At 06:45 AM 08/23/09, you wrote:
I agree with Kevin. You need a little headroom built in for
conditions that could change. Equipment ages and changes it's
operating characteristics. Temperature swings cause the same issues.
Does one need to go overboard? Probably not. But if you happen to be
right on the edge under perfect conditions, you may be unhappy when
something moves a bit out of tolerance.
Chuck
WB2EDV
Or when you want to add a preamp to the system.
From the Antenna Systems page at repeater-builder (under "System Engineering")
>In most repeaters the duplexer provides a certain amount of isolation
>between the receiver and the transmitter (some systems, like those
>that use two antennas, or even two sites, don't use duplexers). If the
>amount of isolation, however it is acquired, is greater than what is
>required (the excess is sometimes referred to as "headroom"), then the
>system design is adequate for the job (see the article Some thoughts
>on Repeater Receiver-to-Transmitter Isolation below). That situation
>is fine until they decide to add a preamp to help out the handheld
>users. Then they discover that the amount of isolation isn't enough.
>They forgot that you need (at least) the same amount of extra
>isolation ("headroom") as the amount of gain the preamp provides,
>since it raises the apparent noise floor as well as the signal of
>interest. In most cases you will have to fight with desense when you
>add a preamp (a top-quality preamp like an AngleLinear will help).
>Always have enough extra headroom in your receiver, transmitter and
>duplexer to handle any of a couple of situations: First, the site
>owner adds additional repeaters to the site, or second, that you want
>to add a preamp later on. If the duplexer is your primary provider of
>receiver-to-transmitter isolation do not scrimp on the duplexer. Next
>to a good antenna and feedline the duplexer is the most critical part
>of a good repeater system. Long ago I gave up on four-cavity duplexers
>(two cavities on each side) on VHF/2m, 222 MHz and UHF, I use the six
>cavity pass/reject type exclusively. Duplexer tuning is very, very
>critical. A return loss bridge is preferred, a spectrum analyzer with
>a tracking generator is the second choice. And don't tune the duplexer
>on the ground, then transport it to the site over a bumpy
>four-wheel-drive road, and expect it to be as precisely tuned when you
>get there. Always have the test gear with you at the site to verify
>final tuning after mounting it in the system rack.
Another situation is when the radio site landlord added another tenant - and
he installs a 330w paging transmitter.
Been there, had that happen.
A reasonable amount of extra headroom is always a good thing.
Mike WA6ILQ
