Antenna analyser – what if the device under test does not have a coax plug on it?

I have written a few articles on fixtures for adapting the device under test (DUT) to an antenna analyser's coax jack.

Antenna analysers come with a range of connectors, the UHF connector is very popular, perhaps less so are N-type, SMA and BNC.

I use a range of fixtures made to suit specific applications, but the most flexible are the two shown in the following pic.

Above are two adapters:

  • at the right, an N(M)-SMA(F) and PCB mount SMA(M) with 3 pins of turned pin (or machined pin) female header soldered to it; and
  • A 500m RG400 N(M) to SMA(M) cable and PCB mount SMA(F) with 3 pins of turned pin (or machined pin) female header soldered to it.

The cable is tagged and numbered to reference the saved OSL calibration files.

Also pictured are a set of three OSL calibration parts, OC, SC and 50Ω chip resistor. These allow calibration so that the reference plane is at the tip of the turned pin male pins, and very short connections to small components is possible.

It is possible to connect smaller coax with zero pigtails as a strip of two male header pins can be pushed into the cut end of the coax. Connection to larger coax is possible with millimeter length conductors.

With OSL calibration

Both adapters are best used with OSL calibration, tedious as it may be to perform, but the cal files can be saved for use again and again.

Without OSL

Not all analyser systems support OSL calibration.

If your PC client properly allows backout of transmission line, that may provide a useful facility for offsetting a fixture such as the cable one pictured above.

I no longer recommend Rigexpert analysers, but if you have an older one that is supported by the now superseded Antscope (1) software, you may be able to back out the fixture as I demonstrate with an AA-600 using its ‘subtract cable' facility.

Above, a scan of the fixture with nothing attached to the pin header sockets scanned from 1-100MHz. The length of the cable subtracted is adjusted to obtain closest to infinite impedance for all frequencies in the scan, ie the smallest cluster at the right hand end of the X=0 axis. The display suggests this fixture is good to about 100MHz when used in this way.

A clip lead example

Clip leads seem an obvious solution to adapting an analyser to many DUT, but they are a very common cause of measurement failure.

Let's use one to measure something we know.

The adapter shown above was made for a specific project, for measuring the impedance looking into wide spaced open wire line at 3.6MHz, the very long wires are a practical necessity. Here it is being used to measure a close to 0Ω load (a small coin). The reference plane is the instrument's N type connector using built in calibration.

Above is a Smith chart view of the sweep which looks a lot like a SC seen through a length of transmission line.

Above, the R,X plot is consistent with a SC seen through a length of transmission line. The coin is 130mm from the reference plane (where a SC is reported properly as close to 0+j0Ω), and the R,X characteristic hints that the adapter looks like 130mm of transmission line with Zo around 200Ω and vf around 0.8.

So, the adapter is a length of transmission line that may introduce significant impedance transformation.

Since it was for use at 3.6MHz with loads of the order of 500Ω, let us look at the transformation. At 3.6MHz the electrical length is λ/500 and it transforms 500+j0 to 499.6-j12.5Ω, measurable but quite acceptable for the intended application.

Whilst suitable for the intended application, it may not be suitable for other applications at higher frequencies, even at 14MHz where electrical length is still less than λ/100.

Measuring something we know, the close to 0Ω load (a small coin), informs.

Direct connection of wires to the analyser jack

A method of obtaining very short connections to the analyser is the connect very short wires directly to the jack though care must be taken to not damage the ‘precious' jack.

In the case of UHF jacks, a 4mm banana plug without the insulating handle and with short wire soldered on can be used for the centre pin, for N types a centre pin from a plug with short wire soldered on can be used (with care). A satisfactory connection to the shield can be made with a bared wire secured against the external thread with a nylon zip tie.


Improvised adapters are a common cause of measurement failure, often unrecognised.

Before using an improvised test fixture or adapter to measure and unknown, measure a known impedance. The simplest known impedance is a SC, and a small coin or strip of aluminium foil can provide a low inductance SC.

Interconnections of as little as λ/1000 can introduce significant error into measured values depending on the scenario.