[Lowfer] common mode chokes, active whip input Z, etc.

[Steve Ratzlaff]

Here are some more comments from Jack Smith K8ZOA that I think are worth 
presenting.
73,
Steve
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"It's been so long since I built up a PA0RDT antenna that I can't recall how 
it performed in terms of sensitivity compared with a 5 to 10 ft whip. But 
the input Z of the PA0RDT amplifier and the one in the Z1501D (and your whip 
amp for that matter) isn't materially different. All can be modeled as a 
meg-ohm or so to ground and 8 to 12 pF shunt C to ground representing the 
input FET junction capacitance and wiring.  Above 15 or 20 KHz, the shunt 
capacitance is the dominant factor in input impedance.

The way the common mode choke works is to keep the noise off the coax shield 
outer surface near the active antenna. By near, I mean 10 to 20 or 30 feet 
(3 to 10 meters). Beyond this distance, the coupling factor between the 
active antenna whip and the coax braid outer surface is small enough not to 
be a major problem.

Noise signals ride along the outer surface of the feed line shield. The 
resulting transmission line, assuming a buried feed line of typical coax 
cable size, has an impedance in the few hundred ohm range. So, if the common 
mode choke puts an "impedance bump" of say 10 times the impedance of the 
noise flow, the noise available to be coupled into the active antenna is 
reduced by 20 dB. If a similar choke is used at the radio room as well, the 
total reduction can be 40 dB or more. (Running the feed line underground 
also helps attenuate the noise flowing on the outer braid surface.)

I think the confusion involving whip amplifier impedance may be a result of 
focusing on the coupling mechanism between the coax shield outer surface and 
the whip amp. If this is electrostatic coupling (capacitance) then, of 
course, the signal that is coupled into the whip is proportional to the 
amplifier impedance; the greater the amplifier input Z, the stronger the 
coupled noise will be, assuming simple capacitive coupling between the coax 
outer surface and the whip. However, and it's a major however, the way a 
common mode choke works is to (a) reduce the level of noise that is capable 
of being coupled into the whip and (b) moving the high noise portion of the 
coupled noise further away from the whip where the capacitive coupling into 
the whip is negligible. This distance is, as mentioned, a few tens of feet 
in my experience."

Jack