Using a Dipole Antenna on a 50 ohm FM transmitter

Question

First of all, while I'm not new to electronics, I'm new to the radio world. I am helping a friend overseas set up a community radio, and am getting everything set up over here before I ship it.

I have a CZE-7C FM transmitter, and I have built a simple half wave dipole antenna tuned to the frequency he wishes to use. However, I have noticed in my research that half wave dipoles have a theoretical impedance of about 73 ohms, where as the transmitter wants a 50 ohm antenna. I've read that an ill-matched transmitter and antenna can damage the transmitter, and therefore I would like to know what further preparations are necessary before I hook my simple DIY dipole up to my transmitter.

Perhaps I just don't know what terms to search, but I can't find any advice. I was planning on using 50 ohm coaxial with BNC terminations to connect them, if that helps.

Answer 1

The output is very likely to be an unbalanced type suitable for quarter wave antennas with a matching section. It's got half the impedance of the dipole i.e. 38 ohms and a simple 12 ohm resistor in series will match it to the 50 ohm output.

You can match it to a half wave antenna but you need to use a balun to drive that type of antenna properly - this will also allow you to get the impedance correctly matched.

Answer 2

I'm assuming that when you say "Community Radio", you are speaking of a transmitter operating in the standard FM broadcast band of 88.1 through 107.9 MHz?

I have a couple of transmitters that I use for festival use and, although I purchased a couple of antennae, I built the ones that I use the most often. FWIW - all of my transmitters have a standard 50 Ohm output.

The antennae that I purchased are two distinct styles: one is a monopole, the other looks like a pair of folded dipoles crossed in the exact shape of a "X".

The monopole has an omni pattern, the circularly-polarized antenna has a figure 8 pattern. I'm guessing that the monopole is actually configured as a half-wave dipole but I didn't look inside the (sealed) pole to see how it was built.

Both of these have an external balun in a sealed package - I haven't bothered to open either of them up to see how they are constructed. I will, someday.

The antenna that I use the most often is a simple dipole made with 2- fairly-long telescopic antennae mounted on a small project box. The hot antenna points UP, the ground antenna points Down. No matching network of any sort - both of the telescoping poles simply connect to a BNC connector.

I found a table of length adjustments that allow me to set the telescoping lengths to match the frequency that I am transmitting at. Interestingly enough, the downward-pointing telescoping antenna (ground) is a different length than the upward-pointing antenna.

The table seems to be fairly accurate - I normally get reflected power of less than 1 Watt with a forward power of 30 Watts. I simply tweak the antenna lengths to minimise the amount of reflected power if the antenna is too close to some other grounded object that messes up the tuning.

I normally mount the FM Transmitter antenna from the very top of the Front Of House mix position. There is usually a structure above the mix position that houses two or three spotlights (with their operators) and I'll simply hang the antenna from the roof of that upper structure.

I haven't gone looking lately, but I'd think that there is a ton of information just a short Google search away.

Answer 3

If you need to match the antenna to the transmitter, why not build yourself a Butterworth Low Pass Filter, equip it with trimmers instead of capacitors and use that to get a perfect match. A LPF can also work as a very good antenna matching device as I have discovered by building a few filters myself.

Answer 4

Why not match the 50 ohms to 73 ohms by using an electrical 1/4 wavelength of 60 ohm cable between the 50 ohm cable and the antenna? The 60 ohms comes from sqrt(73 * 50) and an electrical 1/4 wavelength will convert one impedance to the other. The electrical wavelength is determined from the velocity factor of the cable. Divide the 1/4 wavelength by the velocity factor to get the electrical 1/4 wavelength.