[Drake] Preamplfiers and the Non-Need Thereof

[Barry L. Ornitz]

In my post yesterday, I relied on my aging memory for the 
background noise on ten meters.  When the Digest arrived 
today my numbers looked a little strange so I decided to 
look up the real numbers.

In my copy of Johnson and Jasik's "Antenna Engineering 
Handbook" there is a reprint of a graph showing sky 
temperature as a function of frequency which was part of a 
report submitted to the FCC in 1959.  The graph shows 
minimum and maximum expected values.  Taking the numbers 
for 30 MHz and plugging them into formulas in Kraus' 
"Antennas" I calculated a noise figure for a quiet rural 
area to be approximately 17 dB, while it is as much as 27 
dB for an urban area.

I then looked in Ulrich Rohde's "Communications Receivers, 
Principles and Design" and found a similar graph from CCIR 
Report 258, again for quiet, rural areas.  This graph 
showed the background noise to be approximately 15 dB.  On 
the twenty meter ham band, the background noise is about 25 
dB, and over 40 dB on the eighty meter band.  The value I 
quoted of 3 dB at two meters is correct.

Looking at these numbers, it is easy to see why some 
receivers designed for exceptionally high dynamic range use 
no RF amplifiers at all.  A good double-balanced mixer can 
achieve a noise figure of 7 to 8 dB which is still a lot 
lower than background noise.

            73,  Barry     WA4VZQ     ornitz@tricon.net