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sbilson
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PostSubject: Coax cable length   Coax cable length Icon_minitimeMon Oct 05, 2020 4:16 pm

Does the length of the coaxial cable from the cb unit to the antenna base have any effect on swr's?

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Alan Pilot
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PostSubject: Re: Coax cable length   Coax cable length Icon_minitimeMon Oct 05, 2020 6:02 pm

It shouldn't make any difference but others will say it does.
The length will cause more loss so longer coax needs to be the best you can afford.

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PostSubject: Re: Coax cable length   Coax cable length Icon_minitimeTue Oct 06, 2020 4:25 pm

There are three things that will cause a change in SWR with coax length.

1) As you add coax length SWR will appear to drop slowly.  This is normal, and is due to the losses in coax.  To combat this, the longer the run the higher quality coax you need to use.

2) If adding a small amount of coax, say 3 ft at upper HF frequencies, causes a noticeabe change in SWR, then you likely have a CMC (Common Mode Currents) issue.  Essentially, what is happening is currents are flowing on the outside of the coax, and these currents travel at a different speed as currents on the inside of the coax so when they recombine at an SWR meter you get drastically varying results.  Currents flowing on the outside of a coax almost always have negative results on the antennas radiation pattern, so they should be minimized as much as possible.

3) It is also possible, although not as likely, that a small change in coax length changing SWR noticeably can be caused by a ground loop.  This is less common in general, and mostly shows up in a base installation, although it can happen in cars to.

With numbers 2 and 3 above, there are people that try to tune their setup by changing the length of the coax.  These installations tend to have strange issues that are difficult to trace, from suddenly being "bitten" when transmitting to radio and amplifier transistors poping much more often than they should.

The DB

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PostSubject: Re: Coax cable length   Coax cable length Icon_minitimeTue Oct 13, 2020 3:00 pm

Illustrative: I've put up a 1/2 wave GPA with 20m RG58, which got me a SWR of 1.06. I replaced the RG58 with RG213 and now have an SWR of 1.12.

Now continuing the experiments. The next thing ill try is using a choke balun (RG58 wound on a ferrite core)
BTW, measured with a nanoVNA i got last week.

73

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PostSubject: Re: Coax cable length   Coax cable length Icon_minitimeTue Oct 13, 2020 5:08 pm

Hi Karel,

Just goes to show you that you can't rely on SWR alone as a measuring tool.

The losses for RG58 are higher than the losses on RG213 and that includes on the way back as well as forward.
So in your experiment you will have more reflected power coming back from the RG213 than on the RG58 and hence the SWR discrepancy. From the measurements you gained on your nanoVNA that sounds about right for a 20m run.
When you replaced the coax for RG213 you should have in all reality retuned your antenna, but with such a small difference it's hardly worth it.

It's just one example of how poor or higher loss coax can appear to give you a good SWR.

Another common example is an antenna that been out for several years in all weathers and corrosion has built up on the antenna coax connection, (not unusual for PL/SO239 type connectors). You lose power through that before it gets to the antenna and then lost again on the return for the SWR measurement. You could be hardly transmitting any power from the antenna and yet your SWR all looks good!!

Some measure the SWR at the antenna end in the hope of alleviating coax losses but again it's fraught with possible errors.

It's all fun isn't it?! Very Happy

Keep up with the experiments it's a great way to learn, but stand back sometimes to contemplate your measurements.

From what I have heard the nanoVNA's are a great bit of kit, I may well have to get one myself Wink

I, like others don't worry too much so long as my SWR is less than 2:1 which equates to 10% or less losses but rapidly rises thereafter(!)


All the best,
Victor

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PostSubject: Re: Coax cable length   Coax cable length Icon_minitimeTue Oct 13, 2020 5:38 pm

In fact i did retune my antenna, but couldn't get below 1.12. As i remember the starting point with the RG213 was somewhere in the 1.5.

73

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Victor
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PostSubject: Re: Coax cable length   Coax cable length Icon_minitimeTue Oct 13, 2020 5:54 pm

Still sounds about right Karel.

There does seem to be a 'thing' about obtaining the fabled 1:1 SWR in the radio world. If I've built or tuned an antenna to less than 1.5:1 I'm totally happy!

73

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PostSubject: Re: Coax cable length   Coax cable length Icon_minitimeWed Oct 14, 2020 12:50 pm

Yes, i am happy with any reading under 2, car set up is 1.5 and base station is 1.3 to 1.6 over the 3 antenna i can use.

Had a car set up many years ago with a SWR of 2 for many years, and the radio got me very good reports and when he skip was around would do very good distance , and this was a Cobra 29, night vision model i think it ran for 5 years in the car, sold it on now but i never had a call back from the person who bought it.
So i say relax at under SWR of 2 and enjoy the radio.


Glenn
56CT004       rendeer

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PostSubject: Re: Coax cable length   Coax cable length Icon_minitimeWed Oct 14, 2020 4:38 pm

Interesting discussion.

Results from different COAX....Quality/length etc can be quite surprising. I think if your serious about your hobby,be it Amateur or CB, all we are looking for is a good/efficient feed line/Ant.

That being said you TRY to limit line loss etc and quality/length play a big part in getting a good feed to your Antenna.

I have been looking at a LINE LOSS CALCULATOR and ERP calculator with some very interesting results.

I am putting some images on here to illustrate what I found so hope you can see them.

The images are screenshots of the calculators and the PARAMETERS entered. The images are based on my personal set up and parameters. My coax is RG-8X 25 Meters with an SWR of 1.1.

https://pixeldrain.com/u/hqTQ9uty
https://pixeldrain.cohttps://pixeldrain.com/u/xEB8e6EXhttps://pixeldrain.com/u/CbzHJqX7m/u/6SEffs8W
https://phttps://pixeldrain.com/u/ojHLQ1y6https://pixeldrain.com/u/988ykebSixeldrain.com/u/u4CT1pq8
You cannot SEE  the links as you enter them so hope they are not jumbled up.

Calculations for RG-8/RG-8X/RG-213.

Steph.
https://pixeldrain.com/u/7LPCXY9uhttps://pixeldrain.com/u/u4CT1pq8

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PostSubject: Re: Coax cable length   Coax cable length Icon_minitimeWed Oct 14, 2020 4:59 pm

Thanks fort the pix Steph, very self explanatory. Except for the use of feet Smile . A few weeks ago i looked at a youtube video of DX commander about the difference between vertical and horizontal antennas. The difference wasn't that big which he illustrated with some MMANA designs. Then he mentioned the difference between "bad" and good coax which was a lot bigger.

19-CT-024
73

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PostSubject: Re: Coax cable length   Coax cable length Icon_minitimeWed Oct 14, 2020 5:31 pm

Except for the use of feet?........  Yeah not sure what you meant by that?  The Line Loss calculator was THE LOSS PER 100FT, then on ERP, you enter YOUR ACTUAL  LENGTH, in MY case this was  25 meters= 81.25 FEET.
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PostSubject: Re: Coax cable length   Coax cable length Icon_minitimeWed Oct 14, 2020 6:21 pm

Just a bit teasing, i'm metric, born and raised  lol!

About the same as : "I don't know what a calander is." (second button on the top of the page) Surprised

73

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PostSubject: Re: Coax cable length   Coax cable length Icon_minitimeWed Oct 14, 2020 6:45 pm

Your not much younger than me mate ha ha. Metric......can't be doing with all that business that's down to changing over to metric with the EEC (as it was then ). I can't think in metric for ME it's imperial measurements. I even sent away to the States for some imperial measurement slide rules etc.

I have been an amateur weather forecaster for 50 years and I know ALL the Celsius to Fahrenheit and visa versa by memory.I have my weighing scales on lbs and ounces etc etc.

Don't want to get political but I cant stand ANYTHING to do with the EU.

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PostSubject: Re: Coax cable length   Coax cable length Icon_minitimeWed Oct 14, 2020 7:06 pm

With me it's just thinking metric and convert to inches and foot if needed. Luckily for me wavelength is in meters. Very Happy
Don't get me started about the EU, we'll need another thread for that. for The Netherlands cost and gain is not very balanced.


73

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PostSubject: Re: Coax cable length   Coax cable length Icon_minitimeWed Oct 14, 2020 9:49 pm

Metric means it's lkely more expensive and you can only buy 5 or 10 and not 6 or 12

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PostSubject: Re: Coax cable length   Coax cable length Icon_minitimeThu Oct 15, 2020 2:30 pm

Hi all,

as a follow on from my earlier post WITH LINKS to LINE LOSS CALCULATOR/ERP.

In my own situation my set-up is PRESIDENT McKINLEY (EXPANDED ) SIRIO GAINMASTER elevated to 40ft (BASE of the antenna ). FM UK40 Power out put 25 watts. Back of the box.

I have an 81.25 ft ( 25 meter ) coax feed using RG-8X mini.

Using the Line Loss Calculator:

PARAMETERS: Line loss per 100 feet (standard ) using RG-8X SWR 1.1 would be 1.573 dB.         Power loss would be 7.6 watts  for a 25 watt input.

ERP for MY actual coax length= 81.25 ft would be 1.3 dB  and with a 25 watt input this gives me an ERP of 18.6 watts which is a POWER LOSS of 26%. (This is assuming negligible gain of my antenna )


Using the SAME Parameters for RG-213

Line loss per 100 ft is 1.059 dB. Power loss would be 5.5 watts = 22%

ERP for 81.25 ft coax would be 20.5 watts for 25 watts in,= 18% power loss.


Again using the same Parameters for RG-8 is quite similar results.

If you were to use  RG58  same Parameters  LINE LOSS per 100 ft is 2.00dB and for 25 watts IN you would get just 15.754 watts out  which equates to 9.3 watts loss = 37% power loss.

ERP would be 1.6dB loss, 17.2 watts for 25 watts in, this equates to a 31% power loss.

So it's pretty obvious that whilst not totally useless the quality of your coax CAN result in quite significant power losses.

Steph.
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PostSubject: Re: Coax cable length   Coax cable length Icon_minitimeThu Oct 15, 2020 2:53 pm

redsprite wrote:
Hi all,

as a follow on from my earlier post WITH LINKS to LINE LOSS CALCULATOR/ERP.

In my own situation my set-up is PRESIDENT McKINLEY (EXPANDED ) SIRIO GAINMASTER elevated to 40ft (BASE of the antenna ). FM UK40 Power out put 25 watts. Back of the box.

I have an 81.25 ft ( 25 meter ) coax feed using RG-8X mini.

Using the Line Loss Calculator:

PARAMETERS: Line loss per 100 feet (standard ) using RG-8X SWR 1.1 would be 1.573 dB.         Power loss would be 7.6 watts  for a 25 watt input.

ERP for MY actual coax length= 81.25 ft would be 1.3 dB  and with a 25 watt input this gives me an ERP of 18.6 watts which is a POWER LOSS of 26%. (This is assuming negligible gain of my antenna )


Using the SAME Parameters for RG-213

Line loss per 100 ft is 1.059 dB. Power loss would be 5.5 watts = 22%

ERP for 81.25 ft coax would be 20.5 watts for 25 watts in,= 18% power loss.


Again using the same Parameters for RG-8 is quite similar results.

If you were to use  RG58  same Parameters  LINE LOSS per 100 ft is 2.00dB and for 25 watts IN you would get just 15.754 watts out  which equates to 9.3 watts loss = 37% power loss.

ERP would be 1.6dB loss, 17.2 watts for 25 watts in, this equates to a 31% power loss.

So it's pretty obvious that whilst not totally useless the quality of your coax CAN result in quite significant power losses.

Steph.

Interesting stuff. Do we simply half the fgures if we are using just a 45 foot length of cable?
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PostSubject: Re: Coax cable length   Coax cable length Icon_minitimeThu Oct 15, 2020 3:18 pm

Interesting stuff. Do we simply half the fgures if we are using just a 45 foot length of cable?




Click the link to the calculator and  do the LINE  loss per 100 ft. Click calulate

Then where is say's "Actual Line Length" put in YOUR 45 foot length and click CALCULATE.



Interesting stuff. Do we simply half the fgures ihttps://kv5r.com/ham-radio/coax-loss-calculato
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