GW4ALG's 136 kHz Pages

[ GW4ALG went QRT in February 2007 ]

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Initial Tests of ANC-4 Noise Canceller at 136 kHz

When 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 result. 

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 kHz);
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.

Update: 22/08/2001
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.

 

Picture of ANC-4

Here is a picture of the model 'ANC-4' noise canceller used for the test (Serial No. 010445).  It was manufactured by JPS Communications, Inc., PO Box 97757, Raleigh, NC 27624-7757, USA.

The front panels controls are, from left to right: Power (plus LED); Phase Range [A/B]; Noise Phase; Frequency Range [Hi/Lo]; and Noise Gain.

The rear panel sockets are for: Main Antenna; Radio; 12 VDC; and Noise Antenna.

The ANC-4 is now being manufactured by Timewave Technology Inc.  For more details, see:
http://www.timewave.com/anc4data.html