6m pre-amplifier for K3

It is well known that Elecraft K3 has high noise figure on 6m. The Company sells external pre-amp in order to solve the problem.

I think this weakness is a result of optimizing receiver for high signal level handling capability. Low gain prior to mixer helps

to achieve low distortion numbers.

As pre-amp for 50MHz is rather straight forward task, I built my own.

As usual, I used the best components I had in my workshop. They are perhaps not the best one can buy.

I made two versions. First one, which can be connected to antenna line outside of K3. I wanted to avoid any losses or interference

K3 may introduce between antenna connector and "RX ant out"-connector. Necessary relays and their driving circuits are shown.

The other version is made as simple as possible and to use K3's accessory connectors "Rx ant in" and "RX ant out".

When testing this version with my K3 I learned that K3 generated noise spikes when turning the VFO knob.

That noise was hardly readable without the amplifier. There is a fix for this problem on Elecraft pages and newer versions are ok.

Instead of making the factory recommended mod I decided to use the antenna-line version, which is free from that VFO related noise problem.

In both versions the amplifier itself is the same. BFR91 bipolar transistor is used. Noise figure is about 1.5dB.

A simple filter with air coil in the input of the amplifier provides enough selectivity for most of the cases.

My construction was the ugly style. In RF-circuits good continuous ground and short wires are the key.

Printed circuit can easily spoil the thing if not properly designed.

Amplifier gain is 24 dB, which is a bit more than the Factory sold amp but seems to be fine. High gain helps to push

receivers noise figure down.

22.8.2010

Pekka, OH1TV

updated

25.6.2011