Date: Wed, 18 Dec 2019 23:46:56 +0000 From: Chuck Hutton <[email protected]> To: "[email protected]" <[email protected]> Subject: Topband: Measuring Common Mode Chokes
<In the past, I have simply used my N2PK VNA to measure impedance of the choke by connecting the shield of the coax to the VNA ports. <Recently I've been discussing common mode chokes with others who have a different methodology. <They prefer to do a transmission test through the choke and report the "common mode rejection". <This is done by placing a crossover cable between the VNA output and the choke. The choke output is connected in a normal fashion (center to center, shield to shield) to the VNA input. <This does not seem ideal to me. <First, the choke is being driven in differential mode rather than common mode. <Second, the measurement depends on (varying) isolation between the coax center and shield. So it's not truly common mode rejection. <Am I on thr right track? <A handful of Googles has not netted me any clear summary of test methodology for reporting CMRR. I find a small number of tests reporting impedance. <Chuck ## I measure the actual common mode current with a MFJ-854 clamp on RF ammeter. Good from less than 1ma to 3A. Just clamps around the 213-U coax. Then when swapping CM chokes, any changes or improvements, or worsening is readily apparent at a glance. I measure In several places on the coax run, and all are well marked. https://www.mfjenterprises.com/Product.php?productid=MFJ-854 Switchable In 5 ranges. Jim VE7RF _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
