Background:
CW Reception is both a technological process and a learned skill. The FLEX-6000 and FLEX-8000 Series have several powerful tools that can help merge the two to improve the signal-to-noise ratio (SNR) of received CW signals, making it easier for the best signal processor of all - the human brain - to work more effectively.
Achieving good CW copy in noisy situations depends upon finding a good balance between signal strength, receive bandwidth, and signal processing settings that will allow for the lowest listening fatigue and maximum copy, especially on a noisy band.
The Challenge: 160 Meter CW DX
160 Meters often poses unique challenges for DXing because of the extremely long antennas required, low antenna transmit efficiency, and often very high noise levels on the band. During most of the year, static discharges from thunderstorms are a constant obstacle. Groundwave and skywave propagation transmit high static levels from storms that can be thousands of miles away. This, combined with low signal levels from DX and other long-distance stations makes receiving CW a challenge.
Low Noise / Noise Reducing Antennas:
Using low-noise, and noise-rejecting antenna arrays can greatly improve reception. Very few serious 160-meter operators do not employ "split" antenna systems - transmitting on an RF-efficient antenna, and listening on a different antenna designed to mitigate atmospheric and man-made noise. Actual signal levels may be much lower on this receive antenna, but the signal-to-noise ratio can be many decibels higher, which improves reception. There are volumes of books and articles written on this topic along with a plethora of products available on the market to assist in this aspect of 160-meter work.
Additional Noise Reduction Tricks and Techniques:
After improving your receive antenna, what else can be done to enhance CW reception? Several tricks can be employed. Here are a few:
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Use Good Quality Over the Ear Headphones! Get rid of noise in the shack so that you can focus on the CW Signals! Noise-canceling headphones are effective by removing loud ambient noise in your shack from fans or other noise sources.
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Reduce the Receiver Bandwidth: With a narrower Receiver Bandwidth, there is less noise in the receive bandpass. This results in improved SNR. i.e. better copy.
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Lower the CW Pitch: With lower pitches, (say 500 Hz instead of 700 or 1000 Hz) the brain can pick out the signal versus the noise more effectively. Also, if there is more than one signal in the bandpass, the ability to discern between the two signals is greater at lower audio tone frequencies. This mental trick allows the human brain to discriminate between two signals more effectively.
I have found that 500 Hz is a good frequency. This trick has additional usefulness, which will be discussed later.
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Add some Noise Reduction from the DSP: My experiments have shown that an NR Setting of 15 is a good happy medium between noise reduction and "mushiness" in the current DSP NR feature. (This may change with upgraded NR in future releases of SmartSDR.)
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Add some APF (Audio Peaking Filter): The APF sets an additional audio peak filter for CW reception that is automatically centered on the CW Pitch frequency. The APF slider sets the "Q" of the filter, which affects the gain and bandwidth of the filter. But be careful: Too high can be too sharp and sound a little "ring." I have found a setting of 30 to be good on 160 Meters CW.
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Use the RX EQ (Receive Equalizer): This is a secret that few operators have discovered. If you have set your CW Pitch to 500, turn on the RX EQ and increase the 500 Hz EQ slider for a sharper filter. Then slide all the other EQ sliders down. This adds some additional filtering of noise sidebands outside of the CW bandwidth, making a quieter receive channel, but adds additional peaking at 500 Hz. This can reduce mental fatigue by reducing additional noise in the headphones. (You could leave the 250 Hz and 1000 at about -5 if you wish to soften it up a little.)
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Set the Receive Antenna Preamp Gain Properly: If the Preamp Gain is set too high, all it does is add noise to your signal and reduce the dynamic range of the receiver front-end.
On 160 - 40 Meters, it is often necessary to insert attenuation, rather than gain, into the antenna line, unless the preamp is used to bring up the gain of low-gain low-noise receive antennas.
See this article for how to set the Preamp/RF Gain properly:
How to determine the amount of RF Preamp gain to apply for band conditions – FlexRadio
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Finally. Adjust the AGC: Set AGC to FAST. Then adjust the AGC-T (Threshold) for the best SNR on the CW Signal. On a quiet part of the band, without any signals, begin with the AGC-T control at 50 and slowly reduce it until the background noise drops by -3 dB (roughly half the volume). An optional method is to start with the AGC-T at 10 and slowly increase it until the volume of the noise doubles (+3 dB). The reason for making this adjustment is you want to set the AGC-T at the point just before it starts to increase the volume of the noise. If the AGC is not amplifying noise, any signal at or just above the noise floor will be amplified, making it easier to hear, while the noise stays at a constant volume. Some people can more easily hear an increase rather than a decrease in noise. Depending upon band conditions and signal strength, fine-tune it up or down for the best results in your situation.
For more information, see this video about properly setting the AGC-T:
Flex 6000 Receiver Tuning - How the AGC-T works (youtube.com)
NOTE: If you change any of the settings above, such as the Receive Bandwidth, or if band conditions change, it changes the signal-to-noise ratio of the entire "system." You will need to "tweak" the AGC-T again for the best Signal-to-noise with the changed parameters. It is sort of like driving a sports car with a stick shift. Keep your hands on the controls. In my shack, I use a FlexControl with AGC-T as one of the three AUX settings, so that I can "tweak" the AGC-T quickly when needed.
Each one of these techniques is effective by itself. When combined and working together, they often make the difference between "copy" and "no copy!"