Re: Inverted U terminated low noise antenna

David Cutter

Thanks, Chris they look very encouraging numbers.  I'm not sure how to relate that to the numbers found for chokes from the likes of K9YC and G3TXQ.  They look for resistive loss to the common mode signal in the order of 5000 ohms.  How would your measurements relate?

----- Original Message -----
Sent: Saturday, March 11, 2017 1:35 PM
Subject: Re: [CrossCountryWireless] Inverted U terminated low noise antenna

This morning I built a test jig to make common mode rejection  measurements of the 9:1 current balun and a few other types of balun and transformer for comparison.

Many tests of baluns on the internet just measure the inductive reactance of the balun and quote it as a value in ohms. This doesn't represent what happens in real life as the balun will be connected by coax cable to a receiver or transceiver and the balance of the balun also comes into play.

The test jig I built feeds the RF output signal from the network analyser to an attenuator to make sure that the analyser is correctly terminated.The output of the attenuator is then fed to both antenna terminals of the balun. The coax output from the balun is fed to the network analyser receiver input in the same way it would be used in real life. A low impedance connection is made from the shield of the attenuator to the coax shield on the balun to complete the earth return for the common mode RF input to the balun.

This ensures that the balun is being fed an identical common mode signal on both input terminals and the RF passing through is measured at the output of the balun.

With the CCW 9:1 current balun the common mode rejection figures are:

3.5 MHz 52 dB

10 MHz 42 dB

20 MHz 40 dB

40 MHz 40 dB

50 MHz 32 dB

The figures reduce at higher frequencies due to capacitance across the windings. The 9:1 current balun uses three 1:1 baluns configured in a series- parallel arrangement. Any imbalance on the antenna would only have a small effect on the readings.

I then tested a 9:1 voltage balun using one of the same toroids used in the current balun but wound with the same number of turns as a trifilar winding. The common mode rejection figures are:

3.5 MHz 4 dB

10 MHz 4 dB

20 MHz 4 dB

30 MHz 4 dB

40 MHz 4 dB

50 MHz 5 dB

Effectively we can say the voltage balun as no or very little common mode rejection and would be useless at rejecting common mode noise.

To complete the set I wound a 3:1 transformer (9:1 impedance) using the same toroid core and number of turns on the secondary as before. The common mode rejection figures are:

3.5 MHz 50 dB

10 MHz 45 dB

20 MHz 40 dB

30 MHz 38 dB

40 MHz 35 dB

50 MHz 28 dB

This gave similar figures to the 9:1 current balun with capacitance across the windings reducing isolation at higher frequencies. There are two points to be aware of when using a transformer. The first is that all the RF power flows through the toroid magnetic core. In the current balun only the common mode energy ends up in the toroid. If the antenna connected to the transformer is not perfectly balanced then the transformer isn't balanced and the common mode rejection will fall.

It's been very interesting to make these tests and I now have a test jig to test future designs.



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