The length of coax doesn't effect impedance. Trimming the coax effects what is read on the VSWR meter because what is actually happening is that there is an interference pattern created when you have a mismatch on the end of feedline. This pattern is sinusoidal and changes in voltage and current along the line, in 1/2 wave periods. 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 you are 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 the reflected power into its heatsink. If you put it at a voltage null, I would suspect that the SWR would not get dissipated by the transmitter as much as 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 not going 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 it in a matching stub, or a matching section ie. if you take 1/4 wave of 75 ohm cable put it on the end of 50 ohm cable you will get a match with a 112.5 ohm load. You make an interesting point though, why does the cabling of duplexer's need to be a certain length. I would suspect that its because they are looped and make an inductor. This then is part of the LC filtering, and changing the length effects L. But I could be wrong on that. Jesse On 8/27/07, R. K. Brumback <[EMAIL PROTECTED]> wrote: > > I have heard this point argued for years. "Does trimming the coax > affect the SWR?" If the length of coax has an affect on impedance, then how > could it not affect power out? We strive to maintain 50 ohms at the tail of > all devices to match the end load. GE puts matching networks in their Mastr > II's. I have taken a MFJ-259 and soldered a PL259 only at one end and then > started trimming the coax down and watched the impedance change > significantly with each cut. Duplexers come with precise lengths of cabling. > I have heard that trimming coax only fools the meter. Not being an engineer > with millions worth of equipment I can only make a SWAG (scientific wild ass > guess) as to whether coax length makes a difference in power out. > > Randy > > W4CPT > > > > -----Original Message----- > *From:* Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] > ups.com] *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 the duplexer, 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] <jd1%40broadsci.com> > Broadcast Sciences LLC, Valley Forge PA > v: 610.917.3000 > f: 610.917.3030 > > > 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 > > >
