Topband: Waller Flag Question

[JC]

>>
At -140 dBm and 250 Hz noise bandwidth, the system would require a 1 dB
noise figure front end.  That's about 35 deg K noise temperature.
>>>
 
Tom is as usual 100% right, the RX system gain should be near 1 dB, it means
the preamp at 1.8 MHz should be .5 dB NF  the input filter and the feed line
< then .5 dB att. Together, not each. 
 

To  make things more complicated, when there is no atmospheric noise, like
we have in some winter days, the band noise can drop to 100K, and at that
point the gain of the WF and the NF of the system should be designed to  no
more than 3 dB deterioration  on signal to noise ratio, it means the 1 dB is
not enough, the solution for that is a bigger WF.

>>
Besides that, if the gain is so far negative the coaxial cable will easily
become more of an antenna than the thing we call an antenna.
<<

This is most common problem for all flag . EWE; WF and low gain antennas.
The only way to overcome this is using good quality twist par UNSHILDED
cable, or choke the cable as  much possible. If you know what you are doing.
However detuning any structure or antenna at the same frequency is a must,
it can deteriorate the directivity of the RX antenna to make it useless. No
free beef here.
>>

40 dB gain in front of a receiver is pure fantasy, unless the receiver is
dead as a door nail.
<<  

Tom,  I'm afraid I disagree but agree with some of  that, I am using a 43dB
gain preamp since 2010 with not a single failure yet, but I understand your
point. It is so delicate to implement that most of fellow that try it fail.
Even aluminum enclosure does not shield it enough, 40 dB gain is 10.000
voltage gain, it needs a dual shield with steel to cut magnetic field, the
feed lines must be decoupled over 80 dB, relays must be 100dB or more in
isolation, and much more details that I won't cover.

It is not a weekend project. 


Regards
JC
N4IS