[ARC5] Re-reading Gordon White article...L-2 in a 'command' receiver

[Mike Morrow]

I wrote:

> Here's what the June 29, 1940, ATA/ARA instruction book has to say
> about L-2:
>
> "C-39 (across L-2 in the 190-550 KC receiver only) serves to tune L-2
> to a frequency lower than 190 KC, and by so doing increase the
> amplification of signals at the low frequency end of the tuning range.
> This assists in producing a reasonably uniform receiver sensitivity
> over the tuning range.  The plate to screen capacitance of the RF
> amplifier tube, Type 12SK7, and the wiring capacitance to L-2, perform
> a corresponding function in all other receivers."

Robert wrote:

> Reading between the lines of what Mike reported out of the ARA/ATA manual,
> the gain of the RF stage must increase with frequency.  Why this would be
> so I can't say for certain but given experience with the BC-611, which has
> a very short antenna for its operating frequency range, it could be at least
> partially due to the effective "gain" versus frequency of the antennas
> typically used in aircraft of the period.

Perhaps.  But I suspect the designed RF stage gain vs. frequency in the
'command' receiver is related to why the BC-348-* (except the simplified
Wells Gardner J/N/Q models) have a potentiometer ganged with the tuning
capacitors that increases in value as a higher frequency is tuned.  This
potentiometer is in the cathode of the second RF amplifier, and it reduces
RF stage gain as frequency is raised on any given band.

Here's the description of this feature from the Handbook of Maintenance
Instructions for the BC-224-F/K and BC-348-H/K/L/R
(AN 16-40BC224-2, 11 February 1955):

"(2) The characteristic increase in internal receiver noise, when tuning
 from the low to the high frequency end of a band, has been corrected by
 means of the variable resistor 78. <snip> Here the noise compensator
 resistor 78 is mechanically connected to the shaft of the ganged tuning
 capacitor with an electrical connection to give minimum resistance at
 the low frequency end of the band.  The cathode return lead of the
 second r-f amplifier tube connects to the noise compensator resistor 78.
 The gain of this stage is thereby decreased proportionally as the r-f
 tuned circuit impedance increases (when tuning toward the higher
 frequency end of the frequency band).  This arrangement tends to keep
 the noise level and receiver sensitivity essentially constant over the
 tuning ranges."

It sounds to me like the 'command set' receiver does something similar
by using the reduced stage output while tuning down the high-side slope
of LC resonance.  For this to work well (with a gentle slope), the
LC resonance point would need to be well below the receiver frequency of
operation, otherwise a very large stage gain decrease (with increasing
frequency) would take place at the low end region of receiver tuning.

Thus, the explanation in the BC-348-* manual attributes the problem as
"the characteristic increase in internal receiver noise, when tuning from
the low to the high frequency end of a band" rather than anything resulting
from antenna characteristics.

Mike / KK5F