[Raleigh]

I have a GE 212 AM/FM that uses a metal clip around the power cord that attaches to the FM antenna terminals in the back of the radio. I assume this is a capacitive coupler for the FM signal picked up by the power cord.

When I replaced the power cord, I lost the metal clip that I'd intended to salvage. I made a new one out of some scrap brass sheet, using another radio as a model. The FM works, but not all that well. (There might be other reasons it for it not working well. Another story.)

Here are my questions.

--Could I use a "real" capacitor to couple the power cord to the FM antenna terminals inside the radio, substituting for the metal clip on the same terminal?
--If so, what value would be appropriate?
--How would I attach one or more caps so they would be electrically equivalent to the metal clip wrapped around both wires of the power cord? (one cap connected to one wire of the power cord? One cap attached to both wires of the power cord? Or some other way?

I realize I'd probably need to use a safety cap.

I will almost certainly never use a dipole on the external terminals, so it doesn't matter if I can no longer easy disconnect the power cord coupler from them.

[morzh]

No. Do not even think of attaching anything to the power wires.
Not just a bad idea - THE WORST.

[Raleigh]

Since there are all sorts of things attached to the power wires inside the radio, and we replace those parts all the time in repair and restoration, I find your answer puzzling.

[skyscraper]

Because AC travels through capacitors, and the signals in the radio are referenced to the line, I think using a capacitor of any appreciable value would cause some serious problems. Although that could depend on the circuit.

Getting a good band of metal around the cord (one of mine uses conductive tape) is all I would do.

[BrendaAnnD]

Or, simply wrap several turns of 22 gauge wire around the cord and connect one end of the wire to the FM antenna terminal. GE used that scheme during the 60's.

[Raleigh]

Wow, BrendaAnnD, that's just too simple for me to figure out. Thanks, I'll try it. I figured that imitating the original as closely as possible was the thing to do. Actually, my results aren't very good--very weak signal. Just for grins I tried a dipole with much better results.

I assume I leave the insulation on the 22 ga. wire where it wraps around the power cord?

Against all advice I did try to connect a few different values of caps between power and the FM antenna terminal. I reasoned that when you connect a line filter cap from power to chassis, which I've done on many radios, the cap blocks the AC power, but in theory it couples RF hash ground before it hits the transformer or rectifier. Besides, this radio already has a 47 pf mica between one leg of the power to the ext. AM antenna terminal where anyone could touch it, and the terminal shows no 60 Hz AC at all.

Blocking 60 Hz power but coupling RF to the FM antenna terminal is just what I wanted to do, so I tried it, working very carefully step by step. There really wasn't any result at all with the caps I tried, between .05 uF and 100 pf. When I clipped my test lead by itself to the FM antenna terminal, I got a big boost in signal, but nothing changed when I clipped the other end to the cap that was connected to the power line. The 12 inch test lead gave me nearly as much signal boost as the dipole.

I haven't aligned the FM yet. Maybe the signal will improve when I finish alignment.

[BrendaAnnD]

Remember one thing. When you use the line cord for an antenna, that the line cord then needs to be positioned to pick up the best signal. This always made me crazy, since your house wiring also becomes part of it, so sometimes the cord needs to run at odd angles from the radio.

Also, for FM frequencies, 100pf is MUCH too large. 2 to 5pf is more like it.

[Ron Ramirez]

Philco used a small flat piece of metal clamped around the power cord, well insulated, as an FM antenna on many of their postwar FM radios. Sounds similar to the GE setup.

[Raleigh]

I think it is similar, Ron. I'm just concerned that my home-made substitute isn't performing as well as the original one I lost. If the exact design isn't that critical, maybe it is OK.

I used a similar design from a Panasonic as a model. It's just a piece of flat brass with a slot at the top to go on the terminal. A thin piece of cardboard is wrapped around the power cord, and the metal is wrapped most of the way around the cardboard but not a complete loop around it.

[Ron Ramirez]

Yup, that's how Philco did it. Not the greatest antenna for FM, but it works OK if you're in a city or town with several nearby strong FM signals.

[Mondial]

Another thing to keep in mind when using the piece of metal coupling to the line cord, you want to have a couple of feet of cord from the back of the chassis to the point where the metal coupling is positioned. Since the line bypass caps short the RF signal to ground, there is very low signal on the line cord where it enters the chassis.

By moving the coupling point a quarter wavelength or so from the chassis, the RF signal is at a relative maximum at that point.

I forget the exact dimension, but on the Philco's like the 48-482, there is a rather long wire to the antenna input which connects to the coupling point on the line cord some distance from the chassis.