Crystal Radios Of The 2007 Contest Entrants
Charles Pullen (Chuckster)
This set did a lot better than last year, new ferrite rod coils and a 10k pot for regen control. The circuit is the very simple mpf102 jfets regen from Owen Pooles website, with a separate antenna coil loosely coupled to the detector coil and a 10K pot between the tickler coil and the jfet.
Both coils are 210 uH, the tickler coil is 5 turns added to the ferrite rod and secured with electrical tape. All coils 44/40 litz. I tried a few different jfet's and voltages in the circuit and all were about the same so I stuck with the mpf102 and 9 volt battery although the circuit works very well at 4.5 volts. The beauty of this circuit is decent performance in a circuit that just can't get any simpler.
My 2007 1-AD Contest set was the “Modern Homodyne,” designed and described by G.W. Short in a highly recommended article:
I made several modifications to, and omissions from,
Short's original circuit:
1. ferrite rod coil L1 replaced by basketwound 660/46 litz coil
2. no gimmick capacitor across gate resistor R1 -- not necessary at BCB frequencies
3. no shunt detector diode D3 in the half-wave doubler – simple, single diode detector D4 only
4. no low-pass audio filter -- it eliminates highs, but it is a real detriment to DX listening
5. no final audio amp stage -- not necessary when using sound powered phones
6. no ground plane under circuit board or other shielding (actually, I forgot these, and the circuit seems to work just fine without them).
The Homodyne uses no outside antenna. Its tuning coil serves as the antenna -- in this case it's the 6" diameter (that's right, 6 INCH) litz basketwound coil L1, mounted as a rotatable, tuned loop on top of the set. This coil is all the antenna there is. I'm reasonably sure of no coupling going on to my disconnected outside antenna. The lead-in is several feet away; the set works well in other rooms far away; and the loop exhibits nice directionality. But, larger coils tried, including a 1-meter diameter loop antenna, tended to overload the set and ruin its selectivity.
Feedback works like none I've ever experienced. There is no popping in and out of regeneration. As feedback is increased, the squeal-free tuning range about the desired station narrows, and the volume increases. Further increasing feedback, the squeal-free region closes out, and tuning the station is very touchy -- just like a BFO’ed exalted-carrier set which is always in oscillation. In essence, feedback control R7 can vary the selectivity from broad as a barn to razor sharp, depending upon one’s need..
One drawback to the Homodyne is it is very sensitive to line noise – perhaps because it is so sensitive to everything else as well. During evening listening sessions I can hear the ambient line noise level steadily increase as neighbors switch on SCR light dimmers, cable TV’s, fluorescents, etc. This din usually increases to a point when some idiot switches on something defective, and the noise becomes as loud as an SCR operating in my own house. Unfortunately, the power conduit to the house is right outside the radio room wall. Switching off the main power provides little relief. By then, the noise has been induced into the phone lines, and the cat’s out of the bag. I get a good measure of relief by retreating with the set to an outpost on the electrically quiet lanai (enclosed sun porch) in back. Having only a 6-inch loop antenna makes this feasible.
The directional pattern of the rotatable loop antenna reduces the need for wave traps for all but the strongest interfering signals. I got by in the contest fairly nicely without using any wave traps. Since the loop antenna pattern has sharp nulls off the faces and broad peaks off the ends, directing the antenna to exactly null an interfering station usually leaves enough gain for the weak DX station, whatever direction it may lie in. One really weird effect I haven’t had time to investigate is, on strong local stations and in-house line noise, the loop antenna shows a sharp null off only one face but not the other. This means the loop is showing a cardiod (heart-shaped) pattern instead of the usual figure-eight. One of the real beauties of a loop antenna is that, unlike other traps, it lets you ‘trap’ interfering signals on the same frequency as the desired DX signal.
Without a doubt, the Homodyne is the finest broadcast band DX set I’ve ever operated -- bar none. That includes the likes of the Hammarlund HQ-180A, Collins 51S-1, and others.
How It Works
Drawing extensively from G.W. Short’s article, here is how the set works:
The set is basically a two-stage RF amplifier using a “direct-coupled, complementary-cascade” transistor pair TR1-TR2. All tuning is done by L1-C2, the 6-inch basket coil-variable capacitor tank. Little amplification is done by the 2N3819 JFET TR1 – it serves mainly to match the high impedance input from the tuning tank, preserving the tank’s high Q. Most of the actual gain comes from the PNP germanium AF 239 UHF-type transistor TR2.
Positive feedback* derived at the collector of the AF 239 (‘collector follower’) is applied to the 2N3819 gate (input) to generate strong oscillation at approximately the same frequency as the incoming carrier. The 10 k-ohm pot R7 controls feedback level.
(* In going from 2N3819 gate to AF 239 collector, the phase is inverted twice, for a 360 degree ‘total phase shift,’ thus leaving the feedback in-phase with the input. At least, that’s true at the lower frequencies including the broadcast band. At short waves, phase shift creeps in and must be compensated for.)
An important feature of this local oscillation is that it will lock onto the frequency of the incoming signal’s carrier, even if the tank is not tuned exactly to that carrier. Ideally when the oscillator tank is tuned to the ‘vicinity’ of a weak signal, it will lock on that signal and not on much stronger signals that may lie in adjacent channels.
The front-to-back diode pair D1-D2 serves as a limiter that suppresses the modulation of the fed-back carrier and prevents the level of oscillation from becoming too large. Ideally the feedback consists of un-modulated carrier of fixed amplitude. The 10 k-ohm fixed resistor R6 feeding the limiter allows good clipping and at the same time prevents the limiter from loading down the detector input.
Demodulator / Mixer / Detector
Demodulation (detection) occurs at diode D4 that goes to the output. Incoming signal (carrier-plus-sidebands = RF input) is mixed with the locally-generated oscillation (LO) at the carrier’s frequency. The difference frequency product of the two mixed signals emerges as the so-called ‘intermediate frequency’ (IF) which in our case is simply the audio frequency. Any mixing products of the LO with adjacent channel signals are much higher in frequency and thus out of the audible range. This is how ‘direct conversion’ receivers of this type get their excellent selectivity.
Acknowledgments: Special thanks to Macrohenry for originally recommending the Homodyne circuit to me, and to Brian Hawes for tracking down the special AF 239 transistor as well as the 2N3819 FET for me.