# FM radio in metal-roofed shop



## Joel_BC (Nov 10, 2009)

We've got a converted barn with a metal roof, and my wife uses it as her art studio (sculpture shop). Recently she bought a CD-player/FM-radio unit that will also play music from her iPod. The radio plays pretty well, but is slightly fuzzy (at times) on her favourite station. I believe it's an antenna problem. The unit came with a single-wire "antenna" - the wire being about three feet long that just comes out of a hole in the housing. You can string the wire out and tack it up, and that's the only option provided. But there are no _terminals_ to attach any sort of antenna input by screwing wires in place.

I tend to think that even with this "single-lead" type of antenna, there must be some way to improve it... like connect the wire to some sort of external antenna arrangement.

First question, is this true - even with a single-wire input?

Second, is there an antenna type (either something I can buy or something I can make) that would be recommendable for this situation?

Third, will the metal roof on the building limit the benefit of _any_ antenna placed within the building itself? (We get a couple feet of snow, so I suppose any external antenna would have to be attached to the roof peak, right?)


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## Bearfootfarm (Jul 13, 2006)

> Second, is there an antenna type (either something I can buy or something I can make) that would be recommendable for this situation?


This *might* work:

Sangean ANT-60











The *Sangean ANT-60* portable _*roll-up*_ shortwave antenna will improve the reception of any portable worldband receiver. The antenna wire may be extended up to 23 feet (7 m). 
The ANT-60 terminates with a mini-plug (3.5 mm). If your receiver does not have a mini external antenna jack, an adapter plug is provided to clip the ANT-60 to your radio's whip.


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## agmantoo (May 23, 2003)

FM is frequency modulation and should not be giving the "buzz". I would test the unit elsewhere to determine if the noise is the location or the unit before doing anything else.

PS check to see if you have any flourescent lamps that are not working properly. Radioshack should also sell a RFI filter that may help.


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## flewism (Apr 2, 2007)

Is it a single wire or a loop entering the radio at two points? either way extend it (longer wire). You can add an external antenna anywhere just use solderless terminals to connect to the existing wire. I'd try running a wire and bolting it to the roof making the hole roof the antenna.


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## Joel_BC (Nov 10, 2009)

flewism said:


> Is it a single wire or a loop entering the radio at two points? either way extend it (longer wire). You can add an external antenna anywhere just use solderless terminals to connect to the existing wire. I'd try running a wire and bolting it to the roof making the hole roof the antenna.


Not a loop - just a single wire.

I could try attaching an extension wire between the roof and the unit's wire.

To clarify, there's no "buzz" just a slight lisp to the audio sometimes. The audio comes in well sometimes, then five minutes later will have a bit of a hiss, then will revert to being perfect again. She had a previous radio in the shop, and it was much worse at receiving the station.

My wife's favorite station comes in at two places on the dial, and one of them works better 25 miles southward in our valley. So with our non-mobile, home-based radios, we normally use the _other_ one. So it seems to be a signal-strength issue.


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## 12vman (Feb 17, 2004)

Adding a long(er) wire at FM broadcast frequencies isn't going to help much. (88 to 108 mHz)

The goal is to create some sort of resonance per the frequency. A quick mathematical formula is.. 486 divided by the frequency in megahertz.. This will give you a 1/2 wavelength in inches. Divide this number by 2 will give you a 1/4 wavelength..

Most consumer electronics use a 1/4 wavelength antenna, usually in the center of the band. The center of the FM band is 98 mHz. Take 486 and divide that number by 98. (4.959 inches). Just shy of 5'. (round it!) Divide this by 2.. ~2.5' or 30 inches. If you measure the length of that wire coming from the back of your radio, it should be ~30 inches long. If you lengthen it, it throws things out of kelter and probably won't help much..

Adding an external antenna to a radio like that is difficult! There is no balanced input to connect one.. Bummer.. 

I'm sure that the metal in the building isn't doing you any favors. My suggestion would be to move the receiver near a window in the direction of the station and string the wire antenna around or down the center of it and hope for the best. Moving it around the window area a little may help.. Good Luck..


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## Joel_BC (Nov 10, 2009)

12vMan, thanks. The station is at 91.7 mHz.

It won't really be possible to move the unit close to a window... long story, so to speak, but the radio must remain close to the center of the building.

Could I make some kind of a lenthwise copper-wire coil of some kind (like around a length of wooden 1x2), put _that_ in a window, and then take a lead from from that to the end of the unit's supplied wire?


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## spacecase0 (Jul 12, 2012)

you want a 1/4 wave antenna with coax cable connecting the antenna to the center conductor and your building to the outside conductor, then run it back to your radio, 
just connect the center conductor to your antenna if you have to, but connecting the outside conductor to the ground of your radio will help lots 
if you have any questions PM me as I don't know how to find threads I have posted on here, but I do know lots of radio info...


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## 12vman (Feb 17, 2004)

The feedpoint at the radio is going to be ~300 ohms impedance. With a little soldering, a 300/75 ohm transformer could be connected to the feedpoint by one lead of the two 300 ohm ends soldered to the chassis ground and the other lead soldered to the spot where the existing wire is now. This would give you a balanced external antenna connection.


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## spacecase0 (Jul 12, 2012)

with receiving I have noticed a change if you run the wrong impedance, 
but if you are getting noise on the signal the primary issue is to get the antenna outside


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## Bearfootfarm (Jul 13, 2006)

> The goal is to create some sort of resonance per the frequency. A quick mathematical formula is.. 486 divided by the frequency in megahertz.. This will give you a 1/2 wavelength *in inches*. Divide this number by 2 will give you a 1/4 wavelength..
> 
> Most consumer electronics use a 1/4 wavelength antenna, usually in the center of the band. The center of the FM band is 98 mHz. Take 486 and divide that number by 98. (*4.959 inches*).
> 
> *Just shy of 5'*. (round it!) Divide this by 2.. ~2.5' or 30 inches. If you measure the length of that wire coming from the back of your radio, it should be ~30 inches long.


You changed from inches to feet in the middle of your calculations
It seems it should have ALL been "feet"


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## 12vman (Feb 17, 2004)

Yeah, yer right BFF.. I tried to edit that and the changes didn't take it seems. Thanks for the correction.


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## Joel_BC (Nov 10, 2009)

12vman... Let's go back to the formula, and keep it in inches. If I divide 486 by the favored frequency (91.7), it comes out to 6.30 (rounded to nearest two decimal places). That's 6.30 inches, and divided by 2, that's 3.15 inches. So then what do I do with that?

Do I work with a multiple of it? (say, 31.5 in, or 63 in)

Let's say I could mount 63 in of metal tubing outside the building. Is horizontal placement the best? Can I bring in a single wire lead from that tubing and attach to the radio's single antenna wire? Where should that lead be attached to the antenna... in the middle?

What about how the horizontal metal "antenna" should be oriented toward the location of the relay-transmitter tower? Does that matter?


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## 12vman (Feb 17, 2004)

The formula is in feet. I messed up in my other post and didn't correct it. A receiving antenna doesn't need to be real exact, just close. If you design in the center of the band, it'll work for the whole band..

98 mHz. is the center of the FM broadcast band. (88 thru 108 mHz.) Automobile antennas are ~1/4 wavelength for the FM band, designed for the center of the band.

486 divided by 91.7 gives me 5.299 feet. 486 divided by 98 (center frequency) gives me 4.959 feet. Just round to 5 feet. Half of that would be 2.5 feet or 30 inches for a 1/4 wavelength per frequency. Being your desired frequency is lower than center, you could add a little length I suppose. Maybe an inch or two.. (The higher the frequency, the shorter the antenna in wavelength, the lower in frequency, the longer the antenna needs to be) I refer to an antenna as a length of wire..

The issue that you have is the unbalanced antenna input on your stereo. A 1/4 wavelength of wire is seen as ~300 ohms of impedance. The stereo is designed to accept this impedance. Making the existing antenna wire longer or shorter changes the impedance of the antenna, making it less productive. There must be "harmony" or some sort of "resonance" between the antenna and the antenna feedpoint of the receiver.. 

You'll have to work with the existing length of wire that's on the stereo now or modify the antenna feedpoint of the stereo as I suggested above to have the ability to add an external antenna of any quality. In this case, longer is not always better.. 

The only option that you have is to attempt to "spray" some signal near the receiver, using the existing antenna on the stereo and adding an external antenna system..

The goal is to capture some signal outside of your building and bring it inside using a balanced system. (External antenna, some TV coax, and a way to "spray" signal near your stereo without a hard connection)

If you have an outdoor TV antenna, they make great FM antennas! They are directional and provide some signal gain. Point it towards your desired radio station and you're in business! This would be the cheapest way to go and the easiest. FM hi-gain antennas may not be cheap..

Now we have the antenna figured out. All TV antennas need a matching transformer to connect TV cable to it. The feedpoint at the antenna is 300 ohms and the matching transformer matches the antenna to the TV cable, which is 75 ohms. Now you have a balanced system to work with..

Run the TV cable to the area where the stereo is. Find or buy a set of ElCheepo rabbit ear antenna with a cable type output, like the cable you just ran from the antenna, and connect the cable from the outdoor antenna to the rabbit ears with a double female F-Type connector. (Barrel connector) Place the rabbit ears behind the stereo and measure the 2 rods on the rabbit ear antenna to around 15 inches each from the base. This will give you a 1/4 wavelength dipole antenna at FM frequencies. Position the wire from your stereo near the rabbit ears but do not wrap the wire around the rods. That shouldn't be necessary. The signal coming from the rabbit ears should be collected by the wire from the stereo as long as they are somewhat close to each other. Adjust the rabbit ears and the wire from your stereo for best results but do not change the length of the rods! We want some resonance goin' on.. 

The rabbit ears have a matching transformer built-in, like the one used on the outdoor antenna. It will convert the impedance from the 75 ohm TV cable to 300 ohms and feed the two rods. By adjusting the rods to a 1/4 wavelength dipole antenna at FM frequencies will cause more reactance in the system and give it the ability to emit the most signal that it can. Resonance is the key..

Does that sound like a plan?


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## Joel_BC (Nov 10, 2009)

A little update. Hey! I don't know how long my luck may hold... I tried connecting that single antenna wire (provided with the radio unit) to the electrical ground in the building's line-current electrical system. And it seemed to solve the problem.

In other words, I made "a green-wire connection" to ground. And the lisping sound every time an announcer or singer said the consonant *s* in a word disappeared. I say it _seems_ to have solved the problem because I've only listened for 20 minutes.

Can anyone explain to me _why_ it worked?


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## 12vman (Feb 17, 2004)

Congratulations! You've found a magic proximity that works! It won't be stable (might change from time to time) but easier than installing a bunch of stuff. Many times, experimentation pays off..

Explain why? You simply got lucky! The house wiring length and positioning must be in a fashion to resonate with FM frequencies, making the collected energy high enough to overcome the unbalanced antenna losses, giving you a little more signal to the receiver..


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