Wednesday, August 19, 2015

Copying weak CW signals -- The counter-intuitive solution

How do you copy? (HW?)

AF/RF Gain control
When working QRP stations the signal often fails to register on my S-meter.  My display would often look like the one on the right...  Nothing registering on the S-Meter, even with the preamp enabled, yet the station is there... down in the noise sending my call. 

So as I've been spending more time learning to operate CW QRP I keep getting worked by stations that are able to copy me when I cannot copy them.  More often than not QSB would take their side of the QSO into the noise for a time and I wouldn't be able to copy them.  When the inevitable "HW?" (for how do you copy) would come back I'd often have to say "PSE AGN?" (please send again).  

This occurrence happened frequently enough to where I thought I should ask Paul (AA4XX) what was going on. His response seemed counter-intuitive but as I've come to see it's just the ticket.

Turn DOWN RF Gain and turn UP AF Gain

The radio's RF gain is normally left at 0dB.  It can't be turned "up" past 0dB although most radios have a "preamp" that takes the input from the RF and amplifies whatever is there.  Whereas AF gain controls the audio volume.  When you turn the RF gain down (negative values, i.e. -12dB) the signal is being reduced before it is passed to the AF (audio gain) stage.  As counter-intuitive as it seems, by turning down the RF gain and increasing the AF gain (turning down the input and turning up the output) you can sometimes discern a signal that was previously in the noise.

The human ear is a great discriminator and if the receive signal level is reduced such that the background noise is lessened then the audio level (AF) can be increased to the point where you can hear the signal better.  From what I understand it's a matter of separating the signal from the noise.

KX3 Signal path
I started using this technique this past weekend and it's made a big difference in my ability to copy weak stations.  It has also lessened my listening fatigue because I'm not simply turning up noise, trying to hear the station, I'm separating the signal from the noise.  
Technically I have no idea why this works.  
It would seem that lowering the receiver gain simply to increase the audio gain would just swap one noise for another but it does work with my radio.  It requires me to ride the knobs to keep the station in a comfortable listening range and since my KX3 only has one knob for both AF and RF gain (switched by a quick push) it can be a little tedious.  So I began to make use of the volume control on my earbuds.  

Here's my procedure for copying weak stations:
  1. Turn the volume down on my earbuds/headset (independent from the radio volume knob)
  2. Turn the AF on the radio to maximum.  My headset / earbuds volume control is now acting as my audio volume.
  3. Click AF/RF knob to select RF.  
  4. Turn the RF down until I hear no "noise" and just hear the signal
I have to ride the RF knob during the QSO to control my signal and my independent headphone volume control keeps me from having to keep clicking the knob back and forth on the radio to switch from AF to RF control.
The bands have been relatively quiet for the past few weeks since getting back in the hobby and now that we are on the downward curve of the Solar cycle it's bound to get even worse for the next few years so I may as well learn to copy weak signals, be they QRP or QRO.
I was able to work a few difficult to copy stations (for me) this weekend including N8XMS President of the NAQCC club.  Without the weak signal trick AA4XX suggested to me I don't think I would have been able to work those stations with my present antenna at my QTH.

I hope this helps new CW operators or those struggling to copy weak signals.

Glossary

AF, RF, and IF: AF stands for audio frequencies—usually, sounds that you can hear. RF, or radio frequencies, are the actual frequencies of the radio signals (or TV, or cell phone, etc.). IF, or intermediate frequencies, are generally somewhere between RF and AF. In a receiver, it is usually advantageous to convert RF signals to a lower IF at which is it more practical to obtain gain or selectivity. These IF signals are then further converted to AF and amplified further so they can be used to drive headphones or speakers.
dB (decibel): A measure of signal increase or decrease, or of one signal relative to another signal. In human terms, one dB represents a “just noticeable difference” between two signals (or a just-noticeable increase or decrease). Mathematically, dB is derived from the ratio of two signals. Receivers must handle signals over a huge range—in excess of 100 dB. dBm is a more specific term that means “dB relative to 1 milliwatt.” 1 milliwatt is considered to be a “0 dBm” signal in this case. 
Reduce your power and raise your expectations

73s and 72s
Richard
N4PBQ

1 comment:

  1. Very interesting to read someone else having the problem of being heard by people whose signals are practically unreadable. For a long time, I've blamed the receiver on my ageing TS2000, but I'm beginning to wonder whether there is another phenomenon at play here: actual one-way propagation. I too employ the partial solution of riding the AF and RF controls, but this happens quite frequently to me, even when the received signal is still copyable but much weaker than the report I'm given. (Let's say I give 439 and receive 589 or 579 - in other words, probably accurate and not a 'courtesy' 599.) In my case, most of these QSOs are with European stations to the east of me, but this does sometimes happen with North American stations too.

    Again, no solution at hand but you've raised an interesting question!
    73
    Colin
    GM4JPZ

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