An alternative to the 6M7JHV antenna for 6m / 50MHz
Mast head Low noise amplifier

While we were at it we decided to install mast head low noise amplifier on the stacks and I followed Paul, G4CCZ's ideas.

We used a separate receiver feed to our IC-7800 transceiver to avoid any possibility to damaging the preamp when transmitting. The coaxial relay only has 60dB isolation so we decided to install a small relay to short the input of the LNA to ground while transmitting. The LNA is powered through the receiver coax cable.

There are two big benefits of this arrangement:

  • When the power to the LNA is not present, the unit defaults to a straight through connection that can be used as if the LNA was not present.
  • When the power to the LNA is not present, the LNA is not connected to the antennas protecting it against possible static damage.

- NEW -

In August 2011 I took my old masthead amplifier down (see bottom of page) as I replaced my antenna cables with 7/166" DIN plugs and sockets. In April 2012 I replaced it one that used DIN sockets but more importantly used vacuum relays rather than coaxial relays.

This is capable of handling much more power and should be as reliable as the old one.

I've used a Downeast GaasFET preamp and Jennings vacuum relays in this version

This is the control unit that feeds +27 volts up the receive cable to power
the pre-amplifier and relays.

Make sure you put a diode across the relay coils or you will end up blowing up your preamp as I did!

The unit also switches the linear amplifier.

This is how the preamplifier is mounted on the antenna array.

Old masthead based on Coaxial relays and N-types

o further reduce risk of LNA damage the input to the LNA is shorted to ground using the small coaxial relay.

+12V power is fed to the relays through the receive coaxial cable.

G3WOS's mast head amplifier

G3WOS's mast head amplifier

The LNA connected to the input of the power divider as shown below.

The LNA connected to the power divider

The LNA and power divider connected to the stub mast.



Back to home