Thursday, June 4, 2015

Developing DX-er's Ears

The last three weeks has seen a relative low amount of time behind the radios. Thunderstorms, rain, and flooding have hit Texas and in addition to everything else, the QRN ( static) has been horrendous. That has not to say there has been no DX logged, there has, but it has been at a cost in time, concentration and pounding of the ears to pull it out.

When the lightning has not been so close as to be dangerous to have the antenna connected, the upper bands have been listenable during the day, at least on frequencies above 15 mhz Early in the mornings Europeans and Russians would make it through on 15 and 20 meters, though 20 was often plagued with heavy static. In the late afternoons the regular flood of Japanese stations would be in with fairly good signals, though sometimes even on 15 meters masked by the static crashes. A couple of good catches were logged, including a South Korea on 15 and Kenya (5Z0L ) and Palau ( T88HR) on 17, though it took some good picking to pull out the callsigns This was on 24 May and all on cw. A group on Bear Island Svalbard-JW9JKA- was also heard on 15 meter sideband but it took almost fifteen minutes to identify.

The lower bands were all but useless for DX on phone. DX of broadcast stations on 120, 90, and 60 meters was a total washout and even weaker stations on 49 and 31 meters were tough to pull out.

This past weekend ( May 30 and 31) saw sunshine return to Central Texas, but on the low bands the static was still horrendous because there were still heavy storms in areas within the first skip zone on the low bands, and yes, static crashes are propagated just like desirable radio signals.

The WPX contest would normally have provided a treasure trove of signals, but for me, the event of a grandbaby's birthday precluded a lot of time in front of the radios ( there ARE priorities when there is a three-year-old involved!!) But even the available time on the low bands was still pretty tough. In the evenings, on 40 meters, only the strongest of Europeans were coming in Friday night Texas Time. Only the big club stations in Italy and Croatia were identifiable. Saturday night was a little better but once again only the powerhouses were pulled out. I did log about 35 DX stations through the crashes, notably stations from Poland, Cape Verde, Serbia, Germany, Slovenia, England, Bulgaria and Spain. All had to be in the S-7 to S-9 range to be readable through the noise.

There are ways of hearing things even in these harsh conditions. I have long been a believer that in hunting for weak DX or trying to identify a station in a jumble that the best filter “ is the one between the ears”. There is no physical or software filter in the world that can give totally QRM or QRN free listening. Mechanical or electronic filters can do nothing about two signals that are superimposed or can be so narrow as to strip the intelligence from an AM or Single Sideband signal. And when it comes to static, well that is a whole 'nother story!

The battle to fight static crashes has been going on ever since the days of the coherer detector. While not always continuous, the loud bursts can either totally cover the desired signal or in periods be so much stronger than the desired one that the receiver sensitivity is either pushed down as the Automatic Gain Control operates or that the overall loudness of the static forces the operator to reduce the gain to protect his ears.

In years past, this problem was solved in a simple way. The AGC in the receiver would be turned off and a hard diode clipper in an audio stage would be used to brute force chop off the higher noise peaks.

In some circuits, the level of clipping was adjustable, but in most it was not. There was a switch on the receiver panel marked “NL”, “ANL” or “ Noise Limiter” or just “ Limiter”. Many times if weak signals were involved, the “ bite” of the clipping circuit would not be deep enough and there would still be a considerable difference between the strength of the desired signal being listened for between the crashes and the crashes themselves. With the AGC off to prevent the receiver sensitivity from being driven down by the static and the higher peaks getting past the clipper, most often the “ Clipping” took place within the ears of the DX-er. I am sure that this resulted in more than one case of severely damaged hearing for a couple of generations of hams and listeners! It should be noted that the only way to listen under these conditions is with headphones. Trying to hear and copy through heavy trash with a loudspeaker is a total study in frustration. And if there are other family members in the house within earshot, such can be guaranteed to result in a pillow or something more deadly being thrown your way!

Newer noise blankers were developed in the early sixties that made use of sampling noise pulses usually outside the passband to which the operator is listening. These pulses would then be used to generate short duration signals to turn down the gain of the receiver for just the brief instant of the noise pulse. These can be seen to work by watching the s meter. These work pretty well on short duration pulse noise generated by spark plugs or other electrical “transmitters”. On some receivers, however, they generate some interesting artifacts. If there is a particularly strong signal in the passband that is being sampled and which is not actually heard where the operator is listening, that signal will cover the offending pulses, meaning the receiver won't work to get rid of them and the operator will hear annoying bursts of noise over the desired signal that duplicates the signal he is not hearing. If there is a jumble of strong signals, as during a DX contest, this results in a ragged up and down bunch of noise on the desired signal that is almost more annoying than the background noise that would be there without the noise blanker operating! I have been known to just turn the thing off when that happens.

Other noise limiters or blankers can have other effects;. Some can greatly distort stronger signals, especially phone signals. There are some radios that have adjustable noise limiters, with a control that adjusts how deep the device “ bites” into the signal to allow the operator to set up the best compromise between distortion and noise reduction.

Few of these devices really do much to help with the thundering crashes that accompany spring storms! Sometimes it gets back to the very basic idea of concentration. This means the operator must narrow his mental focus on just what it is he wants to hear. The same kind of effort is often used at a loud, crowded party where there is loud music and dozens of people talking and there is someone with whom you are trying to have a conversation. You simply concentrate on that one person's voice and mentally tune out the noise around you.

This same technique is used in DX-ing. You focus on the one pitch tone of the cw signal or the one voice in the jumble and attempt to mentally separate it from the crowd or from the background noise. It is not easy at first. All those other sounds are distracting, but you must learn to simply ignore them. The development of this ability is really the difference between a good DX-er and someone who just sits in front of the radio waiting for the DX to jump out the speaker. The development of this technique is a good explanation of why a seasoned Dx-er who started out with rudimentary, simple equipment consistently logs more DX than the new arrival in the game whose first receiver is an expensive, filter laden box who depends on those filters to separate the good stuff from the chaff. Sometimes those folks never develop the ability to mentally filter out the trash and sometimes get disgusted with the hobby and drop it because they can't understand why they have bought all the necessary stuff and can't get the good DX into their logs. Sometimes they even get to wondering if the hobby is all its cracked up to be or if the old hands are just making it all up!

Is there a way to tell you how to do this? As I write this, I am mentally scratching my head trying to come up with the words. All that seems to come out is “ concentration”. Pick out one voice and latch on to it, just as you would to one tone in a jumble of cw signals. Mentally listen through the noise and jumble of other signals. Sometimes with AM signals, if they are on exactly the same frequency without loud heterodynes this can be done with practice. With cw, for me it seems to be easier if the desired signal is at a low pitch. If copied at 1000 hz or so and a signal is 200 hz away, there is not much difference in pitch. But if copied at 300 hz, and the interfering signal is 200 hz away, it “ sounds” almost an octave away and is more easily mentally separated. If the interfering signal is much stronger than the desired, this becomes more difficult, especially if it “pumps” the AGC down. Selecting a faster AGC recovery time or, if possible, turning off the AGC altogether may help prevent the receiver from driving the desired signal down in level. If QRN or other man made pulse or broadband noise is not a problem, you may find that turning the noise limiter off will help separate voice signals on the same frequency because the Intermodulation Distortion that would tend to cause the audio signals to seem to blur together may decrease.

By the way, this brings up a point I have heard when people talk about different receivers. Some BCB DX-ers will say that the effect of mentally separating voices or signals in one jumble is easier in some receivers than in others, that some receivers tend to blur them together, while in others, signals though mixed, seem to be more “ separate”. My theory is that the Intermodulation Distortion either in the audio or perhaps even in the Intermediate Frequency stages might be better in some receivers than in others...that the linearity of the amplifiers might be better. That would make sense when some folks talk about how some of the older, high end tube radios seem to “ sound better” or clearer than some of the newer, perhaps budget solid state radios. The old tube amplifier stages, particularly in the older radios tended to be designed with more “ head room” to handle strong signals. To my ears at least, this was the case with the Hammurland and National receivers. But then, that just might be personal preference or what I got used to. I do know from working with broadcast audio for many years that Intermodulation Distortion can make music sound muddy even if frequency response tests with individual tones in an amplifier will show very good frequency response to very high frequencies, while an amplifier with low IM and even less high end response will still tend to deliver music that just sounds “ clearer”.

Back to the issue of filters for a moment. Software filters, crystal filters, and mechanical filters can be very narrow but cannot tell which signal you really want. They can be narrowed down once you pick out the one you want, but you must first determine which one in the pile you want and if you start with the window too narrow, you can't do that. And how can the DSP tell difference between a heterodyne you want to get rid of and a cw signal you want that might be beating against a carrier nearby? Those decisions are best left up to the person twirling the dials! And perhaps the real fine tuning is to be done with that “ filter between the ears”! Good luck in developing your DX-ers ears and fill that log up!

No comments:

Post a Comment