Topband: Multiple Beverage Switchbox

[Joe Subich, W4TV]

> There are two issues here...

The key is how well the shields of the feedlines are decoupled. 
If the feedlines are elevated - or even on ground - the shield 
can act like a "Beverage on Ground" (BOG) and will impress a 
signal on the common point reducing the directivity of the 
entire antenna system. 

If the switchbox is well grounded (low impedance ground), and 
the feedlines from each Beverage have effective (Z > 5K Ohm) 
common mode chokes on both ends to prevent the "BOG effect" 
you can probably get away with switching only the center 
conductor.  If switching only the center conductor, I would 
use a relay arrangement like the Ameritron RCS-8 or microHAM 
six/ten switch that grounds all unused antennas to minimize 
any capacitive coupling across the relays. 

> However, RG6 catv line, is 100% shielded and can be bundled 
> without problems if they are feeding isolated primary 
> windings out at the antennas... 

The primary issue effecting switching is the BOG effect and 
that is independent of the quality of shielding.  It is due 
entirely to the presence of common mode currents on the 
individual feedlines.  The longer the feedline and poorer 
the decoupling, the greater the BOG effect.   

73, 

   ... Joe, W4TV 
  


> -----Original Message-----
> From: topband-bounces at contesting.com 
> [mailto:topband-bounces at contesting.com] On Behalf Of Dennis OConnor
> Sent: Friday, October 09, 2009 8:56 AM
> To: topband at contesting.com
> Subject: Topband: Multiple Beverage Switchbox
> 
> 
> There are two issues here...
> 1. Is your Beverage coax braid grounded at the antenna feed 
> point? If yes, then you MUST switch both sides of the coax, 
> center and braid, at your switchbox, and leave the unused 
> coax(s) floating... 2. Is your Beverage coax floating at the 
> antenna end, i.e. feeding an isolated primary winding on your 
> 9:1 transformer? If yes, then you CAN leave the coax braids 
> grounded together at your switchbox and only select the 
> center conductor one at a time...
> 
> Now CAN and BEST are not identical... Best would be to switch 
> both sides of the feedline...  Standard coax is not 100% 
> shield - it does leak some signal... So the possibility of 
> cross talk is there as the coax runs come together... 
> However, RG6 catv line, is 100% shielded and can be bundled 
> without problems if they are feeding isolated primary 
> windings out at the antennas... I admit, that given I am 
> using RG6 to feed transformers with an isolated primary 
> winding at the antennas, I use a standard MFJ coax switch at 
> the shack end - lazy wins over perfection every time..
> 
> denny / k8do
>