On Thu, 29 Sep 2005 13:35:17 GMT, "Asimov"
wrote:

Hi,

Was wondering if a wideband preamp might have a better noise figure if
its input impedance is greater than the antenna's. It also has a
couple of head to tail 1N4148 diodes across the input for protection.
Would removing the diodes extend the bandwidth or is the extra 8 pF
stray diode capacitance become lumped in with the coax capacitance?
Surely the preamp input has some capacitance too, so how does this
affect the voltage delivered? Please, answer at your convenience.


The diode capaitance is low and really not a factor in preamp gain or
noise. They are non conducting and the capacitance is easily absorbed
into fees and matching systems.

The exact match for any preamp to antenna is a complex as it depends
on device, circuit topology and frequency. Generally with most fets
the ideal transducer impedence for best gain and low noise is higher
than 50 ohms. For most bipolar transistors it's less than 50 ohms.
Getting a reasonable match and incurring low losses in that matching
network are challenges. Every DB of loss at the input contributes
negatively to system noise figure. This includes feedline [coax]
between the amplifier and the antenna.

With all that... Wide band amps generally are of the feedback type
and unless they are of the Norton topology they are noisy. Added to
that is wideband noise has more total power than a narrow band
segment of noise fro the same source. So it goes to say that narrow
band amplifiers generally have the lowest noise.

For HF this is rarely a real problem as most amps have low enough
noise to be useful as the environment is very noisy. As we
transistion to low VHF and higher this case changes greatly. It's
also at VHF and higher that low noise amplification starts to become
significant and more difficult to achieve. Modern high performance
devices however may make this appear simple. For example a wide
band mimic MAR-6 achieves 20db gain 3db noise figure to upper UHF.
However 3db is only ok, as a good Gasfet at 430mhz will be less than
..8db noise figure and even fair number of cheap bipolars can achive
sub 1db with ease.

So it's really an issue of what noise figure you really need and what
bandwidth you need it to cover. For ham purposes this is an issue
for practical consideration for 10m and above weak signal work or
space comms such as EME. For serious weak signal work in the
VHF/UHF realm you have to look at the antenna system and feed
system as part of that noise figure.

Allison
KB1GMX