On Sat, Jul 14, 2012 at 9:28 PM, Richard (Rick) Karlquist < [email protected]> wrote:
> > > On 7/14/2012 2:15 PM, George Dubovsky wrote: > >> All, >> >> I guess it's time for me to put up my "common mode choke" test technique >> for all to shoot at. I have a HP 4815A Vector Z meter, a N2PK VNA, and a >> HP 8753C VNA. I have all the probe adapters for the Z meter, and I have >> been bitten by the small amount of stray C that you just can't eliminate >> (but you should be able to calculate out). I have finally decided that >> my standard test is based on the following: >> >> I made up a board out of .062 FR-4 that is approximately 6" across (and >> about 4" wide). I cut three 50 Ohm microstrip lines full width across >> the 6" dimension. I soldered an N connector on each end (6 connectors >> > > SINCE YOU ASKED, there are a number of unresolved issues here from > a strict metrological viewpoint: > > 1. Your fixture is not an enclosed box. A defined Faraday cage is > recommended. The results might be affected by lead dress or whether > the test bench is conductive or not. Any fixture imposes an arbitrary > and finite spacing between the DUT and the ground plane. In general, > the measured impedance will depend on this. There is no "correct" > answer. > Agreed. In all cases, I placed the unit under test on a wood workbench. In addition, after I had decided on the final balun design for each band, and packaged it in its final pvc enclosure, I attached it to a short section of 2' aluminum tubing to simulate mounting on the yagi boom > > 2. The N connectors should be male and female and you should use > a 2 port type N calibration kit. > Agreed. The 2 females is a luxury I allowed myself at HF, but it is a known source of inaccuracy. > > 3. You used the common short cut of a through cal instead of a > full 2 port cal. This means that you are not calibrating out source > and load mismatch. A common short cut to fix this is to put pads > around the DUT. This can work fairly well. > I always used either 6 dB or 10 dB pads on both ports (right at the test fixture) for these tests. > > The designer of the network analyzer can recognize that the through cal > shortcut is popular and accomodate the user by going to a lot of > trouble to provide well matched ports. Or not. It turns out that > getting well matched ports translates into massive padding. This > has the disadvantage of loss of dynamic range. Then there is the > question of allowing the user to turn the internal padding on and > off. What if he turns it off and then proceeds to do a through > cal anyway? There is no simple answer to this discussion. > > If the environment seen by the DUT in the test fixture is not > the same as it sees in the application, then the measurement > is just an approximation. > I did attempt to simulate mounting on the boom in my testing, including all of the mechanical hardware that would be involved. > > It is entirely possible that you will get results accurate enough > for ham radio since you are working at HF. But the technique > would not be deemed completely rigorous by metrologists. > Ahh, what do I care about the weather... oh, metrologists ;-) I do appreciate your comments, Rick, and I respect your professional credentials. My goal going in to this exercise was to provide a family of CM chokes that would not screw up my YO-optimized yagis and, after measuring some commercial stuff in the junk box, I concluded that I could do better. 73, geo - n4ua _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK
