Posts Tagged ‘crystal radio’
|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.
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|
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!
When it comes to crystal radios, there is nothing revolutionary regarding the CR-1’s basic circuitry but for some odd reason, it has achieved cult-like status as well as high dollar value.
|courtesy: Scotts Crystal Radios|
The article that piqued my interest appears on 'Scott's Crystal Radios' website and makes for an inspirational read, eventually revealing the inside core arrangement of the ferrite-loaded tuned circuits via an actual X-ray of the device! By the way, if you are looking for a nice set of older headphones, Scott's website is the place to visit!
|courtesy: Scott's Crystal Radios|
Scott was eventually able to achieve performance equal to that of his borrowed CR-1, with his own slightly modified versions, all in a similar-sized footprint. Perhaps this is one reason why the CR-1 is so much sought-after, as good performance in a very small package is not the norm when it comes to crystal radios. It's usually a case of ‘the bigger, the better’ when it comes to performance.
A recent search of my junque box revealed several NIB ferrite loopsticks that would allow a potntial reproduction of this interesting circuit.
Several years ago I spent an eye-opening winter learning about DX crystal radios as up to that time I had always believed it would be impossible to hear anything other than strong local signals on a crystal radio. I quickly discovered that there was a very large Crystal Radio Yahoo Group where menbers were working at the leading edge of crystal radio design. I also found that the group sponsored an annual Crystal Radio DX Contest which inspired me to dig deeper.
It wasn’t too long before I decided to join the fun and attempt to build a crystal radio DX-machine but I was in for a few surprises and a long learning curve ... it seemed that hearing broadcast band ‘DX’ on a crystal radio (anything other than loud locals) was not going to be an easy task!
Over the course of several months I tried many types of variable capacitors, tank coil configurations and antenna tuning circuits. I even erected a dedicated antenna system for the various experimental circuits I was putting together ... an 'Inverted-L', 50’ straight up and 70’ horizontal, along with a ground rod connected to several buried radials.
I quickly learned about something I normally didn’t have to worry about when working with ‘active’ devices and that was overcoming system and component losses. In critical crystal radio design, it’s all about minimizing the losses in every stage and every component in the system since there are no amplifiers to help overcome these losses. Your system is only as good as the weakest link. In true crystal radio DXing, no active devices are permitted ... it’s just your crystal radio and the energy generated at some, hopefully far away, transmitter site!
After several months, I eventually ended up with a well-performing triple-tuned set that used lots of 'trapping' because of all of the very strong nearby signals here ... eight 50kW locals!
A description of the learning curve, with several do's and dont's to help new builders, can be found on my website here.
Back then, 80 stations were logged (from my location on Mayne Island in SW British Columbia) over the one-week Crystal Radio DX Contest.
SAN FRANCISCO, CA
100 MILE HOUSE, BC
MERCER ISLAND, WA
DAWSON CREEK, BC
NEW WESTMINSTER, BC
SALT LAKE CITY, UT
SAN ANTONIO, TX
ST. MARIES, ID
MEDICINE HAT, AB
TWIN FALLS, ID
LAKE OSWEGO, OR
ST. PAUL, AB
OREGON CITY, OR
SAN JOSE, CA
LAKE OSWEGO, OR
BRIGHAM CITY, UT
Old notes indicate that there were 14 stations at S9 or higher, requiring heavy trapping to hear anything close to their frequencies.
|My recent interest made me wonder what the situation is today when it comes to the number of strong local ‘blowtorch’ signals, surely the bane of all crystal radio DXers? Although there have been a few changes over the years, a quick scan of the band during the prime DX evening hours found that although one of the blowtorch signals (at 600kHz) was now gone, another had appeared at 1200kHz ... sadly no net difference.|
The top end of the band, always a prime area for good skywave DX, is unfortunately still dominated by a huge signal from KVRI just across the water near the Canadian / U.S. border. If KVRI were silent, the top end would be a wonderfully quiet hunting-ground for new catches. The new local blowtorch (CJRJ) on 1200 kHz will now cause problems for the middle of the band, which was always a good region for DX.
So it seems overall, there hasn’t been a huge change here other than in the middle of the band. It looks as though there are still some good watering-holes to be had but several traps will still be needed in any new system.
Once my present radio-bench project is finished (a '36 RK-39 crystal power oscillator) I’m looking forward to more research and design of a couple of new systems, starting with something similar to the CR-1 as well as some experimentation with toroidal coils. I always find the research and planning phase of any new project more interesting and fulfilling than the actual construction and implementation! Hopefully I’ll have something ready for the fall DX season!
Thanks to VA7MM, I will also have the loan of an original CR-1 next winter to make comparisons to any clone that I might build!
If building a DX-crystal radio is something that might interest you, there are several great websites offering inspiration and helpful info. The links for these may be found at the bottom of my own crystal radio page. As well, there are two active crystal radio groups on Facebook, where daily two-way discussion can be had.
Perhaps, with enough new interest, we can even revive the annual Crystal Radio DX Contest!