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FT736 Noise Figure ?



Hello,

Sometime ago, there was a message via this list on the improvements which
could be realised on the FT-736 by fitting the MuTek front-end kit.  I seem
to remember there being an analysis of the original 736's noise figure,
mostly blaming the helical filters and PIN-diode switch.  A very vague
memory of 3.5dB comes to mind?
Does anybody have a copy of that information?

I'm trying to do a 'proper' analysis of what signal strength is required to
maintain a given BER on the 9600bps downlinks from UO-22 etc.

If a BER of 1 x 10E-4 is acceptable, and is available when Eb/N0 is 12.1dB
(from BER graph available at
http://www.ee.surrey.ac.uk/EE/CSER/UOSAT/papers/icdsc_ws/icdsc_ws.html),
then this converts to a required C/N of 8.9dB for a bit rate of 9600bps and
receiver bandwidth of 20kHz.

If the FT736 *does* have a Noise Figure of 3.5dB, then this converts to an
excess noise temperature of 359K (using Te = (F-1)T0)

The required signal strength ('C') required to provide a C/N of 8.9dB is
able to be derived from;
C = C/N + k + T + B (all in dB)
where k is Boltzmann's constant, T is noise temperature and B is bandwidth.

T(sys) = T(ant) + T(receiver)         (assuming no feedline loss)
and at 430MHz, the effective sky noise temperature at low elevations is the
same as the warm earth : 290K.  We can then use a figure of 290K for system
antenna temperature.

thus T(sys) = 290K + 359K = 649K

Plugging all this into our C = C/N + k + T + B (all in dB) formula gives;
C = 8.9dB + (-228.6dBJ/K) + 28.12dBK + 43.01dBHz
therefore C = -148.57dBW = -118.57dBm

in other words, maintaining a received signal strength into the receiver of
-118.57dBm will give a BER of 1x10E-4.

If a low-noise pre-amp (NF = 0.9dB, gain = 10dB) is used, then Tsys can be
expressed as;
T(sys) = T(ant) + T(preamp) + T(receiver)/G(preamp)         (assuming no
feedline loss) therefore;
T(sys) = 290K + (1 - 10^(0.9/10)) * 290 + 359K/10
T(sys) = 290K + 66.8K + 35.9K = 392.7K = 25.94dBK

therefore when using the preamp, the required signal strength is -120.75dBm
to give a BER of 1x10E-4

So, if the NF of the 736 is 3.5dB, then the use of a (typical) mast-head
preamp delivers a 2dB benefit...  you could almost use an antenna which was
half the size.

Anyway, back to the original question;  does anyone have the measured noise
figure for the FT-736 on 70cm?


Many Thanks,


Chris Hill  VK6KCH





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