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]:[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]:[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.
