On Wed, 26 Jun 2019 03:02:38 -0400, George Cornelius wrote:

+AD4 In article +ADw-qcira8+ACQ-v05+ACQ-1+AEA-dont-email.me+AD4, Frank +ADw-analogdial+AEA-mail.com+AD4 writes:
+AD4APg If your radio has a coaxial connector for the FM antenna, a simple dipole
+AD4APg will match the radio better than a folded dipole. A folded dipole will
+AD4APg work OK but a simple dipole will transfer a little more signal.
+AD4
+AD4 Or use a 75:300 ohm balun. I know, balun losses. But then a dipole
+AD4 of the sort you recommend has issues, too: the balanced to unbalanced
+AD4 transition leads to antenna currents in the feed line. Assuming you
+AD4 have one.

Not sure the OP cares anymore but it's always nice to change the pace from the usual +ACI-TRUMP+ACI-talk.

For receiving, antenna currents on the feedline won't do much beside alter the pattern a bit. I suppose the feed will pick up more interference but I expect this radio and antenna will be in the same room, anyway.

+AD4
+AD4 Also: a folded dipole is a broader band device than a simple dipole.

With the same amount of wire, I suspect that a fan dipole (like this -- +AD4APA ) would have a broader spread yet than a folded dipole. For each leg of the dipole just cut one wire a little long and another a little short. Put about a 10 or 15 degree angle between the long and short wires. But bandwidth of a simple dipole should be OK.
+AD4
+AD4 Another simple one is a loop. I believe the length is 1055/f,
+AD4 making it about 10.5' long. It's lower impedance than a folded
+AD4 dipole and may actually be a good match to a 75 ohm input.

That's a good one. A loop like that could be placed on a door which would allow the pattern to be aimed.
+AD4
+AD4 Shape isn't too important - a circle or a square is common.
+AD4 A square antenna is commonly called a quad.
+AD4
+AD4APg If you want to try something quick and dirty, put the bare end of
+AD4APg a thin wire about 5 feet long into the center terminal of the radio's
+AD4APg coax antenna connector. This is hardly ideal but it's easy and might
+AD4APg work well enough.
+AD4
+AD4 Well, the hams use 468/f for a dipole, 234/f for a 1/4 wave. So for a center
+AD4 frequency of 100Mhz, that would translate to 2.34' .

Well, ya got me there. 5 feet would be more like a end fed half wave dipole which would be a severe mismatch to a low Z input. A couple more feet and it would be a 3/4 wave end fed antenna which would be a better match and give some offset lobes which may or may not be helpful.

If I REALLY want a resonant antenna I check them with a grid dip meter. The formulas usually are off a bit in the real world but, for receiving, they're plenty close. If the receiver has a S meter that meter can be used for tuning the antenna. But a few inches one way or the other won't make much difference.

+AD4 Assuming you have a
+AD4 ground plane. Hmmm. Never heard of anyone building a ground plane antenna
+AD4 for FM radio. Be the first on your block to try it+ACE
+AD4
+AD4 George

I find the top of the refrigerator makes an easy, kinda sorta ground plane for a FM portable.