One can also use 1/4- and 1/2- λ stubs of coaxial cable of the same
impedance as matching networks in conjunction with a "Tee" connector.
"Shorting" or "Opening" the end of the matching stub also makes a
difference, based upon the length being employed.
I believe that VSWR is *one* reason that cabling on a duplexer must be a
function of length, based upon a fraction of the characteristic frequency.
But not being a "duplexer person" I'm not all that familiar with the precise
engineering behind their interconnecting cabling.
Mark - N9WYS
________________________________________
From: Repeater-Builder@yahoogroups.com On Behalf Of Jesse Lloyd
--- snip ---
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 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
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