> there is no practical problem, as I measured a similar setup, in doing
> what you do. One beverage transformer, one ground, one line and simple
> remote switch of the beverages wires, wires diverting @ 60° each 6ft
> from ground. The coupling between the floating wires and the used
> beverage is neglectable and 6 relays of normal quality by far do not
> show unacceptable crosstalk at 2 Mhz even at 450 Ohm. If one has datas
> that say the opposite (read of a poor isolation, better if expressed
> in dB), explainations (i.e. what's the useful practical coupling
> between standard diverting beverages, again in dB) and how they
> measure, please share it.
Hi Mauri,
I have not found a practical way to measure actual antennas except to
translate the measurements from a calibrated test bench to a working
system. How do you measure actual antennas?
Consider two examples:
With capacitive coupling and typical Beverage *resistive* impedances,
as little as 20pF of equivalent capacitance gets you into the area of
20 dB crosstalk. 20 pF is not much, when you consider the wires
outside the relays and everything all added together. If you use the
antenna on higher bands, the problem becomes rapidly worse.
Looking at notes, one commercially manufactured switchbox measured
almost that equivalent capacitance. With 50 ohm termination it would
have an acceptable 45dB isolation. At Beverage impedance levels, it
would have only around 20dB. To me, it is foolish to switch at
several hundred ohms when a low impedance would allow many other
improvements and advantages, like keeping grounds isolated.
Of course while even 20dB is acceptable with a small Beverage, that
number can get much worse easily with some impedance and layout
conditions. One problem is if the source impedance is inductive or
very high (open circuit) or a combination of the two.
Second, if all antennas share a common ground and are terminated to
that ground to minimize coupling through capacitance, the path then
forms a "T" attenuator with the effective ground impedance allowing
coupling. With a typical ground rod, that also can be in the area of
20dB.
While many systems will work, certainly some will suffer harmful
effects that may go unnoticed. The problem is most likely if wiring
is not the best, or with small ground systems using only one or two
ground rods into normal or worse soil. In my opinion it is a better
idea to not ask for a problem when a solution is simple and only a
very small time investment to avoid, even if a few people are happy.
73, Tom W8JI
W8JI@contesting.com
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