FM antennae

[Tim Hancock]

I have a stereo in my metal pole building and the short piece of wire hanging off the back is worthless. I would like to build an antenna that I can mount on or near the roof to be able to pull in FM radio broadcasts with. I get good reception outside with car stereos and portables. How can I wire up and build a simple antenna to the stock single wire that came with my stereo? If 57.3" is the magic length but my roof is over 20' away, how can this work in simple terms?

[Flatbutt1]

Flatbutt = Flatbutt1...I had a logon issue a long time ago and re-registered as Flatbutt1. When the admin tried to fix it Flatbutt started to work again.

Anyway since the full FM wavelength is 114 inches I think any multiple of that will work in an antenna. So figure multiples of 114 inches to the nearest figure of 20 feet and that oughta work.

[slodave]

Originally Posted by Tim Hancock

I have a stereo in my metal pole building and the short piece of wire hanging off the back is worthless. I would like to build an antenna that I can mount on or near the roof to be able to pull in FM radio broadcasts with. I get good reception outside with car stereos and portables. How can I wire up and build a simple antenna to the stock single wire that came with my stereo? If 57.3" is the magic length but my roof is over 20' away, how can this work in simple terms?

The formula is 468 divided by the frequency in MHz. Our FM radio band operates from 88 MZHz to 108 MHz. Since you don't really want to have and antenna for each frequency, we take the mid point or 98 MHz. So, 468/98 = 4.78' = 57.3". This dimension will give the best reception at 98 Mhz and start to have a loss on each side of that freq. Most FM stations are powerful enough that the above will pick up any freq. from 88 MHz to 108. I noticed that in my setup, 89.1 MHz a station my mom listens to, has a touch of static, but is really not noticeable. I could use a different 'mid' freq - closer to 89.1 and it should fix this issue. The dipole would then consist of two lengths of wire each 2.39 feet or 28.65 inches long. This setup will give you a 3dB gain.

The above only builds the antenna, not how to get it to your device (radio). The longer the feed to the antenna. the more of a dB loss. You can add a balun, wrapping the coax four or five times, this will minimize the feed loss, but it will still occur. The cable in my setup is about 50' long and works just fine.

I will try and remember to bring a camera to my parents house tomorrow and get up on the roof and snap a pic.

Dave