EMRFD Message Archive 10246

Message Date From Subject
10246 2014-09-12 07:58:43 Qrp Gaijin Comments on WBR receiver antenna coupling
Dear sir,

I noticed your message about WBR (Wheatstone Bridge Regenerative) receiver antenna coupling in the EMRFD Yahoo Group, here: https://groups.yahoo.com/neo/groups/emrfd/conversations/messages/9676 .

I have been studying this circuit and its operation is more subtle than I first believed. Based on your above message, it seems to me that your understanding of the circuit operation may differ from what I believe is its actual operation, and I believe you would be interested in discussing its operation; thus I am taking the liberty of contacting you.

Let me begin by saying that the following 1928 patent describes in detail the nature of the antenna coupling in the WBR:
https://www.google.com/patents/US1667513 . I have seen no references in online discussions to this patent; I found it myself after a lengthy period of investigation, while attempting to understand the operation of and research the background of the WBR receiver.

The key point from the patent's circuit description is that the antenna signal does not directly excite the resonator at all. Instead it simultaneously goes through both arms of the bridge (which leaves both ends of the capacitor, and both non-grounded ends of the inductor, at the same voltage, thus not exciting the tank), whereupon the signal encounters the input of the amplifying active device. Only after amplification and regenerative feedback does the signal then first appear at the tank.

I'm not sure if you picked up on this point (if you did, please forgive my redundant explanation); your message above states: "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."

Indeed, no antenna power can directly transfer to the resonator, but the antenna power can still flow through the bridge and reach the amplifier, where it is then transferred through the regenerative feedback network into the resonator -- even if the tap point coincides exactly with bridge balance.

I ran a few LTspice simulations and the above-linked patent's description of circuit operation seems correct to me.

I thought I'd pass on this information to you since there seems to be little documentation available on the exact operation of the WBR receiver. The 1928 patent was quite enlightening.

Another interesting point is that the antenna energy is not filtered by the oscillator tank before it reaches the active device. This means that the antenna signals (in N1BYT's design, where the antenna load Z1 is an untuned inductance) encounter the non-linear active device without the benefit of filtering and thus AM blanketing or IMD are more likely to occur than in a receiver where the antenna signals are first filtered by the oscillator's tank. The 1928 patent uses an additional tuned circuit in place of N1BYT's untuned Z1 for additional input signal filtering.

I'm planning to build something like AA1TJ's WBR variant here: http://fhs-consulting.com/aa1tj/40mXCRcvr.pdf (description here: https://groups.yahoo.com/neo/groups/regenrx/conversations/messages/5284). AA1TJ's variant is quite elegant because it achieves balance by using an inherently balanced, symmetrical oscillator (a cross-coupled pair). That seems much more likely to achieve good balance than N1BYT's WBR approach where a single balancing capacitor in one bridge leg is supposed to represent the same impedance as the oscillator's impedance in the other bridge leg.

Best regards,