[HBR] Yet Another HBR Concept

[hbr@mailman.qth.net]

Jim dutifully observes:

> Some possibles:
> 
> 6.55 IF is very near low end of 40 meters. IF feedthrough may be a real
> problem on that band.

There may once have been IF choices that were over half a 
megacycle removed from any strong signals but not these days.   I 
haven't (in a couple of days of looking) found anything noteworthy 
within 100 kcs of 6.5536 and having looked at many other freqs in 
the 5 - 7 Mcs range, I'll settle.

> With VFO at 9.2, premixer output is 10.55 for 80 meters. VFO feedthrough
> may be a problem, resulting in pickup of strong signals in ~2 MHz region.

Otherway round:  19.25 Mcs - 9.2 = 10.05 Mcs premixer output for 
3.5 Mcs.   If VFO feeds through, you'd pickup 2.65 Mcs ... it's a 
question, but another one of those "there are no perfect answers" 
deals.   There are three tuned circuits plus the fact that the premixer 
is balanced for VFO input -- maybe 80 db rejection?   

I haven't checked the 2 - 3 Mcs range lately -- my impression was 
that this used to be ship-shore phone and similar and was not used 
by extremely strong stations.

Premixer is balanced for VFO input and output will be hi Q, tuned to 
10.55

> Other bands need checking

VFO tunes 8.7-9.2 and is subtracted from a multiple of 1.75 Mcs on 
all bands.   

160M: 10 x 1.75 = 17.5, - 9.2 = 8.3 for 1.75 Mcs; 17.5 - 8.7 = 8.8 for 
2.25 Mcs.   On this band the VFO passes through the premixer 
frequency; it's rejected only by the balanced mixing and there will be 
spurs for strong in-band signals.  However the worst of it will be 
above 2 Mcs since that's where the crossover occurs.   Note also 
that the FT-101 dial, calibrated 0-500, doesn't match frequencies 
starting at 1750 -- you have to add 750.   A possible fix is to forget 
the locked oscillator and just use a fixed tuned unlocked oscillator 
(or another crystal) -- on this band: 17.25 - 9.2 = 8.05 for 1.5 Mcs; 
17.25 - 8.7 = 8.55 for 2.0 Mcs.  There could still be spur problems, 
but not so severe (since the crossover is now out of the tuning range) 
and the 500-1000 portion of the dial would read correctly.

It will be necessary to adjust the VFO injection level to the premixer 
carefully to get the full benefit of the balancing.

80M 11 x 1.75 = 19.25, - 9.2 = 10.05 for 3.5 Mcs; 19.25 - 8.7 = 
10.55 for 4 Mcs.  As before -- no obvious problems.

60M There's no direct way to cover this band.

40M:  13 x 1.75 = 22.75, - 9.2 = 13.55 for 7 Mcs; 22.75 - 8.7 = 14.05 
for 7.5 Mcs.   No obvious problems.

30M  There's no direct way to cover this band.   

20M:  17 x 1.75 = 29.75, - 9.2 = 20.55 for 14 Mcs; 29.75 - 8.7 = 
21.05 for 14.5 Mcs.  No obvious problems.

15M: 21 x 1.75 = 36.75, - 9.2 = 27.55 for 21 Mcs; 36.75 - 8.7 = 
28.05 for 21.5 Mcs.  No obvious problems.

10M: 25 x 1.75 = 43.75, - 9.2 = 34.55 for 28 Mcs; 43.75 - 8.7 = 
35.05 for 28.5 Mcs.  There's no direct way to cover more than the 
first 500 kcs and the segment 29.75-30.0 but ... no big loss.   The 
rest could be done if necessary with additional crystals.

An advantage of using the FT-101 VFO mechanism and original 
tuning range is that they already took care of VFO harmonics in the 
ham bands -- there are none.   Additionally, this assembly was 
designed for the same type of conversion scheme so the dial reads 
the right direction.

Walt 
KJ4KV