[Elecraft] Balun testing using K3 and XG3

[John Oppenheimer]

4:1 current Balun measurements can be performed using a XG3 signal
source and a K3 as the detector.

Configuration: K3 RX input, 10 dB attenuator, 50 ohm resistor from
attenuator inside conductor to 200 ohm Balun connector, 100 ohm from
attenuator shield to other 200 ohm Balun connector, XG3 connected to 50
ohm Balun connector.

I found that I get better results with an attenuator attached to the K3
RX input. My assumption is that the RX ANT input does not present 50
ohms to the test configuration.

Set up a set of adapters or connectors such that the Balun can be
inserted and removed from the test configuration.

Adjust K3 sensitivity: Without Balun, set XG3 to 1uV. K3 ATT and PRE
off, adjust K3 RF gain from zero to the first maximum AFV reading
without rolling over the AFV display.

With XG3 off, read and record noise from AFV reading.
Turn on XG3, read and record signal from AFV reading.
Do not turn off XG3!
Insert Balun between XG3 and attenuator
Read and record signal from AFV reading.
Turn off XG3, read and record noise from AFV reading.

I have found that the XG3 output is a little different between power on
cycles. The AFV reading will wiggle a little, +/- 10 or so, choose a
middle value.

Compute the voltage loss of the Balun, which because of the 50/100 ohm
attenuation, will be 2:1 for a perfect Balun.
(Sig - Noise) / (Sig - Noise)

Typical values from my configuration and a home brew Balun:

3.5 MHz : 20 6545 3284 21 = 1.9997 or 2.00
7.0 MHz : 21 6129 3042 38 = 2.0332 or 2.03
14  MHz : 19 6149 2903 22 = 2.1277 or 2.13

I have performed many test and find that they are repeatable to within a
+/- 0.01 ratio.

A current Balun common-mode filter quality test can be tested by
changing the 100 and 50 ohm Balun coupling resistors to any set of
values as long as the sum of the two equal 150 ohms. See "Balance
Quality Test" here:
http://www.w8ji.com/balun_test.htm

John, KN5L