>I do NOT
agree that the ground is needed beyond for whatever it takes to make
the antenna work.<<
Hi
Jim
That's a good point. What is an antenna? Simple question but difficult
answer, let's see a 6m yagi at 100 ft high, it is an antenna for 50Mhz,
however 140ft of feed line is a very good vertical for 1.8 MHz. The feed
line is grounded near the radio, the 1,8Mhz and all BC strong signals
captured by this cable (50MHz antenna feed line) can result in high LF
current around the ground point. Two wires running parallel can share the
same common mode noise; the implication is that you may need to choke every
cable coming into your radio.
Rotor cable is not an exception. I was testing a second WF using a rotor.
When I connected the rotor cable at rotor box, there was a noticeable
increase in the noise floor, why? Let's think about it, my rotor cable was
140ft long, when I connected it to the box, actually I was connecting the
cable to short impedance to ground, and I was actually creating a 160m
vertical with the wire running most of the time close to ground and only 24
ft high at the end. It is not a good antenna for sure but was good enough to
Induce a lot of noise into my 160m RX system, it was necessary to remove
that temporary rotor cable to avoid the noise.
This subject is complicated. The choke with 5 or 10K must be positioned at
low impedance point relative against the ground. A vertical has low
impedance close to ground and very high impedance at the top.
A cable running above ground can show high impedance, like 1K for example,
so a 2 K choke will divide current and attenuate it 2 times or 3db a 4K
choke will do 6db and one 8K choke will attenuate only 9db. It is just a
simple voltage divider.
That's not enough when a preamp is necessary due a low gain antenna, and
that is why a WF is difficult to make it work in practice, it needs 2
preamps and at least 30db gain total.
Grounding the right side of the choke the voltage divider will be 8K against
the ground impedance, let's say 1 ohm, voala! , the attenuation will be 40db
or better on common mode current, and you keep the wires far enough to avoid
mutual inductance or capacitance between them.
Most of the topics here does not impact HF, however on LF it does, in
special if you live in a small lot and like LF your station needs to be very
well grounded and protected for unexpected antennas re-radiating noise, like
rotor cables, AC wiring, and most important the TX antenna itself.
It is possible and doable to build a very effective receiver antenna in a
small lot; the difficult thing is to keep it working well with multiple
antennas and wires around.
Regards
JCarlos
N4IS
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