[Elecraft] K1 on 10 meters: helpful advice

[wayne burdick]

Stephanie (and other interested parties),

I have some additional thoughts on modifying the K1 for 10 meters. I 
also found some old e-mail from another customer who attempted it. 
Between the two, I believe you'll be able to do this successfully.

When I designed the Norcal/Wilderness Sierra, which uses the same I.F. 
as the K1, I realized that a 36-MHz crystal would be expensive, so I 
came up with an alternate mixing scheme where the LO is below the band 
rather than above it. The RX and TX image rejection isn't quite as good 
with this technique, but that probably doesn't matter since the 
band-pass filters are narrow.

Here's the normal conversion scheme (high-side injection):

    RF   28.000 - 28.100
  + IF    4.915
------------------------
  = LO   32.915 - 33.015
+ VFO    3.085 -  2.985
------------------------
  = XO   36.000


Here's the alternate scheme using low-side injection:

    RF   28.000 - 28.100
  - IF    4.915
------------------------
  = LO   23.085 - 23.185
+ VFO    3.085 -  2.985
------------------------
  = XO   26.170

I.e., you can use a 26.170 MHz fundamental crystal. Advantages of this 
frequency vs. 36 MHz include: (1) more power output from the NE602 
premixer oscillator; (2) cheaper; (3) possibly available from 
Wilderness Radio. If Wilderness doesn't have any, you might try ICM. I 
think the ICM part number for the basic crystal type (not including 
frequency) would be #436162.

Low-side injection results in one additional minor issue: the sideband 
gets inverted. This just means that the CW pitch will change in the 
opposite way from the other bands as you tune. VFO readings should 
still be accurate.

Looking back through old K1 design notes, I found an e-mail from 
another customer (Bob Larkin, boblark at proaxis.com) who put his K1 on 10 
meters. He used an overtone 36 MHz oscillator, requiring changes to the 
basic band module. A 26.17-MHz crystal would be simpler, as I described 
above. But Bob's notes on getting adequate power output on this band 
are quite useful. I have attached them below. Let me know if you try 
this! If we have a couple of customers successfully complete the mods, 
we'll offer it as an option kit for use with the 2-band module.

73,
Wayne
N6KR

* * *

The RF filter was designed to have 2.2 dB IL in 50 Ohms and has 3 and 
40 dB
bandwidths of 1.8 and 18.5 MHz. Inductor were the regular 1 uH. 
C16=C20=270
pF, C17=C19=33 pF, C18=1.5pF (if the latter is not available, use 0.75
inches of the gimmick capacitor).

The LPF is an interesting topic. I did not try a direct scaling of the 
other
filters. This might work adequately, and would be good for someone to 
try.
Instead I designed a 2-coil modified elliptic filter. The goal was to 
move
the cutoff frequency far enough above 28 MHz to minimize the insertion 
loss,
and to put an elliptic null at the second harmonic, 56 MHz. The final 
design
cutoff at 34.9 MHz and had a measured insertion loss of 0.17 dB at 28 
MHz
and 65 dB at 56 MHz. L11 is 0.28uH (9T T37-6, over 75%), L12 is 0.22uH 
(7T
T37-6, tight wound--probably should be 8T spread out). C24=94 pF (two 
47 pF
in parallel, one on top, one under), C25=164 pF (two 82 pF), C26=75 pF. 
The
capacitors in parallel with the inductors are mounted below the board 
and
arranged to be flat against the board to clear the bottom board parts.
Across L11 is 13 pF (two 6.8 pF in parallel), across L12 is 36 pF. This 
is
quite a few parts, but the measured performance is great.

Now the K1 was on 10-m, but the transmitter didn't put out any power! 
There
wasn't enough drive to make the Class C Q7 draw current. I made two 
changes
to fix this. Both of these have the potential to alter the performance 
on
other bands, particularly 15 and 20-m. I attempted to evaluate these
effects, but I only have the one K1! Both changes are attempting to
compensate for the drop in gain of Q6 and Q7 with frequency. My 10-m
performance seems to still be limited by drive to Q7.

First, U9 has more gain available. I put 270 pF across R11, boosting the
gain on all bands above 40-m.

This produced a fraction of a Watt, but not enough drive, yet. So, I
measured the impedance looking into pin 8 of J7, in transmit. At 28 MHz,
this was 5+j8 Ohms. So I added an L-network to match this closer to 50 
Ohms.
At the J7-P8 side of D9, I inserted a .12uH (6T #32 on T12-6, tight) 
and on
the J7-8 end put 200 pF to ground.

Now at 13.8 V I get 3W, and at 12V about 1W. All spurs are at least 48 
dBc.

As a by-product the output on 15-m is easily 7W. The power control 
seems to
still be able to work fine and on my K1 the spurs were still essentially
where they were on all bands.



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