Paul Finch wrote:
> Nate and group,
>
> Sorry, I know this will get me flamed but I am getting used to it. A
> isolator does not necessarily make a good installation. A good bandpass
> High Q cavity/BpBr duplexer in most cases will do everything you need done
> without the added headaches of a isolator, especially in high power
> installations.
Don't know why it would get you flamed, it's just a discussion. ;-)
> I learned this from Lloyd Alcorn, one of the owners of Wacom Products and
> also had a long history with DB Products. Unless you are in a extremely
> high RF area you are better off not using a isolator. You must use a cavity
> after the isolator anyway so try it without the isolator and if you still
> have problems you can install a isolator. Nothing lost.
Does having TV 200 yards away, and approximately 15 other transmitters
on the same tower structure as your repeater, another tower 100 yards
away with another 5 or so transmitters, all bands and types of
emissions, and at least 150 miles of line-of-sight coverage 20 miles
away from Downtown Denver count as a "high RF area"? :-)
> The problems with isolators is they drift with age and heat, especially on
> high powered installations like the paging systems I used to work on. We
> never used a isolator unless we were on a Motorola site that required it,
> which is a bad thing. Isolators are great harmonic generators to boot,
> hence the requirement of a good cavity.
Never seen a combiner system made with isolators and mixers have a TX
cavity beyond it on the antenna side... but I guess it could be done?
Of course, usually the systems feeding such a combiner have TX cavities
on them of some sort... not always, though.
But... I'm just an Amateur. And I haven't seen much of anything. I
know there's pros on this list. (Speak up, pros.) Just following in
much larger footprints.
> Again, just giving my opinion from my experience and mostly from Lloyd's,
> he's the smart one!
Heh... I know the feeling.
The current setup at the site that might (or might not) be eating MASTR
II VHF PA's is :
TX -> Pass-cavity (tuned for 1dB insertion loss just to be a "good
neighbor") -> Dual-stage isolator -> Mixer -> Dual-stage isolator ->
Mixer -> TX antenna.
The isolators and mixers are part of a commercially built TX combiner
system. We added the pass cavity in the fall. Lost a second VHF MASTR
II PA up there (one had the built in little Z-matcher and one doesn't)
after the snow fell and the site became mostly inaccessible.
Four total inputs, mixed together in two pairs of TX's, then into the
final mixer. All VHF Amateur band.
Lots of power loss for everyone, 100w in = 25w (6 dB loss) out to the
antenna, but we all don't bother each others transmitters or take up
hideous amounts of tower space either, which is a good thing (TM). 0
The site is 5000' above average terrain toward town, so coverage isn't
much of a problem, and high power out the antenna really isn't necessary
either.
(Heck, I can still work the repeater right now on what's squeaking
through the dead PA from the exciter... it works amazingly well
considerng there's probably milliwatts reaching the antenna up there
right now.)
Receive is a similar setup.
TX and RX antennas are 8-bay Sinclairs mounted vertically with receive
on top. Not a huge amount of vertical separation (the tower's about 80'
high), but some.
The basic goal years ago, was that Amateurs are all sharing the VHF
antennas at the site. It works well when everyone's on the combiner
system. Various experiments over the years pulling any of the VHF
machines off the combiner always result in more problems/headaches than
it's worth up there... in my opinion anyway.
Looking through the diagram, some people want to try a Z-matcher between
the PA and the cavity. Others say it's simply not necessary. One of
the PA's had one, and it didn't seem to help much, but the cavity wasn't
in place when that one died.
EVERYONE says "go measure things!"... of course.
And... we can't get there right now anyway.
So it makes for interesting e-mail fodder and discussion... until we
either get bold and deal with some type of risky or odd transportation
in the snow to the site, or wait until things melt a bit in the Spring!
Good thing I put new shocks on the Jeep last Summer... looks like this
coming Summer is going to give them a workout!
Having fun learning, as always...
Nate WY0X
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