Whats with this FM antenna??

[the dude of dudes]

I recently bought an FM transmitter which came with a bizarre type of antenna. It's on a BNC connector, and is 36" long. The first 12" are coax with a ground shield, then it's spliced onto a 24" length of coax without a ground shield. The antenna doesn't work very well at all, and I think I'll replace it with a 48-55" length of wire. What's the purpose of it being only 36", and two different types of wire spliced together? Is the ground connection on this antenna even necessary?

 

[Hero999]

36" or 914mm sounds pretty long for VHF but it's pretty hard to picture what you're talking about without a picture.

 

[audioguru]

A whip antenna (single piece of straight wire) about 80cm (31.5") long works well for a transmitter on the FM broadcast band. It should be broadside and pointing vertical or horizontal the same as the receiver's antenna.

 

[RadioRon]

Your description makes me think of the type of antenna called a Sleeve Dipole or also referred to as a bazooka feed vertical. These types include a lower section made out of coax to carry the RF up to the feedpoint which is the splice point between the two wires. Usually, the lower coaxial section has a second coaxial outer conductor, or sleeve, which is connected at the splice point, but not at the base of the antenna. Such construction is usually hard to identify since a maker will usually cover the entire structure with heatshrink tubing, but there is usually a lower fat part and an upper skinny part, the whole thing being nearly half a wavelength long.

 

[Hero999]

Have you tried operating it at different angles?

Perhaps you're using it the wrong way round, you need to orientate it to the correct polarisation.

 

[the dude of dudes]

Your description makes me think of the type of antenna called a Sleeve Dipole or also referred to as a bazooka feed vertical. These types include a lower section made out of coax to carry the RF up to the feedpoint which is the splice point between the two wires. Usually, the lower coaxial section has a second coaxial outer conductor, or sleeve, which is connected at the splice point, but not at the base of the antenna. Such construction is usually hard to identify since a maker will usually cover the entire structure with heatshrink tubing, but there is usually a lower fat part and an upper skinny part, the whole thing being nearly half a wavelength long.

This sounds about right.The lower 12" section is coax with 2 connections at the BNC connector. The upper (and yes it is skinnier) section is 24", and actually is not coax. It's a single strand of copper inside a rubber or plastic core. It resembles the center wire in a coax line though. The upper section is covered in heatshrink. So if it is a proper antenna, I still have the trouble of a poor signal. Even setting a radio right next to the transmitters antenna doesnt provide decent reception. Its very staticy and has a pulsing sound in it (does that make sense?). Here's a link to the transmitter:

**broken link removed**

 

[audioguru]

"If it doesn't work, sell it on E-Bay."
Didn't you buy it on E-Bay? No name-brand? What do you expect?

 

[Hero999]

I wouldn't sell anything crap on ebay as I don't want to gain a bad reputation.

 

[the dude of dudes]

Is it necessary to run a length of coax between the antenna and transmitter to give them seperation? Right now its a direct connection, could the transmitter be causing interference with the antenna? It is supposed to be a 5watt transmitter, is the extra power causing a problem?

 

[sam2]

Here is an easy to build antenna. http://www.alpharubicon.com/elect/vertdipoleniet.htm (from the picture on your site you dont have this type of antenna) I built one just to recieve on and it works great( but its good for transmiting too).

You can run this right to your transmiter without a balun( a device for matching impedance between your antenna and feed line) because the impedance is allready matched.

Since VHF is line of site , put the antenna as high as you can and away from conductors( other antennas or anything conductive that will absorb the energy from your antenna). Use string or somthing nonconductive to hang it from. A tree is good and the attic is ok if you dont have a metal roof.
Here is a good site for antenna basics http://kyes.info/antenna/antennadex.html
sam

 

[the dude of dudes]

I tried a 55" length of 20awg connected to the signal pin of a BNC connector and wouldn't you know it, got a good signal! I still cant believe the simplest antenna would work so well.

 

[RadioRon]

I tried a 55" length of 20awg connected to the signal pin of a BNC connector and wouldn't you know it, got a good signal! I still cant believe the simplest antenna would work so well.

I looked at the photos of the original antenna on Ebay and don't like what I see. This is not a correctly constructed sleeve dipole but instead looks like a simple piece of wire fed by a length of coax. I think that this is a bad antenna.

I note that the seller suggests that the transmitter not be operated without an antenna attached. This implies that they have no output amplifier protection circuits and that a poor impedance match may cause the amplifier to overheat. You should try to do a proper job of presenting a good impedance match to the transmitter if you know how. At the very least, build a dipole of the correct dimensions instead of just using your 55inch random wire antenna.

 

[Sceadwian]

Random wire antennas will work fine for short distances, but not for longer ones, the dipole that came with your kit will transmit over a much larger distance but will tend to have a deadzone very close to the antenna because the radio signals are emmited in a sort of toroid with the pole being the center of it. Above and bellow are weakest signals, but at any decent distance away from the antenna or in the same plane as it you should get a good signal.

 

[the dude of dudes]

I looked at the photos of the original antenna on Ebay and don't like what I see. This is not a correctly constructed sleeve dipole but instead looks like a simple piece of wire fed by a length of coax. I think that this is a bad antenna.

I note that the seller suggests that the transmitter not be operated without an antenna attached. This implies that they have no output amplifier protection circuits and that a poor impedance match may cause the amplifier to overheat. You should try to do a proper job of presenting a good impedance match to the transmitter if you know how. At the very least, build a dipole of the correct dimensions instead of just using your 55inch random wire antenna.

I built the simple dipole in your link, and had poor results. I get a crystal clear signal with my 55" wire. With the dipole I get a hum and the bass is distorted. I used a length of coax and slid the ground sheath back over the cable like the article says. I tried overall antenna lengths of 44"-55" with little or no difference. The hum went away if I slid the sheath back into place, so it was essentially just a length of coax. But the bass is still quite distorted. With my single wire I have no ground connection, could this be the difference? Is there a way to improve my single strand of wire antenna to prevent damaging my transmitter?

 

[Sceadwian]

It is worth mentioning that operating this in the United States, or in pretty much any country with an FCC equivalent is illegal.