# FM Transmitter antenna

Discussion in 'Electronic Design' started by mook johnson, Jun 20, 2004.

1. ### mook johnsonGuest

I'm looking at some of the 2 transistor FM transmitter circuits on the
internet and trying to determine the optimum antenna length (single wire
only) to get a range of about 500 - 1000ft. with a 100MHz carrier.

It will be mounted to a small wood or fiberglass object floating on a lake.

2. ### Ken SmithGuest

Assuming the small object floats right at the surface and doesn't stand
much above it:

Put a whip up to about 29 inches (1/4th of a wavelength of the carrier)
above the water. ie: if the object stands 2 inches above the water use 27
inches. "ground" the transmitter by AC coupling the other side to the
water.

If the object is larger make the top of it conductive and measure the 29
inches from there.

3. ### mook johnsonGuest

OK, what would be the next shortest length that would be acceptable?

1/8 wavelength? 1/16th?

5. ### Ken SmithGuest

Put back in proper order

Thats easy, the next shorter is:

(N-1)/(4xN)

Where N is a very large number.

If you go under a 1/4 wave whip, you need to put a coil in series with it.
The shorter the whip the larger the inductance and the worse the
performance.

If you increase the diameter, you can get away with shorter lengths.

6. ### K WilliamsGuest

Of course the longet the better, but the "rule of thumb" for EMI is
that anything less than 1/20th of a wavelength isn't an efficient
radiator may be of use here. In reality there is no answer.
You're going to have to "load" the antenna for any mismatches.