As others have suggested, actually touching a tuned stripline is with a metal probe is not usually a great idea. Striplines and similar RF stages ar easily detuned by metal touching them. Non contact methods may be used and for that I have used capacitive coupled probes but even they can detune a stage and effect the reading on the test eqiuipment.
I would normally be checking the input and output stages of a tuned stripline at an appropriate point to determine the behaviour of the stripline and 'buffer' stages. That may be done via non contact probes, high impedance probes or a low impedance probe if the buffer stage can handle the additional load and its effects taken into account where measurements are made. The cheapest non active high impedance contact probe just uses a coaxial cable with the end split out to braid (earth) and a non inductive 1K resistor as the probe from the centre conductor. Calibration will be non existent but you see the signals and it will work as a relative level indicator on similar impedance circuits. Keep all lead lengths as short as possible at microwave frequencies.
As you will know, your Spectrum Analyser has a 50 Ohm input. Whilst this is common for RF work, it is less than ideal when probing inside equipment as 50 Ohms is a low impedance for many RF stages and will adversely effect their behaviour by loading them. The additional capacitance of a coaxial cable can also severely detune circuits or load them. probing RF circuits at any point other than an active buffer output can be fraught with problems.
As for active probes, I own a few. Commercial offerings are expensive as the active electronics inside them needs to provide a pretty flat response over their operating frequency range. DIY active probes may be built relatively cheaply as has been mentioned. The poor er frequency response of simple active probes may be normalised if a tracking generator or broad band noise source is available. Compensations have to be made for the calibration source signal level of course !
Much depends upon why you want to image the spectrum in an RF stage. If it s for the presence f a signal, that is easy. If you actually want an accurate level measurement, that can be harder to achieve. Relative readings are the mid ground where you can decide if the gain of a stage is not as expected. If you have the same of lower signal level at the output of an MMIC gain block amplifier as that at its input, then there is likely something very wrong with that stage ! I should state that an MMIC is often a 50 Ohm impedance device so you can probe input and output quite easily with just a coaxial cable but remember the effect of the additional 50 Ohm loading and potential mismatch standing wave effects of adding the test lead coax to the circuit.
Finally, the use of an oscilloscope probe in 1:1 or 10:1 mode is tempting but fraught with issues. The oscilloscope probe was designed for a termination impedance of 1 Meg Ohm, NOT 50 Ohms. As such its resistive coax and 10:1 divider is totally wrong for the spectrum analyser and the bandwidth tends to be very low anyway.You can find some active oscilloscope probes that have a switchable 50 Ohm / 1 Meg Ohm output. These will work with a Spectrum Analyser but the bandwidth may be quite limited.
Always remember that active probes are quite fragile and their input signal level limits must be respected or overload will occur resulting in strange outputs (intermodulation) and in some cases, probe amplifier destruction.
Hope this helps a little.
Fraser
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