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I would like to have a go at measuring PIM. ( Passive Intermodulation ) in UHF.

To test my ability to measure PIM, I need to find something that generates PIM.

I either envision a 50Ω device that generates PIM, or a 2-port thru device which can be terminated on one end.

If anybody has any links to PIM sources, or home-brew build ideas, let me know.

Skyler 440
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  • Is measuring PIM so different from measuring any other kind of intermodulation? Are you trying to test your ability to measure intermodulation, or your ability to generate PIM, or that PIM is a real effect? – Phil Frost - W8II Apr 24 '20 at 15:58
  • I want to test my ability to measure it. I'm going to jerry rig a setup with circulators and cavities / combiners to generate two carriers, and then i'm going to look at the 3rd and 5th order 2f2-f1 3f2-2f1 products with a specrum analyzer with the cariers notched out so they don't overload the spectrum analyzer. I need something that makes PIM to see if I can measure it – Skyler 440 Apr 25 '20 at 23:44

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For generating PIM - for testing and calibration, you want a known level signal at the expected intermodulation frequency. This will let you calibrate the whole setup though the losses of the various combiners and filters, and by switching it on and off, measure the level of PIM in your measurement system.

I'd recommend using a signal generator at the correct frequency, feeding that in through a suitable attenuator.

You probably don't want to use a non-linear device to actually generate PIM. Connecting a diode or something to the test point (with an attenuator, and some DC bias) will generate plenty of PIM, but at an unknown level, so you'll have to start by fully characterising your non-linear device.

You can make a low-PIM high power attenuator with a roll of very thin coax. In a 30+30 watt, 900 MHz test set, we used something like this 1.19 mm diameter EZ-47 which has 32 dB/100' attenuation. 50 feet is not a large roll of this cable.

You will need a low-PIM termination for the other side of your device under test, this gives you a way to upgrade your ordinary 50 Ohm termination. Combined with some ordinary attenuators, it also provides a way to measure the final incident power and apply your fake PIM signal.

tomnexus
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Any non-linear device can work. But a good physical analog might be to find a crystal radio kit with a point contact crystal, and use the point contact detector inductively or capacitively coupled to the ends of your terminator.

hotpaw2
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I've confirmed generation of PIM within the 2m band and across 2m/70cm bands using a point contact diode as a mixer.

Two 2m rubber ducky antennas were interconnected by a OA5 diode, with the junction fed by a 600kHz crystal oscillator. The passive setup could intercept a 145.1MHz signal, transmitted by a 2m HT in the vicinity, and retransmit it after mixing it with the 600kHz signal. This was confirmed by the resulting 145.7 MHz signal being received on a second 2m HT nearby. A 145.7 MHz signal transmitted by the second HT was also received at 145.1MHz on the first.

The same test was done using a 2m and a 70cm rubber ducky, interconnected by a OA5 diode, with the junction fed by a 290MHz signal. A 145.5MHz signal transmitted from the 2m HT was received at 435.5Mhz on the 70cm HT and vice versa.

You may go through my blog post, 'Simple Low-cost Demo Repeater', for more details.

vu2nan
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