Wednesday, December 16, 2015

My First Shortwave Receiver

       Everyone will always remember the first receiver that took them into the shortwave bands. I found mine in storage at my folk's house this week, retrieved it and brought it home.  I thought it would be interesting to share what got me started.
       It was a homebrew receiver built from parts that had been part of lab experiments that came with a radio-tv repair correspondence course that my father had taken in the early 1950's.  It was, in fact, watching him solder and work on assignments that first stirred my interest in radio.
       After he completed the course, the materials, including parts and printed lessons were stored away in a closet and when he figured I was old enough to know what to do with them,  I was given the boxes of goodies. I was eleven years old at the time and already DX-ing with table radios and had begun poking around in them, trying to improve performance and hoping to get something going that would tune the short wave bands.
       Among the boxes of parts and papers there were chassis with holes punched and chapters on designs for various circuits used in superhet receivers.  It was a matter of putting the pieces together to get the receiver transferred from sections of lessons to being an operating radio.  There was one experiment that detailed making an H.F Converter to tune from about 6-18 MHz that would convert those signals to be tuned in by a regular broadcast receiver tuned to just above the high end of the broadcast band. That design became the front end of my first receiver to tune higher than the 160 meter amateur band ( I had previously gotten a standard broadcast receiver to go that high by pulling out the tuning slugs and loosening the trimmer capacitors as far as they would go).
       With the help of the head tech and owner of Lawson's Radio and TV repair shop I managed to get the thing assembled and working. I am not sure how many times I had the chassis in the basket of my bicycle for the two mile ride from our house to his shop, but with his help and parts from his parts stash and junk box,  the radio became a working reality.
       The basic radio was a six tube superhet that tuned the broadcast band with one RF stage and one IF stage.  It had an AC/DC power supply with a double section capacitor and choke filter.  The RF stage was modified with a double tuned input aligned to 1650 kHz with another fixed tuned circuit on its output.  The HF converter consisted of a 12K8 pentagrid converter circuit with its own filament transformer supply that lifted B+ voltage from the basic radio.  The converter had its own two gang variable capacitor with the original radio set up fix-tuned to 1650 kHz.

Front and top view of the chassis of the receiver. The main broadcast band tuning capacitor is behind the large drum to the left with the HF converter variable capacitor that provided tuning for the short wave range is on the right. other controls volume, tone and power on-off switch.

Top view of receiver chassis At top left is the edgewise S-meter.  This was actually a milliammeter reading plate current of the 455 kHz IF amplifier stage.  The stronger the signal, the greater AGC voltage applied to the stage and resulting in lower  plate current for the 12SK7 IF amplifier tube.  Thus the S-meter actually read backwards.  The stronger the signal, the lower the plate current indication. To make the meter read correctly,  it was simply mounted upside down! The glass tube to the upper right is the 35L6 audio output tube.  The metal tube to the upper left is the 12SQ7 triode-dual diode tube that served as detector, agc and first AF amplifier.  The glass tube at mid rear of the chassis is the 12SK7 455 kHz amplifier.  The metal tube behind the upper variable capacitor is the 12SA7 Second converter stage. The metal tube below the upper variable capacitor is the 12SK7 RF amplifier for the original BCB set redesigned with fixed coil tuning that became the 1650 khz IF amplifier stage. The glass tube at the rear of the main chassis is the 35Z4 rectifier tube. The metal tube with the grid cap connection above the lower variable capacitor is the 12K8 first converter. The lower variable capacitor tuned the HF converter and was the one used for selecting the shortwave frequency desired.

Under chassis wiring of the receiver. The HF converter chassis is to the left.

Close detail of the upper part of the HF converter stage showing the 12K8 Pentagrid Converter tube, the variable capacitor and the input tuning coil.

Closer top view showing the chassis top near the tuning capacitor for the base BCB portion of the receiver. The coil to the left of the larger variable capacitor is part of the fix-tuned 1650 khz IF circuit.  The two silver cans are the electrolytic power supply filters. The clips on the rear of the chassis are the connections for antenna and ground.  The small transformer on the rear of the HF converter chassis is for filament voltage for the 12K8 converter stage. The other six filaments are in series and operate like a conventional  AC/DC receiver.
Under chassis detail of the HF Converter stage. The large coil is the oscillator coil for the 12K8 pentagrid converter. The small coil to the upper left is one of three tuned circuits between the converter and the rest of the receiver tuned to 1650 khz.

Detail of IF amplifier stage showing the home made IF transformers. Windings were taken from discarded IF transformer cans, slipped over wood dowels and mounted horizontally to allow them to be placed farther apart than originally made to provide looser coupling in an attempt to get better selectivity.  Fixed regeneration was also introduced into the stage to improve selectivity of the stage. Tuning was accomplished with mica trimmer capacitors below the coil and reachable by screwdriver through holes in the chassis from below.

The receiver was not the highest performing beast on the planet,  but it did provide me with a method of listening in on the hitherto un available shortwave  frequencies.  It served for about three years as a way to explore my new world.  I learned a lot about receivers from the project.  In later years, I learned a lot that could have been done to improve it, including such things as converting the AC/DC supply to a transformer supply, which would have required another small chassis to handled the transformer. Voltage regulation could have improved stability.  A manually tuned preselector  could have eliminated the aggravating images that appeared on some places on the dial.  The addition of a 1650 khz crystal between the two tuned circuits between the converter and the rest of the receiver could have greatly improved selectivity. The addition of a beat frequency oscillator could have improved cw reception that was obtained in the original by simply increasing the IF regeneration until the 455 khz stage went into oscillation.  And a Q-multiplier added to the 455 khz IF could have finished the job of making the tuning really sharp.  But that is probably just day dreaming " what-if's"...though to this day I wonder how it would have turned out had I known that at the time and actually made it happen.  In any event,  this receiver which probably looks a little crude to most was the source of many great hours of listening and adventures in the wilds of the short waves.


Is the Band really Dead? Ten Meter Test and More

       I hope this edition will not be two disjointed because it will cover multiple points, but at least as I begin to write, they are all connected.
       This past weekend was the ARRL Ten Meter DX Contest. This is not one of the " Biggies" but is one that is usually lots of fun and almost always provides some surprises.  it is also a good one for SWL's because it is both phone and CW on the same weekend,  meaning all can tune in and experience some of the same prop( any many of the same stations) whether they copy CW or not.
       For hams,  having both modes on the same weekend gives activity even when the band appears to be " worked out", because when one starts running into the same stations with no new ones, one can just go to the other mode.  Its also fun to run into the same station or ops on both modes because all of the other guys are doing the same thing.
       Which leads me to the question in this edition's title: " Is band really dead?"  I never cease to be amazed by ten, and to a lesser extent, six meters. I so often hear or read on internet posts that the bands are dead and that there is nothing to hear,  but upon tuning slowly and carefully, often find there is something there.  There are times on ten meters particularly that when I hear no activity from operators, I can tune up into the  " beacon band" ( roughly 28180-28300) I can hear the QRP beacons identifying their little heads off.  The band is open, just nobody's home.
       Sometimes I think for hams,  the band being dead becomes a self fulfilling prophecy as a result of folks expecting the band to be dead and not even checking it. Also, even if the band is inactive but open for the beacons, nobody finds anybody because all are just listening.  This is one time that the idea of " listen, listen, listen and THEN call" should be abandoned, When the beacons are in and the background noise is up a bit, then is the time to call CQ, not just once, but several times.  It might also be a time to self post one's self on DX Summit to try to attract attention ( though some might discourage self posting this might be an exception)  If not wanting to post yourself,  at least pot beacons being heard.  Either might attract other folks, maybe even DX, to the band.
       In either event, it was with a little trepidation that I tuned up on the band Saturday morning. But I should have known that there is nothing better to open a "dead" band than a good contest.  At 1352 GMT ( 0752 local CST) I immediately ran into HT7C coming into Central Texas at 28019.3 from Nicaragua with a pretty good signal. Only minutes later, a Special Event station from Chile was in at 28005.5 XR90IARU.  The morning was filled with logging Central and South American stations. Interestingly, North American stations were absent.  I could hear the South Americans working them, but they were for the most part inaudible here in Waco.  I did hear K5NA, but that signal was obviously coming in via backscatter.  One European, YT9X from Serbia, propped in at 1444 GMT.
       About  midday, things got a bit more interesting.  Stations from  Eastern Canada began to come in, but the band also opened to the Pacific.  Beginning shortly before noon, several Hawaiians were heard along with the catch of the day, 5W1SA from Somoa who was audible most of the afternoon with signals ranging from S-5 to S-9. No Australians were heard but did hear ZM1A from New Zealand on 28040 at 2227 with a signal just above the noise. About 2315 ( 1715 Local CST) the band snapped shut like the slamming of a door.
       Sunday was almost a repeat, with the exception that by mid morning many US stations were heard.  By noon, a few African stations were coming in, including 6W1SR from Senegal  at 1846.  A couple South Africans were heard.  I am not sure if the band was not open or this particular contest just did not attract that much attention from the continent. I heard US stations working Europeans but heard few myself.  Once again, there were numerous Hawaiians coming in.  By noon,  the Northeast US stations were booming in with amazing signals, quite unlike the day before. Many pushed the S-meter will above S-9.
      All in all, a band that was supposed to be dead was simply unoccupied until the contesters showed up.  I think many times hams, who as a group should know better, don't check the higher bands because they might take a look at some of the Facebook pages and see postings of "bands terrible" and don't turn on the radio.  Some may even fall into the trap of thinking that we are on the down side of the sunspot cycle and the bands are expected to be bad. Sometimes even DX Summit can give one the feeling that nobody's home.  But if we all do that, nobody will really know that the ionosphere is there waiting to be used! Besides, someone has the be the first to post something to SX Summit!!
       The same goes for SWL-ing.  I have really enjoyed the Facebook groups I have discovered in recent days.  They members are fun, they are sharing the love of the hobby with several diverse interests showing up.  With members in different parts of the globe, it is particularly interesting to read about prop from " the other end of the path."
       But often, I will notice posts of " band dead, hearing nothing," or that one of the usual strong band occupants of 31 meters or some other band is not there. You can almost hear the disappointment and the feeling is that the poster will go off and do something else, perhaps missing a real nugget!
        There are times when some of the semi-local stations like Radio Havana, WRMI, WBCQ and others may be weak, fluttery, or almost non-existent.  But then is not the time to turn off the radio and turn on the TV! Before doing that, it is a good thing to check around and see what else is going on down " in the weeds".
       When I see that posted on the groups,  the first thing I do is go check the WWV frequencies.  If on 10 and 15 MHz WWV is weak but WWVH is strong or even covering it, the band is far from dead...just open really, really long! Then is the time to turn on the BFO and start trolling 31 meters for carriers,  then when one is spotted, go back to the AM mode and try to pull something out. Chances are, down in the grass will be a fluttery signal that will have the S-meter dancing a bouncy dance or maybe not even moving.  There those of us in North America will find regional Chinese stations,  All India Radio long path, Vietnamese stations, maybe even some of the big boys broadcasting to somewhere other than North America.  If a semi local station is weak and fluttery or marked with echo, chances are you are hearing it back scatter or multi path. That is not the time to turn the radio off,  but instead time to fill the coffee cup ( or other drink glass!) and pull on the headphones and pull out some new ones!
       If the upper bands are not showing much, keep dropping down to lower and lower bands until you find something.  I usually start the evening with a sweep of the WWV and CHU frequencies and end up with a de-facto prop chart in the log.  A look at the signal strengths for WWV,  WWVH and CHU on 2.5, 5, 10, 15, 20 and 25 MHz and 3.33, 7.68 and 14.67 MHz respectively  gives a pretty good picture of what is happening on the bands.  Then I start with the higher bands and work my way down until I start finding the goodies.  Sometimes a weak signal from WWV does not indicate a dead band, but one that has its legs stretched out long. ( at least for here in your location, things might be different but you can learn over time what it all means)
       One of the other things I have noticed in the Facebook groups, particularly among relative newcomers to the hobby,  its the notion that a simple short piece of wire thrown up along a fence or around a room is adequate for listening.  In a way that is true, at least for casual listening to the power house stations targeting your listening area.  Others in the groups will post suggestions that such is all they need,  and that is true up to a point.
       However,  I think this leads to some of the disappointed postings about the bands being in poor shape. If signals are just a few DB above the local noise level,  if the bands take a shift,  those signals might drop below the threshold of what that indoor or low horizontal wire might be able to deliver above the local noise. Or, a low horizontal antenna might not respond to DX signals arriving at a low vertical angle.   For the new guys,  I would suggest that that for beginning, those kinds of antennas are good for getting your feet wet,  but nothing beats a good outdoor antenna at a decent height.  Old CB antennas can deliver pretty good signals from stations pushing over a million watts of effective radiated power toward you,  but won't do much for hearing a 10 kw station from Africa or Asia if you are in the US. The problem is not sensitivity of the radios, with many of the new ones having really good sensitivity.  Its the ratio of the desired signal to the noise.  Getting your antenna away from the noise and having enough up there to have a decent amount of voltage induced in it by a passing signal is the true answer. 
       And the oft forgotten part is the all critical angle of radiation.  A low, horizontal antenna will respond most to signals arriving at a high angle. Some low angle signals will be heard, but there will be a much weaker voltage induced by that passing signal. Horizontal antennas higher off the ground will respond to lower and lower angle arriving signals. A wire in the clear and not running though tree limbs or foliage will do much better.  Wires running through trees also suffer the danger of having noise picked up by the trees that may be near power lines with other parts of their canopies.
        Vertical antennas or inverted L's will also do much better with the low angle arriving signals.  The ionosphere acts according to the same laws of physics as does a mirror.  The angle of reflection equals the angle of incidence. Signals from a greater distance arrive at a much lower angle than nearby signals. 
       Often when a given band is just about to close, the effective height of the ionosphere is the greatest,  meaning that signals from a greater distance will be coming in. This means that being there for the opening and the closing of a band might be the best time for getting the real DX!
       Speaking of the web, Facebook groups and other things,  a big thank you goes out to Bob Padula in Australia for accepting my article on receivers used through the years and posting it on his excellent site The Mount Evelyn DX Report at    I think I have links he posted there for many of you finding this site.  The increasing number of DX groups brings a new aspect to the hobby, including the sharing of what is going on in real time.
       As always, I welcome comments and other ideas for articles and even DX tips.
       73 and good DX!