[Elecraft] More info Re: Modifing an MFJ 1700B Antenna Switch

[Rod N0RC]

Additional Info

I made one more test of my modified 1700B switch. This time I
terminated the balun with a 220 ohm resistor, and measured the
input impedance with my MFJ 259 antenna analyzer.

For the reference setup:

   MFJ 259-< <---> > 4:1 Balun < 220 ohm resistor

       <---> = ~2ft of RG-8X coax

       The balun is a homebrew device 14 turns on a FT114-43

The input impedance measured 50 ohms +/- 4 ohms, from 4Mhz -
54MHz. The SWR measured < 1.2:1 for the entire range.


Then I put my modified switch in circuit:


   MFJ 259-< <---> >1700B< <---> > 4:1 Balun < 220 ohm resistor

       <---> = ~2ft of RG-8X coax

       The balun is a homebrew device 14 turns on a FT114-43


As I tuned the analyzer from 4-54MHz I noticed the SWR rise and
fall peaking as high as 2.0:1, and the impedance exceeded 80
ohms. For the amateur bands I work one here are the results:

      Band    SWR      Impedance
      40m     ~1.1:1   55 ohms
      30m     ~1.2:1   60 ohms
      20m     ~1.3:1   60 ohms
      17m     ~1.5:1   70 ohms
      15m     ~1.5:1   80 ohms
      12m     ~1.5:1   80 ohms
      10m     ~1.7:1   80 ohms

CONCLUSION

If you are using an unmodified 1700B you may want to make
similar measurements. This is especially true if you are using
resonant antennas, and have added a 1700B to your station
setup. My tests show this switch will change the load into
which your transmitter will work. True you can "tune it out"
with an antenna tuner but with each additional "component" added
to your transmission system, a certain amount of loss is incurred.
So, for the most efficient system you will want to use as few
"components" as possible, and have each component working a
efficiently as possible.

73, Rod N0RC
Ft Collins, CO

----- Original Message -----
From: "Rod N0RC" <rod@n0rc.com>
To: "Low Power Amateur Radio Discussion" <qrp-l@Lehigh.EDU>
Sent: Monday, April 08, 2002 6:37 PM
Subject: Modifing an MFJ 1700B Antenna Switch


> 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:
...