Kevin wrote:
The problem is, like any receiver that has several
helicals in cascade before the first active stage,
the loss that precedes the active stage has a majority
role in the overall NF of the system that follows. It
matters little what the quality of the active stage is,
because the loss has already determined (for the most
part) the system Noise Figure.
Skipp wrote:
Kind of makes it look like helicals are a sin doesn't it...
Hamtronics thinks it's a sin - that's why their receivers have some of
the best sensitivity numbers of all of the stuff we commonly build
repeater from. However, their receivers get hammered in high RF density
locations, and the result is the need of some added filtering ahead of
it. This is where the GLB can and does provide an improvement. I speak
with experience here. I have used many Hamtronics receivers and GLB
preselector/preamps (that I bought new) over the years.
If you are lucky enough to have a really clean repeater site, the
Hamtronics receiver line will work fine with no added preselection. I
bought the GLB's primarily for protection to the repeater receiver when
I was using a frequency agile in-band remote base. Many times it was
possible to link to repeaters on the adjacent channel when I had the GLB
in place. The addition of the GLB wasn't without its drawbacks though.
My Hamtronics receiver would hear at -125 dBm for 12 dB SINAD stock with
no preamp or preselection (the first GaAs device is basically
unprotected). Running an in-band remote base anywhere within a MHz or
so of the repeater input would desensitize the Hamtronics repeater
receiver. After installing the GLB, I could link to repeaters 15 kHz
away from my input with very little desense from the remote base
transmitter. Anything over 60 kHz had no affect on repeater receiver
sensitivity. The link beams were 50 to 100 feet below the repeater
antenna and I normally run 5 watts on the Icom 900 stack.
A 2-Meter version I have lots of pictures of has 1 stage of
pre-selection and four trailing stages. The active device is
an MRF-901. And the coupling for this model is a slightly
different method than some (not all) of the other models I
have looked at.
Are you sure that's not 4 section preceding the device and 1 following?
In more than a few real world situations you might really
need the filter pre-selection a lot more than the most
optimum NF. A practical trade of pre-selection for a slightly
higher noise figure can and does sometimes make the difference
in a usable radio system.
No argument here.... If the site noise figure is worse than the NF of
the GLB, adding it won't hurt your actual effective receiver
sensitivity, and the added filtering can be a real boost. However, I'm
blessed with sites that allow me to realize most of the sensitivity from
a good preamp - with not much ahead of it (now that I don't do much RF
linking).
Kevin
