How can I test a sliding termination - also known as a sliding load?

[DeboraHarry]

I've got a Maury Microwave sliding load of unknown history which is fitted with an APC-7 connector. Is there any quick tests I can do to see whether the device is basically functional or not? A DVM shows 0 Ohms between the inner and outer conductors, which made no sence to me, as I was expecting a resistance of close to but not exactly 50 Ohms.

I then calibrated a VNA with an *N* caibration kit, added an N to APC-7 adapter, and connected the sliding terminaton to that. Whilst the return loss at low frequencies was really bad, when I set the span of the VNA from 2 GHz to 6 GHz, I see a circle close to the centre of the Smith chart, which is what I would have tought. But the fact the DC resistance is close to zero Ohms is worrying me.

 

[biff44]

sliding loads are NOT broadband devices. They are certainly not designed to go down to DC. In the older network analyzers, you would use the sliding load in a calibration of higher frequencies (maybe 1 to 18 GHz), and then use a fixed coaxial load for the low frequencies. The combined calibration would cover the whole frequency range.

 

[BigBoss]

Sliding Tuners have bandwidth,therefore it's normal to see 0 Ohm at DC.

 

[DeboraHarry]

I know they have a bandwidth and that related to the length, as you can only move the load over some distance, and that needs to be a significant fraction of a wavelength. Hence a 1 MHz sliding load would be a bit big!!!

But I thought the basic construction was of partically conductive material which slide as one moved the load in and out. That can't explain why I measure a DC resistance of 0.

 

[biff44]

as i recall, the hp sliding loads had a center rod that had a metal to metal contact with the case at the far end. That is why it appears to have 0 ohms DC. The sliding load material was a hunk of stuff that contacted the center rod, but capacitively contacted the outer coaxial shell. I assume maury is made the same way.

 

[drkirkby]

as i recall, the hp sliding loads had a center rod that had a metal to metal contact with the case at the far end. That is why it appears to have 0 ohms DC. The sliding load material was a hunk of stuff that contacted the center rod, but capacitively contacted the outer coaxial shell. I assume maury is made the same way.

Did HP ever make sliding loads? I know a lot of their cal kits have Maury Microwave sliding loads, so I wonder if they were always made by Maury Microwave, and some have HP badges on them.

Dave

 

[biff44]

HP made none of that stuff directly, but they did buy it from Maury, alford, omnispectra, etc etc.