FM Amplifier

lboucher

Hi Everyone

I am working on christmas lights for next year.
See here
http://www.youtube.com/watch?v=6QhCEZxfU38
http://forums.parallax.com/showthread.php?p=771108

·I want to be able to broadcast music on FM around my house. I bought a cheap 50 mw FM transmitter
·http://cgi.ebay.com/50mW-Digital-PLL-stereo-power-FM-transmitter_W0QQitemZ150319791644QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item150319791644&_trksid=p3286.c0.m14&_trkparms=72%3A1234%7C66%3A2%7C65%3A12%7C39%3A1%7C240%3A1318%7C301%3A0%7C293%3A1%7C294%3A50
I also replaced the dinky 1/4 wavelength antenna with radio shack 1/2 wavelength dipole antenna.
http://www.radioshack.com/product/index.jsp?productId=2062691
It is working out ok, but i would like to get a little more range out of it.

Does anyone know radios well enough to point out a really cheap AMP that would get me up to around 0.25 - 3 Watts.

By cheap I mean 20 bucks.
Thanks for the help.

Lucas

Post Edited (lboucher) : 1/13/2009 2:39:24 AM GMT

Mike Green

You can get in really big trouble for broadcasting a "high power" FM signal without a license.· At the very least you'll get your equipment confiscated if someone complains to the FCC.· Anyone who gets caught selling you what you're asking for will get their equipment confiscated and they'll probably have to pay a substantial fine.

SRLM

There was a Make Magazine article about making an FM transmitter... Anyway, you could just move the module to your front yard. That should be enough for people to see your lights. Also, making a good transmitting antenna can help immensely. Take a look at some ARRL books for more information on that. With the right antenna, you can direct your radio waves in a particular direction and altitude. With that, you can effectively double (or more) your output power in that vector.

WBA Consulting

I am planning on doing the same with mine. I already have a Ramsey FM transmitter from about 8 years ago that has a decent range if I use a good antenna. (Even with just a whip, I can get about 30 feet.) My plan was to make use of some POE parts I have laying around. I was going to feed 48v into Cat 5 with a POE supply along with some sort of audio signal (not sure what will be best for the 80ft it will travel in the Cat5). At the FM transmitter, I will have a POE 12v DC-DC converter that will power the FM transmitter, any necessary adapter for the audio signal to convert it back to line level (if needed), and the dipole antenna. Before I saw my POE box, I was just going to run a Cat5 line out the the FM transmitter and run it off of batteries. (I left it on once and it ran for about 4 days on one 9 volt.)

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Andrew Williams
WBA Consulting
IT / Web / PCB / Audio

Beau Schwabe

lboucher,

50mW should really be plenty from the unit you already have. If your antenna is not properly tuned, then you can take a severe hit on your radiated power.

Is the dipole (1/2 wave) two straight rods going in opposite directions about .75 meter each (1.5 meters total length) or is it a folded dipole antenna (big narrow loop with a 1.5 meter span)

The former is matched to about 75 Ohms, and that's what you want, but you also want the proper length for the transmitted signal which will be determined based on the frequency that you
are transmitting. The later is matched at about 300 Ohms and is definitely not what you want.


Just a straight rod antenna (1/4 wave) will be matched to about 50 Ohms and would be ok if it's the right length, but not as good as the 75 Ohms.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Phil Pilgrim (PhiPi)

If you're pitching your programming to passing vehicles, you may want to retain a vertical antenna orientation in order to match the polarization of car antennas.
If the programming is more for your neighbors at home, then a horizontal orientation would probably be better, although directionality then becomes a factor.

-Phil

Beau Schwabe

Good point Phil

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

potatohead

The Ramsey kits have a good range actually. You can run them a little hot, and get 1/2 mile.

One thing I did, which worked just crazy good, was mount the xmitter in an older car with the whip antenna. It worked just great! I started with a simple dipole. Was OK. The TV antenna on top of the house was quite a bit better. Probably just height. The car ruled, and it was just a lark! Suspect it had to do with the car body and ground.

Still, a Ramsey kit running at 9V would work with full quieting for a block, and was usable a lot farther than that. At 12V... It was a lot better, let's just say that.

It's a lot harder these days to do FM. The IBOC digital radio has brought a lot of extra signals to the dial.

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lboucher

I have the latter with a matching transformer.

Beau Schwabe (Parallax) said...
lboucher,

50mW should really be plenty from the unit you already have. If your antenna is not properly tuned, then you can take a severe hit on your radiated power.

Is the dipole (1/2 wave) two straight rods going in opposite directions about .75 meter each (1.5 meters total length) or is it a folded dipole antenna (big narrow loop with a 1.5 meter span)

The former is matched to about 75 Ohms, and that's what you want, but you also want the proper length for the transmitted signal which will be determined based on the frequency that you
are transmitting. The later is matched at about 300 Ohms and is definitely not what you want.


Just a straight rod antenna (1/4 wave) will be matched to about 50 Ohms and would be ok if it's the right length, but not as good as the 75 Ohms.

WBA Consulting

After discussing this with a few "RF expert/old timer HAM" colleagues, I will try a J-pole first. A J-pole is an omni-directional, vertically polarized, end fed, half-wave dipole. They are easily made (A year ago, I made one for 2 meter out of twin lead for my 2 watt handheld)

I used these plans and made the 2 meter one in about 15 minutes:
http://www.fiu.edu/orgs/w4ehw/j-pole.html


http://www.dxzone.com/catalog/Antennas/J-Pole/
http://en.wikipedia.org/wiki/J-pole

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Andrew Williams
WBA Consulting
IT / Web / PCB / Audio

WBA Consulting

I happened to look at the manual for my Ramsey FM transmitter. It actually has a lot of good info in it regarding unlicensed FM transmitters. Check out FM10A here:

http://www.ramseyelectronics.com/support/default.asp?page=manuals

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Andrew Williams
WBA Consulting
IT / Web / PCB / Audio