Anyone know off hand about Part 15 use on 450+ MHz? Quick web search didn't help much.
Experiments today on a transmitter I was working on indicates that a 1.5 watt transmitter into a dummy load, in my basement, on the opposite side of the house, gives me a solid signal at city hall half a block away. Makes me wonder about the possibilities of such for an STL from the station to the office at city hall where my audio enters the cable TV system for the public access channel.
Of course, wouldn't consider doing it if it isn't legal, but in the engineering world we're apparently doing a heck of a lot of "broadcasting" into dummy loads around the country! I've never tested for any sort of coverage when using a dummy load before in any of the various services and bands I work in. I was quite amused.
Tim in Bovey
When "Type of Use" is "Any", Part 15.209 applies which allows a field strength of 200 uV/m @ 3 meters.
Chances are you may be exceeding that.
This info per:
OET BULLETIN NO. 63
October 1993
(Supersedes September 1984 Issue)
(Edited and Reprinted February 1996)
I have read Part 15 about a thousand
times looking for loopholes of any kind.
I think many of us here have.
There is a big difference between testing
something and using it every day. But I
know you are very smart (just reading
about the things you have done so far
tells me that.) So I know that you already
know what I am saying.
And heaven help you if your link gets into
a public servide repeater or something like
that. But I know you already understand
that, too. 200 uV at 3 meters will do
something, but it's not good enough for
what you want, probably. You would need
an FS meter to know where you stood there
in trems of filed strength.
I'm on the same page you are on,
believe me.
Bruce, DOGRADIO
Dummy loads are most likely sealed against RF leakage but coax cables and equipment cabinets are usually not not. Leakage from the coax and the transmitter itself would explain the strong radiated signal you produced. From working on making a two meter system receiver enclosure RF tight I observed that VHF and UHF can leak like water and you literally have to make the enclosures watertight with metallic connections.
But this is incidental to your project. 433 MHz signals are not a good choice for building penetration and are easily attenuated by obstacles. Unfortunately, other choices such as 915 MHz, 2.4 GHz, and 5.8 GHz have similar issues.
From your previous description of the project it appears that the best and probably most workable common solution would be to use a stock FM band transmitter with a good gain antenna at the receive end. A gain antenna at the transmitter is not going to give any advantage over a simple whip since the legal limit is field strength at a distance and gain antennas function by increasing the field strength in a certain direction and the power to the antenna would have to be suitably reduced to keep the major lobe field strength within limits.
At the receiver end there are no regulatory restrictions on the antenna and the link gain can be achieved there. The problems will be mechanical but if these can be managed then a Part 15 link in the FM band has a good chance of working if you can find a "sweet spot" for the antenna. One trick I used for a VHF receiving system was to place the gain antenna (a Yagi type) above the suspended ceiling with the coax running above the ceiling to the receiver down the hall. It was out of sight and reach and worked well. You do need to be mindful of going through fire breaks with cables as it can be done but must be done properly.
Neil