> Of course, if the shack is on a second floor all bets are off - the
> coax may well not see a low impedance path to ground.
That, of course, is the problem. Even if the "entry window" at ground
level includes a single drive ground rod, the coax may not be even
qualify as poorly grounded for RF resulting in a significant RF voltage
on the end of the uncontrolled "radial".
In addition to common mode chokes at antenna feed points, one needs
to consider the behavior of common mode chokes between the "entry
window" and the ungrounded or poorly grounded (power supply "safety
ground") equipment. Whilst common mode response on the feedline at
the antenna results in pattern skewing, null fill, increased noise
pick-up, etc., the high RF voltage on the equipment is responsible
for much to the "common mode RFI" (RF in audio, misbehavior of computer
controls, keyboard/mouse lock-up, etc.) and in severe cases potential
equipment damage (particularly as electronic equipment design migrates
to ever lower voltage/lower power technology).
Modern solid state design with its low impedance interfaces are much
more sensitive than the tube designs of the 60's and 70's with their
high impedance inputs. USB interfaces (+5V) are much more susceptible
to common mode issues than serial ports operating at +/1 15V and low
power CMOS (3.3V) interfaces are yet another order of magnitude more
sensitive to the impressed voltage. Similarly dynamic microphone
inputs (ca. 10mV peak) are significantly more sensitive than electret
inputs (ca. 150mV peak) but either will be impacted by common mode
issues undetectable otherwise.
... Joe, W4TV
On 4/21/2012 9:58 AM, Steve Hunt wrote:
> If the coax in the shack sees a low impedance path to ground, and it is
> an odd multiple of a quarter-wave up to the feedpoint, very little CM
> current flows; and for any that does flow the shack end is at a voltage
> *minimum* of the CM standing wave. It's very clear if you look at the
> current distributions in a model.
> Of course, if the shack is on a second floor all bets are off - the coax
> may well not see a low impedance path to ground.
> Steve G3TXQ
> On 21/04/2012 14:09, Joe Subich, W4TV wrote:
>> I would disagree with you there. Coax lengths that are an odd multiple
>> of a quarter wave are the worst lengths for common mode problems in the
>> shack as they place the shack at a voltage maximum. Proper grounding
>> will help but with so many shacks on the second floor or the opposite
>> end of the structure from the power (utility) entrance, obtaining
>> proper grounds is uncertain at best.
>> ... Joe, W4TV
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