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 Post subject: whats the best coax for my scanner antenna
PostPosted: May Sun 29, 2022 5:38 am 
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Location: central wisconsin, 54449
Hi to everyone. I have a radio shack pro 2004 scanner. It covers frequencies between 25 mhz to 1300 mhz. I will be putting a discone antenna on my tv tower at about 40' high. Will I be better off with 50 or 75 ohm coax? And what would be the best type of coax with the least amount of loss ? Total length of coax will be approximately 70' before it enters my radio room. I would appreciate any suggestions. My thanks in advance


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 Post subject: Re: whats the best coax for my scanner antenna
PostPosted: May Sun 29, 2022 6:17 am 
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What's the impedance of the discone antenna? Does it have a manufacturer's name or is it homemade?

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 Post subject: Re: whats the best coax for my scanner antenna
PostPosted: May Sun 29, 2022 12:05 pm 
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The antenna is probably designed with a nominal 50 ohm impedance and that will vary somewhat across the wide spectrum of the coverage of that sort of antenna. Also that antenna will only display any gain at the upper UHF part of the spectrum, as I understand it. So you will want minimal losses in the feedline. The amount of loss depends on your budget, more or less. Here is a well known supplier of coax cables for the ham radio people: https://thewireman.com/product-category ... ines/coax/
If you want lowest losses in the upper UHF area such as trunked systems in the 8-900 MHz range, it will cost you a good amount. To make sure your height gain is not cancelled out by losses. The larger coax in the size class of RG-8 is stiffer and heavier but will have the least losses. You could get away with the thinner low loss stuff over a much shorter length but it sounds like you have a long run for a scanner antenna, longer than most people have. You could run smaller sized coax over a short run to the radio from the larger coax for convenience and flexibility but you would have to invest in a couple more connectors and an adapter of some sort. If you were interested in the spectrum below about 300 MHz the losses would be somewhat less and you might not need the super low loss stuff. I guess something in the RG-8 family would be the minimum for lowest losses for such a long run. BTW if it is not on the top of the tower, but side mounted, you would want to keep it well away from the tower itself. A few feet away. And the mass of the tower will probably affect the pickup pattern of the antenna depending on frequency, which is a wide range with a discone. You would want it on the side of the tower towards the general direction of most of your activity.

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 Post subject: Re: whats the best coax for my scanner antenna
PostPosted: May Sun 29, 2022 1:47 pm 
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I agree with all that Wazz posted. In my experience it depends on what you want to receive. For local High and Low band 50 ohm or 75 ohm will work as most modern scanners have no problem with either. I have 2 main scanners that I use to monitor both my local county and the county south of me. Both are 700 Mhz digital simulcast systems.
For the local reception, regular cable tv coax (75 ohm) works fine.
For the distant county I had to get creative. I bought by a 800 mhz yagi antenna and pointed it in the direction I wanted to monitor. I had about 70 feet of old hardline from a commercial tower that I used for the lead in. I hear the distant just as good as the locals.
The latest digital scanners built for simulcast (Uniden SDS 200)work better than the first generation (Uniden BCD 536).
Again, it depends on what and where you want to monitor.
A good forum to check out would be https://forums.radioreference.com/
Hope this helps.

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 Post subject: Re: whats the best coax for my scanner antenna
PostPosted: May Sun 29, 2022 4:03 pm 
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I think that this would be the ideal situation to use a phantom powered wideband low noise preamp at the antenna. Then you can then get away with higher loss but lower cost coax cable.

- Jim


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 Post subject: Re: whats the best coax for my scanner antenna
PostPosted: May Sun 29, 2022 5:20 pm 
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LMR-400 would be a good choice.

DM


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 Post subject: Re: whats the best coax for my scanner antenna
PostPosted: May Sun 29, 2022 5:30 pm 
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CATV or satellite RG6 75 ohm cable will be more than adequate.

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 Post subject: Re: whats the best coax for my scanner antenna
PostPosted: May Sun 29, 2022 7:00 pm 
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The catch is, what sort of connector is on the antenna and scanner? You can get BNC connectors for 75 ohm coax, I know, I have a lot of them that were sold to me as being for RG 58, by crooks. Probably because no one wanted them for their correct usage. That foam core cable TV coax is pretty low loss alright. I am not sure in this instance if there would be a problematic mismatch or not. Maybe not a big deal for receiving. Bad deal for transmitting. Just like I said, you don't want line loss to cancel out the height gains. He might need adapters for that sort of coax to work with the antenna receptacle. No cheap shortcuts, or you will maybe find out how critical a bad connection is at 800 MHz as compared to regular VHF.

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 Post subject: Re: whats the best coax for my scanner antenna
PostPosted: May Sun 29, 2022 8:15 pm 
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For receiving, there really isn't going to be much difference. Especially with a broadband antenna such as a Discone.

The best bang for the buck will be easy to get RG6 as Peter mentioned. Adapters are readily available for different types of connectors.

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 Post subject: Re: whats the best coax for my scanner antenna
PostPosted: May Sun 29, 2022 9:11 pm 
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Another vote for RG-6


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 Post subject: Re: whats the best coax for my scanner antenna
PostPosted: May Mon 30, 2022 2:08 pm 
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Location: Long Island NY
Quote:
CATV or satellite RG6 75 ohm cable will be more than adequate


The vast majority of scanners and scanner antennas are 50-ohm, and at microwave frequencies (1.3 GHz is L-band) the reflections can be considerable with 75 ohm cable even for short lengths. If the run is short (~50 feet or less) I'd use LMR200 cable, if longer then LMR400. Both LMR200 and LMR400 are 50-ohm cables and are available from most places that sell scanners and scanner antennas. You'll get far better performance out of the right cable and connectors than you would by using an inappropriate cable because it is cheap, and then trying to make up for it with some kind of amplifier.

You are aware that the RS PRO2004 is an analog scanner (AM and FM wide/narrow) that dates back to 1986? Things have changed a bit since then; in many areas first responders and government agencies have gone over to digital and trunked radio systems which the PRO2004 will not receive. Cellular calls are all digital and encrypted. The radio will not be compatible with such transmissions no matter what antenna or cable you use. Just make sure there's actually something for it to listen to before investing in an antenna installation.

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Last edited by Chris108 on May Mon 30, 2022 10:11 pm, edited 1 time in total.

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 Post subject: Re: whats the best coax for my scanner antenna
PostPosted: May Mon 30, 2022 9:21 pm 
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I agree with others here: LMR400 (or Belden 9913) for high performance, RG-6 for price (and reasonable performance).

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 Post subject: Re: whats the best coax for my scanner antenna
PostPosted: May Tue 31, 2022 1:40 am 
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I suppose it all boils down to, "How much $$ do you want to spend??"

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 Post subject: Re: whats the best coax for my scanner antenna
PostPosted: May Tue 31, 2022 11:31 am 
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RG-6 is not a reasonable compromise. It’s the wrong stuff to use in a 50-ohm system that operates at high frequencies. If you have to economize, consider RG-8 or RG-58. They are at least the right impedance, though losses will be higher than with the LMR cables.

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 Post subject: Re: whats the best coax for my scanner antenna
PostPosted: Jun Wed 01, 2022 1:31 am 
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Granted, it was at lower frequencies (15 meters) but I did build a dipole for my Heathkit HW-16 and used 75 ohm coax. It worked very well as far as I was concerned and I made a number of contacts with it.

I cannot say it performed any worse than with 50 ohm coax, but how much better if I did I have no idea. It was a temporary installation as I was just a teenager and with little money. The guy who "Elmered" me was the one who suggested it.

We used a Heathkit AM2 SWR meter which he had, however I cannot recall of the short length between the SWR meter and the HW16 transceiver was 50 or 75. I DO remember there were inserts for the venerable PL-259 for either RG-58 or RG-59

Take this as you will. Chew me out if you must, but I'm going by real world experience which I witnessed.
If I committed a technical crime, haul my butt to jail.

I'm done.
You will see no more responses from me on this subject.

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-Steve

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 Post subject: Re: whats the best coax for my scanner antenna
PostPosted: Jun Wed 01, 2022 2:39 pm 
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Location: Long Island NY
Well, Steve, as I see it, we're all in this to learn something. Including me. There are no wrong answers and nobody has the right to "chew out" anybody else. That was not my intent, and apologies if you saw it that way.

In my opinion your Elmer taught you one of the very best and most valuable lessons in ham radio (I've been licensed for 40 years myself), which is how to improvise out of whatever materials you have on hand. Especially in emergencies, but at other times as well, we cannot always go order the exact things we need and wait three days for the UPS truck to show up. Nor is it always feasible economically. So what if your SWR was 2.2 instead of 2.1 because of the mismatch from the 75-ohm coax? That's not going to make a significant difference in who you can contact on 15 meters!

The difference here is, the OP is planning a new antenna installation for his scanner and asked what the best coax for it would be. The scanner in question covers up to 1.3 GHz, and at those frequencies, mismatches over short lengths of cable can cause quite noticeable reflections. So it would be best to avoid them by using a 50-ohm coax in the first place, and preferably one that is intended for microwave frequencies.

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