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Old 05-01-2009, 02:44 PM   #1
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Schematic for fm antenna...


I swear I've seen a schematic for an fm antenna so the cable does not act like a big antenna...cant find it now. Do you know what I mean?

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Old 05-01-2009, 03:59 PM   #2
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Schematic for fm antenna...


What do you mean by the cable acting as a big antenna?

There are a number of antennas that can be used for FM reception - popular is a inside antenna made from twin lead - cheap and easy. See here for one example http://hf-antenna.com/003/

If you are looking for outside higher performing antennas Google for examples of yagi, folded dipole, or log periodic.

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Old 05-01-2009, 04:09 PM   #3
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Schematic for fm antenna...


What I mean is, when I bring the coax to the antenna. I want the shield to act as a shield, not an antenna.
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Old 05-01-2009, 04:40 PM   #4
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Schematic for fm antenna...


Quote:
Originally Posted by rgsgww View Post
What I mean is, when I bring the coax to the antenna. I want the shield to act as a shield, not an antenna.
You need a balun. . .depending on what your antenna is.
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Old 05-01-2009, 05:33 PM   #5
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Schematic for fm antenna...


Quote:
Originally Posted by rgsgww View Post
What I mean is, when I bring the coax to the antenna. I want the shield to act as a shield, not an antenna.
Grounding the shield at both ends should do the trick.
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Old 05-01-2009, 07:27 PM   #6
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Schematic for fm antenna...


In olden days coax was'nt used. Twin lead was usually used as the antenna was balanced! (same potential on both terminals)
The problem with twin lead was that stray signals could be picked up that interfered with the antenna signal.
It was found that the twin lead could be covered with foil that was grounded to stop this interference.
However, coax is better at preventing this interference and is cheaper to manufacture.
Coaxial cable is unbalanced, whereas antenna's are usually balanced so a matching transformer was used to mate these two. These transformers are known as 'baluns'.(sp?)
When FM receivers and TV's first came out, the antenna input was was balanced to match the balanced antenna. So if you wished to use coax, two baluns were required. One at the receiver and one at the antenna.
Then, as coax became more popular FM and TV receivers were designed to work with unbalanced coax. And antennas were made with a built in balun.
When using twin lead a wave trap could be hacked up to short circuit lead interference.
A piece of twin lead about 4 feet long would be connected to the receiver terminals, along with the twin lead from the antenna.
On a TV a channel was selected that had interference on it ( known as a ghost) then a piece foil, such as would come from a chewing gum wrapper, was wrapped around the the 'trap' twin lead.
The foil was slipped along the twin lead until the ghost disappeared.
At the point where the foil ended up, the twin lead was cut off, the conductors stripped and twisted together.
This worked as a permanent trap for a particular channel interference.
With coax, all this has now become redundant.
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Old 05-02-2009, 12:41 AM   #7
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Schematic for fm antenna...


Quote:
The problem with twin lead was that stray signals could be picked up that interfered with the antenna signal.
It was found that the twin lead could be covered with foil that was grounded to stop this interference.
However, coax is better at preventing this interference and is cheaper to manufacture.
All of these statements are wrong. Where did you come up with this stuff?
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Old 05-02-2009, 01:16 AM   #8
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Schematic for fm antenna...


He's right that coax is unbalanced, and twin lead or ladder line is balanced. Antennas are generally designed with feedpoints that require a balanced feed, so a balun is often necessary with coax. Those parts are right.

Coax has the advantage of being easier to run and pull, and is not sensitive to being installed right next to metal objects, since the field is tightly contained, unlike ladder line, which can be affected negatively by being run close to metal objects. Putting aluminum foil around a ladder line would likely cause common mode currents that would increase interference. The spacing of the shield and dielectric in coax is a critical parameter.

Also to the person that said "ground the shield at both ends"... that's probably a bad idea. You don't want a second low impedance RF ground path through a grounding wire.
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Old 05-02-2009, 01:32 PM   #9
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Schematic for fm antenna...


Quote:
Originally Posted by Gigs View Post
All of these statements are wrong. Where did you come up with this stuff?
OK! If I'm wrong tell me why! Shielded twin lead was developed to prevent extraneous signal pickup. Also, unshielded twin lead had to be kept away from any metallic object by at least 6" or it would short circuit the antenna signal. This was unnecessary with shield. It could be strapped directly to the tower rather than use stand offs!
Shielded twin lead is more expensive than coax for the simple reason that it uses twice the copper per foot.

Its fair to challenge a statement, how its incumbent on yourself to prove why its incorrect. Otherwise, you should refrain from making unsubstantiated comment!
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Old 05-02-2009, 02:56 PM   #10
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Schematic for fm antenna...


Twin lead only guides the EM field; because of skin effect it's all external to the wires.
With coax the entire field is contained within the dielectric so coax is more like a waveguide.

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