Basically, your coax, straight or mixed, is just like a hose - the
"gozinta" minus "friction" equals the "gozouta," and the various
impediments are just part of the whole.
The mismatch of VF has no more effect than the SWR - it all comes out the
other end, even though some of it takes a lot of trips back and forth to
get there, losing only what is often called the "copper loss," but is
really the loss characteristic of the design of each individual cable.
Whatever power level arrives at the junction of the two coaxes moves into
the next coax en toto, at a somewhat different SWR and therefore a new
reflection point, but the aggregate process continues. Sure, higher SWR
in the line causes greater loss due to the many trips, but it really
needs to get bad (greater than 5:1) to show.
Yours could be a complex calculator exercise, or a simple dummy load and
watt meter project. Use the pair at the input point of the whole line and
at the output of same, and compare the difference(loss) to the advertised
loss of each, using simple arithmetic to estimate the power input to coax
Allowing for a bit of connector loss, and assuming no defects in the
line, I would guess that, at 40 meters, the improvement from changing the
whole line out for brand new LMR400 would be darn near impossible to hear
or appreciate on the air(about 0.3db in the 160 ft).
BTW, the meter/dummy load technique is good protocol for ANY feedline
installation, BEFORE connection to the antenna. A simple note in the log
of the results to compare new data to some years hence can easily
eliminate the feedline as a suspected culprit on a troubleshooting
Press Jones, N8UG, The Wireman, Inc., Landrum, SC, 29356
Sales(800)727-WIRE(9473) or email@example.com.
Tech help (864)895-4195 or firstname.lastname@example.org
Our 21st year!
On Fri, 26 Jun 1998 18:06:05 -0700 (PDT) email@example.com writes:
>After reading the several emails on the coax subject I
>wonder if you could comment on a setup of mine.
>I have a one year old 100 foot length of RG8 coupled to a
>60 foot length of LMR400 coax using a regular pl259 and a
>I know that the velocity factor is a bit different between
>the two but is is enough to cause a problem.
>The antenna is a Gladiator Vertical monobander for 40
>meters that is mounted about 7 feet above ground and uses
>four quarter wave radials as a ground plane.
>It seems to work OK but I have always been a bit concerned
>about using the two different types of coax for this setup.
>Dan Keefe - KS6Z FISTS Number 3656
>45 Miles North of San Diego - 8 miles East of the Pacific
>Please visit the K6FFF website at:
>also visit K7FFF at:
>When the student is ready, the teacher will appear!
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