[TowerTalk] 50 Ohm/70 Ohm Mismatch

[Jessica Benny]

For another description and photos for making these CATV connectors, see
May, 1992 QST under Hints & Kinks, or the ARRL UHF?Microwave Projects
Manual.

Ray, N6VR

Roderick M. Fitz-Randolph wrote:

> 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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