PROBLEM
Living next to a hospital, I encountered severe powerline hash on my favorite band, 40 meters. The high voltage power lines were clearly visible across the street. Walking around the area with a portable short-wave receiver showed me that no one source was detectable. The power lines everywhere were radiating 120 Hz hash.
EXPERIMENTAL METHOD
Rather than try to draw a schematic, I used components and clip-leads on my
desk, while leaving my receiver on and tuned to 40 meters. I then wondered
if I could use an antenna run parallel to the power lines to cancel out the
noise picked up by my regular 40 meter dipole. To try this out, I wound a
center-tapped RF coil with one set of windings going in the opposite
direction from the other. I then connected the two antennas so that one was
out of phase from the other. Wow! The noise level dropped!
I then found I could use hand capacitance to vary the amount of antenna coupling to that differential coil. Wow again! I could create a null of the noise by holding onto the antenna leads and squeezing them with my fingers. Since I wanted to keep my fingers for myself, I substituted a variable capacitor with more clip leads. I was able to null out the powerline hash.
Then I got brave! I rigged up a 110 VAC line cord with a small mica capacitor and connected the other end to my receiver. The hash level came up. It seemed as if the hash was not only entering my shack from the antennas; it was also coming in through the powerline itself. I then rigged up another variable capacitor to couple a small amount of the powerline hash into my differential coil. Wow even more! I found I could create almost a perfect null of the hash. In other words, by adjusting the settings of the noise antenna AND the minute coupling from the wall socket, a null of 60 db was shown on the S meter.
SOLUTION
What remained was to add additional variable capacitors to increase the degree
of coupling from very minute to very strong. Finally, I drew a schematic of
my big mess on the desk and built my second version on a piece of clear
plastic. Everything had to have floating grounds for this circuit to work.
When I got the clear plastic breadboard version to work, I then mounted the entire assembly into a metal box. None of the variable capacitors could touch the metal, so I drilled 1/2" holes for the shafts. Everything on the plastic breadboard looked like a finished product when put into the metal box.
CONTROLS LAYOUT
- Front Panel (L-R):
- Parallel, 180, Out selector
- Injection
- Null Depth
- 110v Injection A
- 110v Injection B
- 110v Phase
- Injection Range switch
- Rear Panel
- Lift Ground Switch
- Fuzz Null
SUMMARY
By sampling noise from an antenna run broadside to a power line, and then sampling noise from the 110 VAC shack powerline-- it is possible to null out severe QRN on 40 meters by judiciously mixing these noise signals. A differential RF coupling coil was used to create the out-of-phase signals. The entire circuit of the QRM KILLER is PASSIVE. That means perfect signal linearity from very weak to very strong QRN.
Finally, this project shows that you can design something by simply using clip leads and hand capacitance while poking fingers into the circuit at various key locations. The resulting schematic can happen later.
More about these ideas can be found on page 25 of the May, 1942 QST. Yes, QRN has been a problem for over half a century!
73,
RON
Back to Ham Radio Page
Return to George’s Place
E-mail Ron, K9KLT