skipp025 wrote:
The less gain is related to the internal filtering "stages",
not the device. I would expect the version with the GasFet
to have the typical expected NF for that device. Any other
available active devices to have Noise Figures commensurate
with their typical expected values (for those devices).
I doubt it. But without any real NF numbers, it's all
guesswork.
I have measured all the original GLB Pre-selectors and their
performance values are very realistic (no surprises). I wouldn't
expect a surprise/difference if I ordered the GasFet equipped
Simrex Pre-selector.
What you are missing is that ANY losses ahead of the first active stage
add to the noise figure of the system - directly. So, while filtering
exists in the GLB device, so does loss, and this loss is more than what
is experienced when using a quality large diameter cavity. Many times
Skipp you tell us there is no free lunch, and the same applies to the
comparison of selectivity and loss between the GLB and a quality cavity
followed by a good active stage.
Real world test. Take a Hamtronics receiver (no preamp) and do a basic
bench sensitivity test to obtain a baseline. If you find something
around -123 dBm your in the right ballpark. Now install a bi-polar GLB
preselector/preamp in front and measure the sensitivity again - you'll
find you have lost several dB of bench sensitivity - at least 3 or 4
dB. Take the same receiver and add a quality 1/4 bottle with a good
preamp (your choice - something with 1.5 dB NF or less) and do the test
again. Now, the receiver hears at -123 to -127 dBm (dependent mainly
upon the quality of the preamp that follows) because the filter hasn't
severely ruined the system NF ahead of the first active stage. Even
though the GLB has gain, the noise figure of the design has already
determined the sensitivity that will be realized by the receiver that
follows.
The GLB preselector/preamp should not be considered for adding basic
sensitivity, because it's possible (depending on how good the receiver
is to begin with) the opposite will happen - however, it will protect a
receiver that lacks good front-end filtering, like the Hamtronics. Like
everything, the situation helps to dictate what equipment will give the
best results.
Kevin
