[HBR] HR-67

[Helmut Usbeck]

> One little caveat to receiver builders who, like me, wind their own coils
> in RF amplifiers, etc., and that is this: do NOT place your link coil too
> close to the main tuning coil. I did just that with my version of the Mate
> For The Mighty Midget receiver, hoping to maximize sensitivity of the final
> product, above all...
>
> The coils are over-coupled: when I'm trying to listen to AM on 1885-KHz
> with an SSB or CW contest down the band, I can hear the junk all over the
> dial! Now, I agree this receiver isn't the last word in complexity & all,
> but I know if I had separated the two coils up to a maximum of 3/8" apart
> (as suggested by Rocky in his regen books), the set-up would have had a LOT
> more up-front selectivity, where it really counts.
>

Coupling coils used to drive me nuts.  The way I do my RF coils now is to get 
them on frequency (or about) and then instead of using a coupling coil I 
start making temporary taps up from the ground end of the RF coil while 
monitoring (with a scope or a meter) a very low level RF input ( signal 
generator or such).  Go for maximum and then tap back down 1/2 of of the 
signal level being monitored.  You could do pretty much the same with a 
coupling coil by adding or subtracting turns and/or spacing.  It usually 
turns out to be the best compromise between sensitivity and selectivity.  For 
me it's a bit more of a sane cut and try method.

Regards,
Helm.  WB2ADT