2nd receiver, RX Input Protection

[wa9fvp]

I remember talking to someone at the Elecraft booth at Dayton this year about
issues with the RX port and the second receiver.  Even in the manual it
recommends using a band filter.  It prevents high level, out of band RF from
a nearby transmitter from getting into the RX port,  Elecraft also mentioned
that high level in-band harmonics can be an issue.

This may sound strange but to protect my K3S, I found another solution. Sold
by International Crystal Manufacturing, the VFA-1 is a 6 (or 2 meter) RF
amplifier.  Mine was build for 6 meters and was given to me years ago,
before he went SK, by a fellow ham.

I remember that tubes are very rugged and can withstand high RF levels and
static discharge.  There was an article that appeared in Popular Electronics
magazine (Feb 1959) and the 1960 Electronic Experiment Handbook.  Don Smith
W3UZN built a box with a power supply and the VFA-1 attached. He also added
an RF gain control.  I decided to copy his design and try it on my K3S.

Using a vertical 6m antenna and the second receiver, I monitor 6 meters on a
daily basis while working the HF bands at the same time.  I never
experienced 6m front end overload.  Because the VFA-1 has 20 DB of gain,
there's no need to use the K3S preamp.  I have no way to measure the noise
figure but compared the K3S pre-amp, I can hear signals just as good. The
tube is a 6BQ7.

<http://elecraft.365791.n2.nabble.com/file/t344213/DSC00612.jpg>





-----
Jack WA9FVP

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