My First-Ever QSO -- 50 Years After Passing Ham Test

Discussion in 'Ham Radio Discussions' started by G3EDM, Aug 27, 2021.

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  1. G3EDM

    G3EDM Ham Member QRZ Page

    Still calling CQ.

    If you happen to hear me and answer my CQ, do me a favour, send my callsign more than once. I would say the optimum series would be about 30 seconds, just my call for 30 seconds, then DE your call, which does not have to be repeated too many times. If I can hear you at all, I can probably copy your call.

    Because of this issue with images, I am now tuning the receiver + or - 15kHz after each CQ call. This is very similar to how hams would have operated under crystal control in the 1950s or 60s.

    Hopefully these rogue images will be quenched soon thanks to the troubleshooting by @W9BRD.

    73 de Martin, G3EDM
     
  2. G3EDM

    G3EDM Ham Member QRZ Page

    My putative receiver dial frequency is currently set to 7005 and I am hearing EC4C sending CQ, copied at 599 here. RBN says he is sending on 7004.0. So whether or not my receiver is parked on an image, it does seem to be receiving 7005 traffic when the dial is set to 7005. But it's also receiving some traffic sent on other frequencies altogether.... as long as they are approximately 15kHz "up" or "down".

    Edited to add: A few minutes later, still at the same dial position (putative 7005) I am hearing good strong signal of SM6OID calling CQ. RBN shows his calling frequency as 7007.0. So the evidence is mounting that the receiver is usable for QSOs ... even with the image problems and the uncertainty over whether I am tuned to the main frequency, or a heterodyned image of it.

    Edited again a few minutes later to add: Now I am hearing G3VMK calling CQ, again without budging my dial from "7005". RBN has him sending on 7006.0. He is in Cornwall, horizontally at the other end of the country, 482km away so well placed in the skip zone. I will answer his CQ, you never know. Then, return to sending CQ myself.

    73 de Martin, G3EDM
     
    Last edited: Sep 15, 2021
  3. G3EDM

    G3EDM Ham Member QRZ Page

    Now I am hearing DL5NDH sending CQ at a spot around "7002" on my dial. But RBN shows him sending on 7015.0.

    So, based on considerable evidence from my listening today, we can assume that the images are spaced between 12kHz and 15kHz apart.

    Hearing so many hams at one place on the dial does provide a feast in callsign recognition!

    I am stopping transmission for now, as the work day gets going, plus we are expecting a couple of building contractors to do some repairs. Will keep checking this thread during the day.

    73 de Martin, G3EDM
     
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  4. SM0AOM

    SM0AOM Ham Member QRZ Page

    An autodyne detector cannot, by definition, have "images" except for the "audio images" which are +/- the audio beat frequency around the tuned or "BFO" frequency.

    The explanation by W9BRD about your spurious responses being caused by some form of ultrasonic parasitic oscillation either in the detector itself, or in the first audio stage seems very plausible.

    As the autodyne detector becomes a very complex non-linear system when approaching oscillation, a parasitic regenerative condition can easily be triggered in some non-RF part of the circuit.

    "Old-timers" may recall the term "threshold howl" which was a more dramatic AF instability of the regen around the oscillation point. Your problem seems to be an ultrasonic version of "threshold howl".

    The cures to such problems usually are damping out any parasitic circuits by choosing more suitable values of coupling and bypass capacitors.

    You can read more about the subject in the article "Threshold Howl in Reaction Receivers":
    https://worldradiohistory.com/UK/Experimental-Wireless/30s/Wireless-Engineer-1930-04.pdf

    73/
    Karl-Arne
    SM0AOM
     
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  5. G3EDM

    G3EDM Ham Member QRZ Page

    When I turn the AF gain control all the way down (manual "muting" when going to TRANSMIT) there is a residual howl in the headphones, occasionally loud enough to be painful. It is a very high frequency, similar in pitch to the old "whistle" of a CRT TV set. That would be between 10kHz and 15kHz depending on TV system....

    So it may be as simple as quenching the AF stage?? I have a capacitor assortment here, but they are low value ceramics. I don't have high-values or electrolytics. Might as well order some, doesn't cost much and worth a try, before trying the more detailed suggestions made by @W9BRD.

    BTW, I thought that the howl was just microphonics, so I did not worry about it. Perhaps I should have!!!

    73 de Martin, G3EDM
     
  6. W9BRD

    W9BRD Ham Member QRZ Page

    That's the spurious oscillation. Somehow it's modulating your detector carrier. It could be in the AF amp, but my bet is on the detector, now that I've seen that sufficient bypassing is absent at the detector screen and should-be-ac-grounded end of the detector plate choke. If with no antenna connected to the system, tuning in your transmitter's signal results in tuning across a comb centered on the rig's true carrier, you've proven that your detector is self-modulated. Ah: And if you can still hear your rig's carrier as a sort of rushy silence -- I know that sounds self-contradictory -- with regeneration adjusted to just below oscillation, you should be able to discover that the spurs (at least the strongest pair on either side of the carrier) are now gone. In other words, the oscillation occurs -- and can occur -- only when the detector is also oscillating at RF.

    Karl-Arne's mention of threshold howl brings us right into the neighborhood of what's going on, but I'm only up for a bit of a drink of water -- left my receiver on in the other room, and static crashes woke me up :) -- and then back to bed for awhile. I'll explain the mechanism, and why the detector is the likely culprit, in a few hours.
     
  7. G3EDM

    G3EDM Ham Member QRZ Page

    @W9BRD: I've ordered the components to troubleshoot/fix the parasitic oscillation, or have them at hand already.

    Resistors: Large assortment, 1/2W.

    Capacitor assortments:
    • Disk ceramic 1KV, 100pF through .022uF
    • Polypropylene 400V, .001uF through 0.56uF
    • Electrolytic 63V, 1uF through 100uF
    • Electrolytic 450V, 1,000uF through 10,000uF
    That should be sufficient, and go some way to restocking the junkbox.

    73 de Martin, G3EDM
     
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  8. W9BRD

    W9BRD Ham Member QRZ Page

    Just leaning in for a minute: In the meantime, if you can locate a discarded computer power supply, you'll find that it includes a pair of electrolytics with values of 220 to 470 uF at 200 V or so, and a set of line-to-line- and line-to-common-rated bypass caps -- the line-to-line, a plastic rectangle on the order of 0.33 to 1 uF at rated at hundreds of volts ac and even more dc, and the line-to-common, bright blue discs of 0.0047 uF or so at similar voltages -- which can serve as temporary fixes for the problem. One of the big electrolytics would go from the non-plate end of the detector plate choke to common; the 0.33- to 1-uF plastic-rectangular cap would go from the REGENERATION wiper to common; and one of the bright-blue line-bypass caps would go from detector screen to common.
     
  9. W9BRD

    W9BRD Ham Member QRZ Page

    Yes and yes. So while the vanilla-caramel latte is brewing, let's dig in:

    An oscillator -- mechanical or electronic -- must ultimately self-limit its amplitude somehow, or one or more of its components will fail to destruction. Electronic oscillators may amplitude-self-limit by multiple means, but in vacuum-tube circuits, the built-in automatic gain control contributed by grid-cathode rectification charging a capacitance from grid to common usually does it with the user not having done anything special to design-in the behavior.

    A regenerative -- autodyne -- detector is tricky in this regard because for heterodyne reception we commonly operate it with feedback and/or element voltages adjusted so oscillation is not far above the point at which it transitions from not oscillating to oscillating. (The transition point is known as critical regeneration. When we discuss fringe howl, aka threshold howl, in a moment, the fringe -- the threshold -- that's meant is critical regeneration -- the fringe/threshold of oscillation.) What's tricky is that in an operating-point region at and somewhat above critical, the circuit is oscillating but is not yet strongly self-limiting its amplitude. In other words, the oscillatory feedback loop has gain to spare.

    So, fringe howl. Considered in the generic, electronic oscillators operate by creating negative resistance -- a resistance in which, across which, through which, increasing voltage reduces current instead of increasing it, and vice versa. The oscillator types used for classical regenerative detectors -- Armstrong (plate-to-grid ticker), Hartley (cathode tapped on the oscillator tank inductor), Colpitts (capacitive voltage divider across the oscillator tuned circuit, driven by the tube cathode) -- are set up to generate, by means of positive signal feedback, negative resistance at a spot frequency that's determined by resonance in a tuned circuit. We increase feedback -- and/or tube-element voltage(s), whatever -- far enough, and the tube creates negative resistance that's maximum and sufficient to overcome tuned-circuit losses at that resonance point, and we have an oscillator.

    But it turns out that as a tuned-circuit-dependent oscillator transitions into oscillation -- at the point where the frequency-specific negative resistance created by the amplifying device just overcomes circuit, especially tuned-circuit, losses enough for the circuit to go into sustained oscillation -- until we increase feedback enough for the circuit to strongly self-limit the amplitude of it oscillation, the entire oscillator circuit as a "black box" may afford frequency-nonspecific negative resistance that reactive energy storage -- in effect, unwanted resonances -- coupled to the oscillator circuitry may then leverage for simultaneous oscillation at a frequency or frequencies not determined by the tuned-circuit resonance we assume is controlling the show.

    Because an oscillating device that's oscillating but not yet fully amplitude stabilized has negative resistance to spare at critical regeneration, this effect is most likely to occur at and just above critical regeneration -- at and just above the fringe of oscillation -- and so when it occurred in regenerative receivers in the 1920s and 1930s, communications practitioners gave it the names fringe howl and threshold howl.

    Especially for reasons of device physics, and secondarily as a result the circuits and component values commonly used in practical regens, the tendency toward fringe howl is especially strong in triode, as opposed to screen-grid, detectors. (Solid-state triodes can be even worse than vacuum triodes in terms of tendency toward fringe-howling.) Without going to far into the weeds, we can ascribe this to the fact that the anode current of a triode is strongly dependent on anode voltage, whereas in a screen-grid tube, anode current is relatively independent of anode voltage because of the physics of the electronic action of the screen grid. Fringe howl -- which we are now ready to understand as a species of self-quenching superregeneration, likely without any actual quenching -- can occur in screen-grid detectors, but with a difference: Triode-detector fringe howl commonly occurs at and just above critical, and produces a relatively low audio tone of hundreds of Hertz, whereas screen-grid-tube superregeneration commonly occurs well above critical at earsplittingly high, even supersonic, audio frequencies. (So it's not fringe howl per se; but it is self-quenching superregeneration. Seen from another practitioner-named angle, such an oscillator is squegging -- oscillating at more than one frequency at the same time.)

    In short, we can think of fringe/threshhold howl -- and self-quenching superregeneration -- as resulting from an oscillator cyclically seeking, and failing to achieve, and then seeking again, amplitude stability at an audio or super-audio rate.

    As Karl-Arne indicates, experimentation with component values is commonly necessary to minimum tendencies toward this effect -- increasing or decrease RC time constants at one or more device ports; using RC interstage coupling instead of L or LC interstage coupling; using a screen-grid tube instead of a triode; even putting some positive resistance -- an actual resistor of correct value :) -- in series with a parasitic tuned circuit to disallow its leveraging for oscillation by negative resistance -- and more, including increasing or reducing feedback (changing tickler coupling; raising or lowering a Hartley tap; re-proportioning Colpitts-divider Cs) to move critical regeneration to a more readily stabilizable device operating point. In the case of the 1AD4 detector circuit, though, the absence of bypassing at the screen is a flaw that is very likely causing superregeneration, as I think it results in the screen voltage of the tube not being pegged by an RC time constant slow enough to keep the tube out of the amplitude-instability region.

    And BTW, the presence of the 60 H plate choke may or not be problematic for detector amplitude stability; we'll see. (A high-value R can serve as a substitute; we'll experiment.) On the plus side, we may be able to usefully tune that inductance with parallel C to introduce modest, poor man's audio filtering, assuming that the choke's value doesn't vary too much -- swing is the old-school term -- with changes in the dc level through its winding.

    It's all fun and instructional! Regens are a blast...
     
    Last edited: Sep 15, 2021
  10. W9BRD

    W9BRD Ham Member QRZ Page

    You've already proven that superregeneration in an autodyne detector results in reception of multiple copies of the same signal to the detriment of good reception of the expected one-copy-and-only-one-copy signal, so the safe answer to your question is no. :)

    For CW and SSB reception, all we need a regen to do is heterodyne a small chunk of RF spectrum down to audio -- with a bit of intentional frequency error during CW reception (so we hear transmitted signals as tones) and with preferably no frequency error during SSB reception (so transmitted audio sounds normal as received). Superregeneration cannot improve this fundamental process, but it can, and does, spoil the results by creating fake signals that interfere with reception of desired signals.

    I had a "discussion" with superregeneration enthusiasts in a Radioboard thread not long ago, and was directly to read multiple scholarly papers by degreed researchers that proved I was wrong. There's theory and there's fact from usage: Heterodyning on-air signals down to audio with a modulated oscillator is counterproductive on its face. It doesn't matter if the superregen is self-quenched or separately quenched; or whether the quench oscillator is a sine, triangle, or square wave or somewhere in between. A modulated heterodyne oscillator renders monofrequency received signals as combs, period, creating interference that's not actually present on-air. (And BTW, "heterodyning with a modulated oscillator" case also describes the fundamental problem of phase noise in oscillators, the presence of which can spoil reception of weak signals alongside very strong signals whether or not the phase noise is on the strong transmitted signal or the receiver local oscillator, or both.)

    And BTW, I think that the ascription of higher-than-regeneration sensitivity to superregeneration is based on misunderstanding. The discovery and harnessing of superregeneration techniques dates from the days when the term sensitivity evaluated how much signal input was necessary to produce some standard output, whereas nowadays we understand the term as evaluating how weak an incoming signal can be and still sufficiently override the internal noise of the circuit or system under test. Once we understand that a modulated heterodyne oscillator is practically useless for aural heterodyne reception (or for use as the local oscillator in a superheterodyne receiver), superregeneration is out for CW and SSB reception; and once we understand that using a superregen's modulated carrier to envelope-detect multiple copies of an AM signal down to baseband greatly broadens to useless the adjacent-channel selectivity achievable with the technique; we understand that a murmurating cloud of large, menacing question marks should accompany any discussion of howinell superregeneration could be of any use for reception of CW, SSB, and AM signals in well-populated MF and HF bands.

    But that's just me and my half-century of experience with MF/HF reception talking. I don't give a fig about the math if something doesn't actually, usefully work.

     
    Last edited: Sep 15, 2021
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