Hi Wayne,
Thanks very much for the additional information! Most of my
guestimated values for the 2-band board seem to match those that were
in the email you sent; the only ones I was way off on were C16 & C20.
I actually found a cheap source of fundamental 36MHz crystals.
They're about $4.00 each, the only catch is they're surface-mount. I
just solder leads on them and use them in the normal crystal mounting
holes.
Yesterday I was at work most of the day with a lot of time to think
and surf the net, and found one email that I'd missed in my earlier
research:
http://www.ac6rm.net/mailarchive/html/elecraft-list/2003-06/
msg00789.html
Based on that email, I reduced the value of RF-R11 from 82 ohms to 33
ohms. I now get up to almost 3 watts out on 10 meters. I'll have a
go at adding the L-network at J7-P8 as described in the email you
provided and see if that improves things any further.
I also went a little crazy last night with experimentation, and
decided to have a shot at my 'ideal' 4-band filter board: 80 / 40 /
20 / 10 meters. I've got a design in mind to provide 4 separate low-
pass filters, but it will mean some drastic surgery to the filter
board, and I won't have room for the noise blanker any more. As it
stands now, on the test bench, I have a full 5 watts output on 80,
40, and 20 meters, and 2 to 3 watts on 10 meters. The board's two
existing low pass filters are set-up for 40 and 10 meters. So while
I can currently listen on all four bands, I won't be transmitting on
80 or 20 just yet. The key for me though is, I know what is
required, what the values are, and have a good idea how to lay it out
physically, so basically I know it's possible.
Thanks again for the info and suggestions!
Happy holidays and 73 de
Stephanie Maks
va3uxb
On 26-Dec-2005, at 14.44.01, wayne burdick wrote:
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, [EMAIL PROTECTED]) 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.
---
http://www.elecraft.com
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