Crystal Radios Of The 2008 Contest Entrants
Mike Tuggle, Lyonodyne-17, Open Class Entry
"So, the solenoid is king, the spider needs a make-over and the basketweave is done." - from a post on Rap-'n-Tap -
Mike Tuggle, Loop Class Entry
This double-tuned set is built onto a Steve McDonald (VE7SL)-design alt.-az. loop antenna.
The ferrite coil slides on a loop strut to vary coupling. The idea was to see if mainland stations (2400 miles, and greater, away) could be heard here in Hawaii on a passive loop crystal set. The answer is, yes -- just barely. I really could have done with a bigger loop. Maybe next year.
Leading up to this circuit, I tried several designs using zero bias-voltage MOSFETs. A tapped ferrite coil or a pair of 5-inch diameter basket coils was used for gate and drain-source circuits. The basket coils were way over-coupled. None of the MOSFET circuits had promising sensitivity for possible mainland reception. Only by going to an ITT FO-215, kindly provided by Steve Bringhurst, could the big hop be made. This repeats my finding in last year's XSDX: The MOSFETs provide good selectivity but are not all that sensitive.
Twenty-five stations were heard, five of them DX on the mainland. Two of the DX stations were co-channel with strong local stations. I suspect the DX stations had exalted carrier help from the locals. This may be a potentially exploitable property of loop antennas: their characteristically sharp nulls can be directed to knock down strong local stations while, at the same time, leaving some carrier to exalt co-channel DX stations lying anywhere off the null direction. However, complete nulling the local station takes out everything.
I built a new crystal radio this year. I am now using 660/46 litz wire for my coils. One pair of coils for 530 kHz to 950 kHz. A second pair of coils for 950 kHz to 1700 kHz. They are basket weave with a five inch diameter. I wound solenoid coils of many diameters, inductances, and wire spacings with the 660/46 litz. None of them approached the measured Q of my five inch basket wound coils. My calibrated dials are six inches in diameter with two 6:1 verniers in series driving each dial. A third scale on each dial covers 1500 kHz to 1700 kHz. A pair of dual 8-60 pf silver plated porcelain insulated variable capacitors are switched in for 1500 kHz to 1700 kHz. Silver plated trimmer capacitors calibrate them to cover the dial. Overall selectivity and sensitivity are to the point that I really can’t complain.
This year’s contest was fun because the band was in flux all week. The first day had average to good conditions. The next three days were very poor with spotty receptions. The next two days had cold fronts pass through and stir things up. There were some unusual openings to the southwest. The last days were lively during the gray line times with some new stations for my all time heard list. That list is now 469 stations.
Here is a table of my activities during the 2008 contest.
DATE #HRS #STATIONS 1-18 13 168 1-19 13 35 1-20 10.5 18 1-21 7.5 9 1-22 7 15 1-23 9 20 1-24 7 9 1-25 5 8 1-26 6 16 1-27 9.5 3
Total of 87.5 hours and 302 stations.
291 stations identified.
11 stations not identified.
Total points: 556,834
Just in case you haven’t guessed by now, I only eat, sleep, and work the crystal radio contest for 10 days in January.
The set-up is a standard double-tuned arrangement with hobbydyne coupling. I tried to learn from and use the info that folks have so generously posted on R'nT and Dave's forums. Wish I could "give back" as much as I've received.
Antenna: 200' inverted "V", apex ~ 40'. I "sling-shotted" a wire over the highest tree near the house.
Ground is a 4' pipe into damp soil + a wire into a pond.
The set is Double Tuned: Ferrite inductors 10" above table top on empty HDPE containers. Getting the coils up away from the table made a BIG difference.
Detector Coil: Vintage ferrite bars, 9x2, 1/8” sep, 35t cw 330/46, 120 µH. "Battradio" ferrite bars, 2 stacks of 9 (7 high+1 each side) Wrapped with packing foam. This is kind of a 1.25"x1.25", "squarish" coil. I used Used the recent posted Q info with the wire I had available.
Dual gang 10-410pf cap, series-or-parallel (ceramic DPDT switched). The switch in the tank circuit didn't seem to hurt selectivity. Two tuning ranges gave good mechanical bandspread (<25 khz/div @ max f) via Jackson Bros. ceramic insulated cap, + 48:1 vernier, 0-100 dials. The Tuning is VERY sharp - really need the verniers (8:1 dial x 6:1 planetary).
Tuggle tuned ATU: 0.5”x4” Amidon 61 rod, 50t cw 165/46, 203 µH (~best for this antenna). 4" Amidon rods give better Q than 7.5" rods. Separate 365 pf Jackson Bros. caps + verniers. I needed a 48:1 vernier on parallel cap - tuning is sharp.
Hobbydyne: ~ 17pf (ceramic air variable) + 27 mH; choke (Dave Schmarders'). Helped at high end but probably don't need it with these ferrite bars.
Diode: FO-215 diode, rd~258k. (Picked the one with largest rd.). I could switch-in schottky's etc, but this was best overall match.
Audio: Dave's 200kUberformer + 500k Benny, shunted by 1µF. Two USI-UA1611-1 SP elements in series. ( 200k:2.5k).
Calibration: Curve-fitted table of dial settings for every 10 khz of AM band. This is VERY useful for quickly pouncing on DX that pops up then fades.
Op-Aid: Master log of 300+ stations heard over the last 2 years. It really helps having frequencies paired to: ID’s, formats, ZIP’s, area codes, major roads, distance, grey-line, etc.
Spotter radio: YAESU FT-757GX.
I wanted to try something new this contest. My old friend, Lem Morrison, has had a lot of success with air core coils, so I thought I would focus on a similar set for 2008. I had conversations or e-mail exchanges with several friends on a set recommendations. This included Lem, Gil Stacy, Mike Tuggle, Dave Schmarder, and Steve Bringhurst. The set would use 660/46 Litz on some kind of form. Steve's recommendation of HDPE vinegar jugs with a six inch diameter was the final choice of coil form. I used bamboo skewers through the ends of the jugs as part of the supports system. These skewers went into vertical posts of CPVC pipe; The pipes were epoxied into the fiberboard bases. The arrangement is was I call “goal post support”. I set the jugs up several inches from the fiberboard to help minimize losses.
The radio has five basic building blocks: the trap, antenna tuning, detector, audio matching, and headphones. The trap was the easiest part since I used a toroid unit I had built a few years ago. It uses and FT114A-61 toroid with a five turn link opposite the primary winding. The ceramic insulated variable cap is tuned with a 5:1 ratio velvet vernier from a WW2 BC-375 tuning unit.
The antenna module uses a variable with a built-in 2: 1 vernier and is wired in series with the antenna. The antenna connects to the frame, and the stator connects to the top of the parallel tank circuit. This tank circuit consists of a “goal post” mounted Litz jug-coil and a ceramic insulated variable cap with a vernier, also from a BC-375 tuning unit. These verniers have a 50:1 ratio and use a right angle drive approach. A drum made from a peanut butter jar top provides basis for the frequency readout.
In many other radios I used a dual section cap for the combination of antenna and tank circuit caps, but I wanted additional flexibility. The approached used here worked out great. The separate series antenna cap was especially useful during daytime DXing. It allowed me to use a greater value of antenna series cap for higher volume on weak daytime stations as well as reduced series capacitance when needed for selectivity. The base of the unit is fiber board with a from panel made from a black plastic clipboard.
Next is the detector module with the goal post mounted Litz jug-coil, an FO-215 diode, and a Fair Radio “holy grail” variable cap. This unit also uses the same type base and front panel as in the antenna unit. I selected to use neither taps nor a contra- wound approach on the two coils, just continuous wound coils. I did add complexity by adding an additional eight turn winding between the Litz rope for biasing a zero- bias MOSFET. This was switched in and out via a panel mounted double pole ceramic switch. I had trouble with the MOSFET and switching arrangement and finally bypassed it, using only the diode detector.
A panel mounted phono plug allows connection to the remote audio matching unit. A length of video cable joined the detector and audio units. I build a version of Mike Tuggle's double transformer Stanley match, and it worked great. Mine was wired for only two impedance options. I used the high input setting almost exclusively. I also tried a Select-To-Match and single transformer Stanley matching units for use with the in-the-ear units, but the double transformer Stanley Match seemed to work as well or better than those.
The headphones required more attention than any other part of the radio. The many hours of listening in previous contests with heavy sound powered elements had taken its toll on the nerves around my ears. In the previous contests the irritation became quite severe, such that even one of my eyeballs throbbed with pain! I had mentioned this one Rap N Tap,and a received quite a number of suggestions.
Garry Nichols and John Davidson provided me with an in-the-ear Shure unit to test. Steve Bringhurst provided a set of British phones with a wire behind the neck for partial support and a cloth band over the head. I also tried some Koss and Realistic in-the-ear phones. I used both the Shure and Koss phones in the beginning of the contest. They were comfortable and worked fine, but were not nearly as sensitive as my old phones. Steve's phones were sensitive but were not comfortable. I finally settled on a helicopter headset with salvaged SP elements. The headset fit completely over and around the ears. While these were not as sensitive as my old set of phones, they were a reasonable compromise.
I have used a Realistic DX-398 for several previous contests, and I used it for this one, too. A neat trick is to bring the antenna matching unit near the DX-398. Sometimes the DX-398 needed this extra boost the hear the signal the crystal set was receiving!
I like to use computer logging as I go along, so I used my normal home PC for that. A four foot table from Lowe's and a comfortable chair rounded out the listening post.