Here's a silly experiment that's an attempt to find additional uses for the 5886 electrometer tube (typically scrounged from old radiation survey meters). I wanted to see if the tubes could make reasonable front-ends for very high impedance audio amplifiers. My first attempt was a simple grounded-cathode amp and the plate linearity and bandwidth were awful leading me to believe these old tubes might only be good for "D.C." But then I tried a one-transistor feedback amplifier and the circuit came to life. Below are two versions with an added emitter-follower. They perform equally well. The top one is DC-coupled but it does rely on the plate voltage being correct for biasing the rest of the circuit. I'd go with the second circuit in most cases.
The feedback holds the plate voltage fairly constant and the resulting linearity and bandwidth are quite good. I didn't measure the distortion but a sine wave looks great whereas previously it looked like a full-wave rectified sine. The bandwidth is a surprising 400 kHz. The circuit seems to perform just about identically to the same circuit with the tube replaced by an electrometer JFET (like the 2N4117). So why not just use a 2N4117? I only have emotional answers to that question. : ) I suppose this circuit is a bit more immune to static discharge. The input impedance is "crazy high" even without bootstrapping but then it's not exactly low with the FET. If I continue I'm grasping at straws.
Note how little current the filament needs; it's not much more than a typical transistor stage.
I used a 22 megohm to ground the gate but that could just as well be a 100 gigohm or even a neon lamp with its internal radioactive isotope.
Moved to area 50
Moved to Area 50
I'm adding a magnetic receiver to the SID plots
Interesting bias technique
I'm playing with some old clock chips to see if I can find any value in them!
This is the latest active antenna for LF reception, especially my SID receiver that uses WWVB. I've connected the Oddball SID Receiver directly to this antenna without any 60 kHz pre-selection. Previously the antenna was tuned. This antenna blocks the strong AM band signals so it will be interesting to see if the SID receiver can function properly without a tuned front-end (much like a simple SDR).
Here it is in the antenna box, replacing the SID Seizer preamp. The 22 ohm is in series with the base of the second transistor and isn't in the schematic. A jumper probably would work but I'm always concerned about parasitic oscillations and I had a span to jump.
I made a "bias tee" box for two antennas:
The power comes in on the left, connects to the two 10 mH chokes and is delivered to the antenna connectors at the bottom. The signals are passed to the two receiver connectors at the top. One will be for the 60 kHz SID receiver and the "Watering Hole" Receiver at 185.3 kHz. The other is for a broadband whip that's currently driving my Sferics Detector.
After bench testing a couple of things became clear. The circuit has trouble driving the transformer below about 35 kHz, so I reduced the gain at the low end, making sure to preserve gain at 60 kHz. (My goal is to receive 60 kHz and 185.3 kHz at a minimum.) Also, the gain drops off above about 100 kHz, possibly due to the transformer. Lowering the MPSA18 collector capacitor gives the circuit a peaking characteristic that flattens the response nicely. Although the Bode plot below doesn't show it, the actual unit is quite flat from 45 kHz to 300 kHz. The actual plot rolls off at 45 kHz instead of 20 kHz. The peaking is controlled by the 470 ohm in series with the .0047 uF (a lower value resistor giving more peaking). Tweaking the values gives a beautifully flat response dropping like a rock outside the band.
The response is sharply down by the edge of the broadcast band thanks to help provided by the notch. I just fine tuned a 10 pF trimmer (in place of the 7.5 pF in the schematic) until KLBJ (590 kHz) completely disappears.
I added a 75 ohm resistor in series with the output and the circuit has no trouble driving a 25 foot long cable with a high-Z termination. The receivers will have high-Z inputs.
The antenna is back outside with the new circuit installed, awaiting some additional wiring to get the SID detector back in business.
(This project is languishing due to lack of interest!)
These days I'm looking at ways to pick up emissions from the Jovian system. Going against tradition, I've opted to try to make two small loop antennas do the job, instead of the typical long dipole. The antennas are much like the Hula-Loop antenna, only built for outdoors. In use, the two loops are arranged in a line and spaced 1/2 wavelength apart (25 feet for 20 MHz). This spacing and alignment causes the signals from the antennas due to vertically polarized signals along the antennas' line to be out of phase, and easily canceled simply by adding the signals. Alternatively, one antenna can be flipped around to reverse the phase and a differential amplifier can be used to subtract the signals, with the same cancellation occurring. Signals along the other horizontal axis are already ignored by the loops. The result is a pattern that points straight up. Signals from above will exhibit the same phase in both antennas. Here's a couple of very, very preliminary schematics:
This is the schematic for each antenna. This part seems to work fine and may not change much. The schematic below is a possible receiver that has not been tested, except for the two variable-voltage circuits at the top. Those adjust the voltage to the antenna amplifiers for fine-tuning the gain. The phase is also fine-tuned by slightly changing the frequency of one loop to get minimal response to terrestrial signals. I've gotten from 6 to 10 dB of noise reduction. And, if the two in-phase signals add the way they should, that really means over 9 dB improvement over a single loop!
The receiver portion is untested and will require that I rotate one antenna 180 degrees. Looking at it, I need to add termination resistors for the cables. I'd probably eliminate the 1000 uH chokes and bypass the emitters of the darlingtons to ground with .01 uF caps. That would terminate the cables with close to 75 ohms (68 plus a little from the darlington emitter). The receiver is a direct conversion type with a low-level audio output that will need further amplification and processing. Eventually, I want to make a circuit that will "collect" pops and pulses in windows of time in some fashion, perhaps a peak/hold or an integrator. That way, the storms can be displayed on a slow plot, showing several hours of data.
I've been thinking about the circuit above and I've decided that it would be better to have a 1 MHz tuned transformer in the collectors of the differential amplifier, followed by a 1 MHz filter. That would drive another amplifier before driving a detector. The reason to not go directly to baseband is that it would be easy to set the bandwidth of the filter to 50 or 100 kHz, before the detector, to get a higher receive bandwidth.
For now, I've been using an ordinary power combiner feeding a selective level meter for testing the antennas (at 20.1 MHZ). I transmit the audio all over the house with the "Unfair Radio Transmitter." The antenna phasing trick does seem to work. I can get up to 10 dB reduction in background noise during the day, and that's not counting the fact that two antennas should increase the signal strength significantly. Here's a photo of the antennas and receiver in an empty room above a garage:
The camera was inside one loop, facing the other loop. They're slightly out of alignment to reduce a local interference source, but that's gone now, so they're nice and straight, in a near north/south alignment.
There was an "Io B" period on 11/25/2011 that I seemed to have received with this setup. Keep in mind that virtually all of my experiments give "false positives" at the beginning! This will be no exception, I suspect. Most of the time, the receiver outputs a clean white noise sound, but this racket occurred during the expected window of time, and faded right on schedule - seemed real to me. (Well, I think I heard that "noise" again and it might be splatter from a strange-sounding teletype transmission of some sort - not sure yet.) Fine-tuning one antenna could peak the signal or completely null it out, so the cancellation technique does work. Here's a terrible little video where I disconnect one antenna from the power combiner to show how much louder the background noise is without cancellation. Unfortunately, the battery in my camera died right at the end, so you only get a fraction of a second of sound with the second antenna disconnected.
I'm pleased that these electrically small loops seem to work indoors! But the room is a good distance from the electronics in the house, it's a second story above a garage, and there's little metal in the walls.
It was pointed out to me that Jupiter will be invisible to this type of receiver for much of next year. It will be high in the sky at the same time as the sun. It will be late next year before the conditions are good again. So, I'll probably use this setup with the power combiner and selective level meter to determine whether the antennas really work and wait to build the stand-alone receiver for a while.
I moved the solar flare stuff to: Solar Flare Project