I'd forgotten long ago about unusual circumstances such as "fold over". The review of math and info here is very welcome Cal, again Thanks for what you do.
It is so refreshing to listen to the joy in your voice as you explain things useful to other Hams !!! The chase to 100k subscribers WAS taking an unbelievable toll on your personality. I have enjoyed every “NEW” video that has been released. THANK YOU !!!
Great and useful video, thanks very much. I'm really glad you have got off the "chasing subscriptions express". I guess RUclips pushes video makers to do it but for me, it's counterproductive. Particularly when channel owners beg for subscriptions at the beginning of a video...before the viewer has seen any content! 73.
Oh cal,wish I'd blooming remembered you done this before I tuned my cobweb antenna 🙈🙈would've been so much quicker🤣🤣now saved it just incase 👍👍thankyou
Didn't notice you're audio was "broken" until the message came up. I'm sure there's a lot of people that would like to have your broken audio. 😂 Thanks for the "bent over at the end" answer. I couldn't seem to get a straight answer to that question. I don't think it matters that much on an EFHW Horizontal dipole though. 😆 Thanks for all you do Callum. Cheers
Thanks Callum. I use that spread sheet all the time and have been since I got my Classic a couple of years ago. However, I have used it on EFHW, Dipoles, Fan Dipoles. The lot. Thanks for making that available.
Thanks for the tip on the fold over. I have a fan dipole using fold over technique. I am fortunate to have a VNA so have been manually tuning it that way its a bit slow going though. Now i know its about 3x i can make better guesses. Cheers Cal.
Yes, it's a good guess BUT not exact because the further you move away from the point (where the foldover happens), the greater the loading you will experience as you make the foldback longer and longer so that eventually, when the foldover is complete (fully linear loaded) small changes nearer the feedpoint (to your foldover) makes more of a change.. I need to make another video about this!
@@DXCommanderHQ yes please, that also explains the what i am seeing. Be interested to know or measure the efficiency of the antenna as well how much gain i am loosing vs the benefit of a physically shorter antenna. VNAs cant tell you that bit sadly.
@@DXCommanderHQ So if the tuning sheet says to cut 7.28 cm off you really do not want to have a 21.8 cm fold over. How do you figure out what to do in that case?
Awsome! I downloaded the Calculator which is really helpful to have it if you build your own wire antennas. You are a real Genius Lord Callum, thx for your great job. 73 de YFUG 💯🙋♂
So, there's 2 parts of "raising" or "lowering" the swr at any particular band. One is to increase or decrease the frequency of the swr. With this, shortening or lengthening the element (increases or decreases the frequency of the swr along the x axis.) This (along with the calculator) are very helpful resources. I've been using it a lot at tuning my signature 12.4. I simply identify the frequency of the lowest swr part of my given band, identify where I want that lowest swr to be (ie middle if the given band). I plug both into the calculator and adjust the element length accordingly. This moves that lowest swr frequency over to the desired frequency. With the 10m band where there's sooo much foldback, like Callum said, there's limited effect in increasing or decreasing the foldback. In experimenting, I did a foldback of the foldback (folded the end of the foldback back up the element line). Interestingly, this had a much quicker effect of moving the lowest swr point over along the x axis over than just lengthening the foldback. (This came from not really trusting cutting the foldback yet but knowing it needed to be shorter.) I've gotten all of my bands essentially centered in the given bands on my antenna analyzer using these methods. But this doesn't really make any real difference if the swr is too high on the y axis. Which, as far as I can tell, but I might be missing something, these methods don't address. So.... how does one reduce the swr NOT along the x frequency axis, but along the swr intensity on the y axis?
I see you commented on another video and I left my reply there.. If you are stuck, either chat on the Discord Server or drop me an email. Try and make it bullet points because I have a problem reading a lot of text.
I am sure i should be doing some work this weekend, but finding watching your videos more beneficial at the moment. I just wondered if you sell tee shirts with the DXCommander logo on and if so do you do polo neck?
By this calculation you can adjust resonant frequency, but not SWR as it is sum of resistance, what in fact you do not adjust. Correct me if I am wrong. Anyway thanks for nice explanation using Calculator. Best regards Cal.
In the MAIN, your resonant frequency for q/waves and dipoles disregards impedance and resistance (for our science experiments) because we normally know they will always (mostly) be between about 40 and 70 ohms. So best SWR will be best resonant frequency. But I think you knew that already! :)
I agree, this seems to be a pattern with teaching hams, and it can be very misleading. They tend to skip to the end and leave the juicy bits out, or in this case, assume they know it's for a 50ohm designed system. It's a very poor teaching method, as you need to explain the fundamentals for true understanding which will result in better, calculated practice.
Sir question I bought a CZE 15-A transmitter FM radio station on 102.7 FM for our community HOA non profit of course and only announcements for community and community involved no profit. My problem is I should be getting better coverage. Plus a lot of static even not far away. I should get art least a mile to 5 miles on 15 watts. The 1/4 wave Ground plane antenna came with it the company says it's all set. I got 50' coax that I got with it and the connectors already on for this TX snd antenna which I was told are TNC and SL16-J coax connectors. The top of antenna the antennas are pre made and labeled. The one I used is 102. Of course. They assured me it was the best coax and antenna for what I am wanting to do with it. I made sure everything was connected correctly and tight. I had to hire a guy to put up the mast 25' and secure to house and it is above the roof clearing everything to broadcast the whole community easy. But dead spots, statically, sounds so bad. No interference, no loose connections. I am a retired DJ not an engineer. It sounds awful. So tone said it is the SWR setting it should be 1. But the vertical is pre set it does have a screw on it. The horizontal legs crap I did not drink anything but tighten them up. I do not know what I am doing. Can you please suggest what I need to do. Who would I contact to fix this problem? All my engineers are retired or dead, it was a long time ago. In different states. Are you in USA? I am. So don't know if that makes a difference. Please email me if you will Backyardufologist@gmail.com If you will. Thank you!
Check out radiomobile. That will give you mapped precise coverage. Make sure antenna is totally in the clear and you have line of sight to your 5-mile requirement.
Thanks, How do you Calculate this on a horizontal loop antenna, lets say a 80M loop? Grabbed the calculator and added a column for INCHES... hi Hi... W1SWL
Appreciate your videos. I'm just trying to get into 10 meter as a technician talking on 28.3 to 28.5 MHz. I got myself a President Lincoln II + and an MFJ whip (161OT). Was just doing the calculations and I came up with about 104 inches. But the MFJ 161OT is only 98 inches. Also it looks like my SWR is about 7. So I'm stuck not TXing for now. :(
OK, a number of ways out of that. Check what ground plane you have - increase it.. SOunds like that system you have is rsonant at around 33/34Mhz.. You can ADD a little bit of copper wire to the top.. Just wrap it around and stick it out the top?
@@DXCommanderHQ Thank you. I'll have to figure out how to make a ground plane. I currently have it about total 15.66 feet in the air on a metal post. Going to try that copper wire too.
Hello mate. Can you please assist on the SWR matter : where does the reflected power go and if you agree that after the ping pong between the antenna and the transceiver it goes back to the antenna and gets transmitted. I would really appreciate any clarification on this subject. 73 de SV4RIN
Hi I have mismatch readings from the internal meter and the external meter. I don’t know why and I would like to ask if you know how I can get to read the true swr
Measure at exactly same place (say in the shack at the radio) and if mismatch is only slight (say external meter is let's say 1.5:1 and internal rig SWR shows 1.2:1, then ignore. I would suggest if you measure at the same place and your radio is happy, just go with it because your external meter MIGHT be measuring genuine "complex" SWR that frankly your radio couldn't care about.
I’m a simple trucker so can you give me an example of how to use this calculator in this scenario, AM settings: channel 1 (26.965 MHz) = 2.5 SWR, Channel 20 (27.205 MHz) = 1.3 SWR, and Channel 40 (27.405 MHz) = 1.2 SWR. how do I calculate the length to cut or length to increase the whip on a Predator 10 K antenna with a 24 inch shaft in this scenario?
Well, I don't know what a Predator 10K is, I'm sorry BUT you are saying 27.405 is 1.2:1 and you would like it resonant lower down? In which case shorten it say exactly half an inch and check again.
Long ago I tried using a calculation method to cut antennas and it never worked. I had to do it inch by inch experimentally using the at the time standard SWR meter. Today, hams are using a VNA to measure and cut antennas but a lot of hams, especially new hams, don't understand what the VNA numbers mean. So they don't know what to do. Ignorance is not bliss at all!
Hi Callum. I am using an older version of the DX Commander. I have found that the resonant frequency at 10m is 28.554MHz. This gives me a high SWR of 4.48 at 28000kHz and 3.31 at 29700kHz (not much rise to the right). Should I want to move the resonant frequency to a lower part of the band, I believe more wire will need to be added. However, the current length is close to the plastic disk. One solution is to add more wire and fold it back onto it self. Is there a calculation to be used for determining how much fold-back is needed after adding the extra wire? Thank you in advance for your assistance. (Please excuse the length of this message as I know you prefer shorter ones). … Rob WD4IFT
So are you measuring the antenna with an antenna analyser to see what frequency it resonates on to then determine if it's too long or short for the frequency you desire? I had hoped this video was about how to adjust using an swr meter as the instructions for the CB base antenna I have seems to be the opposite of what google tells me. And what my old CB literature tells me.
You don't need a fancy pants meter, you can just see (for instance) that Channel 1 is (say) perfect and channel 40 is (say high), then you know that best frequency is ABOUT channel 1 (as previous), so you would need to lift the freq up about 20 channels.
Antennas confuse the heck out of me. When I put up a wire antenna, I can usually get a good/usable SWR at a specific frequency but it's usually never close enough to 50 ohms. I struggled like crazy with my cobweb. Any pointers?
Am I correct that you would need to double the amount for an EFHW? This is a great calculator BTW, thanks! I did add a cm to in conversion cell though 😉
Thank you for this information. At an SWR of 1.5:1 there is a loss of 4% in radiated power, and this increases somewhat exponentially as SWR increases ( e.g. with SWR of 2.0:1 there is an effective loss of ~ 11% in the radiated power from the antenna). However, if you are unable to physically adjust your antenna, and have to rely on an ATU to bring the SWR down to say, 1.1:1 from 2.0:1 (or higher), does the ATU 'recover' the lost power and allow the antenna to radiate it, or does the ATU simply absorb the reflected power in it's LC circuits and protect the finals of the Transmitter? In my case when I added 8 x 1/8 lambda radials to my base loaded vertical antenna, my SWR jumped from ~1.5:1 to over 6:1 !! (Don't ask why, I haven't been able to figure that out . So I used an external 'heavy duty' ATU to bring the SWR that my TX saw back down to ~1.5:1. But according to the charts, an SWR of 6:1 is a 50% loss in radiated power. Am I correct in assuming that my ATU is simply 'absorbing' the reflected power and it is NOT being radiated by the antenna ? Thank you.
Power is not automatically lost due to an swr reading ,this is the common misunderstanding in radio ,reflected power adds to the forward power and is lost due to feedline attenuation, if for example it was a lossless feedline then there would be no loss even with 20.1 swr, a tuner would allow the radio to produce full power
But something else to consider. The total antenna system. Feedline. Balum. Grounding. Soil type. Height. Use your program as a ballpark figure. (Circle of play in UK? Cricket.) Then measure SWR at the particular location, measure twice, then cut. Also, if SWR before adjusting is 1.5 or less, do not worry about it. I would not cut a centimeter off, that is for sure. And water, fog, humidity, frost/ice will change SWR. Just get the antenna resonant or SWR to less than 1.5, and let the impedence matcher do its magic. Now you mentioned CB? Is it about the 11M band as in the USA? Because in Down Under, they call CB what we in the US call GMRS.
I'd forgotten long ago about unusual circumstances such as "fold over". The review of math and info here is very welcome Cal, again Thanks for what you do.
It is so refreshing to listen to the joy in your voice as you explain things useful to other Hams !!! The chase to 100k subscribers WAS taking an unbelievable toll on your personality. I have enjoyed every “NEW” video that has been released. THANK YOU !!!
Thank you so much 😊
Great and useful video, thanks very much. I'm really glad you have got off the "chasing subscriptions express". I guess RUclips pushes video makers to do it but for me, it's counterproductive. Particularly when channel owners beg for subscriptions at the beginning of a video...before the viewer has seen any content! 73.
Totally agreed.
Oh cal,wish I'd blooming remembered you done this before I tuned my cobweb antenna 🙈🙈would've been so much quicker🤣🤣now saved it just incase 👍👍thankyou
Thank you for responding Callum. This video answered my question.
dobby likes spreadsheets. spreadsheets make the numbers go. spreadsheets are life.
Didn't notice you're audio was "broken" until the message came up. I'm sure there's a lot of people that would like to have your broken audio. 😂
Thanks for the "bent over at the end" answer. I couldn't seem to get a straight answer to that question. I don't think it matters that much on an EFHW Horizontal dipole though. 😆 Thanks for all you do Callum. Cheers
End-feds: Erm, same difference.. Double the result on the numbers.
Thanks Callum. I use that spread sheet all the time and have been since I got my Classic a couple of years ago. However, I have used it on EFHW, Dipoles, Fan Dipoles. The lot. Thanks for making that available.
Glad it was helpful!
Thanks for the tip on the fold over. I have a fan dipole using fold over technique. I am fortunate to have a VNA so have been manually tuning it that way its a bit slow going though. Now i know its about 3x i can make better guesses. Cheers Cal.
Yes, it's a good guess BUT not exact because the further you move away from the point (where the foldover happens), the greater the loading you will experience as you make the foldback longer and longer so that eventually, when the foldover is complete (fully linear loaded) small changes nearer the feedpoint (to your foldover) makes more of a change.. I need to make another video about this!
@@DXCommanderHQ yes please, that also explains the what i am seeing. Be interested to know or measure the efficiency of the antenna as well how much gain i am loosing vs the benefit of a physically shorter antenna. VNAs cant tell you that bit sadly.
@@DXCommanderHQ So if the tuning sheet says to cut 7.28 cm off you really do not want to have a 21.8 cm fold over. How do you figure out what to do in that case?
@@DXCommanderHQ This is great to know. I've got all the parts in the mail to try to make a fully linear loaded EFHW.
Awsome! I downloaded the Calculator which is really helpful to have it if you build your own wire antennas. You are a real Genius Lord Callum, thx for your great job. 73 de YFUG 💯🙋♂
Lovely! Thanks.
Very helpful tool Calum.🍻👍
Thanks for the video Callum - had forgotten about having to triple the result for "fold over". Hoping to catch you on 40m sometime de M7CCQ
So, there's 2 parts of "raising" or "lowering" the swr at any particular band. One is to increase or decrease the frequency of the swr. With this, shortening or lengthening the element (increases or decreases the frequency of the swr along the x axis.) This (along with the calculator) are very helpful resources. I've been using it a lot at tuning my signature 12.4. I simply identify the frequency of the lowest swr part of my given band, identify where I want that lowest swr to be (ie middle if the given band). I plug both into the calculator and adjust the element length accordingly. This moves that lowest swr frequency over to the desired frequency.
With the 10m band where there's sooo much foldback, like Callum said, there's limited effect in increasing or decreasing the foldback. In experimenting, I did a foldback of the foldback (folded the end of the foldback back up the element line). Interestingly, this had a much quicker effect of moving the lowest swr point over along the x axis over than just lengthening the foldback. (This came from not really trusting cutting the foldback yet but knowing it needed to be shorter.)
I've gotten all of my bands essentially centered in the given bands on my antenna analyzer using these methods.
But this doesn't really make any real difference if the swr is too high on the y axis. Which, as far as I can tell, but I might be missing something, these methods don't address.
So.... how does one reduce the swr NOT along the x frequency axis, but along the swr intensity on the y axis?
I see you commented on another video and I left my reply there.. If you are stuck, either chat on the Discord Server or drop me an email. Try and make it bullet points because I have a problem reading a lot of text.
If you only knew how many times I've used this.. big help
Great tip about foldovers
I am sure i should be doing some work this weekend, but finding watching your videos more beneficial at the moment. I just wondered if you sell tee shirts with the DXCommander logo on and if so do you do polo neck?
Soon!
By this calculation you can adjust resonant frequency, but not SWR as it is sum of resistance, what in fact you do not adjust. Correct me if I am wrong. Anyway thanks for nice explanation using Calculator. Best regards Cal.
In the MAIN, your resonant frequency for q/waves and dipoles disregards impedance and resistance (for our science experiments) because we normally know they will always (mostly) be between about 40 and 70 ohms. So best SWR will be best resonant frequency. But I think you knew that already! :)
I agree, this seems to be a pattern with teaching hams, and it can be very misleading. They tend to skip to the end and leave the juicy bits out, or in this case, assume they know it's for a 50ohm designed system. It's a very poor teaching method, as you need to explain the fundamentals for true understanding which will result in better, calculated practice.
Great job Callum!
Sir question I bought a CZE 15-A transmitter FM radio station on 102.7 FM for our community HOA non profit of course and only announcements for community and community involved no profit. My problem is I should be getting better coverage. Plus a lot of static even not far away. I should get art least a mile to 5 miles on 15 watts. The 1/4 wave Ground plane antenna came with it the company says it's all set. I got 50' coax that I got with it and the connectors already on for this TX snd antenna which I was told are TNC and SL16-J coax connectors. The top of antenna the antennas are pre made and labeled. The one I used is 102. Of course.
They assured me it was the best coax and antenna for what I am wanting to do with it. I made sure everything was connected correctly and tight. I had to hire a guy to put up the mast 25' and secure to house and it is above the roof clearing everything to broadcast the whole community easy. But dead spots, statically, sounds so bad. No interference, no loose connections. I am a retired DJ not an engineer. It sounds awful. So tone said it is the SWR setting it should be 1. But the vertical is pre set it does have a screw on it. The horizontal legs crap I did not drink anything but tighten them up. I do not know what I am doing. Can you please suggest what I need to do. Who would I contact to fix this problem? All my engineers are retired or dead, it was a long time ago. In different states. Are you in USA? I am. So don't know if that makes a difference.
Please email me if you will
Backyardufologist@gmail.com
If you will. Thank you!
Check out radiomobile. That will give you mapped precise coverage. Make sure antenna is totally in the clear and you have line of sight to your 5-mile requirement.
Thanks Callum!
Thanks, How do you Calculate this on a horizontal loop antenna, lets say a 80M loop? Grabbed the calculator and added a column for INCHES... hi Hi... W1SWL
For a loop: Multiply result by 4.
Great info Cal. 73
Thanks 👍!
I like the information you give! THANK YOU,
My pleasure!
I don't like cutting wire so I always fold over and I never understood why I'm folding over so much ... No I know thanks Cal!
Now you know :)
Very interesting little program must look that up always frigging with someting keep brain occupied
Big thanks!
Glad it helped!
Outstanding! Just subbed. Thanks again for the awesome content!
Thanks and welcome!
hi i have a 200n and the swr is 1,5 up and 1.5 down on145.500 the coax is 213 rg
Thanks!
Welcome!
Appreciate your videos. I'm just trying to get into 10 meter as a technician talking on 28.3 to 28.5 MHz. I got myself a President Lincoln II + and an MFJ whip (161OT). Was just doing the calculations and I came up with about 104 inches. But the MFJ 161OT is only 98 inches. Also it looks like my SWR is about 7. So I'm stuck not TXing for now. :(
OK, a number of ways out of that. Check what ground plane you have - increase it.. SOunds like that system you have is rsonant at around 33/34Mhz.. You can ADD a little bit of copper wire to the top.. Just wrap it around and stick it out the top?
@@DXCommanderHQ Thank you. I'll have to figure out how to make a ground plane. I currently have it about total 15.66 feet in the air on a metal post. Going to try that copper wire too.
@@DXCommanderHQ Oh I just had an idea. I have another whip that's identical. I think I will just make it a vertical dipole.
Hello mate. Can you please assist on the SWR matter : where does the reflected power go and if you agree that after the ping pong between the antenna and the transceiver it goes back to the antenna and gets transmitted. I would really appreciate any clarification on this subject. 73 de SV4RIN
Yes it does.. And in the process, the lossy coax also gradually eats it up as heat. Depends how lossy!
Hi I have mismatch readings from the internal meter and the external meter. I don’t know why and I would like to ask if you know how I can get to read the true swr
Measure at exactly same place (say in the shack at the radio) and if mismatch is only slight (say external meter is let's say 1.5:1 and internal rig SWR shows 1.2:1, then ignore. I would suggest if you measure at the same place and your radio is happy, just go with it because your external meter MIGHT be measuring genuine "complex" SWR that frankly your radio couldn't care about.
I’m a simple trucker so can you give me an example of how to use this calculator in this scenario, AM settings: channel 1 (26.965 MHz) = 2.5 SWR, Channel 20 (27.205 MHz) = 1.3 SWR, and Channel 40 (27.405 MHz) = 1.2 SWR. how do I calculate the length to cut or length to increase the whip on a Predator 10 K antenna with a 24 inch shaft in this scenario?
Well, I don't know what a Predator 10K is, I'm sorry BUT you are saying 27.405 is 1.2:1 and you would like it resonant lower down? In which case shorten it say exactly half an inch and check again.
Long ago I tried using a calculation method to cut antennas and it never worked. I had to do it inch by inch experimentally using the at the time standard SWR meter. Today, hams are using a VNA to measure and cut antennas but a lot of hams, especially new hams, don't understand what the VNA numbers mean. So they don't know what to do. Ignorance is not bliss at all!
Interesting!
Your content is outstanding. Suppose you have an antenna with a coil, or a hamstick. Is there a mathematical way to tune those?
Erm.. Normally the manufacturer SHOULD have don’t the work for you and suggested increments..
😎👍👍👍
Hi Callum. I am using an older version of the DX Commander. I have found that the resonant frequency at 10m is 28.554MHz. This gives me a high SWR of 4.48 at 28000kHz and 3.31 at 29700kHz (not much rise to the right). Should I want to move the resonant frequency to a lower part of the band, I believe more wire will need to be added. However, the current length is close to the plastic disk. One solution is to add more wire and fold it back onto it self. Is there a calculation to be used for determining how much fold-back is needed after adding the extra wire? Thank you in advance for your assistance. (Please excuse the length of this message as I know you prefer shorter ones). … Rob WD4IFT
Yeah.. I made a video about this.. The title is something like "what happens when you fold back the end of a dipole"..
So are you measuring the antenna with an antenna analyser to see what frequency it resonates on to then determine if it's too long or short for the frequency you desire? I had hoped this video was about how to adjust using an swr meter as the instructions for the CB base antenna I have seems to be the opposite of what google tells me. And what my old CB literature tells me.
You don't need a fancy pants meter, you can just see (for instance) that Channel 1 is (say) perfect and channel 40 is (say high), then you know that best frequency is ABOUT channel 1 (as previous), so you would need to lift the freq up about 20 channels.
I wish you would actively pursue RUclips over the capricious and unwarranted deletion of your M0MCX channel. Don't let the a-holes get away with it!
I made best efforts and for the sake of my mental health, gave up.
Antennas confuse the heck out of me. When I put up a wire antenna, I can usually get a good/usable SWR at a specific frequency but it's usually never close enough to 50 ohms. I struggled like crazy with my cobweb. Any pointers?
OK, so ignore the 50 ohms, for basic "known" ham radio antennas, in the main we can just point to best resonance. The impedance should follow you..
Am I correct that you would need to double the amount for an EFHW? This is a great calculator BTW, thanks! I did add a cm to in conversion cell though 😉
Erm.. I believe so!
If you using an inverted v and it says shorten by 1 ft, is that 1 ft each leg or 6" each leg?
Each leg.
Thank you for this information.
At an SWR of 1.5:1 there is a loss of 4% in radiated power, and this increases somewhat exponentially as SWR increases ( e.g. with SWR of 2.0:1 there is an effective loss of ~ 11% in the radiated power from the antenna).
However, if you are unable to physically adjust your antenna, and have to rely on an ATU to bring the SWR down to say, 1.1:1 from 2.0:1 (or higher), does the ATU 'recover' the lost power and allow the antenna to radiate it, or does the ATU simply absorb the reflected power in it's LC circuits and protect the finals of the Transmitter?
In my case when I added 8 x 1/8 lambda radials to my base loaded vertical antenna, my SWR jumped from ~1.5:1 to over 6:1 !! (Don't ask why, I haven't been able to figure that out .
So I used an external 'heavy duty' ATU to bring the SWR that my TX saw back down to ~1.5:1. But according to the charts, an SWR of 6:1 is a 50% loss in radiated power.
Am I correct in assuming that my ATU is simply 'absorbing' the reflected power and it is NOT being radiated by the antenna ?
Thank you.
No. You can't "recover" the lost power. And remember that half your power is only 3dB..
A tuner will correct the phase of the reflected power to add to the forward power and sent it back towards the antenna
Power is not automatically lost due to an swr reading ,this is the common misunderstanding in radio ,reflected power adds to the forward power and is lost due to feedline attenuation, if for example it was a lossless feedline then there would be no loss even with 20.1 swr, a tuner would allow the radio to produce full power
But something else to consider. The total antenna system. Feedline. Balum. Grounding. Soil type. Height.
Use your program as a ballpark figure. (Circle of play in UK? Cricket.)
Then measure SWR at the particular location, measure twice, then cut.
Also, if SWR before adjusting is 1.5 or less, do not worry about it. I would not cut a centimeter off, that is for sure.
And water, fog, humidity, frost/ice will change SWR.
Just get the antenna resonant or SWR to less than 1.5, and let the impedence matcher do its magic.
Now you mentioned CB? Is it about the 11M band as in the USA? Because in Down Under, they call CB what we in the US call GMRS.
That poor keyboard. :)