75ohm Impedance question K3S and Antenna side.
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Re: VF Varies With Frequency
 
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A typical, Type RG-58A Vf measurement, as a function of frequency, is
shown in second plot: https://www.kn5l.net/transMeasure/ Vf is frequency dependent. John KN5L On 4/1/20 4:45 PM, Mpridesti via Elecraft wrote: > Requested technical comment from a long established coaxial cable manufacturer on this topic. > > This was the response: > > The Vp is not measured at any frequency and is independent of frequency. ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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In reply to this post by Elecraft mailing list
Hmmmm.... I'm seeing great variability in the Vf does/does not vary with F.
This link says yes: https://owenduffy.net/transmissionline/concept/mvf/index.htm Other's say no... My "gut" says yes. Yea! Now I have something to research in order to figure out how/why! My favorite kind of puzzle! I'm guessing that not only can this be demonstrated mathematically one way or another, but also experimentally using say, my VNWA, sig generator, et al. 73, ______________________ Clay Autery, KY5G (318) 518-1389 On 04/01/20 16:45, Mpridesti via Elecraft wrote: > Requested technical comment from a long established coaxial cable manufacturer on this topic. > > This was the response: > > The Vp is not measured at any frequency and is independent of frequency. The only variable in play is the dielectric constant. The Vp represents the speed at which a signal transmits along the cable as a % of the speed of air. Air/vacuum will be the fasted medium to use to transmit an RF signal. The thing is, air is usually not practical since there is nothing to support the center conductor, can be easily crushed and is susceptible to the ingress of moisture. > > We use foamed dielectrics and expanded tapes to get as close to air as possible. Vp will vary based on the specific dielectric used. The vast majority of our cables range in velocity of propagation from 76% to 86%. For a given cable, the Vp will likely not vary more that +/- 1% over it’s length but as I mentioned earlier, it would be best to use +/-2% for planning purposes. > > End response > > Regards, > > Mark, K1RX > > >> On Apr 1, 2020, at 1:43 PM, Jim Brown <[hidden email]> wrote: >> >> On 4/1/2020 7:58 AM, Don Wilhelm wrote: >>> When dealing with a length of transmission line, the use of the '468' factor should not be used - compute the actual wavelength and then apply the velocity factor. >> There is yet another variable -- VF varies with frequency. At low frequencies, it is lower (slower), increasing until it converges to the published value at VHF. For this reason, matching sections and stubs must be measured at or near the operating frequency with an analyzer or as a stub placed in line with a generator and receiver. They should be cut long, then trimmed so that the null in that generator/receiver circuit is heard, or the analzyer reads a short or open. >> >> How much is this variation? For typical transmission lines, it's on the order of 1% from 80M, a bit more for 160M as compared to the published value. If what you're building is a stub to kill harmonics, it's the difference between the CW and phone bands on 80M. >> >> 73, Jim K9YC Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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In reply to this post by Elecraft mailing list
I would not be buying any cable from them.
Wes N7WS On 4/1/2020 2:45 PM, Mpridesti via Elecraft wrote: > Requested technical comment from a long established coaxial cable manufacturer on this topic. > > This was the response: > > The Vp is not measured at any frequency and is independent of frequency. The only variable in play is the dielectric constant. The Vp represents the speed at which a signal transmits along the cable as a % of the speed of air. Air/vacuum will be the fasted medium to use to transmit an RF signal. The thing is, air is usually not practical since there is nothing to support the center conductor, can be easily crushed and is susceptible to the ingress of moisture. > > We use foamed dielectrics and expanded tapes to get as close to air as possible. Vp will vary based on the specific dielectric used. The vast majority of our cables range in velocity of propagation from 76% to 86%. For a given cable, the Vp will likely not vary more that +/- 1% over it’s length but as I mentioned earlier, it would be best to use +/-2% for planning purposes. > > End response > > Regards, > > Mark, K1RX > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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In reply to this post by Elecraft mailing list
Hi Mark,
Whoever you talked to needs to go back to school and study Maxwell's equations. He is WRONG! http://k9yc.com/TransLines-LowFreq.pdf http://k9yc.com/Coax-Stubs.pdf 73, Jim K9YC On 4/1/2020 2:45 PM, Mpridesti wrote: > Requested technical comment from a long established coaxial cable > manufacturer on this topic. > > This was the response: > > The Vp is not measured at any frequency and is independent of > frequency. The only variable in play is the dielectric constant. The Vp > represents the speed at which a signal transmits along the cable as a % > of the speed of air. Air/vacuum will be the fasted medium to use to > transmit an RF signal. The thing is, air is usually not practical since > there is nothing to support the center conductor, can be easily crushed > and is susceptible to the ingress of moisture. > > We use foamed dielectrics and expanded tapes to get as close to air > as possible. Vp will vary based on the specific dielectric used. The > vast majority of our cables range in velocity of propagation from 76% to > 86%. For a given cable, the Vp will likely not vary more that +/- 1% > over it’s length but as I mentioned earlier, it would be best to use > +/-2% for planning purposes. > > > End response > > > Regards, > > Mark, K1RX > > >> On Apr 1, 2020, at 1:43 PM, Jim Brown <[hidden email]> wrote: >> >> On 4/1/2020 7:58 AM, Don Wilhelm wrote: >>> When dealing with a length of transmission line, the use of the '468' >>> factor should not be used - compute the actual wavelength and then >>> apply the velocity factor. >> >> There is yet another variable -- VF varies with frequency. At low >> frequencies, it is lower (slower), increasing until it converges to >> the published value at VHF. For this reason, matching sections and >> stubs must be measured at or near the operating frequency with an >> analyzer or as a stub placed in line with a generator and receiver. >> They should be cut long, then trimmed so that the null in that >> generator/receiver circuit is heard, or the analzyer reads a short or >> open. >> >> How much is this variation? For typical transmission lines, it's on >> the order of 1% from 80M, a bit more for 160M as compared to the >> published value. If what you're building is a stub to kill harmonics, >> it's the difference between the CW and phone bands on 80M. >> >> 73, Jim K9YC >> >> >> ______________________________________________________________ >> Elecraft mailing list >> Home: http://mailman.qth.net/mailman/listinfo/elecraft >> Help: http://mailman.qth.net/mmfaq.htm >> Post: mailto:[hidden email] >> >> This list hosted by: http://www.qsl.net >> Please help support this email list: http://www.qsl.net/donate.html >> Message delivered to [hidden email] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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In reply to this post by Jim Brown-10
I put a shorted 1/4 wave stub — about a foot long on 144 MHz — on the coax to my 2m beam. The beam is right under my HF antenna, and thanks to the stub I can run 1.3 kW on 40-10m with absolutely no interference to the 2m rig, and no effect on 2m SWR. Before I had the stub, hitting the key on the HF rig caused the 2m transceiver to reboot.
The stub is near the radio, although I don’t know if that matters. I just used the published VF for the coax to figure the length. To be fair, I should mention that the 2m beam is vertically polarized and the HF antenna is horizontal. But they are just a few feet apart. Victor 4X6GP > On 1 Apr 2020, at 20:38, Jim Brown <[hidden email]> wrote: > > On 4/1/2020 7:58 AM, Don Wilhelm wrote: >> When dealing with a length of transmission line, the use of the '468' factor should not be used - compute the actual wavelength and then apply the velocity factor. > > There is yet another variable -- VF varies with frequency. At low frequencies, it is lower (slower), increasing until it converges to the published value at VHF. For this reason, matching sections and stubs must be measured at or near the operating frequency with an analyzer or as a stub placed in line with a generator and receiver. They should be cut long, then trimmed so that the null in that generator/receiver circuit is heard, or the analzyer reads a short or open. > > How much is this variation? For typical transmission lines, it's on the order of 1% from 80M, a bit more for 160M as compared to the published value. If what you're building is a stub to kill harmonics, it's the difference between the CW and phone bands on 80M. > > 73, Jim K9YC > > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to [hidden email] Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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In reply to this post by Vic Rosenthal
So I found a formula for feedline length and just need some eyes on it to make sure I applied it correctly.
I want to measure twice and cut once. Then I can use my antenna analyzer with a dummy load on the line to trim it. The columns are band, freq in Mhz, Multiplier to give multiples of the 1/2 wave length. You have to view the email full screen width or the tables wrap making it hard to read. It looks like 111.7694545 feet is the calculated half wavelength for 80m (3.53Mhz), not sure what will happen on 30m as it's closest length appears to occur at 3 wavelengths where the feedline would be about 4.801946936 Feet too long. Not sure what the SWR related losses will be but it will probably be ok. BAND Freq 1 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 80 3.52 111.7694545 223.5389091 279.4236364 335.3083636 391.1930909 447.0778182 502.9625455 558.8472727 614.732 670.6167273 726.5014545 782.3861818 838.2709091 894.1556364 40 7.04 55.88472727 111.7694545 139.7118182 167.6541818 195.5965455 223.5389091 251.4812727 279.4236364 307.366 335.3083636 363.2507273 391.1930909 419.1354545 447.0778182 30 10.125 38.85713383 77.71426765 97.14283457 116.5714015 135.9999684 155.4285353 174.8571022 194.2856691 213.714236 233.142803 252.5713699 271.9999368 291.4285037 310.8570706 20 14.08 27.94236364 55.88472727 69.85590909 83.82709091 97.79827273 111.7694545 125.7406364 139.7118182 153.683 167.6541818 181.6253636 195.5965455 209.5677273 223.5389091 15 21.4 18.38450841 36.76901682 45.96127103 55.15352523 64.34577944 73.53803364 82.73028785 91.92254206 101.1147963 110.3070505 119.4993047 128.6915589 137.8838131 147.0760673 10 27 14.57142519 29.14285037 36.42856296 43.71427556 50.99998815 58.28570074 65.57141333 72.85712593 80.14283852 87.42855111 94.7142637 101.9999763 109.2856889 116.5714015 Formula used: L = (S * W * Vf)/f S 983.5712 W 0.5 VF 0.8 983.5712 L = (S * W * Vf)/f S speed of light in meters or feet W number of wavelengths desired VF velocity factor Jerry Moore Cell: 803-431-1870 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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Hi Jerry,
What coax do you plan to use (manufacturer and part number)? Did you intentionally use 27 MHz in the last row? Your other calculations look good but you carried them to far too many significant digits when the Vf of most coax (especially CATV coax) is only approximate. Cut your coax ten percent too long, leave the end of the coax open circuited. Your antenna analyzer will then set the length very accurately. 73 Frank W3LPL ----- Original Message ----- From: [hidden email] To: [hidden email] Sent: Friday, April 3, 2020 12:53:40 PM Subject: Re: [Elecraft] 75ohm Impedance question K3S and Antenna side. So I found a formula for feedline length and just need some eyes on it to make sure I applied it correctly. I want to measure twice and cut once. Then I can use my antenna analyzer with a dummy load on the line to trim it. The columns are band, freq in Mhz, Multiplier to give multiples of the 1/2 wave length. You have to view the email full screen width or the tables wrap making it hard to read. It looks like 111.7694545 feet is the calculated half wavelength for 80m (3.53Mhz), not sure what will happen on 30m as it's closest length appears to occur at 3 wavelengths where the feedline would be about 4.801946936 Feet too long. Not sure what the SWR related losses will be but it will probably be ok. BAND Freq 1 2 2.5 3 3.5 4 4.5 5 5.5 6 6 6.5 7 7.5 8 80 3.52 111.7694545 223.5389091 279.4236364 335.3083636 391.1930909 447.0778182 502.9625455 558.8472727 614.732 670.6167273 726.5014545 782.3861818 838.2709091 894.1556364 40 7.04 55.88472727 111.7694545 139.7118182 167.6541818 195.5965455 223.5389091 251.4812727 279.4236364 307.366 335.3083636 363.2507273 391.1930909 419.1354545 447.0778182 30 10.125 38.85713383 77.71426765 97.14283457 116.5714015 135.9999684 155.4285353 174.8571022 194.2856691 213.714236 233.142803 252.5713699 271.9999368 291.4285037 310.8570706 20 14.08 27.94236364 55.88472727 69.85590909 83.82709091 97.79827273 111.7694545 125.7406364 139.7118182 153.683 167.6541818 181.6253636 195.5965455 209.5677273 223.5389091 15 21.4 18.38450841 36.76901682 45.96127103 55.15352523 64.34577944 73.53803364 82.73028785 91.92254206 101.1147963 110.3070505 119.4993047 128.6915589 137.8838131 147.0760673 10 27 14.57142519 29.14285037 36.42856296 43.71427556 50.99998815 58.28570074 65.57141333 72.85712593 80.14283852 87.42855111 94.7142637 101.9999763 109.2856889 116.5714015 Formula used: L = (S * W * Vf)/f S 983.5712 W 0.5 VF 0.8 983.5712 L = (S * W * Vf)/f S speed of light in meters or feet W number of wavelengths desired VF velocity factor Jerry Moore Cell: 803-431-1870 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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Hi Frank,
I don’t know the manf or part number. This is ½ hard cable from the cable company that I received as a gift from very fine OM K4MQG, Gary Dixon. I just guessed at the freq for 10m. I don’t use the band so I just put something in. The extra digits are a result of the number used as the numerator in the calculation and I didn’t want to round it because I’ll convert that to inches and add a bit before I actually cut. I used 80% for the vf Thank you Frank !! My antenna purchase (Hustler 6BTV ) is slightly delayed (holding onto cash until after the pandemic). I can still get the radial and coax work done. Jerry – AE4PB 73 .. From: [hidden email] <[hidden email]> Sent: Friday, April 3, 2020 2:27 PM To: [hidden email] Subject: Re: [Elecraft] 75ohm Impedance question K3S and Antenna side. Hi Jerry, What coax do you plan to use (manufacturer and part number)? Did you intentionally use 27 MHz in the last row? Your other calculations look good but you carried them to far too many significant digits when the Vf of most coax (especially CATV coax) is only approximate. Cut your coax ten percent too long, leave the end of the coax open circuited. Your antenna analyzer will then set the length very accurately. 73 Frank W3LPL _____ From: [hidden email] <mailto:[hidden email]> To: [hidden email] <mailto:[hidden email]> Sent: Friday, April 3, 2020 12:53:40 PM Subject: Re: [Elecraft] 75ohm Impedance question K3S and Antenna side. So I found a formula for feedline length and just need some eyes on it to make sure I applied it correctly. I want to measure twice and cut once. Then I can use my antenna analyzer with a dummy load on the line to trim it. The columns are band, freq in Mhz, Multiplier to give multiples of the 1/2 wave length. You have to view the email full screen width or the tables wrap making it hard to read. It looks like 111.7694545 feet is the calculated half wavelength for 80m (3.53Mhz), not sure what will happen on 30m as it's closest length appears to occur at 3 wavelengths where the feedline would be about 4.801946936 Feet too long. Not sure what the SWR related losses will be but it will probably be ok. BAND Freq 1 2 2.5 3 3.5 4 4.5 5 5.5 6 6 6.5 7 7.5 8 80 3.52 111.7694545 223.5389091 279.4236364 335.3083636 391.1930909 447.0778182 502.9625455 558.8472727 614.732 670.6167273 726.5014545 782.3861818 838.2709091 894.1556364 40 7.04 55.88472727 111.7694545 139.7118182 167.6541818 195.5965455 223.5389091 251.4812727 279.4236364 307.366 335.3083636 363.2507273 391.1930909 419.1354545 447.0778182 30 10.125 38.85713383 77.71426765 97.14283457 116.5714015 135.9999684 155.4285353 174.8571022 194.2856691 213.714236 233.142803 252.5713699 271.9999368 291.4285037 310.8570706 20 14.08 27.94236364 55.88472727 69.85590909 83.82709091 97.79827273 111.7694545 125.7406364 139.7118182 153.683 167.6541818 181.6253636 195.5965455 209.5677273 223.5389091 15 21.4 18.38450841 36.76901682 45.96127103 55.15352523 64.34577944 73.53803364 82.73028785 91.92254206 101.1147963 110.3070505 119.4993047 128.6915589 137.8838131 147.0760673 10 27 14.57142519 29.14285037 36.42856296 43.71427556 50.99998815 58.28570074 65.57141333 72.85712593 80.14283852 87.42855111 94.7142637 101.9999763 109.2856889 116.5714015 Formula used: L = (S * W * Vf)/f S 983.5712 W 0.5 VF 0.8 983.5712 L = (S * W * Vf)/f S speed of light in meters or feet W number of wavelengths desired VF velocity factor Jerry Moore Cell: 803-431-1870 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] <mailto:[hidden email]> ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [hidden email] |
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