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Old 09-21-2007, 04:23 PM   #1
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Ohms…Ohms….Say it slowly…Ohms

A couple of questions about Ohm ratings.

First, does anybody know what the Ohm rating would be for radio antenna wire? After poking around at Radio Shack today, I’m guessing 50 Ohm. The reason I ask is that I’m trying to decide what to do about the radio antenna for the am/fm radio. The OEM connection came thru the television antenna, but unfortunately, I can’t repair that. The TV antenna works, but no luck with the radio.

I have a bunch of left over pieces-parts form the OEM setup as well as the remnants of two radio antennae that were mounted to the center and back vent covers up on the roof. Guessing that these were either CB or short wave. Just trying to decide what to buy to make it all work.

Second, what to do about the speakers. I seem to remember reading in a thread here someplace that modern speakers/radios use a different Ohm rating than in the early ‘70’s. And somebody mentioned that mixing old speakers with new radios could fry the radio??? The original speakers are still there, but I’m sure they are junk now, so they will have to be replaced. The original speaker wire is still there as well and looks to be in good shape. It appears to be about 14/16 gauge, stranded copper. That shouldn’t be a problem, should it?

As always, thanks for the help.

Jim
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Old 09-21-2007, 04:32 PM   #2
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Radio antenna wire is usually 75 ohm coaxially wound. Keep in mind that 75 ohms is the impedance, which is the alternating current version of direct current's resistance.

Bottom line is that you can not verify a given setup with a run-of-the-mill ohmmeter.

Back when I was souping up car stereo, speakers were 8 ohm impedance. Nowadays, the standard is 4 ohm. Yes, you could cook a radio expecting 8-ohm speakers if you ran it at high volume for a while on 4-ohm speakers. Dinner-music-level volume should not be a problem.

Your speaker wire description sounds adequate.

Tom
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Old 09-21-2007, 04:50 PM   #3
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Jim, where is the radio located in your Overlander? Is it in the front overhead, or is it in a cabinet along the street side of the coach? I don't know if you've been following that closely, but I had good luck installing a car antenna on the side of our coach.
If you are replacing the radio, maybe you could pick up 4 new modern speakers to go with it.
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Old 09-21-2007, 04:55 PM   #4
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Are you talking coaxual cable or the old twin lead wire. I had the old twin lead wires and that has an impedance of 300 ohms. You can use a BALUN (balanced-unbalanced) matching transformer to conect coaxual cable to the twin lead wire for a proper siganl to the TV/Radio.
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Old 09-21-2007, 05:03 PM   #5
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Yes you can, but

Quote:
Originally Posted by Buttercup
... You can use a BALUN (balanced-unbalanced) matching transformer to ...
I tried that route in hopes of using my television's previously wired, 300 ohm flatlead. For whatever reason, I lost a lot of signal strength. Ended up having to run a new coax line for the television.

On a happier note, I doubt Jim's radio lead was wired with flatlead.

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Old 09-21-2007, 05:06 PM   #6
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=

On a happier note, I doubt Jim's radio lead was wired with flatlead.

Tom
The stuff I pulled out of our ceiling looked like standard radio antenna wire, I'm guessing Jim's is the same, based on the similar vintage of the trailers.
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Old 09-21-2007, 05:22 PM   #7
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Originally, there was flatlead coming from the TV antenna down to what I originally thought was a power booster of some sort. After consultation with my buddies here on the forum, I now believe that the “transformer” was in indeed a balun. I have replaced the flatlead with coax from the TV antenna to the interior of the camper to run my new HD LCD TV (GREAT PICTURE!), the flatlead was completely shot, apparently broken someplace inside the jacket, so it had to be replaced. You can read about it here http://www.airforums.com/forums/f450/hdtv-old-airstreams-34784.html if you need some late night reading material.

Now I’m faced with providing an antenna for the radio, hence the current question. I wanted to use some type of antenna like Terry found recently for his rig, but I can’t seem to find anything suitable. I’m also not really comfortable drilling new holes in the outer skin unless necessary. Since I’ve already got many of the pieces there, I just trying to put something together using new and old.

Jim
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Old 09-21-2007, 05:33 PM   #8
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Terry, I'm going to replace teh speakers with something modern (and that actually works!). And the stereo is in the front overhead, below the control panel.

Tom, you are correct. The wire that ran from the balun to the radio is some type of coax, not flatlead. It's really a strange looking setup.

Give me a minute to compose a reasonable description of it, you'll get a kick out of this weirdness. Back in a few.

Jim
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Old 09-21-2007, 06:05 PM   #9
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So the radio coax is really strange looking stuff. On the radio end, there is one piece of coax that presumably at one time had a standard radio jack on it (all hacked up now). So far so good, right? On the balun end, there were two pieces of coax that appeared to be cobbled into one cable going towards the radio. One cable appeared to acting as the center dielectric providing the signal a path from the balun to the radio, the other piece of coax appeared to be acting as the ground as it was soldered to the outside mesh shield of cable number two on the balun end, then ran towards the radio in a sheath that contained both wires. My assumption was that the two wires were joined inside that sheath and that one wire acted as the signal path, while the other provided the necessary ground/shielding to make a clean RF signal path. When I took the two apart, guess what? No connection between the two pieces of coax.

Strange, eh? All of this leads me to believe that either a) somebody got lazy at the factory, or b) I’m actually clueless or c) I just need to replace the whole run of wire with a little help from my friends here.

Thanks for the help guys.

Jim
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Old 09-21-2007, 06:13 PM   #10
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1000 words

Jim,

Can you post a picture of the setup?

Tom
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Old 09-21-2007, 06:22 PM   #11
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Quote:
Originally Posted by Jim & Susan
A couple of questions about Ohm ratings.

First, does anybody know what the Ohm rating would be for radio antenna wire? After poking around at Radio Shack today, I’m guessing 50 Ohm. The reason I ask is that I’m trying to decide what to do about the radio antenna for the am/fm radio. The OEM connection came thru the television antenna, but unfortunately, I can’t repair that. The TV antenna works, but no luck with the radio.

I have a bunch of left over pieces-parts form the OEM setup as well as the remnants of two radio antennae that were mounted to the center and back vent covers up on the roof. Guessing that these were either CB or short wave. Just trying to decide what to buy to make it all work.

Second, what to do about the speakers. I seem to remember reading in a thread here someplace that modern speakers/radios use a different Ohm rating than in the early ‘70’s. And somebody mentioned that mixing old speakers with new radios could fry the radio??? The original speakers are still there, but I’m sure they are junk now, so they will have to be replaced. The original speaker wire is still there as well and looks to be in good shape. It appears to be about 14/16 gauge, stranded copper. That shouldn’t be a problem, should it?

As always, thanks for the help.

Jim
Jim.

The antenna cable can be purchased with different "ohm" ratings.

The primary consideration for which rating to use, is not the antenna, but the radio.

Determine what the input antenna rating is for the radio.

That dictates the proper antenna, without guessing.

Andy
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Old 09-21-2007, 07:03 PM   #12
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Quote:
Originally Posted by TomW
Jim,

Can you post a picture of the setup?

Tom
No, I've already disassembeld it. I think Andy may be right, just look at the Ohms rating of the radio (if I can find it anywhere in the documentation) and go from there. I was really hoping to use some of the "left-over" parts to piece things back together.

Thanks for the help everybody.

Jim
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Old 09-21-2007, 07:19 PM   #13
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Maybe in CA

Quote:
Originally Posted by Jim & Susan
... Andy may be right, just look at the Ohms rating of the radio (if I can find it anywhere in the documentation) ...
Jim,

I think most modern-day radios are set up for 75 ohm cabling. The Pioneer SuperTuner installed a year or two ago in my Airstream certainly was.

The spec sheet is attached below. Look at \FM tuner\Usable sensitivity & FM tuner\50 dB quieting sensitivity; both call out "75 ohm"

The factory-installed radio wiring in my Overlander plugged in to the new radio with no modification. And I have good reception.

Tom
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Old 09-21-2007, 07:23 PM   #14
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Okay, so its unreadable.

Attached is a zoom-in of the above image.

Tom
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