[aprssig] Trouble with Calculator Serial Cable

[Stephen H. Smith]

gerheim at cox.net wrote:
> It turns out that the serial cable for the HP-50G calculator doesn't respond well to RF.  When the D7 transmits on high power, (5W)  the serial interface locks up and won't function.  It either produces garbage output or doesn't produce any output.  I have to disconnect the cable to get it to "reset".  
>
> If the D7 transmits on low power (couple hundred mW), or if I put some distance between the HT and the serial cable, it'll do OK.  (There are 2 cables in use:  one from the HT to the null modem, and the calculator cable (with integral circuitry) from the null modem to the calculator. )
>
> I'm going on the theory that the cable circuitry (level shifter/inverter) is shielded adequately in its metal enclosure, but that RF is going down the cable(s) to either the calculator or the cable circuitry.  I'll try some Radio Shack ferrite chokes on the cable(s) this evening.  
>
> Any ideas out there? 
>
>   


This is a chronic problem with the TH-D7.   The body of the radio is 
simply TOO SMALL to be an effective groundplane for it's whip.   ANY 
cable plugged into the radio unavoidably becomes a radial or 
counterpoise for the antenna, and is crawling with RF.   

Try installing some ferrite beads or snap-on cores on the cable about 
15-16" from the point where the cable enters the radio.  This combines 
the height of the radio chassis above the serial port and the first part 
of the cable to form a 19" (quarter-wave) radial for the antenna, but 
prevents RF from travelling any farther along the cable.