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Jim says,
No, the SWR being measured is on the load side of the meter. ========================================= Jim, I have difficulty in telling you this without hurting your feelings. Or perhaps you are just joking and pulling my leg. Try the other one - its got bells on. I will assume you are not joking. You clearly havn't the foggiest idea of how the so-called SWR meter works. Although you may be in good company - including not a few professional engineers who have picked up old-wives' tales from radio amateurs. And have been led astray by the meter being called something which it isn't. The meter indicates SWR when placed at the ANTENNA end of the line. It is the antenna which does the terminating. And the meter gives the correct answers only when the line Zo = 50 ohms. If there is a tuner or matching network in the line then the meter will indicate SWR on the line between the transmitter and the tuner - provided the meter is placed next to the tuner. When there is no appreciable length of line between transmitter and tuner (as is the very common case) there is no line on which to measure SWR. But the meter will indicate an SWR - and tells lies. But there's nothing to worry about just because the meter is indicating SWR nonsense. The meter is telling you what you really want to know, its exactly why you placed it there - whether or not the transmitter is loaded with 50 ohms. ---- Reg. |
"Owen Duffy" wrote - Your example demonstrates that a typical SWR meter does not measure, or necessarily indicate the SWR of the (actual) transmission line on the load side of itself. ---------------------------------------------------------------------- ---------- You must have been reading what I've been writing for the last 6 or 7 years. ---- Reg. |
On Tue, 27 Sep 2005 00:24:51 +0000 (UTC), "Reg Edwards"
wrote: You must have been reading what I've been writing for the last 6 or 7 years. Reg, Yes, I have probably read and learned a good deal from stuff you have written, but I have skipped over a lot of what you have written... probably most noticeably when you and others trade kicks to the groin to see who is the last man standing. Back on topic: A point that you hinted at, but might have been overlooked by some is that it can be relatively unimportant that the SWR bridge's sampling line has the same characteristic impedance as the impedance at which its detector has been nulled. For example, a typical SWR meter designed originally for 75 ohms, with a 0.1m long ideal 75 ohm sampling section, but with the detector adjusted to read nil reflected power with a 50+j0 load on the "antenna" terminals of the meter, will in most cases operate just as well as a 50 ohm SWR meter on 7MHz, as the detector will truly show when it has a 50 ohm load, the indicated VSWR for other loads will substantially correct (ie within typical accuracy for the type of instrument), and the insertion VSWR (~1.02:1) because of the 0.1m of 75 ohm line will be insignificant in practice. In many amateur reflectometer designs (and in some commercial implementations), very little attention has been given to the characteristic impedance of the sampling section, and in some cases to the insertion VSWR (that results). I recall testing a relatively expensive SWR meter rated from 1.8 to 150MHz, and noting that whilst it indicated a VSWR1.1 at 144MHz on a good dummy load a Bird 43 ahead of it indicated an insertion VSWR 1.5:1. So whilst it was good at indicating a 50+j0 ohm load on its "antenna" terminals, it was not very capable of delivering that load to its "transmitter" terminals. Owen -- |
"Cecil Moore" wrote On my system, there's a 50 ohm cable from the transceiver to the input of the SWR meter and another 50 ohm cable from the output of the SWR meter to the balun. Each of these cables forces the ratio of the voltage to current in each of the traveling waves to a value of 50 ohms. I have an in-line Autek WM-1 and no tuner. -- ==================================== Cec, You can't measure SWR on a line which is less than 1/4-wave long. Preferably it should be as long as 1/2-wavelength to ensure the max and min voltage points both occur on it. But what do you do with the SWR when you have measured it? Of what use it? All anybody needs to know is whether the transmitter is terminated or is not terminated with 50-ohms. This is very important. And when describing to a novice how your station works there's no need to mention SWR or reflections. They are quite unecessary complications and can be dispensed with except perhaps for trolling on newsgroups. ---- Reg. |
Reg Edwards wrote:
Jim says, No, the SWR being measured is on the load side of the meter. ========================================= Jim, I have difficulty in telling you this without hurting your feelings. Or perhaps you are just joking and pulling my leg. Try the other one - its got bells on. Meaningless babble. I will assume you are not joking. You clearly havn't the foggiest idea of how the so-called SWR meter works. Although you may be in good company - including not a few professional engineers who have picked up old-wives' tales from radio amateurs. And have been led astray by the meter being called something which it isn't. I assure you I have a very good understanding of how SWR meters in their various incantations work and how SWR works. The meter indicates SWR when placed at the ANTENNA end of the line. It is the antenna which does the terminating. And the meter gives the correct answers only when the line Zo = 50 ohms. A SWR meter indicates the SWR of whatever is connected to the load side of the meter relative to the reference impedance the meter is designed for. It matters not if the thing connected to the load side of the meter is an antenna, an antenna with a transmission line, a transmission line with a dummy load on the end, or two popsicle sticks and a bubble gum wrapper. If you are only interested in the SWR of the antenna and don't, or can't, correct for the effects of real transmission line in between the meter and the antenna, then you have to connect the antenna to the load end of the meter directly. If there is a tuner or matching network in the line then the meter will indicate SWR on the line between the transmitter and the tuner - provided the meter is placed next to the tuner. Tuners have nothing to do with the issue. You can put a 1956 Buick on the load side of the meter; you still read SWR of the system at the point of measurement. It is you that is trying to confuse the issue with arm waving. When there is no appreciable length of line between transmitter and tuner (as is the very common case) there is no line on which to measure SWR. But the meter will indicate an SWR - and tells lies. Babbling nonsense. But there's nothing to worry about just because the meter is indicating SWR nonsense. The meter is telling you what you really want to know, its exactly why you placed it there - whether or not the transmitter is loaded with 50 ohms. That is not what I would want to know. I know the output impedance of the transmitter and the line (assuming it is good) impedance. What I want to know is the antenna SWR and the measurement of that is trivial though it seems to have you totally confused. ---- Reg. -- Jim Pennino Remove .spam.sux to reply. |
Owen Duffy wrote:
On Mon, 26 Sep 2005 21:39:27 +0000 (UTC), wrote: Jim, that seems inconsistent with your earlier statemetn "No, the SWR being measured is on the load side of the meter." The load side is the side with the load, i.e. the antenna, on it. In the example you quoted with a 100 ohm load on a 100 ohm line, were the line loss low, and the line long enough to be sure to sample a fully developed voltage maximum and voltage minimum it would be found that the VSWR was 1:1. Not for a 50 Ohm system, i.e. a transmitter expecting 50 Ohms and a meter calibrated for a 50 Ohm system. I am sorry Jim, the VSWR is a property of the transmission line and its termination, and the VSWR on that 100 ohm line with a 100 ohm termination is 1:1. The VSWR could be *MEASURED* on that line by sampling the magnitude of the voltage at different points on the line and it would be found that the magnitude of the voltage was constant, which means VSWR=1:1. No, the measured SWR is relative to the design impedance of the SWR meter which is normally 50 Ohms. If you use a 100 Ohm SWR meter you get 1:1. The SWR is a function of the TOTAL SYSTEM impedance connected to the load side. Replace the 100 antenna with a 100 Ohm resistor and the reading doesn't change. Eliminate the line and connect the 100 Ohm resistor directly to the meter and the reading doesn't change. Yet we would expect the "SWR meter designed for 50 Ohms" to which it is connected (on the load side) to read VSWR=2:1, so is it measuring the SWR on the load side of the meter as you earlier stated? Yep. No, it isn't. The SWR meter in your example reads 2:1 when the SWR on the 100 ohm line is 1:1. Your example demonstrates that a typical SWR meter does not measure, or necessarily indicate the SWR of the (actual) transmission line on the load side of itself. The SWR of the SYSTEM, line and antenna, is NOT 1:1 for a 50 Ohm reference. Owen -- -- Jim Pennino Remove .spam.sux to reply. |
Owen Duffy wrote:
On Tue, 27 Sep 2005 00:24:51 +0000 (UTC), "Reg Edwards" wrote: You must have been reading what I've been writing for the last 6 or 7 years. Reg, Yes, I have probably read and learned a good deal from stuff you have written, but I have skipped over a lot of what you have written... probably most noticeably when you and others trade kicks to the groin to see who is the last man standing. Back on topic: A point that you hinted at, but might have been overlooked by some is that it can be relatively unimportant that the SWR bridge's sampling line has the same characteristic impedance as the impedance at which its detector has been nulled. For example, a typical SWR meter designed originally for 75 ohms, with a 0.1m long ideal 75 ohm sampling section, but with the detector adjusted to read nil reflected power with a 50+j0 load on the "antenna" terminals of the meter, will in most cases operate just as well as a 50 ohm SWR meter on 7MHz, as the detector will truly show when it has a 50 ohm load, the indicated VSWR for other loads will substantially correct (ie within typical accuracy for the type of instrument), and the insertion VSWR (~1.02:1) because of the 0.1m of 75 ohm line will be insignificant in practice. In many amateur reflectometer designs (and in some commercial implementations), very little attention has been given to the characteristic impedance of the sampling section, and in some cases to the insertion VSWR (that results). I recall testing a relatively expensive SWR meter rated from 1.8 to 150MHz, and noting that whilst it indicated a VSWR1.1 at 144MHz on a good dummy load a Bird 43 ahead of it indicated an insertion VSWR 1.5:1. So whilst it was good at indicating a 50+j0 ohm load on its "antenna" terminals, it was not very capable of delivering that load to its "transmitter" terminals. Owen -- Inaccurate crap equipment has nothing to do with the arguement. -- Jim Pennino Remove .spam.sux to reply. |
Reg Edwards wrote:
You can't measure SWR on a line which is less than 1/4-wave long. Because your measurements are taken 1/4WL apart, your SWR value will be in error for all feedlines except lossless ones. SWR can be had at any point on a transmission line. Measure the forward power and reflected power at any point. SWR = [SQRT(Pfor)+SQRT(Pref)]/[SQRT(Pfor)-SQRT(Pref)] The transmission line length must only be long enough such that the V/I ratio is forced to the Z0 value. According to some pretty smart guys I asked, that's about 2% of a wavelength. -- 73, Cecil http://www.qsl.net/w5dxp |
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