> "Steve Bosshard (NU5D)" <[EMAIL PROTECTED]> wrote:
>
> There is a formula in most hand books where the
> characteristic impedance can be determined from
> the inner diameter of the shield and the outer diameter
> of the center conductor.
There is a quick fudge method to determine yes/no
50 or 75 ohm line (if the line is good).
A good 50 ohm line properly terminated will have an
SWR of 1.1:1 (one to one or "flat"). If the line is
75 ohm line, the SWR will rise to about 1.4:1 or 1.5:1
when driven with a 50 ohm source and terminated into
50 ohms. The above is only a relative check/test
when the line is assumed to be good.
Mini Circuits makes a bidirectional 50 to 75 xmfr, which
can be used along with 75 ohm terminations to verify
the 75 ohm segment. If you were using the 75 ohm line
for receive only applications, you could just keep the
Mini Circuits 50/75 xmfr in line.
If you wanted to put rf power into the 75 ohm line, you'd
need to make a matching network. Not hard really, but it's
much more "frequency range" or band dependent.
> I have built many makeshift connectors using an UHF
> barrel, PL258, and slotting the outer conductor of
> the cable with a hacksaw, then driving the barrel over
> the center, applying passivating compound, and using a
> hose clamp to secure the shield to the barrel connector.
> I have used UHF barrels on 1/2" and 3/4" lines with good
> success. Also there is not a whole lot of difference
> between 50 and 75 ohm cable.
I've done similar with a trip to the better local owned
hardware store plumbing section where a number of copper
pipe adapters can be used to adapt different sizes.
Using a 75 ohm line on a 50 ohm system often means an
instant 1.4:1 to 1.5:1 SWR. Relative to the grand scheme
of things... some people can and do live it the mismatch
quite well.
cheers,
skipp
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