|Initial Tests of ANC-4 Noise Canceller at 136 kHz
Bob, G8RW, started getting local QRM problems on 136 kHz from a neighbour's cable TV
set-top box, I offered to test his model ANC-4 noise canceller at my QTH to ascertain its
usefulness on this LF band. I had already experienced significant success using my homebrew noise canceller on LF, so I thought that I might be able
to modify Bob's ANC-4 for use on 136 kHz.
I started by reading the manual for the ANC-4 and noted that the Operating Frequency
Range was stated to be "500 kHz to 80 MHz" with the additional note:
"useable down to 100 kHz". So, before taking the ANC-4 apart, I decided to
try the unit 'as is' to see how it performed on 136 kHz. The results have been very
promising: here's the story so far . . .
The noise canceller arrived in the post from Bob in mid-August 2001. It is a
well-constructed little unit, and is housed in a very robust steel box - the sort that you
could stand on (or even drive over), and yet cause it no physical damage. Upon
arrival, the unit was missing its built-in telescopic antenna. In the Power 'off'
position the 'Main Antenna' socket is connected directly through to the 'Radio' socket,
using the normally-closed contacts of the switching relay built in to the ANC-4. In
the Power 'on' position, the relay operates to switch the noise canceller into circuit.
If transmit RF is detected from the 'Radio' socket while power is 'on', the relay
automatically releases to protect the noise canceller.
The tests were carried out using my single-turn LF loop antenna
(65 m perimeter); a 5-watt transmitter running into a dummy load
(as the 'noise' source); and the ANC-4.
All main antenna connections and DC power connections were made; and a crocodile clip
(also known as an 'alligator clip') was used to connect the noise antenna socket to the
chassis of the 5 watt transmitter. The Frequency Range switch was set to 'Lo'
throughout the tests.
The first test was to ascertain the loss through the noise canceller. Signals on
136 kHz dropped by about two 'S' points when the ANC-4 was switched on - indicating an
insertion loss of about 10 dB. This was not unexpected, and is about the same amount
of insertion loss that I experience with my own homemade noise canceller. Those
having 136 kHz receivers that are only just sensitive enough to hear weak signals on the
band, may need to use a receive preamp to compensate for the loss through the ANC-4.
The next test was aimed at cancelling a weak noise signal, in this case generated by
VFO 'leakage' from the 5-watt transmitter.
The S meter on the 136 kHz receiver was reading S6 with the ANC-4 switched 'ON', and
the 5-watt transmitter switched to 'transmit' (but in 'key up' mode). By advancing
the Noise Gain control, and adjusting the Noise Phase control, it was easy to completely
null out the signal from the transmitter. Instant success! Some apparent
interaction between the two controls was noticed, but this is not uncommon with noise
cancellers; and, rather like using a manual antenna tuning unit having two controls,
adjustment becomes easier with practice.
Now for the worst-case scenario: could I null out the noise signal under 'key down'
conditions? That is, when running the full 5 watts into the dummy load. Well,
not completely - but the results were still impressive. At key down, the S meter was
now reading S9 + 45 dB. Using the Phase Range; Noise Phase; and Noise Gain controls,
the noise signal could be reduced to S9. Occasionally, it would be possible to see
'nulls' down to S5, but, in practice, the carbon tracks of the single-turn Noise Phase and
Noise Gain potentiometers were not capable of maintaining such deep nulls.
Nevertheless, the ability to obtain about 45 dB of noise cancellation was a pleasing
Of course, the buzzing noise that Bob is experiencing from the set-top box (presumably
from the built-in switching-mode power supply), may not be as easy to cancel as my
artificial 'noise' signal. When I return the ANC-4 to Bob for installation at G8RW,
I will be offering the following advice concerning use of the ANC-4:
1) Verify that there is no residual noise present when the main station antenna is
disconnected from receiver (the noise canceller can only cancel noise components entering
the receiver from the main antenna);
2) If necessary, use a receive preamp between the noise
canceller and the receiver (to compensate for the 10 dB insertion loss of the ANC-4 at 136
3) Where possible, try to 'target' the particular noise source for coupling to the
noise antenna socket. It is much better to use a pick-up loop/ferrite rod located
close to the noise source, than to use random lengths of wire that might pick up unrelated
sources of noise, in addition to the target noise source.
Bob called me this evening to say that he has installed the ANC-4 and, using a
simple noise antenna, is already able to null out most of the local QRM. Bob now
intends to experiment with different noise antennas, and build a receive pre-amp to make
up for the insertion loss.