[TowerTalk] Baluns for RX antennas

[Tod Olson]

Craig:

What measurement equipment and set up did you use to determine the Z of the
chokes that you built? Do you have any pictures of the measurement setup? Do
you have any graphs or data beyond the (f,Z) points in your email you might
be willing to share?

I made some indirect measurements in 2010 of Z of the larger diameter core
chokes as a function of frequency . The write up is located at

http://tinyurl.com/6p8jvcl

It might be interesting to perform the same measurements with the same set
up using the smaller diameter cores.


Tod, K0TO


On 4/23/12 8:05 AM, "Craig Smith" <craig at powersmith.net> wrote:

> 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
> 
> 
> 
> 
> 
> _______________________________________________
> 
> 
> 
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>