Popcorn Electronics Email Address Change Reminder -- Thanks!

 Greetings ###

The old email server went down due to Winter storm ~ 2 weeks ago --- and AC power was restored to the property only today. Several radio-related emails were on the incoming server -- and perhaps others may have been missed due to the server shutting down after the U.P.S. battery eventually died.

-  That's why I did not reply to your email message -

The old server is only checked once per month now and will get phased out by the Equinox

Please remove my old email address from your email contact list -- and the previously sent email address that automatically comes up in the TO: line when you compose an email.  The new email account is via GMail.

Click for our current email address   

Many thanks and best regards!

Popcorn Todd 

PNP Transistor Radios — What your father knew but didn't tell you

Emailers have encouraged me to publish my off-cuts. About 50% of my ideas fail. This is a tough hobby grasshopper. I first chose to not publish my VHF PNP experiments from 2024 because few people ever make PNP transistor circuits --  and as the kids say; "like  - who cares?".  Perhaps no one indeed.  But here goes.

I love PNP circuits and the rich history they hold as radio transitioned from tube (valves for you Brits) to solid state design.

Solid state amateur home brew is ancient craft

In the 1960s, using small signal germanium transistors, home brewers were making single side band suppressed carrier transmitters + also receivers with crystal filters, phase-shift networks, & even Weaver networks.  Nothing new under the sun?

Above — Click to see a solid state complete 40 meter band SSB transceiver using a Collins mechanical filter for IF filtration. The A01 transistor and all others were PNP. Still, they got it done without all the fancy programs and test equipment we've got today. I feel humbled by old circuits like this. These images came from a printed book called the Transistor Radio Handbook. This serves as my all-time favorite radio book to read since forever.  Someone has it online here.   I treasure my printed copy.

 

Above — Inspired by this Figure, I decide to make a low current, PNP RF preamplifier for my battery operated weather receiver at 162.55 MHz. Please read - the Philco 2N1742 datasheet - an fT of 150 MHz -- and whoa, look at the current gain. I'll use a few more modern silicon BJTs in my design lol.  

I love silicon transistors -- although I once build a custom fuzz 'stomp box' for a professional guitarist using germanium transistors when I served as his (tube) amp tech.

 

From reading the  Transistor Radio handbook, even in the 1960's, home brew players had enough common sense to stick their projects in metal boxes and understood how to bypass and decouple their circuits.  Somehow, despite easy summations like Mark Montrose's book above, some modern analog builders seemingly feel that the laws of physics don't apply to their projects. Curious.

 

Above — My base schematic. A PNP preamp with tuned input + output, no negative feedback (or neutralization) and a collector current of 2.4 mA. I tried old-school neutralization as shown in the inspiration schematic , but found that tapping the output coil as shown worked much better to stabilize the amp.

Input Tank Circuit 

Above — Sweep of circuit adapted to allow assessment & tuning of the input tank circuit.

For the collector coil, I temporarily placed a FT-23-43 ferrite with 10 turns on it in-situ.  I shunted that coil with a 56 Ω resistor to give a decent output Z for my sweep circuit. Thus, I knew that the frequency transfer response was due the input circuit. I tweaked the trimmer capacitor so it peaked at 162.55 MHz.

Q1 = MPSH10, a 650 MHz part still surprisingly available in Mouser's catalog for 2025. Even with the "temporary" 56 Ω resistor | coil, this amp provides a power gain of  ~ 4.6 dB.

The rest of the circuit

 

Above — "Rookie mistake". To show what might happen if one decides to not include the 39 ohm 'UHF snubber' resistor on the collector,  plus -- not tapping the collector coil as performed in the original schematic.

 

Above — The transfer function of the original schematic as shown with a MPSH10 BJT. Love this.

Most modern builders would never put a BJT as a low noise amp in a VHF project. Plus they would run a lot of current for a high dynamic range.

BJTS at VHF and above bands in the 1960s led to JFETS -->MOSFETS -->GaAsFETS | MESFETS --> pHEMT and so forth over time. But, today, we are making battery powered radio circuitry & using BJTs circa 1963.  Feels refreshing.

 

 Above — What happens  when we stick in a 2N3906 PNP?  Power gain = 2.4 dB

 Above — My fastest leaded PNP part, the BF509 soldered into the Q1 slot.

 

Above — It's fun to read the datasheet of older parts. I've got several PNP SMD parts with an fT up to 9 GHz. Even most of these are obsolete and were collected years ago when parts were real ( not so bootlegged ) and relatively cheaper.

PNP circuits are really not that different or difficult than NPN circuits. I encourage you to experiment with PNP transistors for fun and learning.

Best to you --  Spring is coming!

 

Op-ed. Having fun and receiver performance are not always related -- The fate of analog-biased home brew radio --

 

I'm only guessing the date, but perhaps, sometime in the 1970s -- a portion of home brew and certainly the professional amateur radio industry pursued high dynamic range as the be-all, end-all for receivers.

1. Like most – I appreciate a sublimely quiet, high dynamic range receiver; especially on contest weekends. Imagine our “ideal receiver” -- one that that easily detects 2 or more desired signals with an amplitude difference of 90 or 100 dB with ultra low distortion. Then too, our perfect receiver provides ball-busting immunity to the spurious responses produced by the nonlinear interaction of multiple strong signals coming into our antenna with no sweat. Ooh la la .

We’re talking about amateur radio contest-grade receivers. You generally have to run lots more current, high amplitude local oscillators, spend more money on parts -- and perhaps apply more complex circuits to get towards our perfect, idealized receiver if you home brew this stuff.  Will the pursuit of high dynamic range receivers help our analog-biased home brew hobby survive a little bit longer?  Perhaps for some.

In Canada, from surveys and also from viewing the sea of grey-haired club Hams milling around at the Ham Fest / radio flea markets, the largest demographic are those Hams aged 60-80. Many of these folks still like CW, although the younger Hams don’t. It seems that the youngsters prefer voice modulation modes, plus they apparently favor battery powered, mobile rigs. All these points might prove good goals for home brew designers who hope to attract younger Hams into home brew.

A significant portion of the ‘old-timers’ are inactive – some haven’t fired up their rig(s) for decades. Not even on 2 meters.

I’ve chatted with a few of these ‘nearly, or totally silent folks’ (who still identify as Hams and come to larger amateur radio gatherings for social contact, community and to reminisce ), Why are they silent? The answer seems complex. Partly, it was the Internet which arrived here locally in 1995. Then perhaps -- smart phones, changing tastes, changing family-life priorities, home & yard downsizing, health concerns, the digital electronics age, plus ‘tens of billions or so’ competing leisure activities may have contributed to these folks giving up operating their radio sets. What about home brew? -- I’ve got no idea. All of this and more?

With emails and conversations, some entry-level analog-biased home brewers feel intimated by high performance, contest grade projects, or advanced test procedures. Not everyone wants to go in whole hog and learn stuff like small signal analysis, or C programming, how to solve complex equations in Python, or to measure noise figure

Newcomers face more practical tasks like finding truly knowledgeable, trustworthy Elmers & sourcing affordable parts in their country. Basic skills like soldering, how to make circuit boards and understanding how to wind transformers are all skills they need to learn. As with every other hobby, they must navigate through abundant online misinformation, big meanies, and hucksters.

At 1 time the Ham philosophy to just make do with surplus or cheap gear served us well. Like most hobbies, this morphed into consumerism -- better, faster, newer, more, more, more. Now, in the digital age – some potential builders feel even more like a fish out of water.

 

Further, the asynchronous nature or automation of Ham radio has put off a few people....  I recall 1 recent, very snarky email in particular; “While I slept for about 8 hours, 15 stations heard my beacon. Yippee!     Boy howdy,  has Ham radio ever gotten exciting!!!”

Maybe synchronous communication - actual rag chews on home brew gear will provide us something to talk about?!

For some older builders, having fun, re-purposing and being thrifty were key reasons they liked home brew and even Ham Radio in general. How do we reignite the home brew passion in those turned dispassionate, ‘silent’ 60-80 year olds? Perhaps it's a lost cause? Further, how do we recruit a few younger Hams into building some old-style analog-biased radio gear? What about the recruitment of bored baby boomers into amateur radio in general?

For ‘silent’ older Hams, I conjecture that complex rigs, or projects that require them to learn to compile code into a hex file and/or write some lines of code won’t likely help recruit them back into home brew radio.

Personally knowing about presbyopia first hand, should; or how will they learn how to build with SMD parts as certain leaded parts become expensive, hard to find, or bootleg?

These are topics perhaps the home brew community might think about.

On the other hand – cheap, powerful test equipment lies abundant. For example, low cost, hobby-grade vector network analyzers. Test equipment wise – it’s never been a better time to do home brew radio.

Perhaps simplicity, fun and thrift for newcomers, or re-comers will once again serve analog-biased home brew for a few more years?

With a simple, but good receivers, third order intercept point products normally lie below the receiver noise floor during casual listening. If the front end gets overloaded and pushes the receiver into non-linear operation, perhaps they might switch on an attenuator, or turn a potentiometer to hopefully re-establish linear function – or just have to cope. It’s not the end of the world.   Having fun and receiver performance are not always related.

• Am I active on the amateur bands??

• Am I having fun ??

  Might be the best 2 metrics to embrace. Who knows? Perhaps, I’ve got it wrong and demographics and technological advancements totally determines the fate of analog-biased home brew radio? Perhaps like the horse and buggy, amateur radio analog-biased home brew’s fate is sealed.

   

• Simple, but good gear

• Emphasize and teach quality basic measurement techniques

• Thrift

• Have fun

• Learn at your own pace

• Communicate

These 6 above bold themes will inform Popcorn Electronics video content going forward.

We’ve got a lot of newly or ‘newish’ retired boomers with time on their hands. Some of them might enjoy making some home brew analog radio gear. Kits are also an option.

Best!

My 3.5X MHz Voltage Tuned VFO -- Have I gone crazy?

 

 

My Voltage Tuned VFO - Have I gone crazy?

I built a 3.5X MHz scratch voltage controlled VFO for home brew radios. No PLL circuit -- a free running variable frequency oscillator. Have I gone mad?  Perhaps. Complete design + analysis, plus a short assessment of frequency stability due to temperature changes at 2 frequency bands.

* RESOURCES *

Brad's VFO Story  How to make a temperature stable VFO
https://youtu.be/kcc8V5IMmlw

SolderSmoke Daily News -- Ham Radio Blog
https://soldersmoke.blogspot.com/ 

Iulian, YO3DAC - VA3IUL Website

https://www.qsl.net/va3iul/

Transistor Radio Series -- KO4BB Frequency Doubler -- 3.5X to 7.X MHz

 Greetings ###

I continued to seek a frequency doubler - and started experimenting with emitter-coupled pairs to make the full-wave rectifier. After several promising circuits, I discovered this circuit from Didier, KO4BB.  Scroll down to find Low phase noise common base frequency doubler

I encourage builders to check out his entire web site ko4bb.com

My work is pushing me towards simple, but good, battery powered circuits -- this implies keeping current low to conserve battery power.  This low phase noise doubler draws 3.4 mA quiescent - and delivers 6 - 6.4 dBm output power at 7 MHz from an input power amplitude of 10 dBm at 3.5 MHz. Fantastica.

Above — The doubler designed by Didier, K04BB. I used 2N4401 BJTs that came from the same reel, but did not match them. The 200 Ω emitter degeneration resistors were 1% tolerance parts.

Q1 and Q2 provide a series pass voltage regulator with Q2 as the feedback amplifier. The  Q2 BJT also serves as the voltage reference -- and provides temperature compensation. Ideal for a field mobile radio where temperature changes happen. The bias to the emitter- coupled pair is only 0.85 VDC.  I added a 100 µF electrolytic capacitor to the base of Q1 to boost DC filtration.

I designed my own output network, and because the emitter-coupled BJT pair provides such a high voltage amplitude, I eliminated an active buffer stage. This in turn, helps keep current draw low. A simple 32 turn to 5 turn ferrite transformer goes into a low pass filter plus a final -4 dB 50 Ω pad.

The proof of how accurately the input signal gets full-wave rectified is demonstrated by the amplitude of the 3.5 MHz input signal at the output.

 

Above — The spectrum analyzer transfer function.  I placed a 10 dBm signal on the input and the full-wave rectifier suppressed this by ~ 53.5 dB indicating good balance. The output = 6.32 dBm, perfect to drive a Level 7 diode ring mixer.

Above — Before I tested the output signal in a spectrum analyzer, I connected it to a 50 Ω input 'scope channel. Even here, you may see this doubler functions fairly well. This is the best 3.5 MHz doubler I've built. Big thanks to Didier, K04BB for sharing his design -- it's a keeper.

Best!