13613	2017-02-20 10:28:41	Baris	Measuring coax loss
Message	Date	From	Subject
Hello group, What's your preferred method of measuring coax loss? I thought one can use a return loss bridge, open or short the other end of the coax, get a dBm reading, then get another reading with no coax attached and the DUT port either open or short, the difference would be the coax loss. Would this method be correct, or how accurate would it be? I'd like to hear your opinion. Regards, Baris
13615	2017-02-20 10:40:49	Tayloe, Dan (Noki...	Re: Measuring coax loss
With the cable attached to the proper load (50 ohms for a 50 ohm cable), I would simply measure the power going into the cable vs. the power coming out of the cable going into the load. The difference is the loss. - Dan
13616	2017-02-20 11:05:40	Phil Sittner	Re: Measuring coax loss
Baris- I use an AD8307-based power meter (EMRFD fig 7.13) and a small signal source that consists of an old TTL oscillator can that has a five pole LC filter. First measure the output of the oscillator with the meter and then measure the output after it passes through your length of coax and subtract the two values. The difference is the loss. You measure the length of the coax and use the calculator http://www.qsl.net/co8tw/Coax_Calculator.htm. Your loss should be very close to the calculated value or you have defective cable. Phil, KD6RM
13618	2017-02-20 12:27:11	AD7ZU	Re: Measuring coax loss
Baris, If you do not have a power meter power can be calculated using an oscilloscope to measure the amplitude of the voltage at the input and again at the load. As Dan suggested earlier terminate the cable with a resistor equal to the cable impedance. Measure the peak to peak voltage (Vpp ) Convert to Vrms Vrms = Vpp / 2.82 the 2.82 is 2* sqrt(2) P = (Vrms * Vrms ) / R in this case R is the terminating resistance same value is used for both measurements. Subtract the power at the load end from the power at the source end and that is the cable loss.... or db loss = 10 log10 (P1/P2) or 20 log10 (V1 / V2) ...should be the same result Randy AD7ZU
13619	2017-02-20 12:59:14	Baris	Re: Measuring coax loss
Thanks for the suggestions everyone, the power meter or scope method seems to be the most intuitive one. I have an AD8307 meter I build a while ago but I never got around to calibrating it, in terms of db/mV, so maybe I should look into that, because the scope thing sounds like it can be impractical, especially with roof installations etc. Any idea how the return loss bridge method would work here? In this document I found, http://www.qsl.net/kl7jef/Build%20a%20Return%20Loss%20Bridge.pdf , the author mentions this setup, but I'm confused with the divisi
13620	2017-02-20 14:08:14	K5ESS	Re: Measuring coax loss
Randy wrote “, but I'm confused with the division by two at the end”. Because in the return loss method you’re measuring the combined loss in both directions. Mike N K5ESS
13624	2017-02-22 02:51:01	Baris	Re: Measuring coax loss
I see that now Mike, thank you. I think I'll do some measurements to compare the two methods. The best thing would be to test the coax at the operating frequencies, right? I have a return loss bridge which is the one from EMRFD (with FT37-43 core), I'm also wondering how accurate the measurements will be as I increase the frequency. Baris 2017-02-21 1:07 GMT+03:00 'K5ESS' k5ess.nothdurft@verizon.net [emrfd] <emrfd@yahoogroups.com>: Randy wrote “, but I'm confused with the division by two at the end”. Because in the return loss method you’re measuring the combined loss in both directions. Mike N K5ESS
13626	2017-02-22 09:05:28	Philippe Creis	Measuring coax loss by Larry Jannings
Hello the group, in March 2015, I discovered the article "Quick Coax test" wrote by Larry Jennings in QST Magazine on August 2012. I found his method very interesting. Last year I bought an old/used cable reel with no reference at all, but in a very very good general state. I had no information about length, velocity factor, attenuation by meter, nothing..... just it was a beautifull câble with tinned dual shield, external jacket made with rubber. I applied Larry method with success. I also took advantage of this moment to also measure the real length, capacitance and inductance. To summarize, in order to appreciate the coaxial losses, you measure the Vswr of the câble at the frequency of operation, with the other end left open or closed, and you apply this formula: Matched losses =10log((SWR+1) / (SWR-1)) in dB/100 feet. For example, il you measure an swr of 3:1 , this indicate cable losses of 3dB. But I recommend to read his article, you will understand why there is 10 in the formula instead 20 normally !!!! Have a nice day all, -- 73... Philippe - F4GRT ________________ Thunderbird 45.7.1* Le 20/02/2017 à 19:28, Baris bggungor@gmail.com [emrfd] a écrit : Hello group, What's your preferred method of measuring coax loss? I thought one can use a return loss bridge, open or short the other end of the coax, get a dBm reading, then get another reading with no coax attached and the DUT port either open or short, the difference would be the coax loss. Would this method be correct, or how accurate would it be? I'd like to hear your opinion. Regards, Baris __________ Information provenant d'ESET Smart Security, version de la base des signatures de virus 14967 (20170220) __________ Le message a ete verifie par ESET Smart Security. http://www.eset.com
13627	2017-02-22 10:23:51	K5ESS	Re: Measuring coax loss
Baris, Sorry about the name mix-up. Looked at the bottom of the post instead of the middle. Yes, testing at the operating frequency would be best although if you know the type of coax you could compare your measured results with the published specs for the coax. If they are in close agreement you could use the spec sheet numbers to extrapolate to a higher (or lower) frequency. If your measured values differ markedly from published numbers that might mean there is some anomaly with your coax. I couldn’t put a number on the accuracy you might expect but the performance of your bridge is going to likely decrease as the frequency increases. You should be able to find information on how you can assess your bridge’s performance. 73 Mike N. K5ESS
13629	2017-02-22 13:23:06	AD7ZU	Re: Measuring coax loss
Baris, A couple more thoughts .. A calibrated RF voltage or power measurement isn’t required if you use the same apparatus to measure both ends (load and source). This is because the db loss calculation is a function of the ratio of the voltages or powers and any common measurement error will be cancelled in the calculation. A simple RF probe like the one that’s been in the handbook for decades will suffice if you are only measuring / calculating the db loss of a random length of cable…and wouldn’t necessarily need an oscilloscope just a good quality DMM and the probe. The second point is that coax impedance varies with frequency. It will be close to the manufactures nominal value but likely not exact. If the impedance of the cable is not equal to the termination resistor there will be some error, additionally the terminating resistor will have some associated capacitance but the total error will be small. It would be a good idea to measure the terminating resistor and find one that is very close to the nominal cable impedance. I didn’t know what precisi
13630	2017-02-23 01:16:53	Baris	Re: Measuring coax loss by Larry Jannings
Hi Philippe, I don't have access to that article right now, but I had read the article "Measuring Cable Loss" from QEX (http://www.arrl.org/files/file/Technology/tis/info/pdf/0505qex044.pdf), which basically summarizes the same thing. It talks about the problems associated with that formula and proposes a slightly different method, you may also find it interesting. - Baris 2017-02-22 15:51 GMT+03:00 Philippe Creis Philippe.Creis@wanadoo.fr [emrfd] <emrfd@yahoogroups.com>: Hello the group, in March 2015, I discovered the article "Quick Coax test" wrote by Larry Jennings in QST Magazine on August 2012. I found his method very interesting. Last year I bought an old/used cable reel with no reference at all, but in a very very good general state. I had no information about length, velocity factor, attenuation by meter, nothing..... just it was a beautifull câble with tinned dual shield, external jacket made with rubber. I applied Larry method with success. I also took advantage of this moment to also measure the real length, capacitance and inductance. To summarize, in order to appreciate the coaxial losses, you measure the Vswr of the câble at the frequency of operation, with the other end left open or closed, and you apply this formula: Matched losses =10log((SWR+1) / (SWR-1)) in dB/100 feet. For example, il you measure an swr of 3:1 , this indicate cable losses of 3dB. But I recommend to read his article, you will understand why there is 10 in the formula instead 20 normally !!!! Have a nice day all, -- 73... Philippe - F4GRT ________________ Thunderbird 45.7.1* Le 20/02/2017 à 19:28, Baris bggungor@gmail.com [emrfd] a écrit : Hello group, What's your preferred method of measuring coax loss? I thought one can use a return loss bridge, open or short the other end of the coax, get a dBm reading, then get another reading with no coax attached and the DUT port either open or short, the difference would be the coax loss. Would this method be correct, or how accurate would it be? I'd like to hear your opinion. Regards, Baris __________ Information provenant d'ESET Smart Security, version de la base des signatures de virus 14967 (20170220) __________ Le message a ete verifie par ESET Smart Security. http://www.eset.com
13632	2017-02-25 13:36:13	Philippe Creis	Measuring coax loss by Larry Jannings
Hi Baris, hello the group, Baris, I read your article with a geat pleasure. It really follows on from the one of Larry Jennings on QST. I think I will soon make another measurement of my reel of coaxial, just to practice the differents methods described and well argued. Many thanks Baris for this link, it was very interesting. I really liked it. -- 73... Philippe - F4GRT ________________ Thunderbird 45.7.1 Le 23/02/2017 à 10:16, Baris bggungor@gmail.com [emrfd] a écrit : Hi Philippe, I don't have access to that article right now, but I had read the article "Measuring Cable Loss" from QEX (http://www.arrl.org/files/file/Technology/tis/info/pdf/0505qex044.pdf), which basically summarizes the same thing. It talks about the problems associated with that formula and proposes a slightly different method, you may also find it interesting. - Baris 2017-02-22 15:51 GMT+03:00 Philippe Creis Philippe.Creis@wanadoo.fr [emrfd] <emrfd@yahoogroups.com>:
13633	2017-02-26 13:44:34	Baris	Re: Measuring coax loss by Larry Jannings
You're welcome Philippe, thanks for that article too. I've also built a N2PK VNA recently, so I'll be able to better compare these various coax testing methods soon! Baris 2017-02-25 23:23 GMT+03:00 Philippe Creis Philippe.Creis@wanadoo.fr [emrfd] <emrfd@yahoogroups.com>: Hi Baris, hello the group, Baris, I read your article with a geat pleasure. It really follows on from the one of Larry Jennings on QST. I think I will so
13634	2017-02-26 14:34:37	Baris	Re: Measuring coax loss
Thanks Randy, I'm hoping to try several methods in the coming days and find out how well they match. Baris 2017-02-23 0:21 GMT+03:00 AD7ZU ad7zu@yahoo.com [emrfd] <emrfd@yahoogroups.com>: Baris, A couple more thoughts .. A calibrated RF voltage or power measurement isn’t required if you use the same apparatus to measure both ends (load and source). This is because the db loss calculation is a function of the ratio of the voltages or powers and any common measurement error will be cancelled in the calculation. A simple RF probe like the one that’s been in the handbook for decades will suffice if you are only measuring / calculating the db loss of a random length of cable…and wouldn’t necessarily need an oscilloscope just a good quality DMM and the probe. The second point is that coax impedance varies with frequency. It will be close to the manufactures nominal value but likely not exact. If the impedance of the cable is not equal to the termination resistor there will be some error, additionally the terminating resistor will have some associated capacitance but the total error will be small. It would be a good idea to measure the terminating resistor and find one that is very close to the nominal cable impedance. I didn’t know what precisi