[VHFcontesting] impedence of 8" long bulkhead @ VHF/UHF?

[Robert Cumming]

Don

There are all kinds of "bulkhead"  UHF 
connectors.  I've used the Short variety (83-1F) 
made by Amphenol with good results up to 2M but 
I've heard nothing but horror stories about the 
longer variety on VHF and above.  The quality of 
the connectors from a lot of sources is poor and, 
while ok on HF don't stand up the the demands of VHF+.

Also you stated the cable is 9913 equivalent (I'm 
assuming the Air filled type not 9913F or9913F& 
foam.  Looking at the pictures on you web site I 
don't see any waterproofing on the 
connectors.  For 9913 this is asking for trouble 
especially in a humid environment like a crawl 
space.  I have successfully used 9913 on 50, 144, 
222 and even 432 here in the Hot, Humid Florida 
atmosphere for the past 15 + years but, not without high quality "Coax Seal".

A few observations from Hot, Humid Central Florida.

9913 can work if properly installed and 
WATERPROOFED.   I have used a few runs here in
the humidity of central Florida for over 15 years with no water intrusion.

The outside connection must be sealed with a good 
grade of Coax seal or Butyl Rubber tape and I use 
at least 2 layers of 2" plastic tape followed by 
3 layers of 3/4" of good electrical tape (like
Scotch 33, 88 or 700).  I apply the coax seal 
directly to the connector and the coax then three
layers of tape  extending at least 1" past the 
Coax seal.  I leave the inside end unsealed  in
Air conditioned space so any air entering the 
cable (and it will due to expansion during the
heat of the day and contraction during the cooler 
night) will be dry air.  If both ends are exposed
to the weather it will draw in humidity at night 
or during rainstorms.  Store the cable in "Air
Conditioned" space prior to installation to 
minimize the humidity within the cable.

For runs of 100 feet or less use 9913F7 or LMR400 
flex. and forget about the jumper around the
rotor.  If you want to run 9913 or LMR400 and 
have a mast mounted preamp - mount the preamp
under the rotor and run the flex jumper from the 
preamp to the antenna .  Connectors sized for the
9913 inner connector work fine with LMR 400. 
Another great alternative is 1/2" Andrews
superflex ( I have used this stuff on 2304 for a 70' run).

For runs over 100 feet look for either LDF 4-50 
or LDF 5-50 at area swap meets and
hamfests.   You usually can find the connectors 
at the same place or watch e-bay for connector
bargins.   Another alternative is 75 ohm CATV 
hardline (only 1.5:1 SWR)  Connectors are easy to fabricate.

Again on waterproofing, even "N" connectors will 
draw water into the connector over time when not 
sealed with coax seal - the center chamber in the 
connector has some air space that expands and 
contracts with temperature changes.  I follow the 
Andrews guidelines packed with their waterproofing kits:

Installers should become thoroughly familiar with 
and use the installation tips given here.
Installation Tips
• When applied, the tape must be above 32°F (0°C) 
to ensure adhesion. Keep tape warm by carrying in coat pockets.
• Do not stretch the tape. Apply only enough tension to provide a smooth wrap.
• Smooth each wrapped layer with your hands to ensure full adhesion.
• Do not pull the tape to tear tape ­ always cut 
it.  (Pulled tape eventually unravels, decreasing  protection.)
• Add extra final layers of tape in warmer 
climates where there will be long exposure to 
damaging ultra violet (UV) rays. Twoor three 
extra layers of tape will provide additional UV protection.
• When wrapping tape, overlap the tape to half-width.
• Ensure vent or drain holes at the bottom of the 
antenna or device are not covered with weatherproofing tape.

If you can find them at a hamfest the Andrews 
water proofing kits are a great value - Andrew
221213 Weatherproofing Kit contains:
2 rolls of 3/4" x 66' black plastic tape
1 roll of 2" x 20' black plastic tape
6 rolls of 2-1/2" x 24" butyl rubber tape (coax seal)


Vry 73
Bob Cumming
W2BZY
QRV 160M-9 CM +3CM
from EL98hr


    At 06:13 PM 1/1/2007, k8bb at comcast.net wrote: 
At 06:13 PM 1/1/2007, k8bb at 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
>
>
>
>
>
>
>_______________________________________________
>VHFcontesting mailing list
>VHFcontesting at contesting.com
>http://lists.contesting.com/mailman/listinfo/vhfcontesting