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measuring cable loss
"Owen Duffy" wrote in message ... K7ITM wrote in news:1186788470.852002.260460 @b79g2000hse.googlegroups.com: On Aug 10, 2:28 pm, Owen Duffy wrote: ... I don't think you can compensate for lack of f/b ratio in the coupler, for example because the coupled lines are too long. ... I'm curious what you mean by that, Owen... Tom, I was thinking of several instruments, all of the coupled lines type of construction, that on a s/c and o/c failed to indicate rho=1, and showed similar readings when physically reversed, suggesting it was not just a fwd / rev matching issue, there was something about the coupler that was too dependent on the location of the SWR pattern relative to the coupler. Since they worked better at lower frequencies, the length of the coupler was likely to be a contribution. Owen Would this be a problem for a directional coupler designed for a specific frequecy? Jimmie |
#2
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measuring cable loss
"Jimmie D" wrote in
: "Owen Duffy" wrote in message ... K7ITM wrote in news:1186788470.852002.260460 @b79g2000hse.googlegroups.com: On Aug 10, 2:28 pm, Owen Duffy wrote: ... I don't think you can compensate for lack of f/b ratio in the coupler, for example because the coupled lines are too long. ... I'm curious what you mean by that, Owen... Tom, I was thinking of several instruments, all of the coupled lines type of construction, that on a s/c and o/c failed to indicate rho=1, and showed similar readings when physically reversed, suggesting it was not just a fwd / rev matching issue, there was something about the coupler that was too dependent on the location of the SWR pattern relative to the coupler. Since they worked better at lower frequencies, the length of the coupler was likely to be a contribution. Owen Would this be a problem for a directional coupler designed for a specific frequecy? Jimmie Jimmie, I am talking about the el-cheap inline SWR / Power Meter that is often sold to hams with unrealistic specs. You can / should always test the performance of the kit you are using to determine if you should have confidence in it. There are a bund of notes on testing a directional wattmeter in the article at http://www.vk1od.net/VSWR/VSWRMeter.htm . BTW, for your purposes, if you had a Bird 43 with an element that read upscale on fwd power (250W element for your application), it is all you should need to form a reasonable estimate of line loss and set the transmitter to deliver 100W to the antenna. You might need a smaller slug to make a measurement of RL on a s/c or o/c termination. Owen |
#3
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measuring cable loss
"Owen Duffy" wrote in message ... "Jimmie D" wrote in : "Owen Duffy" wrote in message ... K7ITM wrote in news:1186788470.852002.260460 @b79g2000hse.googlegroups.com: On Aug 10, 2:28 pm, Owen Duffy wrote: ... I don't think you can compensate for lack of f/b ratio in the coupler, for example because the coupled lines are too long. ... I'm curious what you mean by that, Owen... Tom, I was thinking of several instruments, all of the coupled lines type of construction, that on a s/c and o/c failed to indicate rho=1, and showed similar readings when physically reversed, suggesting it was not just a fwd / rev matching issue, there was something about the coupler that was too dependent on the location of the SWR pattern relative to the coupler. Since they worked better at lower frequencies, the length of the coupler was likely to be a contribution. Owen Would this be a problem for a directional coupler designed for a specific frequecy? Jimmie Jimmie, I am talking about the el-cheap inline SWR / Power Meter that is often sold to hams with unrealistic specs. You can / should always test the performance of the kit you are using to determine if you should have confidence in it. There are a bund of notes on testing a directional wattmeter in the article at http://www.vk1od.net/VSWR/VSWRMeter.htm . BTW, for your purposes, if you had a Bird 43 with an element that read upscale on fwd power (250W element for your application), it is all you should need to form a reasonable estimate of line loss and set the transmitter to deliver 100W to the antenna. You might need a smaller slug to make a measurement of RL on a s/c or o/c termination. Owen Well it a done deal, Engineering support came out last night and ran the checks for us while Im on vacation and recovering from minor surgery, Yaaay. They did it the normal way and by measuring the return loss and they decided the "return loss method" worked better. Not sure what better means at this point. accurate enough and easier and faster would constitute better. Jimmie |
#4
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measuring cable loss
On Sat, 11 Aug 2007 07:58:36 -0400, "Jimmie D"
wrote: They did it the normal way Hi Jimmie, Given the long and winding road to this point, it would give me pause that suddenly something became "normal." The remainder of your post is in contradiction to your earlier statement: On Thu, 9 Aug 2007 08:13:45 -0400, "Jimmie D" wrote: The normal procedure for doing this is to inject a signal at one end and measure the power out at the other. For the sake of clarity (normality aside), what you originally described Thursday is called "insertion loss." On Fri, 10 Aug 2007 01:17:31 -0400, "Jimmie D" wrote: Power delivered to the antenna but be maintained with in +- 1 db in this case that power is 100 watts. Power is normally checked at the TX and recorded after allowing for line loss as "power at the antenna". This again defines "insertion loss." and by measuring the return loss and they decided the "return loss method" worked better. A description of the classic self-fulfilling prophecy. I presume you mean this to be "the normal way," but it doesn't really describe a method or procedure (a "way"); instead, it describes an outcome. There are many "ways" to measure a characteristic called "return loss." Some "ways" are more accurate than others. Having introduced this term, "insertion loss," there remains one more term to consider: "reflection loss." This and "return loss" can be found scaled on the common form of the Smith Chart. The distinction to these terms are that "return loss" and "reflection loss" are a single port characteristic (that port being the "load" which, of course, is NOT the antenna, but rather the line and the antenna). "Insertion loss" is a two port characteristic that properly conforms to your original question. ALL such losses are defined by the system within which they reside. This means you have to also characterize the impedances of BOTH the load and the source. This last requirement is often dismissed in this forum where the determination of the source's Z is frequently rejected as being an impossibility (even when it is specified by the equipment designer). When Zsource = Zline = Zload, then many complexities are removed. I have seen others ask you the characteristic Z of the load with no response by you; and I am certain you have no comfortable assurance about the Zsource of your transmitter. However, to this last, it would be immaterial if Zline = Zload. Not sure what better means at this point. accurate enough and easier and faster would constitute better. This, too, simplifies what is an exceedingly difficult determination (of "return loss," "reflection loss," or "insertion loss") for the accuracy you originally suggested. Accurate, easy, and fast are not normally words used in conjunction except in advertising promotions. The accuracy of any power determination is related to the known Z of 1. The load; 2. The source; 3. The detector. At 1 GHz, these determinations are not so easily dismissed as trivial, nor confirmed by dragging a $20,000 analyzer into the shop. The analyzer answers the problem of knowing its own source Z, but it does not answer what that source Z is of the transmitter (again, only a necessity in the face of returned power). Now, given no one has actually correlated accuracy to any metric here, and given that accuracy is determined in large part by the three Zs above; then a little more discussion is in order. Using only two (the detector and the load could be interchanged for the simpler analysis): Zsource = 100 Ohms Zload = 33.3 Ohms view in fixed font: 1 - Gammaload˛ Error = ------------------------------ (1 ± Gammasource · Gammaload)˛ Error = +0.42dB to -0.78 dB These errors are independant of other errors such as instrumentation error (meter linearity, conversion problems, ...) or operator errors (reading the meter - a mirrored scale is required to keep this below 5%). Modern instrumentation (if you have the $$$$) solves some of this, others dismiss it as a trivial concern and rely on name brand (Bird is frequently uttered to achieve perfection). Now, as to the variability in the error wholly associated with just the Zs (providing you can accurately determine them - yes, a game of infinite regress). The allowable error of 1dB is nearly wiped out with some very possible characteristics and you haven't even begun balancing the error budget. With luck (a fictional village where every armchair technician resides) the error induced by mismatches could be 0. That luck demands you know the length of the line (again, with some accuracy - I enjoy the irony here too). The variation built into the Error computation is from not knowing that length (as is common, few know this with enough precision in wavelengths). At 1 Ghz, the characteristic of aproximately 200ft of coax is apocryphal. 73's Richard Clark, KB7QHC |
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