>
> On Tue, 25 Aug 2009 14:59:27 -0400, Tom Haavisto wrote:
>
> >http://www.k1ttt.net/technote/matching.html
>
> >Shows how to match 50 to 75 ohm cable
>
> Not only is this complicated and frequency sensitive (it only covers
> a single band), it is un-necessary.
Whether it's complicated and un-necessary is really a matter of opinion.
Granted, the mismatch loss is insignificant, and if you have an autotuner or
manually tuned amp that you use
all the time anyway, just use 75 ohm cable and let 'er rip. Since the
original post is about "high power" it is
very likely that this is totally appropriate.
But the series section matching technique is a very easy way to match 50
ohms to 75 ohms for people who
want to do that.
With a 50 ohm load, the mismatch on a 75 ohm line can be up to 2.25:1, and
that is sufficient to cause
my barefoot, auto-tuner-less radio to fold back. The problem will get worse
if the load is 35 ohms or 40 ohms, where you'd just
casually use 50 ohm cable, but end up with pretty bad SWR at the radio with
75 ohm cable. I had this problem when I fed a 40m groundplane with RG-11
once. I didn't know about these very useful transformers back then, and
built a UNUN to match up to 75 ohms (and *THEN* I let 'er rip and didn't
worry about the worst case 1.5:1 that I saw) I use a pair of these
designed for 16 MHz (the mean of 14 & 18.1) on either end of a length of
RG-6 to match in and out of my 17/20 m antenna.
While it doesn't affect a perfect match to the 75 ohm line at any frequency
on either band, it does so enough that a random length of 75 ohm cable
will not end up presenting a fold-back-worthy SWR at the shack end, even if
the antenna is like 40 ohms instead of 50.
In the context of the original post, sure, just run it straight to the amp
and load into whatever the impedance is. But if you've got some SWR
sensitive
application without an autotuner available, this can be an awfully
convenient and cheap way to make sure you don't have to worry about the
length of
your 75 ohm line. Even better is a conversion done in two steps. I built
one of those and use it from 5-21 MHz .
73
Dan
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