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#160022 04/28/05 09:44 PM
Joined: Jan 2004
Posts: 615
J
Jps1006 Offline OP
Member
I have a small Sony bookshelf stereo in a basement. The AM antenna that came with it doesnt get all the AM stations I want. I have an antenna on the roof with RG6 between them, and it gets FM great.

Here's my question: can I mount an AM antenna on the roof, split into the RG6 to get down stairs and then split back out to hit the AM terminals on the stereo? Any ideas on how to make a remote AM antenna? Thanks.

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#160023 04/28/05 10:32 PM
Joined: Dec 2003
Posts: 886
H
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No, you can't do that.

What provisions does the receiver have for an external AM antenna? If none you are pretty much stuck with what and where it is. You can't extend the antenna. About all you can do is wrap a piece of wire a few times around it and run it somewhere to see if the reception improves.

-Hal

#160024 04/28/05 10:52 PM
Joined: Jan 2004
Posts: 615
J
Jps1006 Offline OP
Member
The receiver has two spring terminals for an external antenna. It also came with a small AM antenna that is the little wire wrapped around a plastic loop, then the wires twist together until them terminate on the receiver. how much length can I add to the twisting part?

What if I mount the AM antenna that came with the stereo on the roof, then run a cat5 (to get a twisted pair) between the two?
BTW thanks for the quick response Hal.

[This message has been edited by Jps1006 (edited 04-28-2005).]

#160025 04/29/05 12:42 AM
Joined: Jul 2002
Posts: 8,443
Likes: 3
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Jps,
The ultimate aerial for an AM Radio set is a long wire Inverted-L type arrangement.
When AM radio was all they had, radios used one wire to the aerial itself and another to a Ground stake nearby.
However, it must be said that any loop aerial used for AM radio reception will be inherently directive (ie: they must be pointing side-on to the station to be recieved).
For extending wiring to a loop aerial in this fashion, you neeed to use 300 ohm ribbon (Twin-lead) or you will end up with a mis-match between the aerial and the connecting wire and this will result in signal degradation.
Hope this helps. [Linked Image]

#160026 04/29/05 01:25 AM
Joined: Jul 2004
Posts: 9,959
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G
Member
Back in the olden days when radio was AM we used a 100' of wire on insulators. as an antenna. I remember many a night listening to Dick Biondi (WLS in Chicago) from Southern Maryland. We could also get WOR and WOWO


Greg Fretwell
#160027 04/29/05 09:55 PM
Joined: Dec 2003
Posts: 886
H
Member
The last thing you would want to use is God d*^%@ CAT5! If there are indeed spring clips for the AM antenna try a length of 300 ohm twin lead as Trumpy suggests- if you can find it.

-Hal

#160028 04/29/05 10:08 PM
Joined: Jan 2004
Posts: 615
J
Jps1006 Offline OP
Member
what's the matter Hal, tired of people treating Cat5 as the miracle cure for all modern wiring needs? [Linked Image]

I'm just curious though, what's the point of the 300 ohm wire? is it a matter of connectors and terminations or is it a characteristic of the signal that would require the 300 ohm wire?

BTW Dick Biondi is still on the FM oldies station here in Chicago.

[This message has been edited by Jps1006 (edited 04-29-2005).]

#160029 05/01/05 06:12 AM
Joined: Jul 2002
Posts: 8,443
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Jps,
Quote
I'm just curious though, what's the point of the 300 ohm wire? is it a matter of connectors and terminations or is it a characteristic of the signal that would require the 300 ohm wire?
Now Hal may be able to give a better explanation than me, but 300 ohms was the input impedance of a centre-fed dipole antenna, when TV first started.
Hence the use of twin-lead feed lines (Ribbon lead) for TV's at the time.
TV sets that use antennae, to recieve a signal now are still 300 ohms but we use a 75 ohm coaxial cable to feed the TV Set.
Between the antenna and the co-axial cable, you need to have a "matching device" so that the two things "agree" with each other.
We use a Balun to do this, which is a 1:4 RF Transformer.
This is an impedance only, not a true resistance.
Anyone else care to put thier spoke in?. [Linked Image]

#160030 05/01/05 12:57 PM
Joined: Jan 2004
Posts: 615
J
Jps1006 Offline OP
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Oh, Okay, Thanks Mike.

It is starting to make more sense now. This is a weak area of mine and I think I am going to dig up an old electrical theory book I have laying around somewhere.

#160031 05/01/05 04:06 PM
Joined: Dec 2003
Posts: 886
H
Member
Well, I'm not even sure about the 300 ohm twinlead. My only thinking is that the capacitance between the two conductors is low so if anything that would be my choice.

With AM loop antennas such as this the loop itself is part of the input circuitry. It's been a long time since I looked looked at any AM receivers. I do remember that sometimes the external loop is connected across a winding on the input RF transformer, sometimes only one side is and the other connected to chassis ground either directly or through a capacitor. There is usually some "tuning" going on between the loop and the input transformer so the wire and it's length can be a factor in it's operation.

Your best course here would be to experiment and see what works best. I really wouldn't get too hung up on having to use the supplied loop antenna. The receiver may well be designed to use a long wire antenna instead of the loop. Give it a try, connect one side to ground and the other to a piece of wire.

Keep in mind that AM frequencies are much lower than FM and TV and as such the antenna requirements are much different and usually less sophisticated.

I have always found that the AM reception on most stereo receivers (if they even include AM) is pretty poor anyway, certainly not nearly on par with the work that went into the FM section. This is probably because of the ever declining popularity of AM in this country. Not too many people go out and spend $$$ on a stereo system or home theater system and want to listen to talk radio. So don't look for manufacturers to put a lot of effort or research into a better AM antenna.

-Hal

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