Topband: BDR redux

[Doug Waller]

> So I was listening to A45XR in the
> noise initially even though I had AGC Threshold set to S9+12.  For
> weak signals near the noise floor, it also sometimes helps to reduce
> RF Gain to find the S/N sweet spot where the signal seems to separate
> from the noise.  This is only needed in those cases where the signal
> is just barely above the noise level.  This is why Orion's AGC takes
> some seat time to learn to use properly.  Once learned, it is fairly
> easy to adjust and the AGC action is the best I've seen at keeping
> a wide range of signals at comfortable levels.  In the CQ WW on 10
> meters, it was a pleasure to set and forget it, almost never needing
> to ride AF and RF Gain controls like I did with previous radios.
>
>                                                  73,  Bill  W4ZV

The Orion sure sounds like a FB receive rig to me.   I am still using the
trusty FT-1000MP, but with some exciting modifications for topband
reception.

Using a SCAF-1 audio filter with two back-to-back 1N34A diodes in front, the
MP has become a super receiver.  The clipper diodes do not touch weak
signals, letting them freely pass, but all signals and noise over 0.3 volts,
are clipped.  The result is that even with last nights S-9+ QRN crashes, I
easily heard and worked A45XR right after you did.  I am hearing weak
signals that can not be heard without the clipper/ filter setup and A45XR
was no exception.

The clipper diodes create alot of high frequency distortion, which is
entirely removed by the SCAF-1 filter on CW.  I have volume control
headphones which can be turned down for totally comfortable copy on pileups,
contests and lightning crashes.  All signals are of near equal amplitude!
An AF AGC would not work as any strong signal would turn the gain down,
removing the desired weak signal.  With clipping, the weak signal is always
there.

The real weak signal advantage is that the RF gain control may now be turned
down to a lower level than for normal listening.  At these low RF gain
control levels, the MP becomes a super receiver.  Many previous problems,
including NB overload, IF overload and background hiss are now history.
Using this setup with an INRAD 250Hz / 125Hz  I.F. filter selection on CW,
weak sigs are copyable with several 20 dB over signals within 1-2 kHz.
Using two phased loops on a 30ft rotary boom for receive, rather than
Beverages, makes this all the more impressive from a subdivision lot.
Little Guns *CAN* be successful on Topband from a small lot.

If there is someone out there that can devise a circuit for receiver RF
front-ends that will clip stronger signals and noise, while passing weak
signals untouched, most existing modern radios could be instantly
transformed into super receivers.  The chips probably now exist.  Elecraft
recently added two 5¢ back-to-back 1N4148 diodes between their 4.915 mHz
I.F. and existing low pass filter, which clip high amplitude signals and
produce the same results as the MK-5 does with additional pre-selector
filters for each band.

73, Doug / NX4D (nr Orlado, Fla.)