Your typical swr meter does not measure voltage on the line. It is measuring a combination of voltage and current. By just measuring voltage it is impossible to tell which is forward and which is reflected.
By using a slotted line you can find voltage min and max but you must have a line that is at least 1/4 wavelength long. This is impractical at HF frequencies and cumbersome at VHF. 73 Gary K4FMX > -----Original Message----- > From: Repeater-Builder@yahoogroups.com [mailto:Repeater- > [EMAIL PROTECTED] On Behalf Of Ron Wright > Sent: Wednesday, August 29, 2007 9:00 AM > To: Repeater-Builder@yahoogroups.com > Subject: Re: RE: [Repeater-Builder] Duplexers > > I think he quote needs little correction...âWhen you are using a VSWR > meter you are measuring voltageS, not just one voltage, hi. > > You are measuring voltage ratios. > > The SWR "reading" due to losses changes when one moves closer or farther > away from the end of a feedline. The power going out is attenuated, then > the load reflects a portion of this back and gets attenuated again and the > reflected is measured. Moving closer increases the power to the load and > also increases the reflected read at the source showing a higher SWR. > Lengthing the cable does the opposite. > > This is why one can have say 500 ft of RG58 at 450 MHz completly open at > the load end and the swr might read 1.5:1 at the source. Also for long > feedlines with antennas can give deceptive readings at the source. SWR at > the load is much more real. > > 73, ron, n9ee/r > > > > >From: "R. K. Brumback" <[EMAIL PROTECTED]> > >Date: 2007/08/27 Mon PM 12:25:59 CDT > >To: Repeater-Builder@yahoogroups.com > >Subject: RE: [Repeater-Builder] Duplexers > > > > > > >Quote from Jesse: âWhen you are using a VSWR meter you are > measuringvoltage, if you move the meter to a different spot on the cable, > the voltage isdifferent, therefore it gives you a different reading.â > > > >This now makes more sense to me as I once saw a feed line > demonstrationwith voltage and current sleds showing the difference at > different points alongthe line. At some places the voltage was null (as > with any sine wave). I donâtsee how this could happen at the antenna > port of a transmitter unless it wasmicrowave as the cabling from the tuner > to the output connector is not near ½ wave. Also to Alan, I appreciate > your sympathy for us âlittle peopleâbut I do find this very > interesting. And as you can see, the experts sometimesneed a tune up. > >Randy > >W4CPT > > > >-----Original Message----- > >From: [EMAIL PROTECTED]:Repeater- > [EMAIL PROTECTED] OnBehalf Of Jesse Lloyd > >Sent: Monday, August 27, 2007 12:48 PM > >To:Repeater-Builder@yahoogroups.com > >Subject: Re: [Repeater-Builder]Duplexers > > > >The length of coaxdoesn't effect impedance. Trimming the coax effects > what is read on theVSWR meter because what is actually happening is that > there is an interferencepattern created when you have a mismatch on the > end of feedline. This patternis sinusoidal and changes in voltage and > current along the line, in 1/2 waveperiods. You will find max voltage > peaks and min voltage peaks. Also current will go up and down too. When > you are using a VSWR meter youare measuring voltage, if you move the meter > to a different spot on the cable,the voltage is different, therefor it > gives you a different reading. > > > >Now if you put a voltage null at your transmitter, what would happen? > Normally with high SWR your transmitter will get hot because its > dissipating thereflected power into its heatsink. If you put it at a > voltage null, Iwould suspect that the SWR would not get dissipated by the > transmitter as muchas if you put it at a voltage peak. The standing waves > are still there,there is still a mismatch, you will get the same power > out, but its just notgoing to hurt your transmitter as much because of the > heat. > > > >The only time coax length makes a difference to power out is if your > using itin a matching stub, or a matching section ie. if you take 1/4 wave > of 75 ohmcable put it on the end of 50 ohm cable you will get a match with > a 112.5 ohmload. > > > >You make an interesting point though, why does the cabling of duplexer's > needto be a certain length. I would suspect that its because they are > loopedand make an inductor. This then is part of the LC filtering, and > changing thelength effects L. But I could be wrong on that. > > > >Jesse > >On 8/27/07, R. K. Brumback <[EMAIL PROTECTED]>wrote: > >I have heardthis point argued for years. "Does trimming the coax affect > theSWR?" If the length of coax has an affect on impedance, then howcould > it not affect power out? We strive to maintain 50 ohms at the tail of > alldevices to match the end load. GE puts matching networks in their Mastr > II's. Ihave taken a MFJ-259 and soldered a PL259 only at one end and then > startedtrimming the coax down and watched the impedance change > significantly with eachcut. Duplexers come with precise lengths of > cabling. I have heard thattrimming coax only fools the meter. Not being > an engineer with millions worthof equipment I can only make a SWAG > (scientific wild ass guess) as to whethercoax length makes a difference in > power out. > >Randy > >W4CPT > > > >-----OriginalMessage----- > >From: [EMAIL PROTECTED]:Repeater- > [EMAIL PROTECTED] On Behalf Of Jeff DePolo > >Sent: Saturday, August 25, 2007 12:30 AM > >To: Repeater-Builder@yahoogroups.com > >Subject: RE:[Repeater-Builder] Duplexers > > > >> When you put the Bird between the TX and theduplexer, you > >> have changed the > >> length of the jumper cable, which upset the tuning. > > > >Adding a wattmeter or any other length of cable between the transmitter > and > >the duplexer Tx input port has no effect on the tuning of the duplexer. > It > >may change the load Z the transmitter sees, which may make the > transmitter > >happier (or sadder) depending on the resulting Z, but in no way does it > >alter the tuning of the duplexer itself. > > > >Adding or removing cable lengths between the transmitter and duplexer > also > >does not change the VSWR as seen by the transmitter (minimal cable loss > >effects notwithstanding). > > > >--- Jeff > > > >----------------------------------------- > >Jeff DePolo - [EMAIL PROTECTED] > >Broadcast Sciences LLC, Valley Forge PA > >v: 610.917.3000 > >f: 610.917.3030 > > > >No virus found in thisincoming message. > >Checked by AVG Free Edition. > >Version: 7.5.484 / Virus Database: 269.12.6 - Release Date: 8/24/2007 > 12:00 AM > > > >No virus found in this outgoing message. > >Checked by AVG Free Edition. > >Version: 7.5.484 / Virus Database: 269.12.6 - Release Date: 8/24/2007 > 12:00 AM > > > > > > > >No virus found in this incoming message. > >Checked by AVG Free Edition. > >Version: 7.5.484 / Virus Database: 269.12.6 - Release Date: 8/24/2007 > 12:00 AM > > > > > >No virus found in this outgoing message. > >Checked by AVG Free Edition. > >Version: 7.5.484 / Virus Database: 269.12.6 - Release Date: 8/24/2007 > 12:00 AM > > > > > Ron Wright, N9EE > 727-376-6575 > MICRO COMPUTER CONCEPTS > Owner 146.64 repeater Tampa Bay, FL > No tone, all are welcome. > > > > > > > > Yahoo! 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