On Wed, 14 Jan 2009 08:04:39 -0500, n4is wrote:
>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.
YES.
>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.
YES. ANY wire will act as an antenna in the presence of excitation, depending
on its
length and the frequency.
>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.
I like to think of chokes in wires like this as the equivalent of an egg
insulator in
a guy wire. It breaks the undesired antenna into pieces too short to draw
signficant
current. I've discussed this in detail in
http://audiosystemsgroup.com/RFI-Ham.pdf
>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.
I look at a choke on a conductor as simply adding that low Q resonant circuit
in
series. See the RFI-Ham piece.
>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.
Each of these applications needs to be viewed in terms of their fundamental
equivalent circuits, thinking antennas and lumped elements (that is, the wire
is an
antenna, the choke is a lumped parallel resonant circuit.
>Most of the topics here does not impact HF,
I disagree -- again, it depends on the location, length, and orientation of the
wire.
You would have a hard time convincing me that a wire resonating on 20M, 40M, or
80M
can't affect a nearby vertical on those bands. My primary antenna for 160 is a
vertical, and I choke everything anywhere around it. I've also loaded a
vertical at
the same location on 80, and observed significant interaction with nearby
feedlines
before I choked them. http://audiosystemsgroup.com/NCDXACoaxChokesPPT.pdf
documents
this and includes NEC analysis (beginning on page 60).
>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
I disagree. Grounding is not into which noise is poured. When you ground a
conductor
you provide a path for current, and RF current results in radiation. You've got
to
BLOCK the current. That requires either an insulator or a high Z -- that is, a
choke.
73,
Jim Brown K9YC
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|