Not really HIGHLY technical but it may be close

[Texun]

I want to use an OS antenna on a mast for my police scanner. I know scanners, CB, HAM etc. call for call for 50 Ohm coax, but I have some quad shield RG6 which of course is 75 Ohm and would like to know if anyone here has used 75 in place of 50, and if so how did it work?

Google turned up a few old radio forum threads where operators claimed to have used 75 Ohm TV coax with no problem for scanners, but I would like to hear from an AT'r who has seen or used it.

I understand there would be an impedance issue if I were transmitting, and especially so if the content were digital, but the primary use will be VHF analog receive only.

It's not so much the cost of replacing what I have on hand, but I know my quad shield is quality low loss cable, so unless using it would cripple the performance, all I need to do is add a PL259 and a BNC for the ends. If I have to go with RG8 or RG58 my only option would be to order online. R-Shack coax is not a consideration and Lowes and Home Depot don't carry RG8 or RG58.

I only need about 50 Ft max and using my existing RG6 quad I estimate a drop of ~1.25dBmv at 155Mhz.

Can I expect good results using what I have or should I order new RG8 online?

 

[Born2bwire]

You're going to get the same mismatch losses regardless of whether you are transmitting or receiving. Still, a quick calculation and I think you'll probably only lose maybe 5% or .22 dB from the mismatch losses but that is a long run of cable.

 

[bobdole369]

50 ohm cable is what you want.

Using 75 ohm cable gives you a 1:1.5 mismatch right off the bat, so add 1.5*total loss of the system. You are going to lose 1-2dB for any connector - I note a pl259 (very lossy at VHF BTW) - and a bnc, so 4db more loss. If the connector is soldered, and not machine made/maintaining perfect shield - add another db or so.

Is your antenna used for receiving tuned for the frequency you want? swr typically goes 3:1 in about 8 mhz range up at vhf, but it depends on the antenna. That would add (3* (1.5 from the 75 ohm losses above) ) * total loss.

In theory this is a terrible horrible no good very bad idea. FUBAR before you start, soup sandwich, etc.


In practice - it'll work in spades and your intended purpose will be acheived. Replacing the cable with RG8 is a good idea and it'll work ever so slightly better.

my only option would be to order online

Know any ham ops? Any electronics stores in your area?

 

[Texun]

I don't know any ham ops, but I would rather see the cable before I buy it so I know what I'm getting. The nearest large city is San Antonio. I've been away for years so I'm not sure what's around here but I would expect someone to have a shop with quality cable. I don't need heliax quality - just a good low loss cable will work for a short 50Ft run.

IIRC, the insertion loss on PL259 and BNC connectors is flat. Is that correct? I know the attenuation through coax is greater with higher freqs, and I've calculated it and don't expect any problems with a short coax run, but It's been a long time since I've done this and I'm not sure about connectors.

 

[Modelworks]

I would stick with 50 ohm cable designed for the purpose.
http://www.fordyce.org/scannin...ning_info/scanant.html

 

[Colt45]

Got any old thinnet ethernet cables around? They were RG-58 / 50ohm, with BNC ends.

Can't think of anything else that normal folk would have around... everything for TV is 75 ohm.

 

[bobdole369]

Ya you are sorta outta luck short of finding a local store or paying the shack through the nose.

If you check eham.net or qrz.com you can find a very well stocked and nice community where I guarantee someone is selling some 50ohm.

 

[bobdole369]

Can I ask what kind of antenna's going up? (discone, 5/8wave at 150mhz), etc?

 

[Texun]

Originally posted by: bobdole369
Can I ask what kind of antenna's going up? (discone, 5/8wave at 150mhz), etc?

Sure, just don't throw any rocks - it's from Radio Shack.

It's a leftover that I never used but it's not a discone. For my needs I am looking for one that does VHF well, specifically in the 150-160MHz range, and although this one is supposed to be cover 108-1300Mhz, RS claims the antenna does best between 152-470Mhz.

Here is a link to the antenna: Model: 20-176

And according to the guys at strongsignals.net the 20-176 ranks #1 in the range I am mostly interested in. Unscientific reviews rate it very high, and as mentioned at www.strongsignals.net, will also state that it's weakness is above 800Mhz.

Not everything RS sells is necessarily bad. Many of their scanners are made by Uniden and are consistently well received by the scanning community Their scanners may be good but I wouldn't hang their cable on my home if it were free.

I dropped the idea of using my quality quad RG6 as an option. I called a trucker friend who directed me to a friend of his who had some new Beldon 9913 RG8. I've got it cut and waiting for me when I return to the installation location next month. Without going a more extreme grade of cable I believe this should provide good to very good performance with minimal attenuation. The repeaters I need to monitor are all within 40-50 miles max. The terrain is mostly flat and I'll have it 40 Ft above ground and above most obstructions so it *should* work fine.

Thanks to everyone for helping me regain some clarity. I've been out of this for a while and I only want to do this once.

You asked if it was a discone so I don't know if you already have one, are planning to buy one, or if you are just curious, but there is a huge split among users with discone antennas. It's wide bandwidth makes it appealing but that same bandwidth also allows for a lot of noise on the receive side. I never used one myself so I cannot speak objectively on the performance.

 

[bobdole369]

Yup this is true. Back in the day (and even today) the scanner line is not too shabby. The ham rigs they sold were excellent beginner stuff and they were the only retail chain I know of that actually sold an SSB CB rig (truck stops and the like excepted).

I used a discone a few years back, had it mounted up on a 25 foot TV mast. It worked well enough, its wideband-ness was why I used it, but I put a 2 meter quarterwave on the same mast and got stronger signals in the VHF "interesting" band (150-174mhz). But the quarterwave was MUCH worse on UHF (450-470ish), while the discone kept trucking up there.

Just curious - not too many folks into radio as much as say 20 years ago when I started.

 

[Texun]

Originally posted by: bobdole369
Yup this is true. Back in the day (and even today) the scanner line is not too shabby. The ham rigs they sold were excellent beginner stuff and they were the only retail chain I know of that actually sold an SSB CB rig (truck stops and the like excepted).

I used a discone a few years back, had it mounted up on a 25 foot TV mast. It worked well enough, its wideband-ness was why I used it, but I put a 2 meter quarterwave on the same mast and got stronger signals in the VHF "interesting" band (150-174mhz). But the quarterwave was MUCH worse on UHF (450-470ish), while the discone kept trucking up there.

Just curious - not too many folks into radio as much as say 20 years ago when I started.

From what I've read on the discone the reviews are pretty widely split. Some hate it or love it. Most of the negatives come from users who feel that because of its broad coverage, something like 23-1300MHz, the noise floor is too high. However, I'm not 100% sure I buy that.

I'm going to try a quarter wave at 25Ft. that I already have since what I want is in the VHF-HI range. If I find that I want better reception in the 800+ range I'll replace it, but with the changes in trunking and all the rebanding going on I'm not going to do much up there right now.

I've decided to pass on my original thought of using RG6 and have acquired some quality Beldon RG8. As much as I enjoy raising an antenna I really only want to do this once - for now.

I agree. Not many people are interested in radio much now.

 

[Rubycon]

9913 is overkill for a receiving setup but if the price is right...

High VSWR - say 3.0 (!) is 25% reflected power. Receiving signals makes no difference but say you're running 2 meters at 100W and you have a mismatch on your antenna. 25W of power is going BACK to the transmitter and the finals aren't going to be happy with that - at all.

 

[Texun]

Originally posted by: Rubycon
9913 is overkill for a receiving setup but if the price is right...

High VSWR - say 3.0 (!) is 25% reflected power. Receiving signals makes no difference but say you're running 2 meters at 100W and you have a mismatch on your antenna. 25W of power is going BACK to the transmitter and the finals aren't going to be happy with that - at all.

The price was right - free! I have a friend who had some left over after a project. All I need is 50Ft at the most.

The antenna will transmit on 2 meters and 440, but this will be a receive-only setup so at least I won't have to deal with SWR. With good coax I can swap antennas later on if I want to. Wish I could try it out now but this is for my home out of state so it will be another month before I get to actually set it up.