UHF connectors are not 50 ohms. In cases where the length is small in terms of
wavelengths the error is not significant. Hence a single connector won't
matter, but the long bulkhead is too long. The highest frequency that I will
use UHF connectors is 6 meters.
If you would like to calculate the impedance, the simplified formula from my
1943 edition of Fred Terman's Radio Engineers handbook is:
log c/b
Z = 138 -----------------------
square root of E
Where c = radius of the inside of the shield
b = radius of the inner conductor
E = dielectric constant of the insulating material.
You might be able to modify the feedthroughs by substituting a different
dielectric for the insulator, or shaving off some of the plastic from the
existing insulation.
Most feedthroughs that I've seen use polystyrene insulation. E=2.3
Teflon E is 2.1, plexiglas E is 2.8 Air is 1 by definition. Foam dielectric
from a piece of 1/2" hardline might be a good bet, the E is probably in the
low to mid 1's.
73, Fred K2TR
k8bb@comcast.net wrote: Hello,
I have searched the archives for this, but I do not see an answer.
My Question:
What is the reason that I show *very weird* SWR curves at VHF/UHF when
feeding through UHF bulkhead connectors that are 8" ( and/or 4") long when
measured into a known 50 Ohm load good through VHF.
My Situation:
My feedlines enter my house/shack through two bulkhead "panels" connected
with 9913 equiv. between. You can see pictures and a "story" at this web
page http://home.comcast.net/~k8bb/ant.htm which explains why I have this
arrangement and how I went about installing it. Through process of
elimination, I have determined that somewhere between the shack panel and
the outside panel there is occurring what I think is some sort of impedance
mismatch that is causing high and very inconsistent SWR at VHF and above.
My Suspicion:
I think that the bulkhead connectors that are feeding through the panels
(probably the 8" ones?) are not a consistent 50 Ohms throughout and are
causing enough of a "bump" in the system impedance to cause bad SWR readings
on 2m and 70cm. The 4" (UG-363/4INCH) and 8" (UG-363/8INCH) bulkheads were
purchased through The RF Connection: http://www.therfc.com/uhf.htm
Maybe Not The Coax?:
The coax that connects them is 9913 equivalent (JSC Wire and Cable Type
#3500) from a local CB shop. It is not exactly Belden or Times, but I have a
lot of experience with coax and connectors and the stuff "looked and felt"
quite good. (In high school, I worked in a local CB/HAM shop and installed
HUNDREDS of PL-259 connectors on countless types of coaxial cable. I
developed a pretty good sense of what good cable and good connectors were
all about. The connectors are all Amphenol with Teflon.)
Eh? Ideas? Suggestions?
Thanks for reading ... Happy New Year
Don Chisholm K8BB
Pontiac, MI
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