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Friday, September 30, 2022

Sticker News (from Holland)

Lex PH2LB sends this really interesting Vice story about stickers and sticker culture.  Who knew?  The original article is in Dutch, but just hit the Google Translate button and Bob will be your uncle. 


I shared with our Solder-Sticker Community these thoughts: 

Really cool Lex.   This reminds me that we need more stickers.  And maybe a new design.  The IBEW thing confuses Americans -- here it looks like a labor union thing. 

The Color Burst Liberation Army logo (below) is one possible element that we might want to include.  I think our new stickers should exhort people to DO something:  


"LIBERATE 3.579 MHz!"  




Things like that. 

Thoughts?  Discuss.  

Bandsweep with the New Homebrew 40 meter Direct Conversion Receiver

It is inhaling nicely but some improvements are still pending. Click on the video above to see and listen to the bandsweep done on 40 this morning. 

-- The front end consists of capacitive divider input impedance matching circuit, followed by one LC circuit and an FET RF amp.  

-- The VFO is a super-simple Colpitts design by Farhan.   The two feedback capacitors do double duty in the LC tan circuit.  

-- I am using an old variable capacitor instead of the PTOs that we have been experimenting with.  

-- The mixer is singly balanced using one trifilar toroid and two diodes.  We have found out that even with these three simple devices, there is significant variation in how people connect them to VFO, RF in and audio out.  I think we have found the best way to do this:  Be sure to put the VFO on the primary of the transformer, and let this signal turn the diodes on and off. 

-- For the AF amplification, I have one FET, followed by two BJTs.  I have a small audio transformer between the speaker and the final AF amp.  There is plenty of audio. 

You may wonder why, after all the SSB superhet transceivers, I am building a simple Direct Conversion receiver.  Well, we hope to help a bunch of high school kids build one, so we need to be really familiar with how it works.  And I find that as simple as it is, there is still a lot to learn in a project like this. 

Thursday, September 29, 2022

A Bout of Direct Conversion-ism in Northern Virginia -- DC Receivers Under Construction

There I was, minding my own business,  when suddenly I was dragged into the construction of Direct Conversion receivers. 

Here is a video about my latest effort.  But I feel the urge for more simplification -- I may go back to the seminal DC receiver designed by Wes W7ZOI and presented in the November 1968 issue of QST.  It is on page 15 here: https://worldradiohistory.com/Archive-DX/QST/60s/QST-1968-11.pdf

Tuesday, September 27, 2022

SolderSmoke Podcast #130 Feb 5, 2011 Trivia Questions! Recorded on a D-104! Heroic efforts to Suppresssss Ssssibilant SSSSs

February 5, 2011

For many reasons, this is a pretty amazing SolderSmoke episode. We presented some radio-electronic trivia questions. This is the one where I used an Astatic D-104 microphone to record the show. I even describe putting Starburst candies, Chicklet gum, and even Duct Tape on my teeth in an effort to get rid of the sibilant S problem. This was truly heroic podcasting my friends! There is lots of other great stuff in this one. Maria arrives in the shack with both Tyson the cat and Cappuccio the dog! Lots of space talk. The story of Grote Reber's name. Much more: Some important trivia questions (answers will be given in the podcast): What is the connection between radio amateur (and pioneer radio astronomer) Grote Reber and world famous astronomer Edwin Hubble? In the world of radio-controlled helicopters, what is "TBE"? In Britain they have radio "rallies," but at one time they had hamfests." When did this happen? Only one terrain feature on Venus is named for a man. He is man of radio. Who is he? What music did Neil Armstrong listen to on his way to the moon? Plus: -- The Air and Space Museums -- Our music -- The SolderSmoke D-104 -- NASA asks for our help -- Telescope repairs -- A BFO for the Trans-O -- Back on Echolink MAILBAG! Please send reports on the D-104 audio!

Monday, September 26, 2022

Open Circuits: Cutting Open Components for a Look inside -- First Chapter Free


Bob KD4EBM sent me this: 

“…. Eric Schlaepfer (@TubeTimeUS) and Windell Oskay (co-founder of Evil Mad Scientist)’s latest book, Open Circuits, is a celebration of the electronic aesthetic, by literally opening circuits with mechanical cross-sections, accompanied by pithy explanations and illustrations. Their masterfully executed cross-sectioning process and meticulous photography blur the line between engineering and art, reminding us that any engineering task executed with soul and care results in something that can inspire feelings of awe (“wow!”) and reflection (“huh.”): that is art. …”

I saw on Hack-A-Day today that the first chapter of the book is available as a free download: 


Many of our favorite parts are dissected in that chapter.  The innards of a ceramic disc capacitor, for example, are shown above.

Thanks Bob! 

Saturday, September 24, 2022

Talking about Homebrew Radio with the Williamsburg Virginia Radio Club

It was very nice to talk to Dino KL0S and the Williamsburg area radio club.  Thanks Dino! 

Wednesday, September 21, 2022

Nick's "Shelf-17" Al Fresco Homebrew Transceiver -- Frank Jones Would Approve!

This is a really wonderful video from Nick M0NTV.  

Some reactions:
-- Wow, a real celebration of Al Fresco!  And of the wooden chassis -- Frank Jones would be so pleased. 

-- Nick's description of the tales of woe caused by metal boxes is right on the mark, as is his description of the benefits of leaving the circuitry visible.  He's right -- this is an art and science kind of thing. 

-- That's a shelf, but it is FAR from being a shelf of shame! 

-- I am jealous of the S-meter.  I may need to include one of those in future rigs. 

-- Great to hear the shout outs to VK3HN, ZL2CTM, and N6QW -- we are are indeed the IBEW! 

-- I think we can see the N6QW influence in Nick's decision to use a steerable filter/IF amp board.  FB. 

-- I liked hearing Helio PV8AL in Boa Vista.   When I was building simple Direct Conversion receivers for 40 meters, I knew that I had it right when I could hear Helio's roosters in the morning! 

Be sure to watch this video, and to subscribe to Nicks Channel: https://www.youtube.com/c/M0NTVHomebrewing
He has done many other videos providing more details on the various stages that make up this rig. 

I hope to work Nick on 17 HB2HB soon. 

Thanks Nick! 

Tuesday, September 20, 2022

The James Webb Space Telescope -- Background Video and Update Article

The Scientific American video (above) has some really interesting info on how they built the JWST and what its capabilities are.  For example, they tell us that it could detect the heat from a single bumble bee at the distance of the surface of the moon.  How the mirror was made, and how it is adjusted in space is really amazing. 

The Scientific American article provides a good, balanced view of how the initial results from JWST are calling into question the basic models of cosmology: 

Monday, September 19, 2022

Video version of SolderSmoke Podcast #240

Show notes appear in blog post below. 

I had some technical problems that caused audio distortion, but we talked about so much good stuff that I decided to go ahead and post the podcast.  We can't let the perfect be the enemy of the good enough. 

Thanks again to Pete Juliano for his willingness to use a big chunk of his very limited free time. 

SolderSmoke Podcast #240 NIMCELS? Hex DX, CBLA RX, PTO? Solid State HT-37? Exotic caps, Benches, SBE-33, TenTec 540, KWM-1 MAILBAG


SolderSmoke #240 

Monday September 19, 2022


Sorry, I had some technical difficulties so my audio is a bit distorted. 

NIMCELS?  Niche Internet Micro Celebrities. I dunno....  Can our listeners come up with an acronym that describes our place in the universe?  

Sponsors:  Parts Candy.  Carlos in Chicago.  Great leads.  Don't scrimp with a crimp!  You need some good quality test leads on your workbench.   Get yours from Carlos.  Click on the picture on the blog page or go to PartsCandy on e-bay.  

"Techno Wizardry"?   Our lawyers at Dewey Cheatham and Howe will be after them for that!  

Bill's Bench: 

Hex Beam has made me a DXer.  91 countries. Having fun. Mozambique (Long path), San Andres island, and Saudi Arabia, all on Friday. All homebrew SSB 100 watts. 

Put my old 20 meter CW station back on the air:  Barebones superhet and the VXO 6 watter.   WYKSYCDS. This may be one of very few CBLA Receivers in the world!

Been getting active in the Vienna Wireless Society.  Dean KK4DAS is President and a lot is happening in the Club.  Makers group. Lunch with the group on Tuesdays.  Lots of fun.

We are looking at Farhan’s Daylight Again Analog rig.  Using parts from Dean’s 3D printer, we have made PTOs. See blog post for picture.   (But are they really PTOs? Or just Variable Inductors?) 

VFO madness.  Solid stating an HT-37 VFO. Grayson: “You have clearly lost your mind.” Perhaps.  100 MHz FM broadcast RFI. This brought me to capacitors and linear tuning:

SLC, SLW, SLF and Midline/Centerline caps.  Do special caps explain why some VFOs have linear tuning while others do not?  NO, not really.  You can get linear tuning with ordinary caps.  See blog post

Working on the bench.  Reading Adam Savage’s book on workshops and tools. (link to the book in the right hand column of the blog.   Some other workbench links on the blog.  Upgrading tools and test gear.  I have a proper bench power supply. Better side cutters.  Engineering rulers.  Digital calipers. Better solder (I was talking to Ron WA6YOU – a real solder authority - about this.  Kester 60/40 ).



Currently offering seven items:

·  UDVBM-1 (in three versions)

·  PG-TIA Broadband IF Amplifier

·  TIA-AGC IF Amplifier

·  Dual-Gate MOSFETs

·  OA741D Op-Amp (discrete components)

·  8-Pole QER Crystal Filter

·  I2C Rotary Encoder

Three more are shown as coming soon:

·  GP Audio Amp (general-purpose audio amp)

·  HyCas IF Amplifier with AGC

·  M-Cubed (Michigan Mighty Mite kit)

·         Mostly DIY RF offers hardware for hardware-defined radio. That's not a comment on software-defined radio. SDR is cool and useful, and it has its place.

·         In fact, I believe in explicitly making use of technology from all eras (vacuum tubes, discrete semiconductors, and integrated circuits

·         What does "mostly" DIY mean? It means making what you want and using ready-made for the rest. Even the most fanatic DIY'ing home brewer does this without much thought. He uses components he didn't fabricate, electricity he didn't generate, and theoretical knowledge he didn't discover. One can't fight all battles, one has to choose which to fight and which to leave to others. If an appliance-operating Ham doesn't want to fight at all, that's okay, but Mostly DIY RF has nothing to offer him.


I need YouTube viewing hours.  So I am putting some of the best SolderSmoke podcasts up on the YouTube channel.  Please listen/watch on YouTube. Here is the playlist:  https://www.youtube.com/playlist?list=PLv9MEKq1quk-gnxog7eBqY_y7cpQadf6T   

Google feed no longer sending our e-mails.  So please follow the blog, Get yourself a blog feed reader.  I use Feedly.  It is free.   Download, search in Feedly for SolderSmoke, hit subscribe and you are done. https://feedly.com/i/discover Follow Pete’s blog too!  It is easy.

 Pete’s Bench



TenTEC Model 540



Dan KC5VPI getting into homebrewing. No longer WRONG!

Michael AG5VG built subharmonic DC receiver, put it on 40 and 20.  FB.

Mike AA9IL working on old tube stuff including a DX-100 AMD UUHF gear up to 122GHz.

Ed WA4MZS liked the SolderSmoke book. USCG vet.  As is Mike Rainey AA1TJ Semper Paratus!

Clint KA7OEI – The Music of the Beams – My Hex  does not sing!  So far!

Walter KA4KXX – The Knack started early.  Video of getting a toolbox for Christmas. And working on PTOs!  He likes them!

Phillip G4HOJ Also working with subharmonic mixers – using the more complex 4 diode version.

Peter GW4ZUA Built an amazing rig from RADCOM (1980s) Cool VFO with varactor fine tuning and a coil with a dust core that was cut in half to minimize temp effect. Box now holds 3  band version of Pete’s SimpleCeiver. FB.

Bruce KC1FSZ building 25 Watt amp for his Peppermint Bark rigs.  We won’t turn him in to the QRP authorities.

Grayson KJ7UM tested the 12BY7A from my DX-100 VFO.  It was in fact weak. Thanks Grayson.

Charles Smith KV4JT has some really interesting stuff on how to solid state tube gear.   Especially R-390s. https://soldersmoke.blogspot.com/2022/08/trigger-warning-solid-stating-old-tube.html

Gianfranco I0ZY Old friend from Rome.  Amplifier manufacturer.  FB.

Paul WA1MAC getting started again after long hiatus.

Vasily.. I mean Todd VE7BPO.  Always great tech info.  Thanks.

Mark WB8YMV building a superhet with a 455 kc IF.

I sent Farhan some comments on his Daylight Again video: He wrote back “I marvel at their stamina to watch an hour long video of a man talking with a funny accent!”  

Sunday, September 18, 2022

Building Farhan's PTO -- But Is This Really a PTO?

Farhan has a PTO in his Daylight Again transceiver.  This caught the attention of homebrewers around the world.  Walter down in Orlando built one.  Dean KK4DAS and I have built versions also. Dean has a 3D printer and made the coil support used in my version (above).  Thanks Dean! 
I had a little trouble at first because one of the capacitors I had in there was not NP0, so the device drifted a lot.  But once I straightened that out (I put in a Silver Mica cap in its place) the oscillator became very stable. Mine moves about 25 kHz with each turn of the bronze screw -- this is nicely linear tuning.  But I think I will have to use a San Jian counter to keep track of the frequency. The long plastic tube on mine is there to eliminate had capacitance effects. 

One of the guys in the Vienna Wireless Society questioned whether we should really call this a PTO.  After all, bronze has a permeability equal to that of free space. So we are not really changing the permeability.  Frank Harris points out that the bronze screw really acts as a shorted secondary.  What do you guys think?   Is this a PTO, or do we need another acronym to describe it? 

This has been a really fun project.  I never built an oscillator like this before.  I will probably follow up by building the rest of the Daylight Again rig. 

BTW  WA6OTP has a nice PTO design: http://www.wa6otp.com/pto.htm

Thanks again to Dean, and to Farhan.  

Friday, September 16, 2022

Fixing Up An Old Homebrew Rig -- Barebones Superhet and VXO 6 Watter

I'm not exactly sure why I pulled this old rig off the shelf, but I'll write up what I did -- I often use this blog as a kind of notebook.  I can look back and easily see what I did on my last encounter with the rig. 

The receiver is Doug DeMaw's Barebones (aka Barbados) Superhet.  This was my first superhet receiver. I built in in 1997.   The transmitter was my first real homebrew project -- it is the VXO 6 watter from QRP classics.  I built it in the Dominican Republic, probably in 1993 or 1994.  I built the power supply so that I could say that the entire rig is homebrew. 

This rig is getting a bit long in the tooth:   The receiver is built with 40673 Dual-Gate MOSFETs, an some of the transistor cans have gotten rusty.  The frequency readout on the receiver is the top of a coffee can fitted onto the reduction drive behind the tuning knob from a Drake 2-B (not MY 2B!). 

Here are two 2013 videos that I did on this receiver: 

-- I put the crystal filter back in CW mode.  I had widened it so that I could listen to 20 meter SSB, but I decided to go back to its original configuration.  When I built the receiver in 1987, I didn't characterize the crystals -- I just used the capacitor values that Doug DeMaw had in his article.  I pretty much did that again this time, just putting caps that are close in value to what Doug had.  DeMaw used color burst crystals at 3.579 MHz.  So I guess this would be a GREAT receiver for the Color Burst Liberation Army!  

-- I used My Antuino (thanks Farhan!) to check the passband.  Here is what it looks like.  I just put the Antuino across the 10k resistors on either side of the input and output transformers.  The coil cores had become very loose -- I just tried put them in the right place.  I may need to put some wax in there to allow them to better stay in place. I think they could have used toroids instead -- that would have been easier. One of the transformer connections was open -- they don't work well that way,  once I fixed that, the passband looks like this: 

-- Each of the horizontal divisions is 500 Hz.  The passband is not pretty, but it is OK, and I  didn't feel like doing too much work on this to get it in better shape. 

-- The filter peak was a bit lower in frequency than expected.  I found that trimmer cap C3 in series with the BFO crystal would not allow me to lower its frequency sufficiently.  So I moved C3 to a position in parallel with the crystal.  With this mod, I could get the BFO frequency to 3578.69.  This produces a 690 Hz tone when the received signal is at the peak of the IF passband.  Opposite sideband rejection is quite good. 

March 2013 Rebuild of the VXO 6 watter

-- I didn't have to do any real work on the transmitter.  The RF amplifier in the transmitter had served for a time as the RF amp in by 17 meter DSB rig (I had added a bias circuit, which I removed when I put the amplifier back in Class C).  Some time ago I rebuilt the oscillator circuit (which had been literally cut off the board when I used the amplifier in the DSB rig). 

-- I did have to reconfigure the muting circuit -- the T/R switch in the transmitter switches the antenna and also -- through a two wire circuit -- cuts off 12 V DC to the transmitter when in receive mode. 

-- For sidetone I just put a small piezo buzzer through a 1k resistor between 12 V DC and the key line. 

It all worked fine -- I talked to three stations on the high end of  the 20 meter CW band. 

Thursday, September 15, 2022

SolderSmoke (Old Smoke) Podcast #103 -- March 15, 2009 -- From Rome -- QRSS, Knights and Wizards, LTSpice, and an Echolink QSO with Jeff KO7M (the guy with the Piper Cub)

 15 March 2009 

Beware the Ides of March! 
Ostia Beach and Ostia Antica
248 Knights of QRSS. And Wizards! 
ET Phones Home (with QRSS?) 
Possible new grabbers in VK6 and Dubai
Telescope, Satellites... 
REAL QRP QSOs on 80 and 40
Saving an old Toshiba Laptop
-- Piper Cubs and MFJ Cubs
-- Satellite QSOs 
-- LT Spice and test gear
Gene W3PM listens from QE2, HB WSPR rig
Jim AL7V sending parts for my W3PM rig
Jim AB3CV's color burst Gnat
Jason NT7S on Tektronix guys and SolderSmoke
Kevin ZL3KE on old computers
Paul M1CNK's DDS 30-based QRSS beacon
Soeren OZ2DAK on exercise bikes to power beacons

Wednesday, September 14, 2022

SolderSmoke Podcast #85 With Mike Herr WA6ARA June 8, 2008 WIZARD ISLAND!

When I was putting this podcast up on YouTube, I needed a picture of Mike Herr WA6ARA who was my guest on this episode.  I found a picture of Mike activating a summit... THE SUMMIT OF WIZARD ISLAND!   That is Wizard Island (above).  Mike and his wife are shown at the summit (below). 

I really liked talking to Mike, and to listening again to our conversation.  But I realize now I should have asked him about parachutes!  I'd been talking about the great picture of the Phoenix lander under canopy as it descended to Mars.  Mike made and tested parachutes for NASA!  

You can fire up a YouTube playlist that will bring you a steady stream of SolderSmoke podcasts here: 

Here is a report from Mike on the Wizard Island activation: 

Tuesday, September 13, 2022

Great Technical Info and Tribal Knowledge from GQRP

Thanks to Tony Fishpool G4WIF for sending us this link.  


There is a lot of great tech info and Tribal Knowledge on the GQRP page.  This is all related to our discussion of how to set up an electronic workbench or workshop. 

Thanks Tony and thanks to GQRP. 

Monday, September 12, 2022

Workbench and Workshop Tips -- Please Put Suggestions in the Comments

During a recent visit to Barnes and Noble I picked up Adam Savage's book (please use the Amazon link in the righthand column) about Making and workshops.  Adam's book reminded me of the importance of giving thought to the organization and set-up of your workbench or workshop.  I found more tips on YouTube: 

Wow,  watch the KatVoltage video (above).  Unlike the person in a recent unfortunate advertisement, Kat clearly knows which end of the soldering iron to grab.   She is setting up a good workshop -- you can tell from how she is organizing the bench.   I wish her a lot of luck. htt(s://www.youtube.com/watch?v=1NcVzTu7TbE&t=54s

Andreas (the guy with the white glove and the Swiss accent) has some good ideas:https://www.youtube.com/watch?v=inW57njiq1A

The EEVblog guy has a good guide to the basic stuff that you need to build an electronics workshop.  It is a bit dated (2011) but the guidance is still very good (I wouldn't go with the homebrew or kit-built power supplies): https://www.youtube.com/watch?v=R_PbjbRaO2E 

Van Neistat has a very good Top Ten List of things NOT TO DO in a workshop. NO FLATHEAD SCREWS.  I'm with you Van.  NO HOARDING.  etc. https://www.youtube.com/watch?v=3bAZQ9nWybM&ab_channel=VanNeistat  Paul VK3HN suggests we develop a similar list for ham radio homebrewers.  Please put your suggestions in the comment section below. 

Sunday, September 11, 2022

An Especially Good (Old) SolderSmoke Podcast

February 22, 2008    
-- We were in Rome.  
-- I read a 1931 QST ad sent to me by my fellow "Hambassador" David Cowhig, WA1LBP; David was in Okinawa when he sent it. 
-- I describe Wes Hayward's comments about Pat Hawker, G3VA. 
-- I talk about getting on the air with my HW-8 (after fixing it!), re-tubing my Drake 2-B, and putting up a 30AWG antenna in Rome. 
-- I describe meeting up with Roman hams (including amplifier manufacturer I0ZY!) and visiting the local radio club.
-- The Science Museum in Florence, Italy. 
-- DSB from Rome with my NE602 rig. 
-- Tony Fishpool and Graham Firth's Test Gear book. 
-- An important corollary to Murphy's Law. 
-- Listening to SolderSmoke from safari, under the Southern Cross in South Africa. 
-- Ron Sparks calls in from Dubai. 

You can put on a playlist of all the SolderSmoke podcasts here: 

Saturday, September 10, 2022

The Cure for Frequency "Bunching Up" in Analog LC VFOs -- It Is Not So Simple. But we have a good calculator. Comments sought!

Bob's calculator shows good tuning linearity with an ordinary SLC capacitor 

One of the complaints about analog LC VFOs is that they have non-linear tuning -- when you turn the dial (usually attached to a variable capacitor) the space between frequencies is NOT constant.  This is especially apparent at the high end of the frequency scale where frequencies (and stations) appear to be severely bunched together, making tuning difficult.  This problem contributes to the defection of some great homebrewers:  They give up on LC VFOs and they switch to digital VFOs.  Sad. 

But there is hope:  Not all LC VFOs tune this way.  Even on rigs from "back in the day," back when the Si5351 hadn't even been thought of,  some LC VFO rigs tuned linearly.  My beloved Drake 2-B and my almost equally beloved HT-37 are good examples.  How did they do this?  How did they escape the dreaded "bunching up?"  

For a while, I thought that it might have had to do with the use of the series tuned Clapp circuit.  But on further noodling, this didn't seem to make much sense. 

Then -- like others -- I thought that it  must be caused by the adroit use of special capacitors. You see, in ordinary variable capacitors when you turn the dial, the capacitance increases linearly.  But in the LC circuit, frequency changes as the inverse of the square of the capacitance. Thus the bunching up. So the solution must come from the use of the special capacitors that compensate for this, that -- because of the shape of their plates --  produce linear tuning.  With these variable caps, frequencies on the dial are spaced out nicely, there is no bunching up. Great right?  

From Terman, Radio Engineers Handbook, 1943, page 123

Over the years, many hams have jumped to the conclusion that rigs with good tuning linearity MUST be using these special caps. For example, in 2013 a ham posted in the Antique Radio forum this message: 

There are three types of open, variable plate caps;
SLC= straight line capacitance where the capacitance varies linearly,
these are the most common and have half-circle plates
SLF= straight line frequency where the plates are tapered to allow
for linear tuning of the frequency
SLW= straight line wavelength, you get the idea...

SLF and SLW caps have oblong plates.

The effect on tuning a receiver can be dramatic. One example is the
Hammarlund SP series of receivers where the ham bands are very
compressed at one end of the tuning range. They used SLC caps
in the VFO. On the other hand rigs like the Kenwood TS-520
and FT-101 series have linear tuning across each band. These use
SLF variable caps. Most old 1920's battery radios used SLW
where stations were identified by their wavelength.

Well, not really.  

-- I now have several VFOs from the extremely linear-tuning FT-101.  But when you open them up to look at the tuning capacitor, it is NOT a Straight Line Frequency capacitor.  

-- Many of us over the years have built VFOs that are quite linear in their tuning without resort to these special capacitors -- we did it with ordinary Straight Line Capacitance caps.

-- When you look at the "How to build VFO" literature in the ham radio books, you see a lot of good recommendations about using solid, brass-vaned caps with ball bearings at either end.  But never do you see circuits that require the use of SLF or SLH capacitors. If they were the key to tuning linearity, we'd see them mentioned in the literature. But we don't.   

So where does the linearity -- or bunching up -- come from?  

The answer comes to us from a really neat calculator from Bob's Electron Bunker: 


This calculator allows you to select your frequency range, and the tuning range of your variable cap. It then displays for you what the tuning range will look like on your dial.  You can see if there will be bunching up, or if the frequencies will be nicely spread out.  And -- and this is the really cool part -- you can then specify if your capacitor is SLF, SLW, SLC or Midline-Centerline.  This really illustrates the effect of the different capacitor types. 

I used Bob's calculators to do some experiments with various types of capacitors, various frequency ranges, and various combinations of trimmers and padders.   You can see what I did here: 


One important thing to keep in mind:  The SLF caps were made for AM broadcast receivers that were tuning from 540 to 1600 kc.  That is a 3:1 tuning range.  Most of the time in HF ham radio, we are tuning across a much smaller range, say from 5 MHz to 5.5 MHz.   That is a 1.1:1 tuning range. In those cases where we ARE tuning across a wide tuning range -- for example with a receiver covering 3-9 MHz, the SLF cap can help prevent the bunching up. 

But we can have fairly good linear tuning without resort to SLF caps.  Bob and his calculator point out that by narrowing the frequency range of interest, and by using either smaller range caps (ordinary SLC caps), or SLC caps with trimmers and padders, we can achieve tuning linearity.  And sometimes, when you have achieved this nice tuning linearity with a plain SLC cap, putting a fancy SLF cap makes tuning linearity worse. 

One piece of VFO tribal wisdom that is confirmed by all this:  It is better to use a smaller variable cap with a maximum capacity of about 30 picofarads. 

I think we should spend as much time focusing on VFO tuning linearity as we do on VFO frequency stability.  Bob told me that in the old days, the calculations for various tuning linearity scenarios were difficult.  But now we have Bob's calculator.  When building a VFO, just use Bob's calculator, plugging in the numbers to get a preview of what your tuning linearity will be like. If it is bunched up, you can play with the trimmer and padder values to achieve the tuning linearity you desire.   

Thanks to Bob of Bob's Electron Bunker for this great calculator. 

You can see another discussion of "bunched-up" tuning in the comments section of this article: https://www.nutsvolts.com/?/magazine/article/may2015_Whipple

What do you folks think of this?   Please put comments below. 

Thursday, September 8, 2022

Another Free Book: Contra Cross


The free Kindle offer is now on!  

Bezos is letting me give this one away for five days:  September 9, 2022  12 am PDT to September 13 2022 11:59 PDT.   So go ahead and put it in your Kindle.  It is not about radio, but I think many SolderSmoke fans will like it.  

Download it here: 

Please spread the word and -- if you like -- write a review. 

Wednesday, September 7, 2022

Building a Workshop (or a Shack) in a Crawl Space

At first, this video made me think of Michael AA1TJ and his underground Hobbit Hole workshop.  Then I started thinking about my own crawlspace -- it looks remarkably similar to the one in this video.  Hmmm...  But no, I think I will stay above ground. 

One thing that would DEFINITLY not work for us would be the flooring that kind of eats small parts that get dropped.  I would all soon have large numbers of small parts under those mats.  This would drive me nuts.  

But lots of good ideas from Australia in this video.  

Tuesday, September 6, 2022

Michael AG5VG Builds a Sub-Harmonic Receiver and Moves it to Higher Bands

Michael AG5VG built a Sub-Harmonic Direct Conversion receiver.   But then he took it a step further and moved it up from the 80/40 meter version that I had built, and used the same concept to run it on 20 meters using an oscillator on 40 meters (after some re-winding of the front-end coils).   Using a station from Puerto Rico transmitting on 20 meters as an example, he starts out showing how well the receiver works in sub-harmonic mode (with the oscillator on 40), then quickly switches to normal Direct Conversion mode with the oscillator also on 20 (but using only one diode as the detector) -- he can still hear the Puerto Rican station in that mode.  Very cool.  

Good Evening Bill,

I built the Polykov and I attached a picture of it. I also used Pete's pre audio driver circuit from his jessystems.com site. Then I used an lm386 as the main audio driver. I could hear ft8 on the 40m band. Then I hooked the output of my lm386 circuit to a conventional set of computer speakers to really hear it. I am currently using an indoor wire antenna along the ground so it's certainly not optimal. Very fun build and I'll be learning more about it. When I have a better antenna system I'll hook it all up and send a video of it.



I am just using a standard signal generator at 1 vpp output. The volume gets louder with every 100 millivolt I go up, but so does the noise. 0.8vpp was a little low for me so I bumped it up a bit.

The indoor antenna actually did surprising well but I'm looking forward to putting a wire up into a tree I have here. I just recently moved so I have to setup my outside antenna. I live in the San Antonio, TX area. 

I am currently using three stages of audio amplification to be able to really hear it. 1st stage is Pete's pre audio driver, then an lm386, then a standard set of computer speakers. 

I did plenty of playing around with it last night and the doubling function is so cool how it works. When I was around 3.538 MHz, with the variable cap tuned for the 7Mhz area, I was actually listening to 7.76Mhz, the FT8 frequency for 40 meters. I agree with You and Pete in a podcast you did a bit ago, that FT8 is great for seeing if the band is open and checking receivers with! 

The next project is the art of the 3.5 - 4Mhz analog VFO and use it with the Polykov. I am very dependent on the Arduino/Si5351 pair as the code is available and easy to hook up.

Will keep you both updated. 


Two more videos from Michael: 


I think this is a great example of good experimenting.  Michael took the concept, made some mods,  and put the device on another band.  FB.  

Monday, September 5, 2022

Why Do Some VFOs Tune More Linearly Than Others?

This has been one of the major complaints about our beloved analog LC VFOs:   The frequency tuning on these circuits is often not linear.  For given amount of VFO frequency dial turn you can get vastly different changes in frequency.  At one end of the tuning range the frequencies are nicely spaced and tuning is easy.  But at the other end of the tuning range all of the frequencies are bunched together.  This is one of the problems that leads some homebrewers to defect to the sad land of "digital VFOs." 

But wait.  It appears that the old designers found a solution to this problem.  Just look at the tuning dial of my HT-37.  The frequencies are all spaced out evenly.  How did they do that? 

I had been thinking that this success may have resulted from Hallicrafters' engineers using the series-tuned Clapp circuit.  Here the main frequency determining element is a series-tuned LC circuit and not the parallel tuned LC circuit that we see in the more commonly used Colpitts circuit.  

But hold on -- how could that be?  The frequency bunching problem that we attributed to the Colpitts circuit must also exist in the Clapp, right?  I went back to SSDRA where there was a good discussion of Colpitts and Clapp VFOs.  The advantage of the Clapp was said to be in its use of a larger value coil which helped minimize the effects of stray inductances.  But there was no mention of the Clapp offering improved linearity in tuning.  

I have in front of me two transceivers:  The Mythbuster uses a 9 MHz Clapp circuit (see below).  The 17-12 rig uses a Colpitts Circuit.  I checked the tuning linearity of both.  Both appeared quite linear in tuning, with no real difference between the two.  

Then I looked at the tuning capacitor in the Mythbuster 17-12 rig.  It came out of an old Hallicrafters transmitter, probably the HT-44.  I looked closely at the stator and the rotor plates.  Both are curved.  I'm guessing that this may yield a more constant change in capacitance for a given movement of the main tuning dial.  

Next I opened up the VFO on the Mythbuster.  (It is the VFO from an old Yaesu FT-101.)  I couldn't see the stators very well but it appears that their shape is different from the square shape we often see in variable capacitors.  Could it be that this variable capacitor was also made to provide linear tuning?  

Back in 2013 Norm Johnson wrote about all this in the Antique Radios.com forum: 

A capacitor that has uniform increase in capacitance with rotation will have the stations at the high end of the band squeezed together. Another type known as the straight-line frequency variable capacitor has, as you might guess, a characteristic that gives even spacing of frequencies with shaft rotation. These were popular in the 1920's but weren't very good for superhets where you needed to have a dual section capacitor that would tune both the RF and local oscillator, and have them track each other properly. The midline variable capacitor is more compatible with a superhet, and easier to make both sections track properly. This is the type that you see in most receivers from the late 1930's to the end of the tube era. They don't have quite the equal spacing between stations across the band that the old straight-line frequency caps had, but they're much better than the variables that change capacitance linearly with rotation.

I wrote an online calculator that helps in the design of the tuning. It shows what frequency range you'll get with a specific type of variable capacitor, including the effects of padder and trimmer capacitors. It also displays a dial scale that shows how the frequencies are lined up accross the dial.

Steve W6SSP also provided some really good info back in 2013: 

There are three types of open, variable plate caps;
SLC= straight line capacitance where the capacitance varies linearly,
these are the most common and have half-circle plates
SLF= straight line frequency where the plates are tapered to allow
for linear tuning of the frequency
SLW= straight line wavelength, you get the idea...

SLF and SLW caps have oblong plates.

The effect on tuning a receiver can be dramatic. One example is the
Hammarlund SP series of receivers where the ham bands are very
compressed at one end of the tuning range. They used SLC caps
in the VFO. On the other hand rigs like the Kenwood TS-520
and FT-101 series have linear tuning across each band. These use
SLF variable caps. Most old 1920's battery radios used SLW
where stations were identified by their wavelength.
Steve W6SSP

These two variable caps came out of my junkbox.  Both are Eddystones, made in England.  My guess is that the one on the left is SLF.  But could the one on the right (out of an old regen) be SLW?   

The Drake 2-B also has perfectly linear tuning.  I looked at the manual:  "The tuning condenser is of special design..."   I'm guessing that they used an SLF variable capacitor. The 2-B had no need for ganged capacitors -- the "preselector" was tuned via a separate front panel control. 

I looked at the tuning dials on my Hammarlund HQ-100 receiver.   It is fairly linear in its tuning, but not as linear as the HT-37 or the Drake 2-B; on all of the tuning ranges the frequencies seem to spread out a bit at the lower end.  My guess is that Hammarlund used the midline variable described above by Norm Johnson.  The HQ-100 did use a ganged variable cap, with one section tuning the RF amplifier and the other tuning the local oscillator. 

Mythbuster on the bottom.  17-12 rig on the top

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