Topband: Hi Gain Preamps, Noise Figure Comparison (KD9SV 994 x5, W1FB, W7IUV preamps)

[Don Kirk]

Gary (KD9SV) kindly loaned me one of his 994 x5 preamps (sometimes called
KD9SV VLN (very low noise) preamp) so I could compare its Noise Figure
against one of my W1FB preamps as well as 2 of my W7IUV preamps in series
on 160 meters.

Here is a link to a youtube video I created that demonstrates the
differences in Noise Figure between the above mentioned preamps.
http://www.youtube.com/watch?v=JpIiF1Hfovw

The KD9SV 994 x5 preamp did in fact come out on top (best Noise Figure) and
it's a commercially available preamp, whereas my W1FB and W7IUV preamps are
home built.  Nevertheless in my current application (pennants that are
51.6% the size of full size pennants) the W1FB and W7IUV preamp Noise
Figures are adequate and I didn't notice improvement in my signal to noise
ratio on 160 meters when using the KD9SV preamp.  The lower Noise Figure of
the KD9SV would allow me to use even smaller pennant receive antennas that
would have more negative gain without harming my signal to noise ratio.

I ran into a couple of interesting issues when working on this project.
The biggest surprise was that the input impedance of my old HF rig (Kenwood
TS-180s) was no where near 50 ohms on 160 meters (it actually measured 189
ohms).  I added a transformer to the input of the Kenwood TS-180s to bring
it closer to 50 ohms for my testing.

Also as previously reported by Hardy (N7RT) a long time ago, the W1FB
output transformer turns ratio is incorrect for a 50 ohm match, and I
measured the output impedance of the W1FB preamp at 429 ohms.  I
subsequently added a transformer to the output to bring it closer to 50
ohms for my testing.

*Noise Figure Comparison (average of two different measurement methods)*
W1FB preamp: Noise Figure 3.3 dB higher than KD9SV 994 x5 preamp
Two W7IUV preamps in series: Noise Figure 1.1 dB higher than KD9SV 994 x5
preamp

Note 1: my W1FB preamp is not stock, as I made modifications to protect the
MC1350 IC from damage when transmitting.
Note 2: The KD9SV 994 x5 preamp was set at its maximum gain (41 dB) for all
of my tests.

*Summary*
While I suspect (through my measurements) that the KD9SV preamp absolute
Noise Figure value is not the very low 0.7 dB value listed on a commercial
website, it's indeed a very nice adjustable high gain preamp offering a
maximum gain of 41 dB and it has the best (lowest) Noise Figure when
compared with my other two types of preamps.  I highly recommend the KD9SV
994 x5 preamp if you're in need of a high gain preamp on 160 meters.

73,
Don Kirk (wd8dsb)