> "Harold Farrenkopf" <[EMAIL PROTECTED]> wrote:
> I was refering to the resonant frequency of just one loop and
> cable out of the can behaving like a quarter wave shorted stub
> (or 3/4 wave) for determining the cable length.
No problama, for the most part, that's how many/most people
select the cable lengths. If one were able to look at the
entire cable - cable length - star frequency response over a
very wide spectral display... you'd notice the entire star -
cable network loss is significant - skewed or tilted higher
at the higher side/end of the actual graph/display.
In short, the best performance of the network appears to be
with the cable lengths slightly shorter than what you would
think or see without the wideband spectral view or return loss
display. It cam end up being only a fraction of a dB in many
cases, but the loss can add up fast in >4-leg combiner networks
with even modest improper length cables. This problem seems
to be found more often in tx combiners with fairly wide
frequency spacings.
Add in the crappy square box "star" from Telewave and you've
got what I would call excessive unwanted & unneeded tx
combiner network loss.
> 4 dB is high for a single can system isolator type system but
> it may be better than using a hybrid since it does give a pass
> response for TX noise.
I like to beat up some of the tx combiner mfgrs at IWCE about
the actual resultant loss of most tx combiners. But we all
end up on the same "there is no free lunch" page. The actual
results in many cases seem to be an as-built project. When
someone tries to deviate from even the most generic norm, you
can quickly get into trouble as seen with the previous mentioned
South American VHF Tx Combiner System (Alex). The out of the
box tx combiner doesn't work right and there is/was no obvious
reason why... unless you pay attention to the details.
I like the newer TX/RX and Sinclair non critical length tx
combiners, but they are really not that much different in
the loss department. Plus you don't find their bottles (with
the dual jack hybrid ports (what ever they want to call them)
on ebay nearly as often. :-(
> My preference now would be 2 to 3 pass can's plus Q cans
> per frequency into the star so that the isolator doesn't
> see much of the other close frequency energy to cause IM
> problems.
> Harold
Yes, it would be nice... but with our good friends at American
Tower and simmilar "well run companies" charging "very modest"
per rack space & antenna system hardware mounting prices (per
month)... sometimes evil cost vs performance trades have to be
made. With extra rack mounted bottles you simply translate the
combiner system dB signal loss to the cost of being at an AT
site. What's in your walet would be >2dB down... :-(
The most common tx combiner is the single bottle to hub/star
type. The larger the bottle (in most cases), the better (higher
Q). And it all echos back first to the actual frequencies used
and their relative spacing. Then on to what you're willing to
live with. And as always mentioned, your results will probably
vary.
cheers,
skipp
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