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Call Sign : 2-CT-172 noahtoo Posts : 14 Times Thanked : 1 Join date : 2020-10-01 QTH or Location : Meeker, Colorado, USA Equipment Used : Royce 23 channel CB radio Age : 77
Subject: Coax cable length Mon 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 Major contributor
Call Sign : 163-CT-220... Posts : 2608 Times Thanked : 70 Join date : 2019-11-19 QTH or Location : Anglesey North Wales Equipment Used : Yaesu FT-991A,,Yaesu FTDX-10,,Icom ic-7610,,Anytone AT-D878UV PLUS",,LINCOLN II+. Age : 16
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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karelgol Senior contributor
Call Sign : 19-CT-024 / PD0GOL Posts : 169 Times Thanked : 6 Join date : 2020-05-21 QTH or Location : Steenwijk, JO32bs Equipment Used : Yaesu FT-897D, Quansheng UV-K5 porto, CB silver rod as mutiband HF antenna, OSJ-pole for 2m/70cm, MFJ-925 tuner Age : 66
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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Victor CT Directors
Call Sign : 26-CT-3228 / M7VIC Posts : 6138 Times Thanked : 375 Join date : 2019-11-10 QTH or Location : Bedford Equipment Used : Various
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?!
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
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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karelgol Senior contributor
Call Sign : 19-CT-024 / PD0GOL Posts : 169 Times Thanked : 6 Join date : 2020-05-21 QTH or Location : Steenwijk, JO32bs Equipment Used : Yaesu FT-897D, Quansheng UV-K5 porto, CB silver rod as mutiband HF antenna, OSJ-pole for 2m/70cm, MFJ-925 tuner Age : 66
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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glenn dog Major contributor
Call Sign : 56-CT-004 Posts : 474 Times Thanked : 11 Join date : 2019-08-23 QTH or Location : Oulu -Finland Equipment Used : ft-450d-mc kinley - cobra 19dxeu- lincoln+2 - superstar lord - grant 2 - tti hand held- alan42- crt mike cb - zodiac 68 hunting radio hand held- and few PMR 2
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
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redsprite New Member
Call Sign : 26-CT-3697 Posts : 43 Times Thanked : 1 Join date : 2020-06-27 QTH or Location : Hemel Hempstead Hertfordshire Equipment Used : PRESIDENT MCKINLEY.RadioMoble / SIRIO GAINMASTER 5/8.Sigma Venom 1/2 wave. Age : 71
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.
Thanks fort the pix Steph, very self explanatory. Except for the use of feet . 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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redsprite New Member
Call Sign : 26-CT-3697 Posts : 43 Times Thanked : 1 Join date : 2020-06-27 QTH or Location : Hemel Hempstead Hertfordshire Equipment Used : PRESIDENT MCKINLEY.RadioMoble / SIRIO GAINMASTER 5/8.Sigma Venom 1/2 wave. Age : 71
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.
karelgol Senior contributor
Call Sign : 19-CT-024 / PD0GOL Posts : 169 Times Thanked : 6 Join date : 2020-05-21 QTH or Location : Steenwijk, JO32bs Equipment Used : Yaesu FT-897D, Quansheng UV-K5 porto, CB silver rod as mutiband HF antenna, OSJ-pole for 2m/70cm, MFJ-925 tuner Age : 66
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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karelgol Senior contributor
Call Sign : 19-CT-024 / PD0GOL Posts : 169 Times Thanked : 6 Join date : 2020-05-21 QTH or Location : Steenwijk, JO32bs Equipment Used : Yaesu FT-897D, Quansheng UV-K5 porto, CB silver rod as mutiband HF antenna, OSJ-pole for 2m/70cm, MFJ-925 tuner Age : 66
With me it's just thinking metric and convert to inches and foot if needed. Luckily for me wavelength is in meters. 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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Jape New Member
Call Sign : 26-CT-1894 Posts : 22 Times Thanked : 0 Join date : 2020-04-13 QTH or Location : Eastbourne Equipment Used : AT66 Gainmaster
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.
Jape New Member
Call Sign : 26-CT-1894 Posts : 22 Times Thanked : 0 Join date : 2020-04-13 QTH or Location : Eastbourne Equipment Used : AT66 Gainmaster
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?
redsprite New Member
Call Sign : 26-CT-3697 Posts : 43 Times Thanked : 1 Join date : 2020-06-27 QTH or Location : Hemel Hempstead Hertfordshire Equipment Used : PRESIDENT MCKINLEY.RadioMoble / SIRIO GAINMASTER 5/8.Sigma Venom 1/2 wave. Age : 71