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HF Pre-amplifier

Following on from the construction of the Norton amplifier, the real question to be answered is 'is an HF pre-amplifier necessary on 14MHz and if so what should its specification be'?

The full system noise figure factor is given by

f=fa + (Lc-1)(Tc/To) + Lc(Lt-1)(Tt/To) + LcLt(fr-1)

where fa is the external noise factor given by:

fa = pn/kTob

pn = available noise power from a lossless antenna
Lc = antenna circuit loss
Tc = temp of antenna and ground surrounding (in range)
Lt = transmission line loss (1dB in our case)
Tt = transmission line temperature (288k as its quite mild today)
To = reference temperature (go for 288k)

fr = noise FACTOR of receiver
Fr = noise FIGURE of rx = 10logfr (14 dB in our case)

so the noise power in watts from all sources - pre detection is

n = f k To b

where f is as above the system noise factor which is a theoretical quantity taking account of all noise sources.

For the purposes of finding out whether we really need a pre amp we will simplify the maths and assume that the antenna (and transmission line) is lossless, then the total noise will be from the external sources and the receiver is:

n = (fa + fr) k To b
n = (fa k To b) + (fr k To b)
n = Pa + Pr

in dB the noise power in dBW due to the receiver is
Pr = Fr + 10logb - 204
Pr = 14 - 204
Pr = -190 dBW/Hz
Pr = -160 dBm/Hz

And from the CCIR 1982 report 670 (see photo below)




Pa = Fa -204 dBW/Hz or Fa = Pa + 204

and at 14 MHZ galactic noise = 20 dB/kTo
and man made noise can bring this up to 40 dB/kTo

Taking the higher man made noise this puts the external noise at 40 - 204 = -164 dBW/Hz
or -134dBm/Hz.

With just galactic noise the external noise power is -154dBm/Hz.
More noise from galactic source than the receiver (by 6dB). The Norton amp gain may only be a marginal improvement.

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