Subject: Topband: Beverage Feed Line Choke Installation
K9YC:
> So Carlos is using two 5 KOhm chokes with the DXE circuit (that is,
two ground rods).
It would be interesting if someone could define "extremely high
isolation impedance". :-)
These questions are very important to understand the big picture. When we
are thinking about RX antenna we are indeed looking for the best signal
noise ratio. Any RX antenna should improve the N/S+N ratio. Modeling the
antenna can provide a good start point; however the real world is quite
different because the feed line also is part of the antenna and it picks up
noise from every direction.
A beverage antenna, as an example the 300 ft with another 300ft of feed line
can work like a big low 600 ft dipole when only one choke is used close to
the station.
A Flag antenna can become a vertical top load of the feed line, two HF
dipoles with 100 ft line each can become a 200 ft "V" antenna connecting
both shield at the shack ground.
In the real world the results can be very different because we only look at
one aspect of the RX antenna. In the environment different antennas feed
lines, rotor cable, AC return ground etc can be resonant elements coupling
with the RX antenna.
A choke is a key element to break those different wires that together can
interact very badly with the RX antenna, and it is very difficult to foresee
those circuits. The ideal situation is only the current we see on the RX
antenna NEC modeling. The choke needs to work as an electrical switch
disconnecting those wire currents.
Great isolations can be translated to great attenuation. Attenuation is just
a big resistance, wait a minute! Not exactly, we need a voltage divider or a
"PI" or a "T" circuit to drain the common mode current to some place else.
Well, we only know the choke, the other components, part of the attenuation
circuit, are not so easy to see or understand. One 1/4 wave long cable
grounded at one side has a high impedance on the other side, if we double
the frequency it becomes 1/2 wave long and the impedance at the end is now
low.
In the case of a flag antenna we need to remove all vertical pick up by the
feed line and we need at least 2 chokes with the maximum impedance that we
can find available, one close to the flag transformer and another close to
the ground.
Is it necessary to ground the shield after the choke, so we have ground >
choke > vertical feed line > choke near the feed point flag transformers?
In most of the cases yes, it is necessary, in some other cases no. It really
depends of the feed line dimension and anything else connected to the end of
the coax, even a rotor cable or another coax connected near at the ground
point at the shack entrance. Yes, those cables are connected and together
can be a noise pick-up generator. There is no way for us to know it; my best
recommendation is to play safe. One big mistake is to choke only the RX feed
line and forget about all other cables connected to the station ground.
It is easy to understand that we can ground the center of a director to the
boom not affecting its resonance. But one 20m dipole with 120ft coax line
and another 15m dipole with 120ft coax line connected to one ground point
becomes a single 240 ft long 160m dipole. It is hard to see the coax cables
as a 160m antenna.
20m 15 m
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If you can not control the common mode noise you need to eliminate the
problem using chokes in every thing.
It is the same in the case of the Beverage antenna. The feed line is
connected to other antennas coax, including other RX antennas. In some cases
you need to ground the coax near the choke, but in other cases we don't.
Grounding the feed lines can also cause ground loops. The topic is complex
and W1HIS and K9YC are a mandatory reading if you want to build a good RX
antenna.
I use the most high impedance choke available and my RX feed lines are
grounded and runs inside a metal buried conduit from one metal box to
another. Even to go up the tower I'm using the RX line inside a flexible
aluminum conduit. The RX cable only exits the conduit close to the radio.
My place is very noisy and I may be over killing the common mode noise. But
it seems to be getting noisier every day.
Regards
N4IS
JoseCarlos
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