Boosting FM transmitter signal

December 13, 2004 7:25 AM

I need to boost the range of a FM transmitter from CCrane. Folks on Amazon suggest... (more inside)

... changing the position of a diode (easy), sitting it on a metallic surface, and increasing the antennae length. A few questions questions:

a) If its on a metallic surface, for a "ground plane", does that mean all the broadcast waves won't make it below that ground plane?

b) The transmitter will likely be positioned in a ceiling to rebroadcast the audio of a tv to patients who can't get to the common room... What's the best positioning to get the waves out to their rooms? Up high (top floor of hospital, upside down with groundplane up top?), low (bottom floor of hospital with groundplane on bottom)?

c) For the antennae, I can replace it, and the folks on Amazon suggest one of 29 inches. Radio Shack has 16, 28, 30.5, and 34.375 inches... which should I use?

I'm working off of the 4th comment down from this page. Thanks!

P.S. This isn't in the states so I'm going to leave it to the locals to figure out the legal issues and move that diode as needed.

posted by jwells to technology (3 comments total)

The antenna you could replace it with should be equal to this:

wavelength (meters) = c / f, where c = 299,792,458 meters.

If you were to broadcast at, say, 100 Mhz, your ideal wavelength (antenna) would be 2.99792458 meters. You can choose 1/2 or 1/4 wavelength if that's too long, so that would be 1.49896229 meters or 0.749481145 meters, but the transmission won't be as strong. The 28 inch (Why the hell are they putting antenna measurements in inches? That's so nonstandard it SUCKS!) antenna corresponds to a 1/4 frequency of 105.38 Mhz. The 30.5 inch antenna 1/4 frequency of 96.74 Mhz. The others are outside the FM band.

Now, if you want to boost the output power, there's several ways that they generally limit the output. Often it's a resistor in series with the antenna. Lowering the value of this resistance could boost the gain. On some sets they just make sure that even at full output, with the voltage being fed the transmitter, it can't broadcast at a higher power (in that case add more voltage... but don't blow up the transmitter!)

On reading your link, the resistor thing is exactly what this guy is doing.

The metallic surface ensures proper reflection of the radio waves. There's absolutely no point broadcasting into the earth, so the metal sheet stops it. If you need the radio waves to broadcast in all directions (including below) scrap the metal plate. There's a much more complicated reason than this in my HAM radio book but it really doesn't matter all that much.

Note that by reducing the resistance to the antenna you are making the radio's transmitters more susceptible to SWR problems, which a simple rod or single wire antenna will make terrible. SWR is loss in the antenna caused by feedback of the signal to the output stage of the transmitter due to problems like antenna length not matching the broadcast frequency, and the impedance of the antenna being incorrect. SWR is constant, IIRC, but your transmitter can overheat if SWR is high and the output power is also high.
posted by shepd at 8:17 AM on December 13, 2004

Wow, thanks for the detailed answer... just what I was looking for! I think I'm going to have to hope there is a radio person in the place I'm sending this thing to and send along some possible supplies to work with, as I've no idea of where it will go. I figured in the ceiling would be easiest, which is why I was concerned about transmitting below the ground plane. I don't want to make it so the folks on the first floor can't get anything. So it sounds like attaching it to an interior wall on the middle floor, away from other electronics, without a ground plane, would be best. Maybe it could go in a closet or something.

Regarding the antennae length, since their telescopic, could I send the 34.375 one and just have them adjust the length as nescessary? Or is it more of a factor of the amount of metal in the antennae?

I haven't heard of SWR before but if those problems are encountered, could adjuting the length of the antennae solve them?

Thanks!
posted by jwells at 8:46 AM on December 13, 2004

This is the radiation pattern of your antenna, FYI.

For best coverage, placing the antenna horizontal in the center of the hospital would probably work (if the hospital is taller than wider) or vertical (if the hospital is wider than taller). But hey, I am really not that good with this stuff. Someone else probably has better comments than me.

Regarding the antennae length, since their telescopic, could I send the 34.375 one and just have them adjust the length as nescessary? Or is it more of a factor of the amount of metal in the antennae?

It depends only on the length. Adjusting the antenna to the appropriate length is the right way to solve the problem. :-)

I haven't heard of SWR before but if those problems are encountered, could adjuting the length of the antennae solve them?

Yes, but I wouldn't worry about it too much. The amount of power the transmitter is likely able to output even at full tilt is not likely enough to fry anything. It's probably about 1 watt maximum, and that just won't generate enough heat, or, well, that's the hope.

However, I know the warranty on my CB Radio is insta-void if I don't check the antenna SWR, and it's only 4 watts.

Here's an overly complicated overview of SWR.
posted by shepd at 9:51 AM on December 13, 2004