[Elecraft] Re: K1 on 10 meters: helpful advice

Mon Dec 26 16:30:08 EST 2005

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, 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.
>
>
>
> ---
>
> http://www.elecraft.com
>