transmitter is a PART, and can be used as such by attaching other PARTS giving the whole project a bona fide home builtiness.
It is not so bad, however, to have a sargent-at-arms here in the forum pointing out what can and can't be done (mostly can't).