On Oct 19, 2007, at 3:30 PM, [EMAIL PROTECTED] wrote:
> There is an additional factor that can cause deterioration of
> repeater coverage when PA power
> is significantly increased. It's broadband noise. Increasing PA
> power increases the intensity
> and coverage of the induction field which risks stimulating close
> aboard rusty joints and even
> dissimilar mental joints into behaving like broadband generators
> which will end up as noise on
> your own receiver channel. It doesn't help that it also ends up on
> every other receiver channel
> at the site. The potential consequence is to turn the repeater into
> an alligator. And adding
> additional cavities to the receiver and/or the transmitter is an
> exercise in futility because the
> junk you experience is dead on-channel. Sure, some sites may be
> well enough maintained
> to preclude this result, but the maintenance at my site has dropped
> to just about zero in
> recent years and cranking up power would produce a cure that's
> worse than the disease.
>
> Bruce K7IJ
Additional to this comment, if there's any passive or active IM mixes
at the site, cranking up the power on sites where there are any other
repeaters with the same offset (e.g. other hams) can create serious
headaches if there are also very strong user input signals present.
More power from one repeater adding to a standard mix of...
Your input
+/- Your output
= Negative/Positive offset
+/- Their output
= Their input
(Flip the signs as needed.)
Here's an example just sticking in two local repeaters that are
experiencing a problem when high-level input signals hit one of
them... just as an example, but any frequencies will work...
442.575 input
-447.575 output
--------
- 5.000 MHz
+449.350 output
--------
444.350 input
In the local case, something at the site is actively amplifying the
442.575 input -- it takes very little power from very far away to get
the mix started. The two repeaters have to be on-air at the same
time, of course, because the outputs are part of any mix like this.
Gotta go find it. We can get it to happen with another 447.xxx
repeater's input at the site, also, and there's relatively new
reports of two 449.xxx repeaters bothering each other, also.
This annoying first order stuff can create some bad juju, and it can
happen with ANY group of repeaters running the same offset... VHF
+-600 KHz splits, UHF +-5 MHz splits...
Add in someone running without an isolator and/or some other active
(versus passive) junction/amplification of the input signal... we're
not sure yet. (A 10W signal into a 10dB Yagi from almost 20 miles
away is enough to start it. Lovely, eh?)
Passive IM + Big Power = Big bad mess for all, not just you.
The joy of hunting mixes and mixing sources. (Sigh...)
--
Nate Duehr, WY0X
[EMAIL PROTECTED]
