> Your math looks sound, Jeff, except I thought 1/2" Heliax
> wasn't quite so
> lossy @ 450 MHz (~1.5 dB/100 ft IIRC?).
> >Scenario 1: Preamp = 0.5 dB NF, antenna feeding 200' of
> 1/2" Heliax (3
> >dB loss @ 450 MHz @ 1:1 VSWR)
1.5 dB per 100 ft versus 3 dB for 200'. I think we're in agreement on this
one... :-)
> Realistically
> though, not many
> people connect the input of their preamp directly to 200 ft. of
> line.
When I wrote up the scenario I was thinking of a typical weak-signal home
station. Joe Ham's weak signal station at home could easily have 3 dB of
feedline loss. Joe Ham isn't running inch and five-eigths up their tower.
30 feet or so in the house/shack, maybe 100' buried between the house and
the tower, 70 feet up the tower, probably not an unreasonable scenario.
Without knowing, or having a means of measuring, return loss of the preamp,
one might assume that the best preamp for their weak signal station is the
one with the lowest advertised noise figure, and I think the math shows that
is not always the case. Which is why I originally asked the question - is
it really fair to judge the merits of two preamps based soley on noise
figure without knowing how good of a match they present? My answer would be
no.
> Usually there's a filter ahead of it that has loss &
> can be tuned to
> somewhat tune out the mismatch.
For a repeater installation, maybe, but not for weak signal work where you
typically don't have anything between the antenna and preamp except for
feedline to avoid any additional losses that would directly impact the NF.
Even if you purposely added a filter or some other matching device to the
system, you're then adding more losses.
In the repeater case, without test equipment to try to match the filter to
the preamp input, most repeater techs (including those that do it for a
living) don't have a means of looking at return loss except at transmitter
power levels. A typical repeater owner just buys the preamp and installs it
before the receiver. Sure, it will likely make the receiver more sensitive,
but it could be better if the filter wasn't being detuned due to the
mismatch. And back around we come to my original premise - if the preamp
had a better match even at the expense of slightly increased NF, the
resulting NF could be improved.
> All this sort of muddies the
> waters a bit,
> but theoretically in your example a preamp with a matched
> input would in
> fact yield lower system NF given the same NF specs, due to additional
> induced feedline loss ahead of the preamp as a result of the
> mismatch, not
> due to the reduced power transfer between the feedline & preamp.
OK, I agree there that the NF spec of the preamp is irrespective of the
mismatch, but the system NF will vary as a function of loss increases due to
mismatch.
I'd like to get my hands on a modern NF meter and, combined with VNA
measurements, compare a few off-the-shelf preamps. One of these days...
--- Jeff
