Basic RF question about AM/FM antennas

[rtarbell]

On the back of my Sony stereo unit, there are two antenna hookups - one for an AM loop antenna, and another for an FM antenna. The AM antenna has two wire connections to the stereo, while the FM antenna only has one connection to the stereo (the other end of the antenna is just left "hanging", like with most FM antennas). Why is this?

[murat]

I wonder the same!

[gmphadte]

The ground connection for the FM is meant for connecting the antenna cable shield in case u are setting the antenna outdoors. It could be the rooftop/terreace. If u don't use the shielded cable, the total cable acts as an antenna and its orientation creates mess of the reception.

Gajanan Phadte

[Kent Smith]

I don't know why some companies don't have a connection for the ground side of the FM antenna. They seem to assume that everyone is happy with connecting a few feet of wire and dangling it on the wall. Fortunately, if you have twinlead or coax from an antenna, you can connect the ground side to a screw on the back panel, and it will probably find the circuit ground (check with ohmeter). You could also check the AM antenna terminals with an ohmeter to see if one side is grounded and use that.

[OzMikeH]

FM is around 100 MHz, a quarter wave antenna is about 70cm.

am is about 0.5 to 1MHz, a quarter wave at 750kHz is 95 metres!

Manufacturing a little AM radio with an external 100m roll of antenna wire is entirely impractical, so AM antennas consist of many loops crossing the same space sort of like a transformer secondary winding.

Portable AM radios have a very long antenna wire wound around a ferrite rod inside.

Quarter wave antennas are even too impractical for transmitters, many transmission towers have a large capacitor to ground at the top (the big disc top hat) this electrically lengthens the antenna.

[Conrad Hoffman]

That's odd; most radios I've see have a single connection for the AM antenna, and two or three connections for the FM. The outer FM connections are 300 ohm impedance, and you typically connect a local twin-lead loop there. It's 75 ohms from the center (which is usually ground) to either outside screw, and that would be used with 75 ohm coax to maybe a balun and whatever antenna was out on the roof. The AM connection is for a long wire antenna, and one generally wants a local ground connection for that to work best. I have a ground rod just outside my window for that purpose.

[murat]

Thanks guys. In my case it looks a bit different I guess: My tuner has screw terminals -apparently for twinlead cable (for FM antenna). Now, if I look at inside the tuner, I see that a twinlead cable is connected to the screw terminal and going somewhere in the tuner. However, there is a third cable, which is also connected to one of the screw terminals with a solder lug and going somewhere else in the tuner (I couldn't take the case apart completely to see what's going on). Now this picture suggest that one may choose to connect this third cable or not, because the solder lug is screwed to the terminal from outside and may be left out. Can someone tell me when should I connect it or leave it out? Thanks…

[OzMikeH]

Keep stuffing wires in and moving them about until you get the station you want clearly.
Impedance mismatches are usually not a big deal with receivers.