I am certain that if you measure the outside diameter of the rod, the inside
diameter of the body and calculate the impedance of the resulting
transmission line it WILL NOT be 50 ohms. UHF (misnomer) connectors are not
50 ohms. What you are doing is inserting a short length of a non 50 ohm
transmission line in series with your 50 ohm coax. As long as a wavelength
at your operating frequency is much greater than 8 inches no harm no foul
but when you get into the VHF and UHF region those bulkheads introduce a big
(relative to the wavelength) impedance bump. My advice, UHF connectors are
for HF an maybe 6 meters. Above 6 meters everything is N connectors here.
73
Jim, W4KXY
-----Original Message-----
From: vhfcontesting-bounces@contesting.com
[mailto:vhfcontesting-bounces@contesting.com] On Behalf Of
k2kw-8@adelphia.net
Sent: Sunday, January 14, 2007 3:19 PM
To: k8bb@comcast.net
Cc: vhfcontesting@contesting.com
Subject: Re: [VHFcontesting] impedence of 8" long bulkhead @ VHF/UHF?
Any speculation on the cause of the SWR problems? Could the metal used be a
problem? E.g. stainless steel is used in many antennas (and touted for it's
strength), but it's a horrible conductor and is worthless IMHO for antennas.
Kenny K2KW
---- k8bb@comcast.net wrote:
> 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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> VHFcontesting@contesting.com
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