Right length for table radio antenna

The AM is usually a loopstick inside the radio. That is for FM. The length really doesn't matter much.

On AM, the loopstick antenna is part of the tuned circuit and is sensitive. On the other hand, FM pretty much tolerates anything. Unless you want to f ind the correct ground and impedance and make a proper antenna like those T type ones, just don't worry about it. One wire going to the screw that is the farthest away from ground, any length you like.

If a wire has one end, then it has another. That ought to be adequate...

Jonesy

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There is a lot of literature on the "correct" antenna length for a table ra dio - but what you need to keep in mind is that where inches/cm matter is f or FM. For AM and SW, the typical response would be "the longer the better" - and within broad limits, that is the case.

Will help you with the specifics.

Peter Wieck Melrose Park, PA

Micky-

As others have stated, it is not critical. For nearby stations, it may work without the wire!

The wire you want to replace would ideally be one quarter wavelength long. But that length would be "ideal" at only one frequency. So whatever you choose is a compromise.

Suppose the FM band covers the range of 88 to 108 MHz. The middle of the band would be 98 MHz. A wavelength would be 300/98 = 3.06 Meters. A quarter wavelength would be 76.5 CM or 30.1 inches.

Fred

Fred:

When would it be appropiate to use a half-wavelength or quarter as the dimension of the antenna being built?

KMan-

Perhaps I was mistaken in saying a quarter wave wire was "ideal". My main point was that the length was a compromise to begin with, so is not critical.

Relative to the body of the receiver, a half wave wire fed from one end has a high impedance. A quarter wave wire has a relatively low impedance. How the wire connects to the receiver would determine which would be best.

Suppose the radio has a coil and capacitor parallel-tuned circuit with one side connected to the body of the receiver. If the wire connects to the other "high impedance" side of the tuned circuit, a half wave might be better. If the wire connects to a tap on the coil, or to a smaller coil wound over the first, a quarter wave might be better.

I suspect the typical table radio with a wire coming out the back, has the wire connected to a relatively high impedance point. In other words, the designer may not have put that much thought into it.

I have an old Heathkit receiver that has a balanced 300 Ohm input for its FM tuner. A 75 Ohm unbalanced antenna could be connected between one of the 300 Ohm terminals and ground. I use a single wire connected to one terminal, and can pick up many stations.

Fred

Interestingly, if the input is a *very high* impedance (a FET gate, say), then the antenna length almost does not matter -- if the impedance were truly infinite, then a point antenna would be fine. The E-field will induce the same voltage in the antenna no matter how long or short it is.

Impedance matching is only important when the input needs power, not just voltage.

Isaac

Yes, there's a local station, WYPR 88.1 Baltimore, that just about every radio gets.

30 inches is a lot longer than the wire was. I'm glad I asked. Even 15 inches, if 1/8 wavelengths are useful, is longer than the wire, i'm pretty sure. (Someday I may find the wire on the floor in the corner and I'll measure it. )

Besides 88.1t there are two other stations, I only get easily on the car radio (no matter what car I have) and on a 250 dollar table radio (KLH maybe), but not on some fancy digital tuner/preamp. Some other table radios get one or both, at least if tuned perfectly, and it doesn't seem to matter how expensive they are, so I've taken to buying used radios for $3 to see if they will get those stations.

(But this radio I bought new for my mother because it had digital tuning and I thought it would make it easier to change stations. )

Anyhow, the stations I want are in DC, maybe 35 miles away, on 88.5 (WAMU) and 90.1 (C-Span) so I think I'll use 88.5 in your formula above and see how that goes. Maybe I'll trim a little off until I reach the 90.1 length.

Several projects ahead of this, no space on the work bench, so I won't be able to post back for a couple months or more.

Thanks and thanks everyone.

BTW, a friend just gave me another radio, second hand, that indeed does have a wire about 34 inches coming out the back, so some makers care about this. I think that's what it shows, even if it's not 30.

He bought it at a Goodwill store to dock his iphone or ipad but he says it doesnt' work. I don't have one to test with, but it also doesn't get many radio stations. It's also missing its AM antenna and the only source I have for a plug is maybe my auto burglar alarm from the previous car, which I'm saving for the next car. For the record it's an Imode model IP200A but I don't really plan to fix it

I posted mostly to say that its antenna was 34 inches, not just 15.

One reason those radios have short antennas is the front ends overload near powerful transmitters.

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