Topband: Waller Flag Question

[Tom W8JI]

> 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.


JC,

The problem is gain and noise figure, not shielding.

Let's assume a receiver with 250 Hz bandwidth has a MDS (3dB S+S/N) of -135 
dBm. This is a 15 dB receiver noise figure.

A 0.5 dB noise figure front end amplifier with NO other losses would 
produce -149.5 dB MDS. That is the absolute maximum MDS sensitivity 
obtainable with 250 Hz BW and 0.5dB total input noise figure.

If we include the receiver's noise figure, 14.5 dBm gain would result is a 
system composite noise figure of 3.44 dB. Increasing amplifier gain (with no 
change in amplifier 0.5 dB noise figure) results in the following system 
composite noise figures:

14.5dB = 3.44 dB
20 dB =  1.55 dB
25 dB =  0.86 dB
30 dB =  0.62 dB
35 dB = 0.54 dB
40 dB = 0.51 dB

At someplace around 20-25 dB, you get into system limits. The improvement 
from 30 dB to 40 dB is only 0.11 dB. No one will notice that.

This of course varies with the receiver, but few receivers are worse than 
this example.

Let's say we have an input stage NF of .5 dB with 15 dB gain. In order to 
have a cascade NF of .7dB the second stage has to have about a 2 dB NF.

All of this is peanuts.       A .6dB noise figure is a 3 dB MDS of -149.4 
dBm, while a 2 dB NF is a MDS of -148 dBm with 250 Hz BW.

There is a point where inevitable system flaws make using an antenna with 
such negative gain to require less than 1 dB NF impossible for "copy this 
plan".  This is why Beazley's out-of-phase small horizontal elements were 
mostly met with didn't work. The problem with models is we can build perfect 
systems that we cannot repeat in the real world.

Again my example of the small commercial loop I have. It limits by loop 
internal noise by many  dB at my location, and common mode on the cable is 
very evident. If I moved the same antenna to a location with 20 dB more 
external ambient noise floor, it would limit on external noise.

It seems unlikely most compact antennas are being used in locations so quiet 
they need 30 dB gain, or .6 dB NF.   If they had room, they would not have 
as much ambient site noise.

I'm not being disagreeable, just describing the practical limits.

73 Tom