Proper Calibration of open ended coax ?

[kk4vb]

I wish to use a 5' piece of RG-8X coax between the AIM 4170 ( UHF connector end ) and common mode baluns ( open end ) for choking impedance measurements.

I made an adapter which allows the use of the Standard Cal Units on the ends of the open wires, but This seems to create a 12 pF error... related to the capacitance
of the "Open" Cal Unit..  I am concerned that open wires pick up RFI and may be creating invalid calibration files..  I have spent hours .. into the 9th day now, of trying to get
consistent common mode measurements of a 'simple'  balun.

1) What the correct method for calibration ?

2) Is there some way that the content of the Cal File can be graphically viewed to verify its integrity ?

Thanks
Alex
KK4VB

[Bob]

Hi Alex,

I would recommend using a shorter cable, if it's practical, maybe two feet long. Theoretically, the cable can be any length, but that can lead to repeatability problems.

The position of the cal loads and the cable have to be repeatable. A shift in position of even a few inches can affect the open circuit cal values. When using gator clips, be sure they are spaced consistently. You can use a resistor with leads for the standard resistor.

Baluns are sensitive because their equivalent internal capacitance is only a few pf, so a small shift in the measurement cable or clip leads can cause a big change in the apparent inductance and the resonant frequency.

Generally, when measuring inductors (baluns) be sure they are not close to any conductive surface, like a metal table top. If the balun or choke will be mounted in a metal enclosure, the final measurements should be made with it mounted in its operating position.

The best way to verify the cal data is to scan a load with known parameters. I usually do a scan of the cal resistor itself for a quick check.

--73/Bob

[kk4vb]

Hi Bob,
Thanks for the advice...

I use banana plugs & sockets for all test leads.

Question : I made adapters to use the Standard Cal Units for the open ends..
Do you recommend the use of these or just a short & open test leads and a 50 ohm ( carbon ) resistor ?

I get Dramatically Differing Results and cannot discern which, if Either, is valid.


73s
Alex

[Bob]

Hi Alex,Also
You can make your own cal loads with good results in the HF band.
It's better to use two 100 ohm resistors in parallel rather than a single 50 ohm resistor.
Try scanning some loads that you know, like an air core inductor or some store-bought inductors with 2% tolerance.  Also try some good quality capacitors along with  good RF resistors.
Try it right at the RF connector (after calibration at the connector) and then at the end of the cable. The results should be essentially the same in either location
--73/Bob

[kk4vb]

Hello Bob,

Yes, it is becoming evident that external lead calibration is in no way a 'trivial' .

Please clarify these comments by you associated with another topic:
How a small capacitance can be added to the 'Open Condition' for a choke measurement setup with test leads :

>> Note that when doing the open circuit calibration step, it is important to attach the open circuit load.
>> Don't leave the RF connector open. The open circuit load has several picofarads of capacitance that affect the final result.
>> Leaving out this small capacitance upsets the
>> calculations so the final answer is less accurate. You can see this effect by calibrating with and without
>> the open circuit load and then measuring a known good quality resistor. The equivalent capacitance will be different.

Thanks to Steve Hunt ( G3TXQ ) I have some choke measurement scans performed via the VNA2180 and have been attempting to
'tweak' the calibration sequence of my AIM4170 to duplicate the plots with limited ( marginal ) results.

I Do Believe the AIM4170 is capable of making valid measurements of choke impedances and I believe that there are a large number
of AIM owners who are attempting to do so..

I have a suggestion that might work (?) :
1) Offer a Standard Impedance Calibrator  :  a wire wrapped toroid with a resonance in say the 10 MHz range
   (  or maybe Impedance Calibrators for specific frequencies  ) with known measured impedance characteristics. 
2) Offer these plus an associated Calibration File whereby a user could install the Impedance Calibrator in his test lead configuration and thus
3) Have the AIM4170 measure the Known reference device and develop the software to Create a Calibration File For that particular setup Automatically .

73s Alex
KK4VB