[Elecraft] Modifing an MFJ 1700B Antenna Switch

[Rod N0RC]

After reading Sam's (AE4GX) article about the 1700B:
http://www.qsl.net/ae4gx/MFJCoaxswmod.html (introduced in a QRP-L
posting 04-Apr-2002) I got to wondering about 1700B. I always
felt it had an effect on my antenna tuning. So, I did some
science. First, as a reference I used this setup:



      
   XCVR-< <---> > 4:1 Balun <==== ANT
      
       <---> = ~2ft of RG-8X coax
      
       <==== = ~25ft of 300 twin lead
      
       ANT   = "random length" doublet 
               in attic ~20ft elevation
               
       XCVR  = 746pro, K2 with KAT2
       
       The balun is a homebrew device 14 turns on a FT114-43 
       
       
Here are the Tuning results:


      746pro    K2
80    NT        ~3.1:1        NT = No Tune
40    NT         1.1:1
30    <1.5:1     1.1:1
20    <1.5:1     1.0:1
17    <1.5:1     1.3:1
15    <1.5:1     1.3:1
12    <1.5:1     1.7:1
10    <1.5:1     1.1:1


Then I put the unmodified 1700B in circuit and made another set
of measurements:

      XCVR-< <---> >1700B< <---> >4:1 Balun <==== ANT
      
      746pro    K2
80    not meas. not meas.
40    not meas. 1.5:1
30    <1.5:1    1.3:1
20    <1.5:1    1.6:1
17    NT        1.3:1
15    <1.5:1    2.4:1  !!!
12    <1.5:1    1.2:1
10    <1.5:1    1.0:1      

Then I modified my 1700B as Sam did. Also, In Sam's pictures and
in my switch I noticed some capacitors in the SW. They were
connected from the coax center lead to ground on the COM leads:

   ANT INPUTS               XCVR INPUTS
   >--+                     +---<
      |                     |
   >--+                     +---<
      |                     |
   >--+--+-<    COM    >-+--+---<
      |  |               |  |
   >--+  =     "47M"     =  +---<
      |  |      cap      |  |
   >--+  V      to       V  +---<
      |         GND         |
   >--+                     +---<
   
I removed the caps and made a final set of measurement:

      1700B in Circuit
      746pro    K2
80    not meas. not meas.
40    NT        1.1:1
30    <1.5:1    1.2:1
20    <1.5:1    1.2:1
17    NT        1.1:1
15    <1.5:1    1.2:1
12    <1.5:1    1.7:1
10    <1.5:1    1.1:1          

CONCLUSIONS:

With Sam's mod. and removal of the caps the 1700B is a better
switch for my station setup. Your mileage may vary. I encourage
experimenting if you have this switch, MFJ 1700B.

I can't see the logic of having the caps in the original design.
The only possible reason is a "lumped constant" to even out the
impedance "bump" imposed by the original switch design.
      

73, Rod N0RC
Ft Collins, CO