[TowerTalk] 50 Ohm/70 Ohm Mismatch

[Roderick M. Fitz-Randolph]

Greg Kopp, KG8O wrote:
>
> In response to a question I posed about 1/2" hardline, I have been given
> the suggestion to look at local cable companies for 75 ohm hardline.
>
> What I need to know, is for a run of 200 feet plus, how can I make sure
> that I match the cable to 50 ohms at either end? Are N connectors readily
> available for this kind of hard line?
>
> Please pardon my ignorance here, this is the first time I have tried
> anything like this.
>
> 73 de Greg KG8O
__________________________________________________________________________
Your run is almost perfect for the 50 ohm - 70 ohm transformation.

The velocity factor of CATV foam filled aluminum exterior coax is 0.81.

If you want it to be half waves or multiples there of at 1.775, 3.550, 7.100,
14.200, 21.300, and 28.400 MHz, use the formula:

   984/1.775 X .81 and it will give you the answer of 554.366 feet.

That is the length of a full wave length at 1.775 MHz.

Half that for a halfwave and you get 277.183 feet.

Multiply that by the velocity factor of the coax (.81) and you get

277.183 X .81 = 224.5 feet!

If you make your CATV aluminum sheathed coax 224.5 feet long it will
act like a linear transformer and if you feed it with 50 ohms, you
will get 50 ohms out of it at the other end on the frequencies listed
above.  The minor difference between 1.775 and 1.8-2.0 MHz is minuscule
and it will be great for all the other bands!!!!  Dunno about WARC.
Haven't done any calculations for them.

Now for the next problem: connectors.  Go to your local Lowes Store (or
Home Depot or whatever) and look for a 1/2" to 3/8" pipe coupling.  It
is made by Anderson-Barrows.  Its Anderson-Barrows designation is U50.
The description on the outside of the package says:

   Tubing to Female Pipe Coupling, Part No. BP966-P, 1/2" x 3/8"

Remove the inner (loose) tubing and discard it.  Place the sleeve end
over the 1/2" aluminum tube and move it back a ways.  Take a pipe cutter
and cut the aluminum at 7/8" back from the end and then (by taking a
utility knife and cutting through the shield along the axis of the coax,
using care to not cut oneself) remove the 7/8" aluminum shield and cut
away the foam from the center conductor while being careful to not scrape
off the copper plating of the center conductor.  When you have accomplished
that, you are ready for the next part.

Now take the previously-obtained Amphenol barrel connector (designation
83-1J... that's important), carefully examine both ends. On one end you'll
see a small c-ring insert.  Taking a narrow-bladed hacksaw, cut the end of
the barrel connector along the axis of the barrel connector so that the
blade bisects and cuts the middle of the c-ring insert.

Voila!  The insides now fall out!  You'll find you have two plastic inserts
and a center conductor.  The barrel center conductor will fit over the end
of the coax center conductor.  Solder it to the coax center conductor so
that the closest part of the barrel's center conductor from the aluminum
jacket on the coax is 7/16"

Insert one of the plastic inserts back into the barrel so that the end that
was not cut will capture the insert. Take the end of the barrel that was cut
by the hacksaw and screw it into the threaded end of the U50 coupler.  Now
take the U50 outer section and (after first putting Penetrox A around the
outer portion of the aluminum jacket from the edge to about 3/4" back)
slide the sleeve portion up and screw the outer portion into it.  As it is
being screwed together, two things happen:

(1)  The compression fitting clamps down on the aluminum sleeved coax.

(2)  The end of the barrel's center conductor will come right up snug with
     the end of the barrel.

You are now an expert!  You can go fit the other end with much greater ease!

It works great.  Be sure and waterproof it with Liquid Electrical Tape or
Scotch 33 or Scotch 88 plus some coax seal on top of that, plus another layer
of Scotch 33 or Scotch 88 on top of that and you have yourself one Hell of
a connection!

Good luck to you.  I am using this scheme between my shack and my tower base
which is 225 feet away.  It works great and has for 6 years now.

Rod, N5HV
w5hvv@aeneas.net



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