The more cautious Part 15 operators have found that the very small field strength allowed for FM is simply too insufficient for serving even a very near-by public.
We even fear that ANY presence on the FM dial might attract unwanted attention and several of us have talked about avoiding FM altogether.
But I want to explore one last idea to use FM for a Part 15 purpose.
As it is presently set up, two of our AM transmitters receive their audio from an FM Studio-to-Transmitter-Link (STL). This is convenient and uses presently existing certified FM transmitters, but the omni-directional nature of their radiation wastes most of the bitty-power in unneeded directions.
What we need are ultra-directional antennas between the transmitters and receivers, with the FM power reduced to an absolute minimum to get the job done.
As we know, directional antennas concentrate all the power into a highly focussed beam, therefore very little power would be needed to make it work.
In most directions the side-radiation would be negligible.
Is the engineering to accomplish this a realistic idea?
Probably the most effective gain (directional) antenna for the FM band would be the Yagi design which consists of elements of approximately 1/2 wavelength appropriately spaced. This will be physically large. Another design is the coaxial colinear (or stacked dipole) which is also large but gives omni-directional coverage.
But the situation on FM under Part15.239 is that the maximum field strength is limited in any direction so concentrating the energy radiated with a gain antenna will not increase range if this limit is observed. One might as well radiate omnidirectionally observing the FS limit and use a practical antenna.
One thing to consider for a link is a modulated LED/phototransistor link. These are line of sight and can provide a frequency response up to about 25 kHz and with lensing can give a range of many tens of feet. They are not expensive nor difficult to build. A drawback is that if the beam is interrupted the signal is lost.
Neil
Baby monitors and cordless phones have been pressed into service as STL devices also.
On the plus side, not to many people monitor the baby monitor band.
If the two AM transmitters are installed in the same premises with the studio, couldn't they each get their program input from a (low-Z) wired path from the studio gear? That would eliminate all concern about compliance with §15.239.
Hard wired audio hookups have been avoided because there is no hole drilled in the baseboard from the source location, but now it seems like it might be worth planning some woodwork surgery.
But, exploring the impossible also has appeal, so I have one more FM directional question:
is there a one-way directional antenna for FM? (no back lobe)
This depends on the definition of "no."
Here is a link to an FM antenna with a single main lobe, but it still has some radiation in the opposite direction, and others. Also probably it is too big to install inside a home, and wouldn't produce its published patterns if it was, due to the nearby wires in the home.
http://www.winegarddirect.com/pdf/spec_HD6055P.pdf
I love it!
There is a space in the attic right up above the main control desk where it could be mounted.
Ironically, I'd have to drill a hole to get the audio up there.
The verge of madness comes in handy for projects like this.
is good, but to echo Neil's comment in reply 2 above, the maximum field from this directional antenna still could not exceed 250 µV/m at the peak of its main lobe.
Its free space gain at 98 MHz is shown as 8 dB above a (1/2-wave) dipole. So the matched input power it would need to produce the maximum field is only ~40% of what a dipole would need. That value would be 0.4 x 0.000 000 0114 watts = 4.56 nanowatts.
Probably not many "Part 15" FM transmitters can be accurately adjusted for either of these output powers.
There's always another problem at the end of every solution.
The enthusiasm has been closed to preserve or conserve energy.
Neil's idea up in Post # 2 for an LED/Photo Transistor link is a welcome idea, but let me toss in some logistical considerations which I think can be overcome.
The transmission starting point is directly left of the control desk, a bamboo tripod on top of a footstool which now contains the FM STL transmitter which serves two AM transmitters, one downstairs and one in the next room on the same floor. The tower can be adapted to hold an LED Transmitter.
The doorway in the signal path is the most used in the house since it leads to the kitchen, but the LED beam could be mounted high so that it passes through the doorway above head level.
The Photo Transistor Receiver could be mounted correspondingly high on a cabinet which now holds a toaster oven and FM STL receiver. That could solve one hop of the link.
Now, to get the signal to the basement AM transmitter, the FM transmitter could feed a 1/4 wave horizontal wire, placed under the tower near the floor, putting lobes downward and upward. Of course there would also be side-to-side lobes but we're looking at the buildings next door on either side, so the outside field would probably be nill.
Many years ago, I transmitted a legal (15.239) FM
signal to a neighbor friend whose house was
about 80 feet from mine.
His receiving set-up was a sensitive analog FM
tuner wired to a good sound system. It was in
his living room, and he had a 300 ohm dipole
receiving antenna tacked onto the wall.
And he actually did listen to my station. Also,
I used a mono transmitter so the the little
signal would maybe have less noise, or maybe
none at all.
In the world of Part 15 FM, I think the
above example is probably the only
working scenario.
Bruce, The Dog Radio Group
My attempts to keep FM going involve even less range than Bruce's example, because I'm trying to stay in-house.
I've decided to keep the C.Crane in service, located 1.5 feet from the radio, which I use for audio editing projects, but with one major change: I now turn the transmitter OFF when I am not editing!
The commercial FM band above 92 mHz is the real battleground of competing formats, each trying to grab ratings numbers and advertising results. These are the NAB member stations, with no use for misqito-sized competition from Part 15 stations.
Perhaps it's safer to move Part 15 FM activity down into the non-commercial band, which is populated with lower power stations in many locations, except for the few 100kW NPR outlets. Of course the rules don't make an exception either way with regard to Part 15, but perhaps the "educational band" is a more neighborly place to hang out.
What is your view?
I think unless you are transmitting FM
just to the house next door or the next
few apartments - well - I really think
that is all that can be done.
The only way anybody could hear the teeny
signal at a distance would be is they had
a really great FM DX set-up. And maybe an
antenna on a tower. And of course with a really
good car radio - the signal could be heard
500 to 600 feet away. With a mono transmitter.
That just made me think of something else.
I'll be right back.
Bruce, Radios and Dogs