Yes, smaller cores can conveniently be used for RX antennas. I built up some
small common mode choke assemblies for use with my Clifton Active Whip antenna.
Here is what I ended up with along with some performance data:
My main application was 160 and 80 meters, so this design was optimized for
that, but it does offer useful results from 500 KHz up to 30 MHz. My system is
50 ohms, but similar results should be attainable for 75 ohm systems. I built
two of these filters, each in a small insulated box. Each choke is 6 turns of
RG-174 on two type 31 "large beads", FairRite PN 2631102002, Mouser PN
623-2631102002. I use RG-58 for the main transmission line segments.
The SRF of these chokes is about 2.4 MHz, so they will operate below resonance
on 160 and the BCB and above resonance elsewhere. I measured > 7.5 Kohms
impedance at resonance, but it is probably higher due to my measurement
limitations. The impedance vs. freq is roughly as follows:
500 KHz 1.4 K ohms
1 MHz 3.5 K
5 MHz 4 K
10 MHz 2 K
20 MHz 1 K
30 MHz 0.6 K
Each of the insulated boxes for the chokes have BNC connectors for the
transmission line. I also brought out a connection to the coax shield to screw
terminals on both the input and output sides of the choke. This allows for an
optional connection to an earth ground rod at either location for optimization
of the CM rejection in the final installation.
I located the first choke about 40 ft from the antenna and the second one about
40 ft later right before the transmission line comes into the house. There was
a ground rod at each location so that I could see the effects of earth
connection to the chokes. I used the carrier of a local BCB station (800 KHz)
as a signal source for the testing and measured the carrier level on my K3/P3
setup with a 50 ohm terminator in place of the antenna so that I was measuring
the CM signal.
With the chokes only (no grounding) I saw a 26 dB reduction in the CM signal.
I then tried 9 different combinations of the ground rod connections. 8 of them
were inferior to the chokes only, with degradations of from 5 to 20 dB. One
combination (ground connection at the input of the choke near the house) gave
me an additional 2 dB CM rejection for a total of 28 dB. So this is what I
ended up using.
Overall, I'm quite happy with the result. As seen above, you may probably
dispense with the ground connections for some applications.
73 Craig AC0DS
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