[CQ-Contest] switchbox for 2-Radio contesting - HELP NEEDED

CP2235 at aol.com CP2235 at aol.com
Tue Sep 23 18:27:00 EDT 1997


in the next WWDXCW I want to try single op all band with 2 radios.

I want to build an antenna switchbox with which I can select from 10 antenna
inputs and connect each of the 2 transceivers with one antenna at a time.
I want to be able to connect each TRX to each ANT. All the unused ANT inputs
will be on ground potential. The switchbox will only be used for 160m-10m

I thought of the following concept:
each antenna input leads to the first relay which switches the antenna either
to ground or to the input of the second relay which selects between TX1 and
Each transceiver input leads to 10 relay outputs.

My questions are:

1., power rating of the relays:
my thoughts are: 
- no hot switching of the relays
- max. SWR of antennas connected: 3:1
- max CW output 2000W  (Have only 800W PA but want to have a security 

this results in the following conclusions: (are they correct??)
maximum and minimum feeding impedance of the antenna cables
Zmin = Z * s = 50 Ohm * 3 = 150 Ohm
Zmax = Z / s = 50 Ohm / 3 = 16 Ohm

resulting in: maximum voltage and current:
P = Umax^2/Zmax   -->   Umax = sqrt(P * Zmax)  =  sqrt(2000 * 150)  = 550V
P = Imax^2/Zmin    -->   Imax = sqrt(P/Zmin)  =  sqrt(2000/16)  =  11A

I found a relais in the electronic mailorder catalogue which is for PCB
mounting, rated at Imax = 15A, maximum switching voltage 380V/250v= (I THINK
THIS REGARDS HOT SWITCHING - am I wrong??), maximum power rating 3500VA.

Does this sound like the thing I need???

Any other hints for a good relay?

2., I want to install the 20 relays on a PCB with short, wide connections on
this PCB and switch only the inner conductor of each coax. 
I am concerned about the following things:

   a) how bad does this box affect the SWR?

   b)   how bad will be the losses??

   c)  how good will be the isolation between the different antennas 
        and transceivers? After all I want to be able to listen on one
        transceiver (and antenna) while transmitting on the other 
        transceiver (and antenna)...

   d)  which steps can be taken to improve a) - c) ??  e.g. big grounded
        sections on the PCB, etc...?

3., which kind of cabinet would you use? plastik is bad because no shielding?
     metal cabinet will affect SWR?  lighter cabinet preffered, so can take
     on expeditions... should I use aluminium or is steel sheet a better

4.,I would use SO239 (PL) connectors, as far as I know you really do not need
N-style connectors on HF < 30MHz. Any objections?

All your comments are welcome. Will summarize and publish results.

Thank you

73 Con DF4SA               Cornelius Paul              CP2235 at aol.com

CQ-Contest on WWW:        http://www.contesting.com/_cq-contest/
Administrative requests:  cq-contest-REQUEST at contesting.com

More information about the CQ-Contest mailing list