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Readers may recall my summertime blog, "Finding Your Best Crystal Radio ‘DX Diode'". It described a theoretical method I tried in order to see which of several dozen diodes might emerge as the best of the show, or in crystal radio DX circles, the ‘Holy Grail Diode’!

The grading system revolved around the combination of several factors … the diode’s measured forward-voltage (Vf), the weakest signal level detectable from an RF oscillator (whose level could be varied) and the diode's current when detecting a fixed-level signal on 1220 kHz. These values were used to derive a numerical ranking that I called ‘Vdx’, which would hopefully rank the best overall performers!

It’s not unreasonable to think that diodes with a very low forward voltage (Vf) combined with the ability to detect the weakest signal from the signal generator, might likely be the best diodes in the bunch … or are they?

These tests capacitively-coupled the oscillator signal directly into the crystal radio’s antenna tuner, which then coupled them into the detector stage. Using the methodology described in the earlier blog, the 48 diodes under review were narrowed to the ‘top 10’ likeliest best performers.

This time, actual ‘on-the-air’ signals would be used to compare diodes against each other in real time. A closer look at the top candidate diodes was made over several days and evenings as the days grew shorter and darkness arrived earlier.

My DX set has provisions for comparing a current good performing diode against two others.

 

In this arrangement, shown above, the current ‘best diode’ is mounted in the center switch position so that an immediate comparison can be made between it and the other two in real time.

A weak groundwave signal from Seattle, with a slow fade rate, was used initially but nighttime skywave signals were found to be most useful. Several  hours were spent tuned to 2800 watt CKBI in Prince Albert, Saskatchewan on 900 kHz. Their C&W format meant that most of the time they were broadcasting music, which I've found is always much easier for the ear-brain to detect in the noise floor than are spoken voices.

So what did I eventually find out? My original ranking methodology concluded that the best overall diode of the many dozens was the Sylvania JHS 1N3655A, a 40-year old microwave mixer diode.

The observations of the CKBI signal strength were by ear-brain only and no actual levels were measured since signal levels were usually too weak to measure on my detector's micro-ammeter. Measurements may yet be undertaken using an oscilloscope or by using an audio amplified output to compare signal voltage levels.

So … would my diode-ranking order and testing methodology hold up when actually using the diodes in a hi-end, low-loss crystal radio system when connected to an actual antenna?

I started ‘A-B’ comparisons against what has always been a reliably good performer mounted in the center position, a fairly modern twenty-year old  1N34A.

As noted above, the #1 rated diode (with my Vdx rating of 66) was the 1N3655A microwave mixer diode. Although it did not produce the loudest signal (diode current) compared with others, it had an exceptionally low Vf of .18V and its weak-signal detection level was good although not the lowest. Like a few others, it detected the nearby UHF data stream ‘clicks’ from a nearby Wi-Fi modem, often a characteristic of a good performer.

I was somewhat aghast when my #1 ranked 1N3655A was immediately outperformed by the modern 1N34A, ranked 44 out of 49! The 1N3655A was not just poorer than the 1N34A, it was very poor by comparison ... hmm ...was my selection process really that far off?

Diode #2, also with a low Vf of .197V was also poorer than the 1N34A, whose Vf was an unimpressive .375V.

And so it went for the most part, with my top 10 choices! Most of them were equal to the 1N34A but nothing stood out while listening to real on-air signals until I got to the three ‘curiosity’ diodes, originally tested at the very end.

Surprisingly, the D18 produced a noticeably better signal than the 1N34A and was moved into the #1 position.

 

These last two were both made in the 50s … was there something different about the way they were made? Was the germanium different back then? How did they perform so well when their Vf was so high? It almost appears the opposite of what might be expected.

Seeing the above behaviour, I couldn’t pass up the chance of testing the 48th ranking diode, a beautiful black NOS Rogers 1N34A, probably another product of the 50s. Its Vf was truly discouraging, at .401V and the reason I had mostly ignored it in the past. It was put up against the D18.

 

I was astounded to find that not only was the vintage black beauty better than the D18, it was a LOT better! The ‘just barely’ detectable CKBI signal popped out of the noise to become one that was easy to hear! I had to rock the ‘A-B’ switch back and forth many times just to enjoy the big difference!

Out of curiosity, I tested the last-ranking diode (Vf of .444V) and it truly was deaf, with not even a sound emitting from the phones … so at least I got that one right!

What is obvious now is that the method I used to rank the diodes was flawed. These results have brought up several questions for me that I had never considered previously … far more questions than answers!

Takeaways? I've found that there's a lot about diodes that I don't know and need to learn about! I’ve learned that a diode’s Vf value is not an indicator of its weak-signal detection capability in crystal detectors (in spite of what some You Tube videos might try to convey). I've learned that when detecting a weak signal, the diode is operating below its Vf value which helps explain why a high Vf value does not mean a poor detector or a low Vf does not mean a good detector. Low Vf values were a well considered number when ranking my diodes … an apparent mistake.

Further to this, the diode is operating within its ‘square law region’ when detecting the weak signals we seek. When operating in this region, it means that increasing the input signal by 5 times (for example) will increase its output by 25 times. Similarly, decreasing the input level by 5 times will result in a 25 times drop in output. The importance of reducing as many losses as possible in the antenna tuning stage along with the detector stage itself can certainly pay fast dividends when it comes to weak signal detection. Conversely, ignoring system losses will very quickly reduce performance.

Also ignored in my system was diode capacitance, diode operating impedance, reverse leakage and no doubt some characteristics I'm not even aware of. Diodes with lower C will have fewer losses than those that are higher. I wonder how much of a factor was this in my overall unexpected results! The diode’s internal resistance when detecting a signal is a factor that I did not consider. The method of determining this value is complex but it may explain some of what I noticed.

There appears to be something different with older diodes that makes them great performers … larger junction? Germanium quality?

A final take away ... with enough knowledge, one can measure every tiny detail about a given diode without actually using it. No doubt a ranking list of diodes going through such rigorous scrutiny could zero-in on the top few. What's the BEST diode to use? It's probably the one that seems to work the best in your particular detector, until a better one comes along ... but it appears you can't go too far wrong with a very good 1N34A ... even in 2024!

 

 

The first time I was involved with a crystal radio DX contest was about 20 years ago when I built a well-performing crystal receiver for the Yahoo Crystal Radio Group's annual DX contest. It was a great learning experience and taught me much about circuit losses and how to overcome them. I originally built several sets but was unable to hear anything other than local stations until I eventually figured things out ... the system was only as good as its weakest link or links!

Fast forward to more recently when I obtained and wrote about the Heathkit CR-1 Crystal Radio, a simple but very well-designed tuner that has become popular with collectors. Using the CR-1 re-kindled my interest in the DX contest activity of years ago and when talking with two other amateurs that had an interest as well (one had also been in the earlier contests sponsored by the Alabama Crystal Radio Group), we decided to bring the contest back once again. The Facebook Crystal Radio DX Contest Group was formed last fall, a set of rules drawn up and the contest date set for the first week of January of this year. This gave interested participants plenty of time to build something they could use in the contest.

I spent all of December designing and constructing a new contest radio, hopefully one with enough selectivity to get around the 15 local flamethrowers (10-50kW S9++ signals) that plague the band for me and eventually drove me from crystal radio activities.

The new radio makes use of several 'traps' to null strong signals ... two are in the antenna line while one is loosely coupled inductively to the detector tank circuit. The two inline trap coils are wound with Litz wire on ferrite toroids (R40C1) while the third is a basket-wound Litz coil (660/46) on a 4" diameter form.

The antenna tuning stage also uses the same ferrite material but in the rod / bar form. I wound a low-end as well as a high-end coil for the tuner using the same high-count Litz as on the big trap coil. The low-end coil is wound on a bundle of three rods while the high-end coil uses a single rod.

Antenna tuner

The detector stage uses another Litz coil with this one being solenoid-wound on a 4" diameter form. Both the antenna tuner and the detector use excellent quality hi-Q ceramic insulated air variable capacitors (18-360 pfd). All components that handle RF are insulated from the plywood bases in order to reduce losses. Moving a capacitor from the plywood to the insulated standoffs makes a noticeable difference, something learned the hard way years ago but actually measured while using the new radio.

Detector stage

 

The detector also has provisions for comparing various diodes as not all diodes are created equal ... not even all diodes with the same number! When testing and comparing diodes of the same type such as the popular 1N34 germanium, every once in awhile one of them will turn out to be noticeably more sensitive than the others. In my built-in B-A-C diode test module, the hottest diode is always mounted in middle-position A, making it easy to quickly compare by switching to the left for B or to the right for C. So far the best one I have found is the vintage Russian D18 germanium diode but an old 1N34 removed years ago from a 1950s-era Heathkit has given it a good run for the money! I've still several hundreds of early germanium diodes, pulled from old diode matrix boards years ago, to test against the D18 as well as numerous Schottky diodes.

Also on board the detector module is a Selectivity Enhancement Circuit (SEC) that increases selectivity by unloading some of the diode's effect on the detector coil, similarly to tapping the diode further down the tank coil. It uses a small butterfly capacitor seen to the right of the main tuning capacitor in the photo above. I found it extremely effective when needed and is well worth the addition to a high-performance tuner.

The detector stage is followed by an impedance-matching transformer for the sound-powered headphones. This stage also houses a 50uA meter to measure diode current / signal strength levels.

 

 

The meter can be switch-bypassed to prevent needle-bounce on stronger signals. It is particularly helpful when using the traps to null a signal to the minimum level.

 

 

The three traps utilized have been very effective in eliminating what I had originally perceived as an impossible DXing situation.

Here are the daytime-power signal strengths of my 15 line-of-site blowtorch stations that, without trapping, very effectively block most sections of the band. Anything over 50uA is ear shattering and problematic, usually requiring the use of all 3 traps:

             KVRI 1600 50uA
             KRPI 1550 100uA
             CJVB 1470 40uA
             CFTE 1410 350uA
             CHMB 1320 100uA
             CJRJ 1200 400uA
             CKWX 1130 300uA
             CKST 1040 90uA
             CKNW 980 150uA
             KGMI 790 100uA
             CHMJ 730 450uA
             CBU 690 650uA
             CISL 650 200uA
             CJWW 600 100uA
             KARI 550 100uA

Overall I was very pleased and surprised at the good performance of the new radio. During the contest period I identified and logged 92 unique stations in 16 states / provinces. More than one station was logged on 9 different frequencies as the propagation varied from night to night.

Highlights of the DX Contest were hearing WHAS in Kentucky (2,007 miles), WJR in Michigan (1,970 miles), KXEL in Iowa (1,556 miles), WCCO in Minnesota (1,423 miles) and CBW-990 in Winnipeg, smack up beside local blowtorch CKNW-980! Additionally, hearing Washington state 250 watter KFLD-870 and 250 watt KWBY-940 in Oregon were great surprises.

I found the use of a spotter radio (Sony ICF-2010) to be very useful in locating signals to target and to zero-beat with an RF signal generator. The generator’s tone-modulated signal can then be tuned in and the xtal radio and antenna / detector stages optimized. 

From here, any pest signals are then tuned to and individually nulled using the traps while watching the signal meter. Antenna and detector stages are then re-tweaked before disabling the generator and listening for the desired signal. 

Often it is heard immediately following the above tuning procedures but if not, monitoring the frequency for several minutes often allows time for the weak signal to fade up to audible levels. 

Comparing programming audio with what is heard on the spotter radio will confirm hearing the correct signal as will comparing audio to the station’s own live-feed on the internet.

Due to the larger and much better antenna (inverted-L 70’ x 100’) on the crystal radio, I would often hear good audible signals on it and not on the spotter (something that I found surprising) so often times it was productive to just tune around the band on the crystal radio, tweaking stages as required.

I’m looking forward to further improvements of the tuner as well as to the next DX Contest whenever that will be scheduled ... hopefully you can join in as well!

                    FREQ UTC    STN    LOCATION    MI      

BCB Ferrite Loopstick

Regular blog readers may recall my two previous blogs, on the Heathkit CR-1 crystal radio receiver.













This very much sought-after radio is a well engineered ‘double-tuned’ set utilizing a series-tuned antenna tank circuit coupled to a parallel-tuned detector tank.



Both coils are wound on the same 1/4” diameter tubular form containing two ferrite slugs ... one for the antenna coil and one for the detector coil. The coils have been pre-wound and fixed on the form, about 20mm apart while the slugs have been waxed in place to set each inductance to the desired value.

courtesy: Scott's Crystal Radios

I do wish that I'd had enough sense when I was a kid to buy myself a CR-1 as it seemed like they were dirt-cheap.




The $7.95 even included a set of headphones! Of course, $7.95 to a 12 year old was probably a lot of money, being about $70 in today’s currency!

My previous experience with homebrew DX crystal radios (ones that can hear stations other than strong locals) had taught me that they required large coils and ‘hot’ diodes. The CR-1 has neither of these yet it performed exceptionally well during the few weeks of evening tests a few months ago. I was able to log 50 stations, as described in the earlier blog ... and began to see that, just maybe, requirements may not be as rigid as I had always thought, when it comes to building DX sets!

When I discovered several ‘new-in-the bag’ broadcast band ferrite loopsticks in my junk box, I realized there might be an opportunity to allow me to make something very similar to the CR-1 circuit.



These are the same loopsticks used in the crystal ‘Rocket Radio’ of the 50s or in various transistor radios of the day.



I breadboard-mounted the two loopsticks so that the distance between the antenna coil and the detector coil could be adjusted, allowing some control over coupling and selectivity ... something not available with the stationary CR-1 coils.


Using the same antenna, headphones and external wavetraps, proved once again the excellent performance available from a very small and simple hi-Q coil system ... a DX machine without huge coils and expensive Litz! A total of 51 stations were logged over a two-week period, one more than was heard with the CR-1 and with a few ‘almost’ heards still waiting for one of those really good propagation nights. Having the ability to adjust the coupling was very helpful and made some of the weaker stations a little easier to detect. Stations in RED are local strong signals while those in BLUE are skywave propagated DX signals:



Mounting one of my old HRO 'PN' vernier dials on the main tuning capacitor provided plenty of bandspread, with each dial division corresponding to ~ 2kHz. It was very easy to locate any given frequency within the broadcast band once the dial was calibrated.



Soon after, I ran across a post by Zoltan Pap on Facebook’s ‘Crystal Set Radio Group’, describing his unique use of an old 455kHz I.F. transformer in a crystal tuner. I thought this was a rather brilliant idea and dug out an old I.F. can from the junkbox to see what it might offer.



The old I.F. can had two litz-wound (10 strand) tank coils, fixed in place over two adjustable ferrite slug cores ... in reality, something very similar to the, now very difficult to find, ferrite loopsticks used above.

The two inductors measured out at ~ 700uH - 1.1mH as the slugs were tuned from one end to the other. I was aiming for something close to the inductance used in the two CR-1 tank coils ... approximately 380uH.




A sufficient number of turns were removed from both coils to yield the needed inductance and both coils on the CR-1 breadboard clone were replaced with the old I.F. can coils.






In just a few minutes of tuning through the band, it was very easy to hear and separate all 16 local stations (RED in the above log). A few hours after sunset (on a not-so-good night) yielded quick copy of KPOJ (620kHz) in Portland, Oregon (231 miles) as well as CHED (630kHz) in Edmonton, Alberta (534 miles), demonstrating that even this old 1940's I.F. can could be turned into a crystal radio DX machine!

I don't believe the 'Q' of this pair of coils is very high, compared with the smaller loopstick, as its selectivity appears to drop off above 1000kHz. I'll try separating the form into two halves so that the coupling can be adjusted. The experiment is still under way but if you want to play and can't lay your hands on the pricey loopsticks, old I.F. cans are often much easier to find and probably a lot cheaper.

If you’re a regular blog reader, you will likely recall my description of  “The Enigmatic Heathkit CR-1 Crystal Radio” a few weeks ago.




Back then I mentioned that I was ‘eager to get my mitts on one’ and that I had arranged to borrow a CR-1 from another VE7 who was fortunate enough to own one.

A few weeks after posting the blog, I received an e-mail from Larry, WB5OFD, in Texas.

"Reading thru your blogs the other night ... discovered your article on Crystal Radio reception reports. I am in the process of disposing of a lot of radio gear I have collected over the past sixty years and in that pile is a Heathkit CR-1. Yours for free if you would like to have it."

Needless to say I was overjoyed, both at the opportunity to actually own a CR-1 myself and at Larry's exceptional generosity!

Larry went on to explain that he had been in the Air Force and his little CR-1 had been all around the world with him, from Alaska to Turkey ... but from its fine appearance, you would never know it.

Larry's gift!

He was happy to pass it on knowing that it was going to a good home. I am most appreciative of this kind gesture from a fellow radio amateur, knowing that these things are not too easy to find ... and are somewhat pricey!

As can be seen in the schematic diagram above, the CR-1 is a simple double-tuned crystal receiver, utilizing a series-tuned tank circuit for antenna-tuning, coupled into the detector tank circuit. The detector diode, a 50’s-era 1N34, is tapped down on the tank for headphone impedance-matching and to reduce circuit loading. Reducing the load on the tank circuit improves selectivity but diminishes sensitivity. Crystal radio design is always a trade-off between these two critical characteristics.

Although I had heard good things about the CR-1, I must admit that I was somewhat skeptical ... just how good could an unmodified CR-1’s simple double-tuned design really be? I was about to find out.

My listening location, on the eastern shoreline of Mayne Island, puts me directly across several miles of saltwater from sixteen exceptionally loud 'blowtorch' signals whose antennas are located near the water on the other side of Georgia Strait. Six of these stations run 50kW ... 24/7. All of these signals are wide and strong, being well-over S9. It is a difficult location for crystal radio DXing as separating weak DX signals from the blowtorches can be challenging.

My previous experience with crystal radio DX is well-documented on my website here. Back then, I quickly adopted the standard protocols to help hear DX. This included the use if a separate ‘spotter’ radio to first find signals that might possibly be strong enough to be heard on the crystal detector. I also used an RF signal generator that let me temporarily put a weak tone-modulated carrier on the frequency of a station that I was trying to hear. Using the tone, the antenna tuning as well as the detector circuit can be optimized for maximum signal. I also used a 100 microamp meter in series with the headphones to make peaking these circuits accurately. The same protocol was used for my CR-1 DXing as well.

Since there are so many very strong signals here, I have added two inline L-C traps on the antenna lead.


My first trap was made from a ferrite bar loopstick inductor salvaged from an old transistor radio.





The second trap is made with a ferrite toroid and Litz wire and produces deeper nulls than the ferrite bar. The bar will soon be replaced by a second toroid trap.



The traps allow me to significantly null any strong signals that could be covering up a nearby weaker signal. For nulling, I set the signal generator on the frequency of the pest signal and then tune the trap for a null while watching the meter. Once everything has been tuned, I’ll often just sit and wait for the desired signal to fade up to a detectable level on the crystal radio and then confirm its audio match to what can be heard on the spotter radio.  Very often, a signal initially too weak to be detected, will quickly pop up in signal strength to an easy-copy level for several minutes, before dropping below the threshold of diode detection level once again.

I am presently using a pair of RCA WWII sound-powered ('Big Cans') phones, impedance matched to the CR-1’s output with a multi-tap audio transformer. I have also used a nice set of extremely sensitive Western Electric 509Ws, manufactured in the late 20s. These are also impedance-matched to the CR-1's output. On weak signal tone tests, I can see only a very tiny improvement with the RCAs versus the old 509Ws as both are very sensitive.



There is a large variation in propagation quality on the broadcast band, especially this far north on the southern edge of the auroral zone. The difference from one night to the next can often be quite dramatic. On most nights the band favors the north-south path while on geomagnetically quieter nights it’s the east-west path that dominates. The band needs to be in good shape for any worthwhile hope of DX on a crystal radio.

On one of the recent better nights, of which there have been very few of lately, one of the stations in Alberta was so strong that it needed trapping! This was something I saw quite often with my previous DX set but I didn't expect to see it with the CR-1.

For crystal radio DX, propagation is the best helper. Small incremental improvements (in terms of db losses) can be made in any part of crystal radio's systems but on nights of good propagation, tens of db improvement will magically appear, thanks to Mother Nature!

When in Turkey, Larry had the opportunity to connect the CR-1 to the large FLR-9 circular antenna array used during the cold war for HF direction-finding of targeted signals. Covering 1.5MHz to 30MHz, the FLR-9 consisted of ninety-six 120' towers, suspending 1056 vertical elements ... all over a 1500' diameter ground screen! His notes show that he logged the BBC, Italy and West Germany on the CR-1 while using the array!

The FLR-9 array in Augsburg, Germany

Although the antenna system connected to my CR-1 is not nearly as impressive as an FLR-9 array, it is very capable on the broadcast band. I’m using my 630m inverted-L, bypassing the tuning and 50-ohm impedance matching system, essentially feeding it as a top-loaded vertical wire. The (somewhat slanted) vertical wire is approximately 70’ which is then attached to a 3-wire 100’ long tophat. The antenna is very close to the ocean and parallel with the beach. The CR-1 ground system consists of about 60 buried radials, varying in length from 30-60’. The basic antenna is self-resonant at around 1200kHz.

Over the past few weeks, I have been spending a few nights patrolling the band between 9:30 and 10:30PM, to see what might be heard with the CR-1. So far, I've logged 50 different stations ... far more than I had expected to hear.

The log below shows all of the stations heard. The stations in red are all local line-of-sight transmitters and are extremely strong ... all are well over S9 on my Sony spotter radio. The stations shown in blue are all ‘DX’, with the furthest so far being KOA in Denver, at 1100 miles.



The log illustrates just how much the blowtorch signals prevent weak-signal detection, even with traps! The stations logged on 1510 and 1530 were only possible when the 1550 blowtorch lost their audio for about five minutes one evening! Selectivity becomes increasingly more difficult towards the top end of the band and, unfortunately, there is a larger concentration of strong locals (who seem to delight in over modulation and splatter), making reception up there extremely challenging.

There are still some lower-band signals that I have yet to log and they have been gradually growing stronger as the nights get longer. As well, the region above 1600kHz may still provide a few opportunities over the next few weeks, if the loud local on 1600 can be sufficiently trapped ... the next few weeks will tell if there’s anything left in the CR-1’s tank!