Cb coax extension

Submitted: Friday, Apr 17, 2015 at 19:58
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Due to more equipment being mounted on the roof, I am looking at re-running the cb coax via the rear of the vehicle which will mean I need to start over with a longer piece of coax. The supplied length from memory was 5m, by looking and estimating would say I need to replace it with an 8 or 9m length. I have had a google and not found a very direct answer so the question is, is an 8 or 9m run an issue? Doing the terminations is not an issue and obviously the cable will be swapped out like for like (75 or 50 ohm).
I am not overly concerned if there is a slight drop in performance but would like to know if it will happen.
Thanks
Tim
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Reply By: Notso - Friday, Apr 17, 2015 at 20:03

Friday, Apr 17, 2015 at 20:03
We had a base station set up in a residence on a cattle station. I reckon the cable would have been 40 or 50 feet from antenna to radio.
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Follow Up By: Notso - Friday, Apr 17, 2015 at 20:03

Friday, Apr 17, 2015 at 20:03
Sorry, it worked fine but can't comment on how much the signal was attenuated.
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Follow Up By: Member - Stephen L (Clare SA) - Friday, Apr 17, 2015 at 20:25

Friday, Apr 17, 2015 at 20:25
Hi Notso,

Yes you can run the coax over that distance for a base station mounted UHF Radio, but not the standard coax that you get when you buy a standard vehicle mounted antenna. You would have used the coax that is around three times the diameter and designed to be used over long runs.



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Reply By: Member - P and JM - Friday, Apr 17, 2015 at 21:09

Friday, Apr 17, 2015 at 21:09
Hi Tim,

The coax length you are quoting will be ok for HF radios (27MHZ) and must be RG-58 coax @ 50 ohms.
If it's for UHF radios (476-477MHZ) you will need RG-213 @ 50 ohms.

RG-58 is approx. 5mm in diameter and RG-213 is around 12mm in diameter.
Your local electronics store will be able to assist you. Hope this will help you.

Regards P&J
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Follow Up By: Tim - Friday, Apr 17, 2015 at 21:28

Friday, Apr 17, 2015 at 21:28
I would prefer to keep the cable around the 5mm size, 12mm holes are a bit much and obviously the bending radius on the smaller cable would be much smaller. What length (ball park) do you think I could go out to keeping the cable diameter the same as it is? It is for a UHF. I might be able to move the head unit towards the rear slightly to gain some extra length.
Tim
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Follow Up By: Frank P (NSW) - Friday, Apr 17, 2015 at 21:36

Friday, Apr 17, 2015 at 21:36
I just installed a Uniden 5W UHF in the BT50. Antenna cable run was 5m. I had it tested. SWR was ideal at 1:1. Measured output was 3.5 watt. I was told the loss of output power is as much due to the quality of the radio as it is cable run.

Me, I dunno, just quoting what I was told by one who knows a lot more than I do. (Runs a comms shop and is a ham radio freak, so I figure he knows a bit of what he is talking about.)

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Follow Up By: Member - P and JM - Friday, Apr 17, 2015 at 21:57

Friday, Apr 17, 2015 at 21:57
Tim,

It's up to you, but 9 meters is approx. 30 feet and that is a long run of RG-58 for UHF. The closer you can get the radio to the antenna the better so as to keep the coax cable as short as possible.

With a 9 metre run you could loose around (+ or -) 1 watt of output power from the rig. Make sure you check/retune the antenna for the lowest VSWR you can get.

Cheers P&J
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Follow Up By: Nomadic Navara - Friday, Apr 17, 2015 at 23:41

Friday, Apr 17, 2015 at 23:41
Quote "I just installed a Uniden 5W UHF in the BT50. Antenna cable run was 5m. I had it tested. SWR was ideal at 1:1. Measured output was 3.5 watt. I was told the loss of output power is as much due to the quality of the radio as it is cable run."

What was the meter that was used to measure the power output and where was the measurement made? Something like a Uniden should put out its full 5 W rated power. The power meter will be as much of the problem as the CB radio. You will also suffer some power loss from the cables, the connectors are likely to add a dB of loss.
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Follow Up By: Frank P (NSW) - Saturday, Apr 18, 2015 at 18:02

Saturday, Apr 18, 2015 at 18:02
Peter,

Thanks, I was hoping to get some input into this. But I can't help much - I don't know the brand of the meter or anything like that, or its calibration, or if indeed the test was done correctly. I have no knowledge in this area apart from some googling last night and must accept the advice I am given.

The measurement was made at the back of the radio with a patch lead only a few inches long.

I, too, thought that Uniden was a good brand and expected it to deliver nearly all of its advertised 5 watts. 3.5 was a bit of a disappointment.

However, I read somewhere (maybe here!) that if the SWR figure is good (which mine was), 3.5 watts vs 5 watts won't make much difference to range. To my simple mind that does not equate, but if that's what the experts say, I will accept it.

I might try to get a second opinion just to see if different test equipment makes a difference.

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Reply By: Allan B (Member, SunCoast) - Friday, Apr 17, 2015 at 21:36

Friday, Apr 17, 2015 at 21:36
Hi Tim,

You should use RG58 cable (50 ohm) for UHF radios. This will have a typical loss of 0.46 dB/m.
Clearly, the longer the cable, the more signal attenuation however, within reason the cable length is not critical.
For best results, purchase a good quality cable and carefully & correctly fit the connectors.
Your antenna is almost certainly pre-tuned so re-tuning of the antenna/cable combination will not be necessary.
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Reply By: Nomadic Navara - Saturday, Apr 18, 2015 at 00:02

Saturday, Apr 18, 2015 at 00:02
Tim, I would not use 70 O cable. Use quality cable like Belden brand and not the cheap Taiwan cables that are frequently available at "good" prices. If you can get a double shielded cable like Belden Duo-bond you will have about half the loss.

Carefully measure out your cable length and use this calculator to calculate the total loss of your intended cable run. If you are getting your antenna up in a favourable position I would not be worried about 3 - 5 dB loss as the better antenna location will make up for the cable loss.

Another chart for quick look up is this one.


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Reply By: GarryR - Saturday, Apr 18, 2015 at 08:05

Saturday, Apr 18, 2015 at 08:05
the rg-58 will be some loss, i never run rg-58 more than 3-4mts, then change to rg-213u for low loss up to 30mts then heliax over 30mts. Your local comms shop will be able to supply and install fittings for once you have run your cable to where ever you wish. I installed my radio (Icom 440n) under the drivers seat, ran the coax and power cables, then had the tech fit off the ends and test. The rg-213u is a lot stiffer to work with, and sharp bends should be avoided
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Reply By: Member - Boobook - Saturday, Apr 18, 2015 at 17:16

Saturday, Apr 18, 2015 at 17:16
8 - 9 m will result in a loss of about 4 - 5db with Rg 58 at 477mhz. That is quite a bit and will limit your reception.

RG 213 is a great for less loss but is way too bulky for cars and unnecessary for that cable run length.

Good practice would be to use LL195 low loss cable. This looks and feels exactly like RG58 and is 50ohm. The loss would be about 2db. It is a high quality, low loss version of Rg58.

You should be able to get it at most Ham radio supplies. Strictly Ham sell it for $2.00 a meter.

LL195 cable by the meter.

All RG58 connectors will fit LL195.
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Follow Up By: Member - P and JM - Saturday, Apr 18, 2015 at 18:23

Saturday, Apr 18, 2015 at 18:23
Hi Boobook,

Thanks for the info on the (possibly new) LL195 coax. To me the double shielding is the way to go for HF and short runs on UHF.

I have enough coax to see me out, but it's very handy to know about the LL195, at a very reasonable price as well.

Cheers P&J
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