# Indoor FM Aerials

## Use and Abuse

The advice over on rec.radio.amateur.antenna is that for an indoor aerial there is no point using anything more sophisticated than a half-wave dipole. This is because of the proximity of conductors like iron girders and wood. If you want an omnidirectional aerial then this can be achieved by mounting a half-wave dipole vertically.

There are two forms of half-wave dipole. A simple half-wave dipole has an impedance of about 75 ohms and is just two bits of wire of an appropriate length. I use a "bunny ears" TV antenna with the ears adjusted to be in a straight line.

A folded half-wave dipole has an impedance of 300 ohms and is made from 300-ohm twin-lead with the ends shorted. You can buy these very cheaply.

The appropriate length depends on the frequency of the station you wish to pick up, but is also affected by nearby conductors. In practice, it is not critical; try:

Length (in metres) = (0.95.c) / (2.f)
where:

• c = speed of light (3x10^8 metres/second)
• f = frequency (in Hz)
• and the "0.95" compensates for end effects

So, to pick up a station at 100 MHz, you need a half-wave dipole with a total length of about 1.425 metres. Here is a calculator for other frequencies (requires JavaScript):

To ensure that the tuner receives the maximum signal, it is important to match the impedance of the tuner to the feed cable, and the feed cable to the aerial. Coax cable has an impedance of 75 ohms, so you could use 75-ohm coax feed cable feeding a 75-ohm simple dipole aerial. Unfortunately, the simple dipole is balanced and the coax is unbalanced. Connecting the two causes losses which can be avoided by using a "balun" (balanced to unbalanced transformer). 75-ohm versions of these are bulky, so you might want to use a 75-ohm to 300-ohm balun, substituting a folded dipole (which is also balanced) for the simple dipole. (Note that a 75-ohm to 300-ohm transformer is not necessarily a balun.)

For what it's worth, I use the 300-ohm input on my tuner connected to 300-ohm twin-lead feeder cable (which is balanced) connected to a 75-ohm simple dipole. I have purchased a 75-ohm to 300-ohm transformer (which happens not to be a balun), but have not bothered to fit it as things work just fine as they are.

Finally, Brian Beezley describes how to make a balun using 75-ohm coax feed cable. He has also analysed many types of (mostly outdoor) aerial.

Webmaster: / martin.leesestanfordalumni.org