Home |
Search |
Today's Posts |
|
#1
|
|||
|
|||
On Tue, 28 Jun 2005 23:17:15 -0500, Cecil Moore
wrote in : Frank Gilliland wrote: Impedance matching of an SWR meter is generally unimportant since most SWR meters used for HF have a directional coupler that is much shorter than the operating wavelength. Point is that they are usually calibrated for Z0=50 ohms and are in error when used in Z0 environments differing from Z0=50 ohms, e.g. Z0=75 ohms. The point is that the error is insignificant when the directional coupler is much shorter than the wavelength. The error is even more insignificant when there are a host of variables and confounds between the SWR meter and the transmitted field that can (and frequently do) affect the objective -- field strength. It's much simpler (and just plain logical) to measure the field strength directly instead of measuring an abstract value halfway towards the objective and relying on nothing more than speculation that the rest is working according as expected. ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
#2
|
|||
|
|||
"Frank Gilliland" wrote in message ... On Tue, 28 Jun 2005 23:17:15 -0500, Cecil Moore wrote in : Frank Gilliland wrote: Impedance matching of an SWR meter is generally unimportant since most SWR meters used for HF have a directional coupler that is much shorter than the operating wavelength. Point is that they are usually calibrated for Z0=50 ohms and are in error when used in Z0 environments differing from Z0=50 ohms, e.g. Z0=75 ohms. The point is that the error is insignificant when the directional coupler is much shorter than the wavelength. It is the directional coupler that is balanced for a particular value of Z0. Tam/WB2TT The error is even more insignificant when there are a host of variables and confounds between the SWR meter and the transmitted field that can (and frequently do) affect the objective -- field strength. It's much simpler (and just plain logical) to measure the field strength directly instead of measuring an abstract value halfway towards the objective and relying on nothing more than speculation that the rest is working according as expected. ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
#3
|
|||
|
|||
On Wed, 29 Jun 2005 17:28:01 -0400, "Tam/WB2TT"
wrote: It is the directional coupler that is balanced for a particular value of Z0. Tam/WB2TT ***** Correct james |
#4
|
|||
|
|||
Frank Gilliland wrote:
Point is that they are usually calibrated for Z0=50 ohms and are in error when used in Z0 environments differing from Z0=50 ohms, e.g. Z0=75 ohms. The point is that the error is insignificant when the directional coupler is much shorter than the wavelength. Nope, that's not the point at all. It is true that a 50 ohm SWR meter designed for HF may not work on 70 cm but the error I'm talking about is the calibration error in a 50 ohm SWR meter designed for HF and used on HF in, for instance, a Z0 = 450 ohm environment instead of its calibrated-for 50 ohm environment. It works perfectly in a 50 ohm environment at the HF frequency of operation. Here's the proof using a 50 ohm SWR meter: XMTR--1/2WL 450 ohm line--SWR meter--1/2WL 450 ohm line--50 ohm load The 50 ohm SWR meter will read 1:1, nowhere near the actual SWR XMTR--1/4WL 450 ohm line--SWR meter--1/4WL 450 ohm line--50 ohm load The 50 ohm SWR meter will read 81:1, nowhere near the actual SWR An SWR meter calibrated for 450 ohms will correctly read 9:1 in both cases. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= East/West-Coast Server Farms - Total Privacy via Encryption =--- |
#5
|
|||
|
|||
On Wed, 29 Jun 2005 16:37:10 -0500, Cecil Moore
wrote in : Frank Gilliland wrote: Point is that they are usually calibrated for Z0=50 ohms and are in error when used in Z0 environments differing from Z0=50 ohms, e.g. Z0=75 ohms. The point is that the error is insignificant when the directional coupler is much shorter than the wavelength. Nope, that's not the point at all. It is true that a 50 ohm SWR meter designed for HF may not work on 70 cm but the error I'm talking about is the calibration error in a 50 ohm SWR meter designed for HF and used on HF in, for instance, a Z0 = 450 ohm environment instead of its calibrated-for 50 ohm environment.... There lies our misperceptions; I was not referring to using an HF SWR meter designed for coax and plugging it into 450 ohm ladder line. ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
#6
|
|||
|
|||
Frank Gilliland wrote:
There lies our misperceptions; I was not referring to using an HF SWR meter designed for coax and plugging it into 450 ohm ladder line. But I specifically stated above the Z0 environment was different from 50 ohms. The same type of error happens when one uses a 50 ohm SWR meter in a 75 ohm coaxial line. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
#7
|
|||
|
|||
On Wed, 29 Jun 2005 22:39:10 -0500, Cecil Moore
wrote in : Frank Gilliland wrote: There lies our misperceptions; I was not referring to using an HF SWR meter designed for coax and plugging it into 450 ohm ladder line. But I specifically stated above the Z0 environment was different from 50 ohms. The same type of error happens when one uses a 50 ohm SWR meter in a 75 ohm coaxial line. If that were true then the mere existence of standing waves could render any measurements worthless. Regardless, I did the experiment a long time ago -- take a 50 ohm SWR meter and plug it into a 75 ohm line -- it gives you almost the same measurement (in fact, I didn't see -any- difference at all). Any small error you might see is, as I said before, insignificant, especially considering the reason you are measuring SWR in the first place. The objective is simply to get the reading as low as practially possible. If you feel the need to quibble about a couple tenths of a point on a ratio then maybe you're spending a little too much time playing with the calculator instead of the antenna. ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
#8
|
|||
|
|||
Frank Gilliland wrote:
Cecil Moore wrote: But I specifically stated above the Z0 environment was different from 50 ohms. The same type of error happens when one uses a 50 ohm SWR meter in a 75 ohm coaxial line. If that were true then the mere existence of standing waves could render any measurements worthless. Regardless, I did the experiment a long time ago -- take a 50 ohm SWR meter and plug it into a 75 ohm line -- it gives you almost the same measurement (in fact, I didn't see -any- difference at all). Please run it again in the following configuration: Xmtr--1/4WL 75 ohm line--SWR meter--1/4WL 75 ohm line--50 ohm load The SWR meter will read 2.25:1 when the actual SWR is 1.5:1 Xmtr--1/2WL 75 ohm line--SWR meter--1/2WL 75 ohm line--50 ohm load The SWR meter will read 1:1 when the actual SWR is 1.5:1 Any small error you might see is, as I said before, insignificant, especially considering the reason you are measuring SWR in the first place. A 50% error in SWR reading is NOT insignificant. The objective is simply to get the reading as low as practially possible. If you feel the need to quibble about a couple tenths of a point on a ratio then maybe you're spending a little too much time playing with the calculator instead of the antenna. A 50% error in SWR is NOT a couple tenths of a point. -- 73, Cecil http://www.qsl.net/w5dxp ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
#9
|
|||
|
|||
On Thu, 30 Jun 2005 07:36:21 -0500, Cecil Moore
wrote in : Frank Gilliland wrote: Cecil Moore wrote: But I specifically stated above the Z0 environment was different from 50 ohms. The same type of error happens when one uses a 50 ohm SWR meter in a 75 ohm coaxial line. If that were true then the mere existence of standing waves could render any measurements worthless. Regardless, I did the experiment a long time ago -- take a 50 ohm SWR meter and plug it into a 75 ohm line -- it gives you almost the same measurement (in fact, I didn't see -any- difference at all). Please run it again in the following configuration: Xmtr--1/4WL 75 ohm line--SWR meter--1/4WL 75 ohm line--50 ohm load The SWR meter will read 2.25:1 when the actual SWR is 1.5:1 Xmtr--1/2WL 75 ohm line--SWR meter--1/2WL 75 ohm line--50 ohm load The SWR meter will read 1:1 when the actual SWR is 1.5:1 I'm not going to argue this -- either you can play with theory and speculate about the results, or you can do the experiment yourself, observe the empirical evidence, and -then- use theory to explain the results. When you get around to doing the latter give me a holler in rrcb since I'm done cross posting on this topic. And BTW, the best location for the directional coupler is at the feedpoint of the antenna. Barring that, the next best place is at the transmitter. Regardless of it's location, you should -never- leave the coupler floating with the coax or you will end up with results like what you describe above. ----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups ----= East and West-Coast Server Farms - Total Privacy via Encryption =---- |
#10
|
|||
|
|||
But I specifically stated above the Z0 environment was different
from 50 ohms. The same type of error happens when one uses a 50 ohm SWR meter in a 75 ohm coaxial line. If that were true then the mere existence of standing waves could render any measurements worthless. Regardless, I did the experiment a long time ago -- take a 50 ohm SWR meter and plug it into a 75 ohm line -- it gives you almost the same measurement (in fact, I didn't see -any- difference at all). Any small error you might see is, as I said before, insignificant, especially considering the reason you are measuring SWR in the first place. The objective is simply to get the reading as low as practially possible. If you feel the need to quibble about a couple tenths of a point on a ratio then maybe you're spending a little too much time playing with the calculator instead of the antenna. I'm running RG-6 out to my 2 meter antenna. I put my cheap RS HF meter inline to see what I'd read. I got my expected 1.5:1. Wattage read 1/2 of what the radio is rated for. It gets out and I'm not worried about it. |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|