Well, here is the conclusion: The bulkhead connector(s) I am using are
causing the SWR problems at VHF and UHF.
While visiting his QTH this past weekend, Tim Duffy, K3LR, was kind enough
to check an assembly (bulkhead-jumper-bulkhead) on an HP Network Analyzer.
Sure enough, the bulkhead connector(s) do not behave above HF. At 10m and
below, they exhibited good impedance and SWR. However, above 30MHz, the
bulkhead connectors were unstable and clearly were the cause of my weird SWR
readings. The 10" connector was worse than the 4" connector, but both were
totally unacceptable. (The jumper itself was fine.)
Next, I cut one open. I was pleasantly surprised to find them "physically
competent" ... I think. While I was not sure what to expect, this is what I
found:
- The center conductor is a heavy-duty, solid rod stock and consistent
throughout the entire length of the 10" bulkhead - no thin wire or coil
spring. (I once had a bad experience with a 90degree connector.)
- The "dielectric" was present only at each end of the connector, where the
center pin was received at each end - it was air dielectric throughout the
entire middle.
I have no question that these will be fine (reliable) at HF with 1500 watts.
Thanks to the many responses I received from this list. Each part helps to
put together the whole in understanding the solution when troubleshooting a
problem. Special thanks to Tim Duffy for his time and attention and to the
other individuals who offered to do the same.
I took some digital photos of the cut-open connector that can be see at:
http://home.comcast.net/~k8bb/bulkhead.htm
73!
Don Chisholm
K8BB
----- Original Message -----
From: <k8bb@comcast.net>
To: <vhfcontesting@contesting.com>
Sent: Monday, January 01, 2007 23:13
Subject: [VHFcontesting] impedance of 8" long bulkhead @ VHF/UHF?
> 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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