December 2016 11
the desired 50 ohms resistive. It can do this within limits,
depending on the range and power handling capabilities
of the capacitors and inductors. There are also some
theoretical considerations to the matching range.
The problem you describe sounds like some kind of
RF incursion, though it’s hard to exactly diagnose it from
afar. If the impedance being matched is far from the system
impedance of 50 ohms, the voltages or currents inside
the tuner can get quite high. This, in turn, causes there to
be strong electric and magnetic fields which can induce
voltages and currents in parts of the tuner that don’t expect
In addition, if there are high RF fields in your shack
(where the radio is for you non-hams), those fields can
induce variations in the power supply voltage and current
to the tuner, as well as actually signal the tuner to switch
bands through its band-switch control inputs.
A trick that we can use to minimize those effects is to
add inductance to the power supply lines — and perhaps
elsewhere inside the tuner — to limit the induction of
these unwanted voltages and currents. There are at least
two things to employ. The first is to use clip-on ferrites to
increase the inductance of the power supply wires. Try
to get the biggest ones you can find, and wrap the wires
around the ferrite as many times as you can.
Since this particular tuner is powered off the AC
mains, it may be difficult to get more than one turn with a
thick power cord, but try as best you can. If you are using
band control wires to the rig (the transceiver), give them
the same treatment. If you can find out what the ferrite
mix is, try to get one that provides a large relative µ, the
magnetic permeability when compared to a vacuum, at or
below 30 MHz. That will give you the highest inductance. I
usually put those ferrites everywhere I can think of. They’re
typically pretty cheap.
The other thing is to try to either shield or AC short
those band switch connectors (labeled Accessory Sockets)
on the back, if you aren’t using them. It’s mechanically
hard, but putting a metal cap over them would provide
some shielding. The better thing might be to attach a
capacitor from the band switching control voltage input
(pin 13 according to the manual) to the chassis. Perhaps a
ceramic disc capacitor around 0.01 µF would do the trick.
Give these tips a try and write back to the Q&A
column to let me know how you fared. NV
Have a question for Kristen?
Email it to Q&A@nutsvolts.com
Comments are also welcome!