I'm looking at getting a preamp that's based around a BF964S.

These appear to have about 1dB noise figure at 200mhz. It's to be used on
2m.


Does anyone know of a suitable alternative that I could swap it with that
has less noise?

Sam

I'm curious, what sorts of things do amateurs do that really need
better than a 1dB noise figure on 2 meters? I can understand it on
higher frequencies, but I'd have thought that galactic noise, at least,
would be a limiting factor on 2 meters.

Looking to be educated,

Tom

In article . com,
"K7ITM" writes:
I'm curious, what sorts of things do amateurs do that really need
better than a 1dB noise figure on 2 meters? I can understand it on
higher frequencies, but I'd have thought that galactic noise, at least,
would be a limiting factor on 2 meters.

Looking to be educated,

I also question (a little) the need for better than a true 1dB NF
when using a terrestrial antenna in the 2M band. On the other hand,
if the transistor is speced to be able to supply 1dB NF, but isn't
working as well as desired because of preamp design, high return
loss, etc.

It isn't really HARD to get 1dB NF at frequencies
below 1GHz (or even higher), but it is sometimes tricky to get a good real
world NF while also providing good return losses (good impedance match),
at high signal levels, especially while trying to minimize DC power usage.

My own high-end jelly bean for near-ultimate performance below 1GHz,
moderately high power for reception (10mw - 100mw), reasonable noise match in
the 50-75Ohm range, but perhaps a little tricky due to high frequency
behavior would be the HP f54143 (Hi IP3, reasonable noise match, it has
lots/too much of UHF or higher freq gain, but is moderately well behaved).
This part costs less than $2... (I bought my supply for between
$1.50 and $2.00 each.) I'd also consider using moderate current SiGe
transistors while biasing them at about 10ma to 20ma (which is generally a
reasonable range for a good 50-75ohm noise match.) Either the SiGe
transistors NESG2101 or the PHEMT (54143) will give better than 1dB NF
with a generally fairly wide noise match. (SiGe is generally better than
most alternatives for oscillators -- off subject -- also, due to their
generally very good 1/f characteristics and generally high Beta, which
translates to generally low current noise -- esp in the low freqs where
part of the AM/PM noise comes from.)

Given the very wide signal strength range, I'd definitely consider OIP3
of at least 25-30dBm, and try to design for no more than 10-15dB gain.
It is very possible that even a very highly performing preamp will work
worse than no preamp at all!!!

John

On Sun, 18 Dec 2005 17:15:17 +0000 (UTC), (John S.
Dyson) wrote:


I also question (a little) the need for better than a true 1dB NF
when using a terrestrial antenna in the 2M band.

clip

Given the very wide signal strength range, I'd definitely consider OIP3
of at least 25-30dBm, and try to design for no more than 10-15dB gain.
It is very possible that even a very highly performing preamp will work
worse than no preamp at all!!!

One reason for attempting to get a low noise figure as possible, even
for terrestrial working, is that you can then afford a larger loss due
to input band-pass filtering.

The unloaded Q to loaded Q ratio of the input filter determines the
filter loss and the unloaded Q is mainly defined by the filter
construction (LC, Helical, 1/4 lines etc.). A 15:1 ratio would give
about 0.5 dB loss, while 5:1 ratio attenuates the passband by 1.5 dB.

Assuming a unloaded Q of 100 for an LC filter and loaded Q of 20 would
produce a 7 MHz bandwidth (at -3 dB points) with 1.5 dB loss in the
middle of passband and assuming 0.5 dB preamplifier noise would give a
total 2.0 dB noise figure, with a nice attenuation of broadcast FM and
VHF TV signals that otherwise might overload the preamplifier.

Paul OH3LWR

Is that galactic noise measured on a wide bandwidth antenna?
What if it is very narrow band?

regards -
Henry


"K7ITM" schrieb im Newsbeitrag
ups.com...
I'm curious, what sorts of things do amateurs do that really need
better than a 1dB noise figure on 2 meters? I can understand it on
higher frequencies, but I'd have thought that galactic noise, at least,
would be a limiting factor on 2 meters.

Looking to be educated,

Tom

It's because I've got about 3dB of aerial loss and filtering loss, that I
can't get rid of.

But thanks for the advice on the devices, I'll take a look at them and see
if they would substitute, and how I would need to modify the circuit if not.


Sam

don't even try!
the ambient noise levels on 2m are such that even a 5 to 6 dB noise
figure won't limit your MDS.
it's just going to complicate your life, because the amplifioers able
to provide a lower than 1 dB NF tend to become unstable. the trouble
isn't worth it.
Saandy 4Z5KS

On Wed, 21 Dec 2005 00:27:31 +0100, "Henry Kiefer"
wrote:

"K7ITM" schrieb im Newsbeitrag
oups.com...
I'm curious, what sorts of things do amateurs do that really need
better than a 1dB noise figure on 2 meters? I can understand it on
higher frequencies, but I'd have thought that galactic noise, at least,
would be a limiting factor on 2 meters.

Is that galactic noise measured on a wide bandwidth antenna?
What if it is very narrow band?

At 2 m the Sun is quite noisy, in addition the noise from the stars in
our Milky Way galaxy will contribute quite strongly to the background
noise levels with noise temperatures of several hundred Kelvins if the
antenna is pointing at the galactic plane.

However, if the antenna is pointing away from both the Sun and the
Milky Way, the noise levels are lower and thus, if you try to receive
a satellite in that direction, a low noise preamplifier will help. Of
course, this can be more easily observed on 70 cm and 23 cm, in which
the "cold sky" noise levels are even lower.

Paul OH3LWR