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Old 22nd November 2011, 02:46 PM   #1
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Default Simple FM Antenna

Here's my 98-cent DIY FM antenna for my garage stereo. It is FAR superior to the dipole I was previously using. With the dipole, some stations were very scratchy. This DIY antenna completely fixed that issue, and provides clear reception across the FM band.

I ran across this design years ago, and don't recall the specified measurements. They don't seem too critical, but I probably wouldn't have made the antenna smaller than 14" wide. The backing board shown here is a piece of 18x13 scrap. The 'bridge' at the center of the board is 2" PVC pipe. The wire is a 15-foot long single strand of scrap 18 gauge -- folded in half, the midpoint of the wire was at the top right screw.

The design works well. It isn't hard to envision it as an artistic and functional piece on an indoor wall, made with better materials.
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Old 22nd November 2011, 02:53 PM   #2
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Cool! Have not seen that one before. The bridge is the to keep the 2 halves from shorting at RF?

Would be nice to get Tubelab George over here to comment.
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Old 22nd November 2011, 03:01 PM   #3
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Default Bridge

The bridge keeps the wires separated where they cross. The wire used here is insulated, so there wouldn't be a 'short' if they did touch.
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Old 22nd November 2011, 03:07 PM   #4
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OK, I was thinking some sort of RF short, not electrical contact between the wires. Why do they need to be separated, do you think?

Also, have you tried any other orientations? Could it go on the ceiling, for example?
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Old 22nd November 2011, 04:42 PM   #5
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Default Directional? Yes.

It is directional.

The antenna is on a west wall. The main broadcast antenna complex in my area is on a hilltop WNW of my location.

Rotating it 90 degrees to a north wall, it works almost as well -- at least according to my subjective listening -- and still much better than a dipole oriented either EW or NS.

Mounted on the ceiling, regardless of how I rotated it, the reception was much worse than a dipole.
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Old 22nd November 2011, 11:05 PM   #6
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Interesting! Thanks.
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Old 23rd November 2011, 02:10 AM   #7
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Do you connect this antenna to the 300 or 75 ohm input on your reciever?
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Old 23rd November 2011, 04:23 AM   #8
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Good question. Many receivers have two sets of antenna inputs, marked 75 and 300 ohms. My old JVC has a only a single set of spring-loaded push-in terminals, marked 75 ohms. The dipole antenna I was previously using had a single wire at each end (75 ohm), and was not a folded dipole (300 ohm).
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Old 24th November 2011, 01:07 AM   #9
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Quote:
Would be nice to get Tubelab George over here to comment.
First off, I AM an RF engineer in my day job. I have worked for Motorola designing two way radio transceivers, cell phones and other RF devices for the last 38 years. In all that time I have avoided the black magic art of antennas. I don't know too much about them, but I have made a few. Most of mine are laughed at by "experts" but work for me.

Look at the TV antenna that I made out of two coat hangers a piece of wood and a muffler clamp. Some of the readers said that it didn't have a proper RF match and I should use a network analyzer. Well network analyzers don't come cheap, and I am watching TV for zero bucks.

DIY TV Antenna 2.0

I used a Radio Shack FM band yagi on a rotator when I listened to FM. The FM band in South Florida has degenerated into sheer noise in the last few years. I don't even have my tuner connected up now.

I have seen a design similar to this used for ham radio except that the feed line was connected in the center where the wires cross.

The FM band has a wavelength of about 120 inches. A dipole should be about 60 inches, fed in the center. It is possible to make a loop dipole with 60 or 120 inches of wire with each end connected to the feedline. You can then experiment with how you arrange the wire.

Most antennas are designed to operate in "free space." This means that there should be no conductive or RF absorptive objects within a few wavelengths of the antenna. This is not possible in indoor or many outdoor locations. It is also not possible in a portable radio or cell phone. That's why those antennas are designed by engineers with PHD's and field solving simulators. I have neither.

It was amusing that we were having problems with a GPS antenna that was designed by experts several years ago. I made one that seemed to outperform their fancy ceramic hockey puck. They were not amused when I revealed my antenna. A mildly modified paper clip!
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Old 24th November 2011, 01:21 AM   #10
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Default Re: similar design

"I have seen a design similar to this used for ham radio except that the feed line was connected in the center where the wires cross."

I think I recalled this design from a ham radio site or book, so you're probably right about that. There may have been variations, but I don't recall one that was center-fed.

That said, it could very well be that the original was a mast-mounted structure rather than board-mounted wire. Regardless, this one is cheap and it works.
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