Dave Richards AA7EE

October 15, 2016

Comparing the Weak Signal Performance of a WBR Regen with a K2

The WBR seems to get a bit of a bad rap with some people for it’s sensitivity. A comment on the last post from a reader called Simon, reminded me that some WBR builders have experienced poor sensitivity. Based on my experience, this design does seem to be fairly deaf on AM, but the sensitivity on SSB/CW is fine. I think there are two reasons why some builders experience low sensitivity –

1) They follow the schematic from the original QST article, and do not include an audio pre-amp immediately before the LM386. In this case, the receiver is not necessarily insensitive – it’s just that the low audio is limiting what you can hear.

2) The value of Z1, the inductance between the coil tap and ground, is not high enough. In the original WBR design by N1BYT, this inductance was a 1-inch length of #20 solid copper wire. I followed this direction with my first WBR (for 40M) and it worked well. The WBR was tackled as a group build in the QRP-tech Yahoo Group, as I have mentioned in this blog before, and some builders experienced low sensitivity. The fix was to replace the 1-inch length of wire with an inductor wound on a toroid. Builders in the group found the optimum value of inductance to be somewhere between about 0.2uH and 1uH. I went lower with my 30M WBR, and found that a value of 0.03uH  (3 turns on a T37-6) worked well.

Of the above 2 reasons, my suspicion is that 1) is the main one for most builders.

We regen fans do get a bit braggy about the performance of our sets. I could never make the claim that my regens perform as well as a superhet, for several reasons. Obviously, the strong signal handling of regens is pretty poor, and the bandwidth is wide. When a regen is adjusted close to the point of oscillation, the nose of the response curve becomes quite narrow, but the skirts are still broad. Also, it’s a small difference, but the fact that a regen listening to SSB or CW hears on both sides of the oscillator, as opposed to a superhet, which only hears on one side of the LO, gives the regen an immediate 3dB disadvantage. Basically, for a given signal, a regen is listening to twice as much bandwidth as it needs to (a doubling of power is an increase of 3dB). It’s not a big difference, but it is there.

Having said all that, I am constantly surprised by how much my regens do hear. I remember one evening, a few years ago, when the Russian K beacon was coming through very, very weakly on 7039.3KHz on my K2. I was amazed to discover that I could also hear it on my WBR. Admittedly, I had to strain to copy it on the WBR, and the fact that it was sending the same letter over and over again – and I knew in advance which letter it was, all helped. However, the fact that it was marginal copy on the K2, combined with the fact that I could copy it at all on my WBR (albeit even more marginally) was an eye-opener.

With all that in mind, here’s a 3 minute video of my K2 and 30M WBR side by side, both tuned to the same weak signal, as I swap the same antenna between both receivers. Hope you enjoy it. PS – no cats in this one!

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