On Fri, 8 Jun 2007 15:21:06 -0700 (PDT), Eduardo Araujo wrote:
>2 Even if we are at the right point 200 or 300 ohms
>may not be good enough.
The first place to put a choke is at the point where you want
common mode current to be zero -- where the coax connects to the
antenna. In general, you want the impedance of that choke to have
a very large resistive component. I would try to aim for 4,000
ohms. At your operating frequency, 1.8 MHz, and at the frequency
of the broadcast station, that requires a lot of turns through a
#31 or #78 material.
There are two ways that noise and broadcast signals can get into
your radio. One is common mode on the feedline, and the choke
suppresses that. The other is the antenna itself, which puts the
noise inside the coax (a differential mode signal). A high pass
filter tuned for about 1.7 MHz will take care of that part of it.
If you are using a power amplifier, the filter should go between
your transceiver and the amplifier.
73,
Jim Brown K9YC
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