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Thursday, December 31, 2020

So Many Wonderful Things on W7ZOI's Site


There he is.  Wes Hayward, W7ZOI in 1957.  I had never seen this picture before.  I found it on Wes's recently updated "shackviews" web page: http://w7zoi.net/shackviews.html . 

There are so  many treasures on that page, and on all the other portions of Wes's site.

Some highlights for me: 

-- Wes's description of the station in the above picture. 

-- On his page about Doug DeMaw, Wes mentions that after Doug edited Wes's 1968 article about direct conversion receivers, Doug built some himself, experimenting with different product detector circuits. Having used Doug's mixer circuit in many of my rigs, and having recently experimented with different product detectors for my HA-600A, I kind of felt like Doug was watching over my shoulder, guiding me along as I experimented. 

-- Wes's use of a digital Rigol oscilloscope.  Makes me feel better about giving up on my Tek 465. 

-- The page about Farhan's visit to Wes, and the awesome gathering of homebrew Titans that ensued... 

-- Wes's meeting with Chuck Adams.  

Thanks Wes.  Happy New Year and best of luck in 2021!  

Tuesday, December 29, 2020

Ross Hull and Clinton DeSoto -- Ham Radio and R/C Planes

Rogier PA1ZZ sent me a video that linked Ross Hull and Clinton DeSoto to the development of today's killer drones.  While it looks like that is quite a stretch (I'm sure Rogier would agree), the involvement of these two radio guys in the early development of radio-controlled planes is very interesting.  

An article, “Radio Control of Model Aircraft,” by Ross A. Hull and Roland B. Bourne, was the first to describe in detail a working radio system suitable for model aircraft.  They wrote: “Most hams are usually far from being one-hobby men and one discovers, almost invariably, an interest in the other sciences and the crafts. A common interest in ham radio, aeronautics, model building, and photography is almost the rule. We happen to be built that way and our interest in aircraft led us, this summer, to take an active interest in this problem of radio control.” (QST, October, 1937) 

More here: 


And here: 


Tuesday, December 22, 2020

Listen to this! The Kearsarge Mountain Transmission System -- Winter Test Transmission -- 9670 kHz

This is a bit mysterious, but I think that is part of the allure.  Last week reports began to surface on reddit about a shortwave test transmission from a group called the Kearsarge Mountain Transmission System.  Kearsarge Mountain is in New Hampshire. Thomas K4SWL over at the SWLing Post Blog posted a link to a SoundCloud recording of the transmission:     


I don't really know much about the origins of this recording, but -- like Thomas -- I really like it.  It is EXACTLY what you need to have playing in the background as you work on some piece of radio gear.  Trust me on this:  If you are a radio person, you WILL like this.   Have a listen at the link above. 

Here are some excerpts from the reddit posts on this: 

Notice of a KTMS winter test transmission beaming to Europe/USA on 9670 kHz at 2300 UTC on 17.12.2020. This transmission will consist of interval jingles, engineer test signals, rare re-media mixes of cult radio favorites, strange tones, and vox. Thank you for your continued attention as the KTMS crew continue working to bring the SWL community unique transmissions.

Our aim as the programmers of the KTMS Test Transmission is to entertain, edify and get SWLs excited about the possibilities inherent in radio. The crew of KTMS consists of amateur radio operators, community and college radio aficionados, antique radio restorationists, low budget radio astronomers looking for alien life and in general people who love the hobby of radio in all its many faceted aspects.

People who put their own funds towards creating radio on the shortwaves are helping to keep the shortwaves interesting and alive and bringing programming that is original and idiosyncratic to a medium that is often otherwise overrun by nationalists, spies, and fundamentalist preachers. They also help to keep shortwave stations on the air -something that ain’t so cheap. Rather, our view, is that by injecting some creative energy into a project such as our KTMS Winter Test Transmission we are giving back to the SWLing community by 1) creating a show for any listeners who happen to catch it, 2) giving the listeners a mystery that doesn’t revolve around numbers, 3) to do our part to keep the airwaves a little less stodgy.

We view this Test Transmission as an exercise in conviviality, surreality, and genuine love of all things radio and shortwave. 2020 has been such a serious minded year we aim to lighten that load with a bit of old fashioned fun.

For anyone who wishes to enjoy a nice winter evening around your set or online SDR we welcome you to get warm with us around the glowing tubes.

Thanks to all who tune in, and until then 73s.

Monday, December 21, 2020

Observations from the Mars 2020 Opposition


These are some of the drawings that I made during the Mars 2020 opposition. I would go out to my back porch with my 6 inch Dobsonian Newtonian telescope.  I would look closely at Mars, making mental note of what I was seeing. Then I would go to the shack and immediately draw what I'd seen. I would then look for relatively recent images of Mars made by people with more experience and better equipment, and I'd compare my drawings with their images -- this enabled me to understand what I was seeing.  Also useful was the Mars Profiler of Sky and Telescope magazine -- you just plug in the date and time and it displays the part of Mars that is facing us. 

I was especially pleased with the drawing and image above because I'd been reading a lot about the Hellas impact crater in Kim Stanley Robinson's Mars Trilogy.   (SCP should be SPC = Southern Polar Cap.)  

I came to think of this as the "M of Mars."  I think it is just Mare Cimmerium, Mare Hesperia, and Mare Tyrrhenum. 

bcastropics.com made this illustration of how Mars's apparent size has changed

Saturday, December 19, 2020

A Very Clever Way to Build a Moxon for 17 and 20 meters

Jim AB9CN sent us Dan Zimmerman N3OX's page in response to my plea for help in SolderSmoke #227.   I think this is a very clever and attractive way to cover both bands with a single Moxon.  


Of course there are other ways to do this -- other listeners wrote in with confirmation that it is indeed possible to nest a 17 meter element inside a 20 meter Moxon (as is done routinely with Hex beams), but this requires a lot of cut-and-try tuning of both antennas.  More in this in future posts.  Thanks Jim, and thanks to all who wrote in. 

I kind of like the N3OX approach.  I think his design makes it more of a workbench electronic project, as opposed to a mere wire and coax antenna project. But hey, that's just me. 

Friday, December 18, 2020

Grayson Evans KJ7UM's New Blog -- Lots of Great Technical Info


Our friend Grayson Evans KJ7UM, author of "Hollow State Design" has launched a new blog.  It is filled with great technical info.  The blog has links to the articles Grayson wrote for Electric Radio magazine.  

Tuesday, December 15, 2020

SolderSmoke Podcast #227: Solar System, SDR, Simple SSB, HA-600A, BITX17, Nesting Moxons? Mailbag

SolderSmoke Podcast #227 is available: 


Mars is moving away.  Jupiter and Saturn close in the sky.  And the Sun is back in action – Cycle 25 is underway.  Also, the earliest sunset is behind us.  Brighter days are ahead.

Book Review:  “Conquering the Electron”   With a quote from Nikola Tesla. 

No real travel for us:   Hunkered down.  Lots of COVID cases around us.  Friends, relatives, neighbors.  Be careful.  You don’t want to be make it through 10 months of pandemic only to get sick at the very end.  SITS: Stay In The Shack. 

Pete's Bench and Tech Adventures:  

 Backpack SDR  keithsdr@groups.io

 Hermes Lite 2

 Coaching SSB builders

 G-QRP talk  

 A new source for 9 MHz crystal filters

 Bill's Bench: 

Fixing the HA-600A Product Detector.  Sherwood article advice. Diode Ring wins the day.  Fixing a scratchy variable capacitor.  Studying simple two diode singly balanced detectors.  Polyakov.  Getting San Jian frequency counter for it.


Fixing up the 17 meter BITX.  Expanding the VXO coverage.  Using it with NA5B's KiwiSDR. 

Resurrecting the 17 meter Moxon.  But WHY can't I nest the 17 meter Moxon inside a 20 meter Moxon?  They do it with Hex beams.  Why so hard with Moxons? DK7ZB has a design, but I've often heard that this combo is problematic.  Any thoughts?   I could just buy a 20/17 Hex-beam but this seems kind of heretical for a HB station.

Suddenly getting RFI on 40 meters.  Every 50-60 Hz. Please tell me what you think this is (I played a recording).  


Dean KK4DAS’s Furlough 40/20


Tony G4WIF  G-QRP Vids.  Video of George Dobbs. 

Grayson KJ7UM Collecting Radioactive OA2s. Why?

Pete found W6BLZ Articles

Rogier KJ6ETL PA1ZZ lost his dog.  And we lost ours. 

Steve Silverman KB3SII -- a nice old variable capacitor from Chelsea Radio Company. 

Dave K8WPE thinks we already have a cult following.

Dan W4ERF paralleling amps to improve SNR. 

Jim W8NSA -- An old friend. 

Pete Eaton   WB9FLW    The Arecibo collapse 

John WB4GTW old friend... friend of: 

Taylor N4TD HB2HB  

And finally, we got lots of mail about our editorial.   No surprise: Half supportive, half opposed.  Obviously everyone is entitled to their opinion.  And we are free to express ours.  It’s a free country, and we want it to stay that way. That is why we spoke out.

Yesterday the Electoral College voted, finalizing the results.  All Americans should be proud that the U.S. was able to carry out a free and fair national election with record turn out under difficult circumstances. And all loyal Americans should accept the results. That’s just the way it works in a democracy.

We are glad we said what we said. It would have been easier and more pleasant to just bury our heads in the sand and say nothing.  But this was a critically important election and we felt obligated as Americans to speak out.  We'd do it again. And in fact we reserve the right to speak out again if a similarly important issue arises.  

Thursday, December 10, 2020

Pete Juliano's Presentation to the 2020 G-QRP Club Convention -- Building SSB Transceivers

It is a great day my friends.  G-QRP has posted on YouTube the presentations made at their 2020 convention. TRGHS! Above you can see the awesome FB contribution of Pete Juliano N6QW.  I watched it live on Zoom, but have been waiting patiently for the opportunity to share it with SolderSmoke readers and listeners. 

There are many other great videos of convention presentations on the G-QRP YouTube channel.  Here is the link to the channel: 
Be sure to subscribe and give many "thumbs up" to the presentations. 

I see that our friend Alan Wolke W2AEW has a presentation on the NanoVNA in that channel. FB Alan. 

Thanks to G-QRP for running this event and for putting these videos on YouTube. 

Saturday, December 5, 2020

Adding 10 kHz of Coverage to My BITX 17

Solar Cycle 25 is underway.  The Solar Flux Index and Sunspot numbers are up considerably.  I have dusted off my old BITX17 transceiver.  This time around I am using it in conjunction with a waterfall display provided online by NA5B's KiwiSDR receiver, which is located about 9 miles east of me. This SDR receiver allows me to see the entire 17 meter band.   It was this panoramic display that made me pay more attention to the fact that the Variable Crystal Oscillator (VXO) that I am using in this rig prevented me from tuning the lower 10 kHz of the 17 meter phone band (18.110 -- 18.120 MHz). 

I use two crystals switched by a relay to cover the band. One is at 23.149 MHz, the other at 23.166 MHz.  The crystal filter is at 5MHz.  With a coil and some caps I could move the frequencies of the oscillator enough to cover 18.120 to 18.168 MHz (top of the band). 

When I first built this thing, I kind of wrote off the lower 10 kHz of the phone band.  I couldn't get the oscillator to work that low, and I was already satisfied with the top 48 kHz.  But the NA5B waterfall often showed SSB stations in that lower part of the band.  I wanted to talk to them.  So I started thinking about how to do this.       

Looking at my schematic (above) I remembered that most of the frequency lowering was done by L1, a 3.2uH toroid.  I figured that to go a bit lower, I would just have to add inductance.  But I didn't want to lower the frequency provided by BOTH crystals -- I just wanted to bring the frequency with the lower crystal down a bit.  

In my junkbox I found a 1 uH coil.  I disconnected the lower lead of the 23.149 MHz crystal from its connection to the relay.  I soldered the 1 uH coil between the crystal and the relay (see picture above).  This moved the lower limit down to 18.087 MHz. 

Now crystal one provides 18.087 - 18.144 MHz 
         crystal two provides 18.137 - 18.167 MHz

So now I have the whole phone band.  Bob is my uncle. TRGHS. 

This was a very quick and satisfying little fix. As Pete says WYKSYCDS: when you know stuff you can do stuff.  Indeed.  And as I re-build and repair gear that I built years ago, I am often reminded that as time goes by, we learn more. We end up knowing more and being able to do more.   

I am also planning on rebuilding my 17 meter Moxon;  this time I will make it better and stronger.         

"Radio, Radio" By Elvis Costello and The Attractions

Wow. At one point in the video Elvis C. climbs onto a radio chassis and sings while standing between two Thermatrons.  FB OM.  

And here is an interesting article about Elvis Costello's music, and opposition to Fascism. 

Friday, December 4, 2020

The Terrible Collapse of the Arecibo Dish: Climate Change, Hurricane Maria, and Funding Cuts. Also: China's New Dish

From https://www.thewrap.com/watch-crazy-footage-of-the-arecibo-observatory-collapse-goldeneye-video/  :

"Alas, over the 2010s it was battered by a series of severe, climate change-linked tropical storms and hurricanes, culminating in terrible damage inflicted by Hurricane Maria in 2017. Unfortunately the 2016 election led to a government unwilling to fund repairs. Though new sources of funding were cobbled together late in 2018, in late Nov. 2020 it was determined there was no way to safely repair the telescope and the National Science Foundation announced it would be decommissioned.

The decommissioning was supposed to proceed after NSF determined the safest possible method, but physics had other plans. So it is that on Dec. 4, the whole thing up and collapsed with almost no warning."

More info (from NSF): 


Here is a good 2017 article that discusses the electronic and mechanical arrangements at Arecibo, and the budget cuts it was facing.  The article seems to almost predict the collapse: 


Here is a comment from someone who worked there and heard the collapse: 

Jonathan Friedman, who worked for 26 years as a senior research associate at the observatory and still lives near it, told the Associated Press news agency of the moment the telescope collapsed on Tuesday.

"It sounded like a rumble. I knew exactly what it was," he said. "I was screaming. Personally, I was out of control... I don't have words to express it. It's a very deep, terrible feeling."  https://www.bbc.com/news/world-us-canada-55147973?fbclid=IwAR3RuwzTfJmqInrOOFB-nctknDzyB_VSr_qdNrjg9LbbxUnAbynKBv9stPQ

Here is an interesting WIKIpedia article on China's FAST dish, with comparisons to Arecibo: 


Comparison of the Arecibo Telescope (top) and FAST (bottom) dishes at the same scale

Adios Arecibo

Wednesday, December 2, 2020

Radioactive Regulator Tubes -- OA2s! Who knew?


Pete found this amazing collection of articles by master homebrewer Ed Marriner W6BLZ.  They were published in a West Coast radio newsletter during the 1950s and 60s.   The above is from this collection.

I know that this kind of article upsets some people.  They will write in saying that they have using OA2s for 50 years with no ill effects.   Others will sarcastically say that we should just try to avoid eating the OA2s.  I know. Don't blame me.  Don't blame Pete.  You can blame the Navy. And Raytheon. And Cobalt 60 and Nickle 63. 

Monday, November 30, 2020

Adam N0ZIB's Direct Conversion Transceiver

This is obviously very cool, but looking ahead I think Adam should think about adding one more mixer, changing the bias on the TX amps, and adding a mic amp.   Boom:  A Double Sideband Transceiver.  

Pete wrote: When I was in the US Navy and a particular unit did something outstanding – the Command ship would raise the Bravo Zulu Flag  for a job well. Don’t know if you can see it there in MO but I have raised the BZ flag to you. Outstanding and congratulations.

Bill and Pete: 

Just finished a DC transceiver using Arduino nano, SI5351 (my sincerest apologies, Bill), diode ring mixer and lm386 audio amp.  The transmit portion is a two-stage class AB pre-amp (from EMRFD page 2.32), which is driving an IRF510 final (biased at 2.08 volts) from Pete’s design. Output is about 5watts into a CWAZ low pass filter, based on the design from here: https://www.arrl.org/files/file/Technology/tis/info/pdf/9902044.pdf

I’m using a manual TX/RX switch which is doing multiple things. It brings the Nano A1 LOW, offsetting the transmit frequency 600 Hz for CW, grounds the audio input to prevent deafness (learned that one the hard way), and it engages a relay that switches the antenna from the receiver to the transmit, and also turns on the transmitter stages. Keying is through the first stage of the pre-amp.  

I still have some tidying up to do, and I’m not sure the LPF works so well using two component inductors instead of all toroids, but I finished it today and made my first QSO into Ontario almost 1000 miles away.  It’s been great fun! 


Sunday, November 29, 2020

Friday, November 27, 2020

6EQUJ5 -- SNR, the Big Ear Radio Telescope, and the "Wow" Signal


This Hack-A-Day article explains the significance of 6EQUJ5 on the paper readout of the Big Ear radio telescope.  It is a signal-to-noise readout. 

The article also has interesting information about the radio telescope that was used.  

I have on my shelf John Kraus W8JK's wonderful book "Big Ear Two -- Listening for Other Worlds."  John Kraus is the guy who built the Big Ear.  In a reminder of how new radio technology really is,  Kraus got his start in radio as a ten year-old boy in 1920.  He ripped the wire out of the ignition coil  of a Model T Ford to make a tuning coil for a crystal radio.  He took the earpiece out of the family telephone.  His father gave him a chunk of Galena.  He used the crystal radio to listen to the early broadcasts of WWJ in Detroit. 

Thursday, November 26, 2020

VK3YE's Super Simple Phasing Receiver

Really simple, really nice.  I like the innovative way they achieved the RF quadrature: they did it by splitting and phase-shifting the RF signal, not the VFO signal.  I also like Peter's use of the AM broadcast signal to demonstrate the sideband suppression. Then, SSTV for icing on the cake. 

Sunday, November 22, 2020

Wrapping up the HA-600A Product Detector Project -- Let's Call Them "Crossed Diode Mixers" NOT "Diode Rings"

This has been a lot of fun and very educational.   The problem I discovered in the Lafayette HA-600A product detector caused me to take a new look at how diode detectors really work.  It also spurred me to make more use of LTSpice.  

In the end, I went with a diode ring mixer. Part of this decision was just my amazement at how four diodes and a couple of transformers can manage to multiply an incoming signal by 1 and -1, and how this multiplication allows us to pull audio out of the mess. 

But another part of the decision was port isolation: the diode ring mixer with four diodes and two transformers does keep the BFO signal from making its way back to into the IF chain.  This helps prevent the BFO signal from activating the AGC circuitry, and from messing up the S-meter readings. LTSpice helped me confirm that this improvement was happening:  in LTSpice I could look at how much BFO energy was making its way back to the IF input port on the diode ring mixer.  LTSpice predicted very little, and this was confirmed in the real world circuit. (I will do another post on port isolation in simpler, singly balanced diode mixers.)  

At first I did have to overcome some problems with the diode ring circuit.  Mine seemed to perform poorly with strong signals: I'd hear some of the "simultaneous envelope and product detection" that started me down this path.  I also noticed that with the diode ring, in the AM mode the receiver seemed to be less sensitive -- it was as if the product detector circuit was loading down the AM detector.  

One of the commenters -- Christian -- suggested putting some resistance into the input of the diode ring circuit.  I put a 150 ohm pot across the input, after the blocking capacitor. The top of the pot goes to the capacitor, the bottom to ground and the wiper to the input of L1 in the diode ring circuit (you can see the circuit in the diagram above).  With this pot I could set the input level such that even the strongest input signals did not cause the envelope detection that I'd heard earlier.  Watching these input signals on the 'scope, I think these problems arose when the IF signals rose above .7 volts and started turning on the diodes.  Only the BFO signal should have been doing that.  The pot eliminated this problem.   The pot also seemed to solve the problem of the loading down of the AM detector.  

With the pot, signals sounded much better, but I thought there was still room for improvement.  I thought I could hear a bit of RF in the audio output.  Perhaps some of the 455 kHz signal was making it into the AF amplifiers.   I looked at the circuit that Wes Hayward had used after the SBL-1 that he used as product detector in his Progressive Receiver.  It was very simple:  a .01 uF cap and 50 ohm resistor to ground followed by an RF choke.  I can't be sure, but this seemed to help, and the SSB now sounds great. 


One suggestion:  We should stop calling the diode ring a diode ring.  I think "crossed diode mixer" or something like that is more descriptive.  This circuit works not because the diodes are in a ring, but because two of them are "crossed."   From now on I intend to BUILD this circuit with this crossed parts placement -- this makes it easier to see how the circuit works, how it manages to multiply by -1, and to avoid putting any of the diodes in backwards.

I prefer the bottom diagram


I'm left wondering if the engineers who designed the HA-600A were aware of the shortcomings of the product detector.  It is really strange that my receivers lacks a 12V line from the function switch to the product detector. And it is weirder still that the detector works (poorly) even with no power to the transistor.  What happened there?  

When you look at the HA-600A manual, you can see a hint that maybe they knew there was a problem.  For CW and SSB, the manual recommends leaving the AF control at the quarter or halfway point, then controlling loudness with the RF gain control.  This would have the effect of throttling back the RF gain (and the potential for product detector overload) when strong signals appear.  MGC in addition to the AGC.  Any memories or insights on this would be appreciated. 

The Wizard of Horseshoe Bend: VK2FC's Wonderful Projects

Google led me to VK2FC's amazing site.  I was digging up info on product detectors and I landed on Glen's description of his version of the W7ZOI Progressive Receiver.  Glen's website provides a very detailed, board-by-board description of how to build this great receiver.  I now want to build one. 


Glen's site has many other projects.  Check them out: 


And here he is, the Wizard of Horseshoe Bend: 

Thanks Glen. 

Thursday, November 19, 2020

Diode Ring Magic


I continue to work on the product detector of my Lafayette HA-600A.   This work has caused me to brush up on my understanding of how mixers really work.   

I think one of the most interesting mixer circuits is the diode ring.  With just four diodes and one or two transformers, this device manages to take an incoming signal and multiply it by either 1 or -1 depending on the polarity of the local oscillator signal.  That is pretty amazing.  

Alan Wolke W2AEW did an excellent video on this: https://www.youtube.com/watch?v=junuEwmQVQ8

Inspired by Alan, I took my most recent homebrew diode ring mixer (with transformers from Farhan, diodes from Jim W8NSA, and a PC board base from the CNC mill of Pete N6QW) and hooked it up to two signal generators and an oscilloscope.   I had the local oscillator at 10 MHz and the signal oscillator at 7 MHz.   You can see my results in the pictures (above and at the end).  You can see the resulting difference frequency (3 MHz) in the broad up and down pattern.  And you can see the sum frequency (17 MHz) signal in the faster oscillations.  All you would need is some filtering to separate them out.  

I really like the RSGB Handbook diagram (above).  I think the bottom schematic with its crossed diodes really explains how the phase reversal takes place:  when the LO turns on D1 and D3 (the horizontal ones), multiplication by 1 takes place.  But when the LO turns on D2 and D4 (the crossed  diodes), up goes to down and down to up, creating phase reversal, or, in math terms, multiplication by -1.  

At a more basic level, mixing takes place whenever -- in a non-linear circuit -- one signal is controlling the gain or attenuation experienced by the other signal. A complex waveform results, a waveform that contains sum and difference products.  A circuit like the diode ring, that alternately multiplies by 1 and -1, is non-linear in the extreme, and the multiplication is controlled by the LO.  The results can be seen in the diagram's complex waveforms, on Alan's Tek 'scope, and on my Rigol.  And in those complex waveforms you can SEE the sum and difference frequencies. That is really cool. 

Tuesday, November 17, 2020

A Diode Ring Product Detector for the HA-600A? Problems.

Pete advised me to try this a week or so ago, but it took me a while to follow through and try it out.  

I got the two diode, one transformer product detector working well, but with it a new problem arose: 455 kHz energy from the BFO was leaking past the product detector back into the S-meter/AGC circuitry.  This showed up in the form of a constant S-3 reading when I switched to SSB/CW.  This was annoying. 

I figured the problem was that the only signal really being balanced out was the IF signal going into L1 of the product detector.  I took another shot at putting the BFO signal into this port, with the IF signal going into the unbalanced potentiometer port.   This did indeed take care of the BFO leakage S-meter problem, but once again the SSB did not sound great -- I think the old problem of simultaneous envelope and product detection returned.  

This was obviously a port isolation problem.  I remembered that the diode ring "doubly balanced" configuration has much better port isolation.  So on Sunday morning I built one, first in LTSpice and then on the bench.  

For the bench model I used some PC board pads out of Pete Juliano's $250,000 CNC machine.  For the toroids I used two trifilar coils wound by Farhan's dedicated staff in Hyderabad.  The diodes were sent to me by Jim W8NSA.  So there was lots of soul in this new machine. 

The circuit worked in LT Spice and at worked well when tested on my bench with my FeelTech (for the BFO) and HP8640B (for the IF signal) sig gens with my Rigol 'scope watching for the audio out.  

But I ran into some problems when I popped the new board in there in place of the old product detector:  The 455 kc BFO leakage problem is gone and the S-meter is where it should be, but...

-- I'm seeing a return of the old simultaneous envelope and product detection problem.  SSB was sounding scratchy again and indeed, when I removed the BFO signal from the diode ring circuit I could hear SSB signals making it into the audio amplifiers.  These signals sounded just like AM signals as heard through an envelope detector without a BFO. 

-- The diode ring circuit also had a very bad effect on how the HA-600A worked in AM mode.  It seemed like the new circuit was loading down the diode AM demodulator.   SW broadcast signals sounded awful in the AM mode until I disconnected the IF input to the diode ring circuit (this input is NOT switched -- it is always connected, even in the AM mode). 

So, for now, am back to using the two-diode, single transformer, singly balanced product detector with IF signal going to the balanced (L1) port and the BFO going in through the wiper of the 100 ohm pot.  

Any suggestions on how to overcome the problems with the diode ring circuit?  

Monday, November 16, 2020

Shortwave Listening

I was listening to Glenn Hauser's excellent shortwave listening show on WRMI this week, and he mentioned a technique that I had not been aware of:  listening for the harmonics of distant stations. This is apparently being done for medium wave broadcast stations, but also for stations in the lower frequency range of the shortwave bands.  If propagation makes it highly unlikely for you to hear the main frequency, perhaps propagation would be better for the the second or third harmonic.  This method is discussed here:   


Here is some more background information on Glenn Hauser: 


Here is a real treasure trove of articles and recordings about the history of shortwave radio: 


Sunday, November 15, 2020

HB-2-HB Contact! N4TD's Amazing Homebrew 20 and 40 meter SSB Transceiver


Oh man, this doesn't happen every day.  Using my homebrew BITX 40 DIGI-TIA,  I had just finished a great QSO on 40 meters with Greg W8GP, talking about Heathkits and homebrew radio.   Then I got a call from Taylor N4TD.  He told me he is a SolderSmoke listener.  Then he just casually mentioned the really big news:  HE TOO WAS ON A HOMEBREW RIG! 

I told Talor that this a rare occurrence.  He agreed. 

Taylor told me his rig is for 20 and 40.  It is a single conversion design with a 9 MHz IF and a crystal filter.  Using a Ukraine-made PA board, it puts out 50 watts.  

Taylor sent me some pictures and wrote: 

Hi Bill,

It was great to work you homebrew to homebrew.  As you said, that doesn’t happen very often.  I used a modular architecture for this radio.  The module size is the ExpressPCB miniboard  size, so they are less expensive and all the same size so they can be moved around.  All the boards are homebrew except for the final amplifier module.  The PA module I got from 60dbm in Ukraine through eBay.  I had tested this module before and found it to be solid, and it was more economical than building the PA from scratch.  It delivers 50W+ and has been reliable through all my sometimes abusive testing.  

73 Taylor N4TD

Taylors rig -- Top view

Taylor's rig -- Bottom view

Saturday, November 14, 2020

The Dish -- Virtual Tour -- New Indigenous Name

Thanks to Peter VK2EMU for this update on the Parkes radio telescope.  Parkes is the subject of our favorite movie about an antenna:  'The Dish."   If you haven't seen it, well, you are just wrong. 

The video update is very nice, with an interesting juxtaposition of old and new test gear.  

But the coolest thing that Peter sent us is the story of the Parkes Radio Telescope's new indigenous name: Murriyang in the Wiradjuri language: 

Thanks Peter. 

Friday, November 13, 2020

How Does My Singly Balanced, Two-Diode, Single Transformer Product Detector Really Work?


As young James Clerk Maxwell used to say, "What's the go of it?"  and "What's the particular go of it?"

I studied this circuit carefully when I was using it as a balanced modulator in my DSB rigs.  I wrote up my conclusions in my book "SolderSmoke -- Global Adventures in Wireless Electronics." 


When I was using it as a balanced modulator, I had the RF "carrier" signal going into L1. This RF signal was 7 dbm, enough to switch the diodes on at voltage peaks.  With the "center tap" of L2/L3 grounded for RF, this meant that when the "top" of L2 is negative, the "bottom"  of L3 is positive.  In this situation BOTH D1 and D2 will turn on and conduct. 

When the top of L2 is positive, the bottom of L3 is negative and neither of the diodes is on.  Neither conducts. 

So we have the RF signal turning the diodes on and off at the frequency of the RF signal.  

Audio from the microphone and mic amplifier is sent into the center tap connecting L2 and L3.  The level of this audio is kept low, below the point where is could turn on the diodes.  The center tap IS grounded for RF by the .1uF capacitor, but it is NOT grounded for AF.  That is key to understanding this circuit. 

In essence by turning the two diodes on and off at the rate of the RF signal, the audio signal is facing severe non-linearity through the diodes.  We could say it is alternately being multiplied by 1 and 0.  This non-linearity is what is required for mixing.  We therefor get sum and difference products:  Sidebands.  At this point, Double Sideband.  

The way the transformer is set up means the RF carrier signal is balanced out:  Even when the two diodes conduct, the top of R1 and the bottom of R2 are of equal and opposite polarity, so there is no carrier signal at the junction of R1 and R2 (they are actually a 100 ohm variable resistor that can be adjusted to make SURE they balance out).  So the carrier is suppressed and all that remains are the sidebands:  Suppressed Carrier Double Sideband. 


What happens when we use this circuit as a product detector in a receiver? Let's assume we are working with a 455 kc IF.   If you run a 454 kc 7 dbm BFO signal into L1, it will turn the diodes on and off as described above.  But you will NOT be able to put the 455 kc IF signal into the center tap of L2/L3 -- that center tap is GROUNDED for 455 kc.   So you will have to run your IF signal into the resistors, and take the audio output from the center tap of L2/L3.   This works.   I tried it in my HA-600A.  But there is a problem: Envelope detection.  

In this arrangement, we are balancing out NOT the 455 kc IF signal, but instead we are balancing out the BFO.  We don't really NEED to balance out the BFO -- it can easily be knocked down in the audio amplifiers, and IT is not responsible for the problematic envelope detection.  We DO need to balance out the IF signal, because if that gets through we can get simultaneous "envelope detection" and product detection.   And believe me,  that does not sound good.

So I tried putting the IF signal into L1, and the BFO signal into the resistors (as shown above).  I took the audio  from the junction of L2/L3.  This seemed work better, with envelope detection greatly reduced. 


But how is this circuit mixing in this configuration?   The strong BFO signal is still controlling the diodes, BUT, with the BFO signal coming in through the resistors,  when the top of R1 is positive the bottom of R2 is ALSO positive.  In this situation D1 will conduct but D2 will not.  The IF signal is facing a big non-linearity. This will result in sum and difference frequencies.  The difference frequency will be audio.  But with D1 and D2 turning on and off in a very different way than we saw in the balanced modulator, how does the mixing happen?  

I think the answer comes from the summer 1999 issue of SPRAT, the amazing journal of the G-QRP club.  Leon Williams, VK2DOB wrote an article entitled "CMOS Mixer Experiments."  

Here is Leon's 74HC4066 circuit: 

I think those two gates (3,4,5 and 1,2, 13) are the functional equivalent equivalent of the two diodes in our product detector. In Leon's scheme the VFO is supplying signals of opposite polarity.  Ours is providing only one signal, but the fact that the diodes are reversed means that they act just like the gates in Leon's circuit.  The transformer is almost identical to the one we use in the product detector. 

Let's look at the output from Leon's circuit: 

"VFO A" going high is the equivalent of the BFO going to its positive peak and D1 conducting. 
"VFO B" going high is the equivalent of the BFO signal to its negative peak and D2 conducting. 

Take a ruler, place it vertically across the waveforms and follow the progress at the output as the two signals (RF A and RF B) are alternately let through the gates (or the diodes).  You can see the complex wave form that results.  The dashed line marked Audio Output shows the difference frequency -- the audio.  That is what we sent to to the AF amplifiers. 

One concern remains:   

What happens when the 455 kc IF signal getting to L1 get so strong that IT also starts to turn the diodes on and off?   I think this will result in distortion, and we can see this in LT Spice.  

Here is the output waveform when the If signal at L1 is kept below the level that would turn on the diodes: 

Here you can see it with a much stronger IF signal:  

The output waveform becomes more of a sawtooth. 

How can I prevent this from happening?   I know AGC should help, but the AGC in this receiver doesn't seem to sufficiently knock down very strong incoming signals. 

Does my analysis of these circuits sound right? 

Monday, November 9, 2020

Improving the Product Detector in the Lafayette HA-600A

Diode product detector on the left, BFO amp in the right

As noted in an earlier blog post, I didn't like the sound of SSB and CW when using the product detector in my Lafayette HA-600A.  It just did not sound right.  The receiver sounded fine on AM with the diode detector.  But when I switched in the product detector, it sounded bad.  The BFO was fine.  The problem was there even when I used an external BFO.  And SSB sounded great when I just coupled some BFO energy into the IF chain and used the diode detector to listen to SSB.  My suspicions were focusing on the very simple BJT product detector. 

Steve N8NM built the HA-600A product detector both in LTSpice and in the real world.  It worked fine in both versions.  Steve even put the product detector into his S-38 receiver -- he reported it worked well there.  

I too built the thing in LTSpice.  Then I went and rebuilt the circuit on a piece of PC board.  I connected the new circuit to the HA-600A, using my external FeelTech sig generator as the BFO.  IT STILL SOUNDED BAD ON SSB.  

At this point I started Googling through the literature.  I found a promising article by Robert Sherwood in December 1977 issue of Ham Radio magazine entitled "Present Day Receivers -- Problems and Cures." Sherwood wrote: 

"Another area that could use additional work is the product detector.  As the name implies, its output should be the product of the two input signals. If BFO injection is removed, output should go to zero. If this is not the case, as in the Heath HW series, envelope detection is also occurring, which causes audio distortion." 

I checked my circuit.  When I removed the BFO signal from the product detector, envelope detection continued.  In fact, with the receiver in SSB mode, and with the BFO disconnected, I could listen to the music of WRMI shortwave.  It seemed that Sherwood was explaining well the problem I was having: Simultaneous envelope and product detection was making SSB sound very bad in my receiver.   What I was hearing just seemed to SOUND like what you'd get with a mixture of product and envelope detection: "scratchy" sounding SSB.  This also seemed to explain why SSB would sound fine when using the diode detector with loosely coupled BFO energy -- in that case it would be envelope detection only, with no ugly mixture of both kinds of detection.

So I built a better detector.  I had had great luck with the two diode one trifilar transformer singly balanced design used by both Doug DeMaw and Ashhar Farhan. I built the circuit using one of the trifilar toroids given to me by Farhan, and connected it in place of the original BJT product detector.  With the FeelTech Sig Gen as BFO, I got good results -- most of the signal disappeared with I disconnected the BFO.  Looking at the circuit, I realized that I was balancing out not the IF signal but instead the BFO signal.  To minimize envelope detection I needed to put the IF signal on the balanced input of the product detector (to L1 in the diagram above).  When I did this, envelope detection seemed to disappear completely and the receiver went silent when I disconnected the BFO.  

Finally, I needed to find a way to use the BFO in the HA-600A with the new product detector.  Obviously I needed more BFO signal -- I needed about 7 dbm, enough to turn on the diodes.  I converted the outboard product detector board into a simple amplifier and put it between the HA-600A BFO and the BFO input port of the new product detector.   This works fine. 

A few issues remain: 

1) The output from the HA-600A BFO through the above BFO amp (and across the 50 ohm resistor) is NOT a pretty 455 kc sine wave.  But the peaks of the distorted wave appear to be enough to turn on the diodes, and when I look at the voltages across each diode (on my two channel 'scope) I see mirror images -- one is on when the other is off.   Is this good enough? 

2) Moving the BFO input from L1 to the junction of the two 50 ohm resistors (that is actually a 100 ohm pot) has big implications for how this mixer works.  With the BFO energy going through the toroid, BOTH diodes are being alternately turned on and turned off.  But both are on, and then BOTH are off.  With the BFO energy going in through the other side, one diode turns on when the other is off.  I think the mixing result is the same, with AF coming out of the output port, but the way the mixer works in this configuration is very different. Does this sound right? 

Designer: Douglas Bowman | Dimodifikasi oleh Abdul Munir Original Posting Rounders 3 Column