EMRFD Message Archive 9676

Message Date From Subject
9676 2014-01-21 14:41:24 Glen Leinweber WBR Regen antenna coupling
Some builders have increased Z1 to couple more RF input to the resonator
of this
regen (described in August 2001 QST). Different builders claim to have
Z1's impedance - at considerably different values, while others achieve
results as is. Z1 is a tiny coupling inductance, in the 50nH ballpark.

Two factors affect coupling from antenna to the bridge resonator:

First is choosing the centre-tap on the resonator's inductor, forming
one branch of
the bridge. Coupling increases when the tap-point moves away from the bridge
balance point. Should the tap point coincide with bridge balance, no antenna
power can transfer to the resonator. This aspect of coupling is
mentioned by the
author in the context of antenna-to-oscillator coupling.

Second factor is the impedance of Z1 - the tiny coupling inductor that
grounds the
resonator mid-tap. A larger value will couple more antenna power to the
The author cautions that a low impedance is more desirable .

This is yet another example where experimental methods are required.
Armed with
theory, proceed to tweak and fettle coupling for this regen. Most have
chosen to
tweak Z1, but might consider tweaking the tap-point instead.

This WBR regen has shown me that many regens couple antenna-to-resonator far
too tightly. Strive for optimum coupling only near the "sweet spot"
where regeneration
is near the oscillating point.
10243 2014-09-11 04:18:24 qrp.gaijin Re: WBR Regen antenna coupling
Hi folks,

I hope I might continue a topic raised in January this year on the list by Glen Leinweber concerning the WBR Regen antenna coupling (see original message below, or in the list archives as message number 9676).

I have been running several LTspice simulations and attempting, without success, to determine exactly what the benefit of the WBR antenna coupling arrangement actually is. The WBR taps the oscillator tank inductor at a center point and lowers that point to almost, but not completely, RF ground through impedance Z1. It is inevitable that some oscillator energy will appear at Z1 -- unless, as mentioned in message 9676, the tap happens to be at the exact point of bridge balance, in which case no energy can be transferred to the oscillator's resonator from the antenna.

So, in spite of all the talk about balance in the WBR, some unbalance at the tap is apparently necessary in order to couple in antenna energy. And if the tap is unbalanced, then oscillator energy will also leak out back to the antenna.

My question is, how is the necessarily-unbalanced WBR center tap any different than a low-impedance tap on a normal coil (one that is grounded at one end instead of at a center tap)? If we couple in antenna energy into a low-impedance tap on a normal coil, we have low (but still present) oscillator energy leaking out to the antenna, and low coupling of energy from the antenna into the tank inductor. I don't see how this is fundamentally different than the almost-balanced-but-necessarily-unbalanced WBR scheme. What benefit does the almost-balance of the WBR bring?

As a practical note, I'm interested in building a WBR for use with a 1-meter whip antenna, which is different than the original WBR design which apparently was intended for use with a full-size antenna. It is this attempt to adapt the design to a 1-meter whip antenna that has lead to my attempts to understand the precise operati
10244 2014-09-11 07:46:21 Roelof Bakker Re: WBR Regen antenna coupling
Hello all,

I have not build the WBR yet, so I can't comment on it.
However, I have build a "normal" regenerative receiver with an antenna
coupling and feedback link.
This receiver works from 200 - 600 kHz and is intended for use at LF.

The measured oscillator output at the antenna port is -50 dBm.
I don't think that this amount of power is something to worry about.

Roelof Bakker, PAoRDT
10245 2014-09-11 08:28:22 qrp.gaijin Re: WBR Regen antenna coupling

I'm not sure about LF, but my experience at HF is that a short whip antenna (say, 1m) connected directly to the top of the tank gives a good signal strength but causes hand capacitance issues as the RF gets radiated from the short whip antenna and back into the circuit. I think this radiation also is a cause of some hum modulation I'm observing (when using varactor tuning, which makes the set more sensitive to these issues). Connecting the antenna to a lower tap reduces antenna radiation but equally reduces receiver sensitivity. So I was wondering if the WBR approach can offer both good sensitivity (with a short whip antenna) as well as good reverse isolation to prevent the oscillator signal from radiating out of the antenna.

I just finished another series of LTspice simulations. I just can't see the benefit of the center tapping scheme. Briefly, here's what I tried and found:

1. Test 1: WBR-style receiver with tap dividing the inductor into equal 0.5 uH and 0.5 uH inductances. Tap raised above RF ground by Z1 impedance of 0.05 uH. Tuning capacitance (spanning both inductors) was 200 pF. Regeneration was adjusted to give an oscillation amplitude of 10 mV peak (measured as an AC voltage, referenced to ground, at the end of the inductor that is connected to the active device) at 7071.8 kHz. A 1 uV signal at 7070 kHz was inductively coupled into Z1. The regeneratively-amplified input signal heterodyned with the oscillator signal leading to audio-frequency ripples (that, if detected, would become the AF beat note) riding on top of the oscillator's signal. The peak-peak voltage of these AF ripples was 84 uV and is an indication of how much the input signal is ultimately available at the tank. The AC voltage at the inductor tap (above Z1) -- which is an indicator of how much oscillator energy escapes to the antenna -- varied from -12 uV to 11 uV.

2. Test 2: Unbalanced inductor with 0.9 uH and 0.1 uH halves (0.9 uH connected to active device). Z1 kept at 0.05 uH. Regeneration was re-adjusted to give an oscillation amplitude of 10 mV peak at 7633.4 kHz. A 1 uV signal at 7632 kHz was inductively coupled into Z1. The peak-peak voltage of the resulting AF ripples on the oscillator signal was 790 uV, indicating more influence of the input signal on the tank. However, AC voltage at Z1 also increased, varying from -42 uV to 39 uV, indicating increased oscillator energy leaking out to the antenna.

3. Test 3: Unbalanced inductor of Test 2, but with Z1 reduced to 0.0135 uH. This reduces the AC voltage swing at Z1 to span from -11 uV to 11 uV, comparable to the balanced Test 1. This means that the oscillator energy escaping to the antenna, in this unbalanced case, has been adjusted to be approximately equal to the oscillator energy escaping to the antenna in the balanced Test 1. (On second thought, this statement is likely in error: the *voltage* at Z1 is the same as in Test 1, but the Z1 impedance has changed.) The question is, in this unbalanced and low-radiation configuration, how much does the input signal affect the tank? The oscillator frequency was determined to be 7634 kHz with a peak oscillation amplitude of 10 mV, the same amplitude as in Test 1. Again, a 1 uV signal was inductively coupled into Z1, at 7633 kHz. The peak-peak voltage of the resulting AF ripples on the oscillator signal was 439 uV, indicating more influence of the input signal
10247 2014-09-12 08:59:53 robert.hair Re: WBR Regen antenna coupling
I'm not at all familiar with the WBR Regen so my comments may be of little or no value to your activity, but I do have some experience in reducing antenna radiation from regenerative receivers.
I have worked for many years in the design of transmitters and receivers for automotive remote keyless entry and garage door openers.  In the early days superregenerative receivers were used in most of these systems and are still used in some.  I used, as did most other designers of superregenerative receivers, an RF amplifier between the antenna and detector.  Its purpose was not primarily to increase gain, but to reduce antenna radiation from the detector.  This RF amp added little cost or circuit complexity and usually only a modest increase in receiver current (typically 1 to 2 ma.).  The issue here was regulatory compliance - without the RF amp it was difficult to get receiver emissions low enough to be legal.  Most often a common base amplifier was used for its good reverse isolation and some designs used a cascode configuration for the same reason.
Rob  N8WNG
10248 2014-09-12 13:30:04 Glen Leinweber WBR Regen antenna coupling
When pushed past the infinite-Q point, the regen tank
oscillates, and couples power back out the antenna. Your antenna is trying
to match into a negative impedance in this case.
Negative impedance? - gives me visions of my antenna actively sucking
radio waves out of the ether, or perhaps a random wire with oodles of
gain ;-)

I think the standard rule still applies: try to match your antenna impedance
to see a conjugate impedance looking into your receiver. Varying
radically alters the tank impedance along with its transformed version
at the
antenna terminals. It would help to have variable input coupling to
optimize this
matching, but few regens go this far (they're hard enough to set up
What to do? Set up very loose fixed coupling so that tiny antenna
signals couple
efficiently to the very very high impedance at the regen tank. This is
how we
set up a regen for good sensitivity. Tiny antenna voltages are stepped
up through
the loose coupling to become very large voltages across the tank. Very loose
coupling gives very high gain - but only when regeneration is set very
very close
to the oscillating point where tank impedance skyrockets.
The WBR coupling scheme potentially can be adjusted from loose right
down to zero coupling (when the bridge is balanced). Most others can only
be set from loose to looser.

A short whip can be viewed as a small capacitor coupling to radio space.
small capacitance is one way of loose coupling, and the whip can connect
directly to the hot side of the regen's tank. A longer whip may couple
too tightly,
and would be better tapped down the tank's coil, or capacitively tapped in.