Re: FM transmitter wanted with more power
- From: John - KD5YI <groups5munge@xxxxxxxxxxx>
- Date: Sat, 18 Jun 2005 01:09:12 GMT
Mac wrote:
On Fri, 17 Jun 2005 04:42:01 +0000, John - KD5YI wrote:
Mebart wrote:
You should be able to get much more than 15 feet range and still be within the law.
Ramsey used to make a very high quality part 15 transmitter, but it is pricey.
With 100 milliwatts and a 3 meter antenna, you should be able to do 300 to 500 feet with a decent receiver.
I'm assuming you want a real FM transmitter, capable of doing stereo with good frequency stability (quartz based frequency).
If you just want someting to make noise with, there are many schematics and products you can buy and operate legally. But, you will have to chase the signal up and down the FM band unless you get one that uses a quartz crystal to determine it's transmit frequency.
GL.
Actually, the rules permit 100 mW and a 3 meter antenna (combined with ground and feeder) on the AM broadcast band (525 - 1705 kHz), but not in the FM broadcast band (88 - 108 MHz). For the latter, the rules state:
[Code of Federal Regulations] [Title 47, Volume 1] [Revised as of October 1, 2004] From the U.S. Government Printing Office via GPO Access [CITE: 47CFR15.239]
[Page 816-817]
TITLE 47--TELECOMMUNICATION
CHAPTER I--FEDERAL COMMUNICATIONS COMMISSION
PART 15_RADIO FREQUENCY DEVICES--Table of Contents
Subpart C_Intentional Radiators
Sec. 15.239 Operation in the band 88-108 MHz.
(a) Emissions from the intentional radiator shall be confined within a band 200 kHz wide centered on the operating frequency. The 200 kHz band shall lie wholly within the frequency range of 88-108 MHz. (b) The field strength of any emissions within the permitted 200 kHz band shall not exceed 250 microvolts/meter at 3 meters. The emission limit in this paragraph is based on measurement instrumentation employing an average detector. The provisions in Sec. 15.35 for limiting peak emissions apply.
It is paragraph (b) above that is the stickler. It takes only about 20 nanowatts into an efficient antenna (say, a quarter or half wave) to get that 250 microvolts/meter at 3 meters. Of course, you could use a kilowatt into a really bad antenna so long as you don't produce a field greater than 250 microvolts/meter at 3 meters.
Sorry to be the bearer of bad news.
John
In the AM case, wouldn't it be pretty hard to push those 3mW into a 3 meter antenna at those frequencies? I'm not saying it couldn't be done, mind you.
--Mac
Hi, Mac -
Did you mean 100 mW? Yes. It's very difficult when the FCC limits your entire antenna installation (ground, feeder, and antenna) to 3 meters:
"Sec. 15.219 Operation in the band 510-1705 kHz.
(a) The total input power to the final radio frequency stage
(exclusive of filament or heater power) shall not exceed 100 milliwatts.
(b) The total length of the transmission line, antenna and ground
lead (if used) shall not exceed 3 meters."This makes the radiating system very low in efficiency. The whole idea (as far as the FCC is concerned) is to limit the field strength thus limiting the ability to interfere with licensed stations. It makes sense to me.
Nevertheless, I've heard that the range achievable on the AM band far exceeds the range achievable on the FM band when both meet the FCC rules.
Cheers, John .
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