Our first commercial 11 metre dish earth station built in 1984 had a
4GHz 60dB gain, 33 Kelvin, electrically cooled two stage parametric LNA
at the feed horn. the cable to the receive system was LDF4-50 with 20dB
loss to a six port passive splitter.
The LNAs despite being pressurised constantly with dry air with a small
bleed hole had to be purged of frost every six months by turning the
cooling into heating and increasing the outflow of air. After 12 hours
of cool down they had to be retuned, pump frequency and power to achieve
60dB flat gain over 500MHz bandwidth. Times have changed.
Regards,
Mike VP8NO
On 05/07/2019 10:08, Wes wrote:
That isn't quite right either. The preamp gain must be much higher than
the following losses to minimize second stage degradation. (Line loss
degrades the NF of the second stage)
Wes N7WS
On 7/5/2019 5:35 AM, Martin wrote:
That's not quite right. The overall system noise is determined by the
preamp noise figure when mounted as close to the antenna as possible.
Cable losses (=noise) after the preamp can be neglected, as long as
the gain of the preamp is higher than the losses in subsequent
components. So the cable AFTER the preamp can be pretty lossy. OTOH,
too much gain from the preamp easily overdrives your transceiver's RX.
A good balance between gain vs. cable losses is mandatory. The noise
figure of the pramp is crucial.
If you are satisfied with your transverter's noise figure , mount it
close to the antenna via a short run low loss cable, if you can. This
saves the expenses for a preamp.
This applies to the RX path. I'm not talking cable quality or output
power for TX.
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