Topband: Waller Flag Question

[Tom W8JI]

> The on-line materials about Waller Flags says that a modest size WF would 
> need about 40 dB of gain to boost the signal to a usable level.  One of 
> the postings says that cascading two preamps of 20 dB each seems to add 
> extra noise, and they talk about developmental work being done on a single 
> preamp of 40 dB.
>
> Is there now such a 40 dB preamp?  Is it made commercially? 
> Alternatively, are there plans on-line somewhere?  Has someone actually 
> used it in a high-RF urban/suburban area, with multiple 50 kW AM BC 
> stations?  With what results?
>

At some point we have to be realistic.

A typical receiver is somewhere in the -130 dBm sensitivity range, depending 
on bandwidth and other things.

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.

If you need a 40 dB amplifier (or even close to 20dB) into a normal good 
receiver, you will never get the noise temperature of things in front cool 
enough to let external noise set noise floor. The issue isn't connecting two 
amplifiers in cascade, the issue is the limit of noise temperature.

The only place negative gain antennas that require more than ~ 20 dB gain 
with a normal receiver at a quiet location will work into the external 
ambient noise floor generated outside the antenna is in a location blanketed 
with strong local noise.  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.

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

73 Tom