Crystal Radios Of The 2007 Contest Entrants
The variable capacitor on the antenna coil is from a radio in that I remember being in my sister's room in the 1970's, (which I dismantled for parts sometime at the end of the 70's, and made my very first crystal set with), and the detector coil is wound around a ferrite rod from the first radio I ever owned as a teen age boy, also back in the 1970's, and died about 4 years ago so I scavenged the ferrite rod from it.
The basic radio is mostly built from parts literally scavenged out of other radios over the years. The only "new" parts are the bogen 725 transformer, FO-215 diode (thanks Dave Schmarder) and sound-powered headphones.
Strong memories in this set for me, and hearing stations fading in and out really brings back memories of the days when I'd listen to my radio, hearing far away stations and caught up in the wonder of it.
2007 CRYSTAL SET CONTEST SET
The rig this year is all new. It is double tuned with 660 litz spiderweb coils. Both capacitors use reduction drives for easier tuning. The detector coil was changed at the last minute to a contra-wound configuration…the jury is still out as to the impact this made. I think I may use separate plug in coils for HI and LO band next year. The antenna is the same as last year… 100 foot end fed up about 25 feet. Headphones are sound powered elements in a reused headset with a bogen transformer that has switched impedances.
My short wave set is very simple.
It consists of a tank circuit composed of a small Johnson roller inductor with a small 100pf silver plated variable cap. I connected another small variable as an antenna trimmer and included a small trimmer to ground. It has a diode selector arrangement including a single generic Radio Shack germanium (black band) and choices of one two or three paired Schottky IN5711s. I found it necessary to add a bias attachment for the IN5711s.
I had an outboard Benny setup that plugs in between my matching transformer and the set. Therefore I added a tiny watch battery to the wiper and one end of the rheostat. The bias really perks up the IN5711 diodes.
My headphones are Western Electric sound powered matched with a UTC Ouncer transformer.
The antenna trimmer proved to be ineffective. So, I tried Steve B's cap string which helps somewhat.
Naturally I wasn't quite ready when the contest started. So I got through the first couple of nights with the diodes still on clip leads.
I used a couple of small door knob caps for the antenna cap
string which is overkill, but I sort of like the look of it.
2007 Crystal Radio Contest Radio Set Description
My 2007 contest set is the same as my 2006 contest set. The set has three coils which slide on a polystyrene tube. From left to right they are wave trap, antenna coil, detector coil. All three coils are space wound (one wire width spacing between turns) copper wire; #24 ga. for wave trap and detector coils and # 23ga. for the antenna coil. All three coil forms are 3" diameter polystyrene tube. The inductance of each coil is approx. 251 uH.
A single Jackson 365 pF VC is used on the wave trap coil and a single Jackson 410 pF VC is used on the detector coil. A dual Jackson 410 pF VC is used on the antenna coil in a Tuggle arrangement. The diode is a FO-215. A Calrad transformer and a "benny" is used in the detector circuit to match the SP headset - USI 1614 - I am using. The antenna is a 3-wire flat top aprroximately 75 feet long and 13-18 feet high. Ground is a series of three 8 foot copper plated steel rods driven all the way in the ground four feet apart.