Topband: 4. MFJ noise canceller

[Donald Chester]

I took some measurements today with my MFJ 1025, using a General Radio 
1001-A rf signal generator, an external calibrated attenuator and a 75A4 
receiver.  The built-in attenuator setting were compared with the external 
unit, and readings  were consistent with each other.  I measured the signal 
level directly to the 75A4, using the S-meter as a signal strength 
indicator, then inserted the MFJ unit and took another measurement.  Here 
are the results:

On 160, the MAIN antenna shows a loss of 10 dB compared to straight through. 
  The AUX ant shows a loss of 13 dB.

On 80, there is a 3 dB loss thru the MAIN antenna channel, and 10 dB loss 
through the AUX.

On 40 there is 3 dB loss thru MAIN and 8 dB thru AUX.

I get a good null on the noise, using an indoor 5' X 5' shielded coax 
rotatable loop as a noise antenna and 500' Beverage as the main antenna.  
The AUX gain is set to max and the MAIN gain setting is between 5 and 6.  
The problem is that the signal must be at least moderately strong in order  
for the noise cancellation to be useful.  The unit's attenuation drops weak 
signals below the combined noise floor of the noise cancellerand receiver.

I attempted to use a preamp (an  old Ameco nuvistor unit with  tuned input) 
with the noise antenna, but cannot get a null with the preamp in operation.

I read the suggestion on te web site about removing the two inductors and 
the resistor that make up the high pass filtor on both antenna inputs to 
inmprove low frequency response.  I am sure that would help, but it seems to 
me that the loss thru the AUX channel  is excessive even  on the higher 
frequenciel.  According to the manual, the AUX channel is supposed to have 
more gain that the MAIN, but at the expense of introducing more internal 
noise.

If anyone else has measured signal loss through your MFJ unit, were your 
results consistent with the above figures?

Don K4KYV

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