A Broadband
Preamp for the SWA Nuller
By VE1ZAC
The SWA nulling device has been a big hit in the shack… with one
exception. The antenna works great but signal levels are typically down 15 dB or
so. This isn’t a big problem, but it entails switching the Pro 3 preamp in and
out while trying different antennas. I found this kind of irritating and
decided to add a broadband, no tune preamp to the box which would bring the
levels back up to normal. If everything worked right, it would be something I
could leave on all the time while using the controller.
After a short
search through my reference material, Low
Band DXing (
Here is the
circuit reprinted (without permission,
but everybody uses the same circuit) from Doug DeMaw’s book:
These transistors
are not garden variety, but are widely used in CATV circuits and UHF radios. A
scan through DigiKey produced a low priced unit in surface mount only, the
KST10 that had suitable characteristics. Many folks recommend the 2N5109, and I
did find them at Mouser Electronics. They are favoured because they can deliver
an improved IMD and third order intercept. What does that mean
? …..basically fewer occurrences of broadcast stations
that occur in strange locations, like the middle of the 160M ham band.
I assembled an
experiment to try and work out the KST10 circuit, as I had it on hand first.
Since it is a surface mount unit, I just soldered it to a chunk of copper clad
PC board as a temporary holder. This worked great.
I elected to use
the transmission line transformer method to match the output. (The right hand
circuit) The books recommended a 43 mix toroid for
the transformer with a u of about 40 or so. This material would be fine, but I
had on hand ferrite cores from the construction of the nulling
device with a u of about 800 to 1000. I thought they would provide improved low
end performance. Having a 43 mix core as well, I made two and tried them both.
The ferrite core was far superior and worked fine to beyond 10 MHz.
Experimental setup:
I simply stuck the
circuit together on a protoboard, figuring if it
worked OK here, it would only improve when I assembled
it on a small piece of board and put a shield around it. There is a regulated
12 volt supply inside the nulling box, so power could
come from there when installed.
With this small
SMT transistor, the emitter resistors were changed to 20 ohms and 120 ohms and
the power supply has a 50 ohm resistor in series. These values provided just
the right bias for the transistor with no overheating and had just the right
gain.
How does it work ?
Just great ! The unit adds
about 10 dB to the signal on all bands, including 10 MHZ. This is very handy as
the output from my antenna on 10MHz was really low, and almost unusable. It
doesn’t overload anywhere, and it makes switching antennas in and out,
including the SWA Beverage, a breeze. I found the unit really is linear from 1
MHZ to 10 MHz.
This really
finishes the SWA antenna project off nicely and makes the unit a pleasure to
operate.
I will make a
further report on the finished unit and the trial with the 2n5109 transistor
when it arrives.