Ken (or anybody who wants to comment),

You seem to be an antenna guru... so I hope you don't mind a dumb question:

I'm looking for a roof mounted AM/FM radio antenna and all I can find on the web are "marine" antennas. Do you know of a simple way to construct an antenna that will pick up mainly AM radio over long distances, and what cable is needed (RG-6)?? My Father-in-Law needs better reception of The Grand Ole Opry at his weekend home in the sticks

And to tie this into HDTV... since I've already got the rooftop antenna for OTA, can I add something to it for radio reception without messing up the DTV reception?

As an old ham radio guy I can tell you that the best AM antenna is a long wire.....with or w/o insulation. By long I'm talking 50 ft. or more. The end of the wire can terminate at the back of the radio. Or you can even take a small wire from the radio and attach it to a window screen. In other words keep it simple. For FM I'd take a small length of twin lead (old tv wire) about six feet long and twist the ends together, then cut the bottom wire in the center and scrape away the insulation and attach a wire to these two and run it to the back of the radio to the FM antenna jack. This antenna needs to be attached to the wall parallel to the floor and it will be highly directional so you need to experiment. It's really just a large loop antenna. Send me a note direct and I'll draw it out for you if you have any questions.
Hugh

Having made many of the fm antennas as described, I always cut to length of 5' which is approximately the center of the fm band wave length. Just a suggestion. You can also make 2 of these and mount these centered at 90 degrees to each other (like an X) and then connect as mentioned tying one lead of each antenna to one down lead. Repeat with the remaining 2 antenna wires to the other down lead. This gives a multi directional sensitivity. Good luck!

Bob: The FM dipoles described above will work, but for long distances you need more gain. Most high gain VHF TV antennas will work nicely. The FM band is just above TV channel 6, and most TV antennas have gain in the FM band.

For AM, a 30-50 foot long-wire is best. Longer will pick up more signal, but will also pick up more noise. For AM frequencies, atmospheric noise dominates, not receiver noise. The key to a good long distance AM antenna is how high it is, not how long it is. The next important factor is eliminating household electrical noise, which is very troublesome at AM frequencies (light dimmers and motors). RG6 is not very useful at AM frequencies. TV antennas are worthless at AM frequencies. However if the TV antenna is high, its RG6 shield might be a good AM antenna. I haven’t tried this, but a couple clamp-on ferrite beads between the AM tap-off and the TV would likely help.

Gene: Your X antenna is bi-directional, radiating at 45-degrees off-angle. There is a way to make it omni-directional by introducing a 90-degree phase shift to one dipole, but there is a 3 dB penalty for doing that.