Topband: Feeding multiple beverages

Tom Rauch
Fri, 29 Nov 2002 20:56:17 -0500

> That brings up a question or two:  Assuming the use of say, 75 ohm
> coax, what is a good method for selecting/switching various receiving
> antennas *in* the shack?...i.e., it is a great advantage in contests
> to turn a single rotary switch or have "express" push-button
> switching...but other than a deluxe matrix of  coaxial relays, what's
> a good method to handle all the different antennas and perhaps allow
> the connection of any antenna to any receiver?

I use relays, because it is easier than running a dozen RF leads with 
potential loose or broken shields to a box that gets moved around.

Most small DIP relays are excellent for 1.8-30 MHz. Larger relays, 
especially those with flexible wires for armature connections, are 
usually the problem.  

With relays you can use whatever switch you desire, and place it 
where ever you like.

Most problematic is bringing the high level transmitting antenna into 
the switching system. Attenuating the level down to "Beverage level" 
it before it gets to the switching system is a good idea. 
> Assuming that an unselected antenna should *not* be left 
> "floating", would it be best to just short it to ground or terminate
> it in it's characteristic impedance?

It might be better to terminate, ground, or float...depending on many 
things. It all depends on what you are doing. None of this is much of 
a problem with low impedance lines, unless you mix low and high level 
systems. (Consider the small coupling required to "contaminate" a 
small loop antenna that is 40dB below a TX antenna's signal pickup on 
> So, one could easily bench measure cross-talk with some common
> equipment such as a RF oscillator, a attenuator and a receiver, right?

That's what I do.
160 is generally not a problem unless you have really low signal 
level lines next to high level lines, or make a mistake. My point is 
as you raise the impedance levels of the switching system, things 
become progressively worse.

I wouldn't bring high impedance leads into a switching system unless 
I was comfortable I knew what to do, nor would I use a common ground 
point for signal grounds of multiple antennas.73, Tom W8JI