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From: Pat Barthelow[SMTP:aa6eg@cv.tmx.com]
Sent: Tuesday, July 07, 1998 9:50 AM
To: Pete Brunet
Cc: towertalk@contesting.com
Subject: Re: [TowerTalk] N connector impedance
Pat,
I have been using the "banana peel" method of attaching a UHF barrel to 3/4
inch 75 ohm CATV cable for about 25 years now. I started by simply taping over
the splice and putting a stainless hose clamp over the top of the tape. It
worked fine for years, but eventually water did penetrate ( I suspect after uV
broke down the tape). I next tried penetrox and boy was that a disaster! It
seems that the penetrox made a path between the center conductor and the
shield. Eventually with 1500 watts, the splices shorted out an caused no end
of problems. I am now using straight silicon grease and so far after a couple
years, NO more problems.
73/Mike N7ML
HI Peter,
You did not mention the frequency you were operating the cable
at....However, in most cases, it still won't matter. Common antenna loads
found in amateur installations, can vary somewhat, even from a design
impenance of 50 ohms, simply from interaction with their environment.
Height above ground, and distance from nearby objects all come into play.
In any event, a 50 ohm load to 75 ohm surge (or characteristic) impedance
of the attached coax, will cause no problems in most situations. The
resulting 1.5 to 1 SWR is usually quite acceptable. At VHF
or UHF, or higher frequencies, with small, lossy cable, or, with extremely
high power, some concerns may
develop, due to extra losses caused by the reflected waves due to the
mismatch, or by very high voltages, and currents found at different
locations along the
coax. These probably pale in comparison to the grief caused by mixing 75
ohm. and 50 ohm N connectors at the ends of the transmission line. Look
at your local cable company for 75 ohm N fittings, and notice the REALLY
NEEDLE FINE points of the center conductor. It will not mate correctly
with the standard 50 ohm fittings that it might encounter. A 50 ohm male
N fitting encountering a 75 ohm N-chassis fitting, will look like it OD'd
on
VIAGRA! It is probably not worth the trouble to seek and use the 75 ohm
fittings.
We, at N6IJ, have recently found a quick, simple termination to
our 75 ohm surplus CATV coax
(.75") used in our longer runs at the site. Simply hacksaw the aluminum
jacket, in quarters, about 1" along the axis of the coax, and use an Xacto
knife and Needle Nose pliers to tear out the foam dielectric, abt 3/4"
deep into the clean cut end, and insert a Barrel UHF fitting, hose
clamping the jacket tightly to the barrrel, and seal with tape. Use
NoAlox between the barrel and the inside of the jacket....Works like a
champ!
73, DX de Pat, AA6EG/N6IJ
"The Contest Station from the Government"
Marina Amateur Radio Contest Station; N6IJ
599 DX Drive
Marina CA 93933
aa6eg@tmx.com
On Tue, 7 Jul 1998, Pete Brunet wrote:
>
> Hi, I want to know whether to use 50 or 75 ohm N connectors on some hardline.
> First some background might be helpful.
>
> As part of building our antenna system we are using some existing 3/4 inch
> hardline that was previously used in a satellite system many years ago. I
> assume this was a receiver system, but it is possible that it could have been
> used for transmission too. We assumed this is 75 ohm cable, however using a
> TDR shows a value close to 50 ohms and using a micrometer and the formula Zo =
> 60*ln(D/d) / e^.5 where D=.675 and d=.165 gives 51 ohms. The location of our
> radio room is not the same as the satellite room so we had a contractor run
> cable over to existing hardline and patch into it. This new cable is 75 ohm.
> (I wonder how much of a problem we are going to have if the original cable is
> 50 ohm?)
>
> I assume we need 75 ohm N connectors at the radio end of this new cable. Is
> that a correct assumption?
>
> Thanks, Pete
>
> =====
> Pete Brunet -- IBM Special Needs Systems, 11400 Burnet Road, MS 9448, Austin,
> TX 78758
> (512) 838-4594 voice, (512) 838-9367 fax, brunet@us.ibm.com, ws4g
>
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