Which coax to use

The actual impedance match between the antenna and the radio, is not hugely important in a receive only situation, and it is unlikely, with the type of listening you will be doing on a scanner, that you will notice a 75 ohm / 50 ohm mismatch.

Much more to the point, I would assume that your police scanner is VHF, whereas your CB antenna will be cut for 27 megs, so its performance at VHF will be very poor, and its impedance at those frequencies, will be nothing even close to either 50 or 75 ohms.

It is generally considered very bad practice to swap backwards and forwards between cable types when extending.

Arfa

You should use the 75ohm (Cable TV impedance) cable. RG-59 is OK for a short run, but RG-6U is lower loss and better shielded. If you have the choice, I'd use the (thicker) RG-6U. Using 50ohm cable provides an impedance mismatch on both ends so will reflect and partially cancel your incoming signal. The result may not be noticeable on the scanner depending on your distance from the transmitter.

Bob

At a pinch, 50 Ohm cable will work, but if you want to get the best out of your scanner you need the right impedance cable, and it's not particularly expensive so why cut corners? I'm surprised though, comms equipment and scanners are more frequently 50Ohm IME.

Also, a CB aerial is inappropriate for a scanner. It is tuned to 27MHz and will be very poor at anything in the VHF or UHF range. I'd recommend you get a proper multiband/wideband scanner aerial, they can be bought for reasonable money, even brand new. The "Discone" is a popular choice, though it's rather untidy looking and stands out like a sore thumb for its fairly small height. There's more 'subtle' aerials out there!

If the CB aerial is a plain 1/4 wave whip (about 9 ft) then you could easily chop it down so it's resonant at your desired frequency, but it wouldn't be an ideal solution because it would only work well on one band and optimised for a narrow frequency range..

Dave

Bad advice, Bob. Extensions should match the feedline.

I agree that impedance should match ...Could you explain why you interpreted my response that he should mismatch the feed. The OP stated that the unit he was using called for 75 ohm RG-59 coax in his installation instructions, but had used 50 ohm coax instead. RG-6U is matched to RG-59 (75 ohms) or so I had always thought, but has better loss and shielding specifications. I must have missed something in the original post, so would appreciate an explanation since I would love to learn from this exchange but your feedback is not sufficient to understand what I missed or you mean here. Thanks.

Bob

your

It depends whether you assume he's going to replace the 50 ohm stuff which is apparently already in place. To me, the OP is unclear.

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interpreted

instructions,

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feedback

a

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For receiving applications the differance in the mismatch between a 50 ohm and 70 ohm is not enough to notice in most applications. The antenna and input to a reciever over a broad range is no where near either of the two except a few frequencies.

RG 6 cable is normally less lossey than the rg 58 or rg 59 cables. It is usually cheeper and easy to find at many Home Depot type stores. The only problem with it is the shield is not usually copper and you can not solder to it. It will have to be a crimped or friction fit.

Depending on the type of CB antenna, he is probably wasting his time with it anyway.

Because it could be interpreted that way, and to me it sounded that way.

Just so, and anything he adds to that feedline should match it. Now if you are saying he should tear out the old feedline and replace it with 75 Ohm cable, that is another matter, and it is unlikely to have any noticable effect on scanner performance, but it will likely render the setup unusable for a CB transceiver. And no, the CB antenna will not perform well for scanner reception, but may be better than none. And if he replaces it with a good scanner antenna (discone?) the feedline should be chosen to match the antenna, and the receiver probably won't care.

Your post sounded more like use 59 or 6 no matter what.

Sorry for the confusion and thanks for the reply. I was definitely trying to say that the poster should follow the instructions and use the correct 75 ohm cable not the 50 ohm one he happened to have laying around from an old CB radio. My assumption was that the antenna that came with the scanner or that he would be using would be the correct one for the frequencies being monitored and would be 75 ohm as well since that is what the manual called for.... I was just adding that the RG6 was better all round compared to the RG59 and that it really was not much more expensive since it is used heavily in the CATV business.

Bob

First double check your scanner manual. I think most of them are 50 ohm.

For best performance, your antenna, coax, and radio should all have the same impedance (or baluns to match). You'll have serious problems with a 50 ohm to 75 ohm mismatch, so replace the entire cable or use an extensi> I am hooking up an outdoor antenna to my police scanner. I just

I just dug out the manual for my Radio Shack PRO-2030 scanner (Don't laugh, it was free) and it has a 50 ohm input impedance, like every other scanner I've owned or repaired. The thing that bugs me is that a lot of scanners use the Motorola plug which was made for 93 ohm coax. I've never calculated the impedance of these connectors, but they don't appear to be 93 ohms.

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Michael A. Terrell
Central Florida

That Mot thing is not a controlled-impedance connector; it was designed for use with very short antennas on car radios for the AM band. Impedance was not the issue, it was lowest possible shunt capacitance between the antenna and the front end of the radio. If that cable happens to be 93 ohms, it's an accident. If it was necessary for that impedance to be maintained, it would be necessary to make sure the center conductor was actually in the *center*, which it's not with "standard" car radio cable.

R-S used those connectors because they're cheaper than anything else on the market -- except for RCA plugs. And anyhow, a mismatched section isn't very important if it's short compared to a quarter-wavelength. Even 450 MHz is not high enough for that to matter much. You're probably going to get more loss in a reasonable length of coax than anything the connector will cause.

Bottom line; it's just not very important for the connector's impedance to match the input impedance of the receiver.

Isaac

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so

feedback

When you've spent years working on systems were every connection is critical, its hard to ignore mismatches. This includes TV stations, CATV systems and $1M+ turn key telemetry systems. BTW, car radio antennas were designed to use RG-62 coax, which is 93 ohms.

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Been there, Done that, I've got my DD214 to prove it.

Michael A. Terrell
Central Florida

Earlier in my career, I, too, "spent several years" in positions that included designing 50 KW AM and FM transmitters, UHF comms, and HFC networks.

In the context of the OP's question (scanners), mismatch on one connector or a short piece of coax just isn't very important.

And AM car radio cable is not "coax", because the inner conductor cannot be properly centered -- it can't be "coaxial" in that big hollow plastic tube. The stuff may be about 93 ohms, but that's not important. What

*is* important with an antenna that is *very short* compared to a wavelength is lowest possible shunt capacitance, and the construction technique of that cable favors that.

FWIW, that low capacitance bit was far more important with vacuum tube front ends. With super-high-impedance FETs, things get a bit less critical. Note that AM car receivers don't always have that "antenna trimmer" to be peaked at 1600 KHz these days; with tubes, it was necesssary.

In fact, you can show that for a zero shunt leakage feedline, and an infinite-impedance RF input, the voltage at the RF amp input is not dependent on antenna length *at all*. That is very different from the case where the input is impedance matched.

Isaac

50 KW? Try a 5 MW UHF TV transmitter site. Ch 55 in Orange City, Fl. A Comark transmitter, and a 1749 foot tower.

Tell that to the people who make the cable. It is listed as "coaxial Cable" which simply means that one conductor is inside the other. BTW, IBM used the same RG/62 93 ohm coaxial cable to connect terminals to their mainframes, and I think it was the early ARCNET networking that used it as well.

I serviced car radios back in the '70s and all the solid state radios had a trimmer as well. The trimmer was to trim the capacitance of the coaxial cable so that it didn't detune the tuned circuit at the input of the RF amplifier. I have almost every volume of the Sams Photofact AR series manuals here, and they all used that design. have you noticed that the newer "No tune" design doesn't work as well? They are easy to overload, and are useless when you want to listen to a distant station at night if a local station is a couple channels away.

A car radio antenna is more of a sensing probe than an actual antenna due to the fact that it is so short.

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Michael A. Terrell
Central Florida