Addendum to "Antenna Feed-Line Project"

or

"Trouble with Bulkheads!"

I recently had some SWR problems using a VHF/UHF FM rig in my shack and an antenna on my house. It turns out that these bulkhead connectors that I use as feed-throughs for my feedlines have impedence problems above HF. Below are two email messages to the VHF-Contest Reflector outlining the situation and the conclusion.

My original question to the VHF-Contesting reflector at contesting.com:

----- Original Message -----
From: <k8bb@comcast.net>
To: <vhfcontesting@contesting.com>
Sent: Monday, January 01, 2007 23:13
Subject: [VHFcontesting] impedence 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

My conclusion and answer:

> 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 seen at:
> http://home.comcast.net/~k8bb/bulkhead.htm
>
> 73!
> Don Chisholm
> K8BB

This is what a 10" bulkhead looks like after being cut open with a tubing cutter: