RAN Technology

Using rectifier diodes as varactor/varicap substitutes

Technical 0 Comments 03/02/2019 

Modern-day solutions for RF part obsolescence

Posted By: Robert Nickels (ranickels)

As anyone who likes building radios knows, it's getting harder and harder to buy RF components.   One of the last casualties was the 3-section pi-wound RF chokes made by Hammond, a company that has done more than most to support the vintage radio and audio hobbies.    The Toko variable inductors long favored by QRP enthuisiasts have become all but unavailable, and variable capacitors are expensive, scarce, and only a few values are still being made.   Collins shut down it's mechanical filter business about the same time International Crystal closed it's doors, and more modern components such as ceramic resonators and filters are rapidly being discontinued by companies such as Murata.    And while you can still buy some popular parts such as varicap tuning diodes and J-310 MOSFETs in SMT packages, thru-hole packages have become obsolete.

So what's the homebrewer to do?   Of course it's always possible to scrounge up the parts needed to build a  one-off homebrew project for yourself,  but not everyone has a deep junk box or the ability to find vintage parts.   That's why my designs are always based on parts that can be purchased new or at least are reasonably available from present-day sources.   To do so often means innovating alternatives to vintage parts  or applying new technology to do what used to be done in different ways.

Tuned circuits for receivers and low-powered oscillators and amplifiers are a good example.   While inexpensive trimmer capacitors can be found, slug-tuned inductors and transformers aren't being made anymore except for low-voltage rated types for solid state circuits.   Only a very limited selection of air variable capacitors can be found outside of junkboxes, surplus, and hamfests.      Varicap tuning diodes are a good alternative for tuned circuits, but currently stocked parts are in the low picofarad range and only available in surface-mount packages.

Regular power diodes like the 1N4007 and 1N5408 can be used effectively as varicap or tuning diodes, taking advantage of the inherent voltage-dependent capacitance property of a back-biased PN junction.   Applying a DC voltage causes the thickness of the depletion zone to increase, increasing the capacitance proportionally.    Unlike varicap diodes that are sold for this purpose, the base capacitance and amount of capacitance change that is possible are not specified by the manufacturer, so these are not precision devices, but homebrewers can compensate for the minor part to part variations that exist.

I recently characterized a dozen each 1N5408 and 1N4007 diodes to get a sense for how consistent their properties are.   Rather than measuring capacitance directly which is somewhat error-prone,  I put each diode into a Colpitts oscillator circuit and measured the resulting frequency at 0.5 volt increments from 0 to 5 volts DC.    The resulting data can be seen below.   (A 10uH inductor and some additional parallel capacitance was present in the test fixture).

Since I had two different lots of 1N4007s from different suppliers I tested both lots to see how closely they track.   While it is obvious that the part to part variation is not what would be expected from a precision device, both of these common rectifier diodes provide a useful tuning range and both  are so widely used it is likely they will be available for a long time to come.

Lastly, since I'd read about using LEDs as tuning diodes I compared a variety of types against the two rectifier diodes.   While some do work fairly well, I also tested a few "short lead" T1 LEDs from a recent purchase and found that they produced no capacitance change at all!   For this reason I decided to stick with the rectifier diodes, but LEDs and other diode types may be worth evaluating further.

   Some pieces of vintage ham gear are not really unique, but are so rare as to almost never be seen.   The Multi Elmac PS-500 power supply is an example, it was sold as an AC power supply for the A54 and A54H transmitters in the mid-50s. There's nothing special about it - just a big transformer with 5 volt rectifier, 6.3 volt filament, and 500 volt secondaries.  Two nice chok...  READ MORE
- Robert Nickels (ranickels),  11/14/2019 

My experience with the Nouveau 75

a new AM QRP transceiver from the 4SQRP group
Category: Technical
Mini-review of the Nouveau 75 by W9RANI like it, warts and all!OK, maybe not quite that "mini"...here's a few of my impressions of this innovative kit from the 4 State QRP Group.I've been interested in amplitude modulation of a class E PA for some time and after Dave Cripe said to stay tuned for the next 4SQRP offering, I wasn't too surprised to see what it was, and bought on...  READ MORE
- Robert Nickels (ranickels),  11/12/2019 
   International Crystal Company of Oklahoma City was certainly best known for its crystals but the company was also a not-insignificant player in the CB radio manufacturing business, and also had a pretty extensive line of experimenter kits.   The kits typically had one to three tubes on a PC board and provided a single function, such as an RF amplifier, or transmitter, although some were ...  READ MORE
- Robert Nickels (ranickels),  08/02/2019 
   Equipment manuals and handbooks of the 1950s and 60s were filled with complex oscilloscope screens and discussion of ripple and as a result proper alignment of a phasing rig eluded most hams.    But a clever New Zealand ham, Fred Johnson ZL2AMJ was able to see what everyone else had missed, and published a short note in the NZ ham magazine in 1972 that makes the process simple and r...  READ MORE
- Robert Nickels (ranickels),  07/20/2019 
    Finding myself in need of a tool to remove a bad bearing on a blower from a linear amplifier I first tried the usual automotive type gear puller but it wasn't able to get underneath this small 5/8" OD bearing.   So I improvised a tool that took 10 minutes to make, cost a few cents, and had the job done in no time at allThe secret is wedges sold at the home improvement stor...  READ MORE
- Robert Nickels (ranickels),  07/16/2019 

W9RAN Field Radio

Vintage light aircraft radios repurposed
Category: Technical
In the early days of aviation radio, transmissions originated from the ground using the longwave transmitter also used for homing,  and pilots acknowledged by wagging their wings.  It didn't take long to realize the benefits of having transmitting capability onboard the plane as well, and the first generation of aircraft radio used the low HF band.    Given today's...  READ MORE
- Robert Nickels (ranickels),  05/11/2019 
   Watch the video  trailer for the 2019 monster movie that relies on a 1950s vintage Hammarlund SP-600 and Collins 30L-1 amplifier....  READ MORE
- Robert Nickels (ranickels),  04/20/2019 
   I find the show-biz association of the founder of Allied Radio, Simon Wexler, to be rather fascinating.  What started out as a supplier of crystal set parts led to a family that became a major force in the entertainment industry and in Chicago business...including many recognizable names:"Allied Radio (now known as Allied Electronics) is a company with a long history. On August 6, 1...  READ MORE
- Robert Nickels (ranickels),  04/14/2019 

Hazard E. Reeves - audio pioneer and defense contractor

Making crystals in midtown Manhattan during WWII
Category: Historic
 Hazard E. Reeves (1906-1986) may not be a household name, but his work is well known to every moviegoer.   He was an American pioneer in sound and sound electronics, and introduced stereophonic sound for motion pictures. He met Fred Waller at the 1939 World's Fair and saw the potential of Waller's "Vitarama" system which he invested in and which soon became the mo...  READ MORE
- BRADLEY STONE (NB9M),  04/12/2019 
    In his book "In Touch with Leo", the success and growing backlog of CB-100 orders is what drove Leo to his banker, Lou Ross, who asked him why he didn't anticipate the number of orders and required capital.  Leo responded "Lou, never on God's green earth could I have believed such an influx of orders!".    More funding was needed than the bank ...  READ MORE
- Robert Nickels (ranickels),  03/18/2019 

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