Hi
Today I did some testing with an RF bridge on the Spec An.
The aim was to test the fixture and the effects of a coil in the fixture.
I started off testing the Suhner 50ohm BNC load. This also showed the performance of my DIY RF balanced bridge. The one I use is shown here:
https://www.epanorama.net/blog/2017/10/06/rf-bridge-for-antenna-measurements/. They are available from multiple suppliers. If fitted mine into a cast aluminium box that is a really good fit. Doing this creates problems with internal resonance that mangles the performance. I fixed this by gluing lots of small ferrite beads onto the interior surfaces of the enclosure.
The first image (1) below shows the test setup laid out on the bench.
The RF bridge was connected to the test fixture and not to the coil for all tests.
All tests were done from 0 to 500MHz
The Spec An was setup to display reflected power and VSWR. 100% reflection from the load end gives a trace at 0dBm. When the load absorbs nearly all of the power, the reflected signal is very low. Much less than )dBm.
The first trace below (2) shows how well the Suhner BNC 50ohm load works. This was connected directly to the DUT port of the RF Bridge. This is at least 40 years old so it was made long before Aliexpress, Bangood and others were around.
For comparison, the next trace (3) shows the response of the cheap SMA Aliexpress 50ohm load. This was connected directly to the DUT port of the RF Bridge. The response of the cheap Aliexpress load is arguably better than the Suhmer. The Suhmer had the disadvantage of needing a cheap BNC to SMA adapter to connect to the bridge. This could explain the results. The test data shows that both test loads work well.
The next trace (4) shows the effect of adding the coil fixture between the bridge and the 50 ohm load. No coil was fitted to the fixture. In this configuration, all of the energy from the tracking generator should pass through the fixture and be fully absorbed by the 50ohm load. Ideally, no RF should be reflected back from the fixture. The first tests showed the 50ohm loads reflected almost no RF. For practical purposes, all of the reflection is from the test fixture. The reflection trace shows the RF test fixture has exceptionally ordinary performance. By the time it gets to 250MHz, almost all of the RF is reflected. Less than 1dB makes it through to the test load. So when I am looking at a coil response trace at 200MHz, only about 2.5dBm is the coil under test.
The next trace (5) shows the effect of adding a coil to the fixture. The coax connected to the coil is terminated with a 50 ohm load. This looks at the effect of wrapping a coil around a wire carrying current. Any voltage induced in the coil is going to be absorbed by the 50ohm load connected to the coil via the coax.
To test the effects of the coil and connected load, I removed the test load from the coax. Any voltage induced in the coil would be reflected back by the open circuit at the end the coax.
Spurs can be seen (6) in the trace where the resonated back to the coil. The resonance in the coil and coax was induced back to the wire in the test fixture. This shows that what is connected to the coil does affect the current passing through the middle of the sensor coil, but not by much. If there was no resonance, it would be less obvious that there is any difference.
So I really should try and make a better test fixture. By the time this one reaches 100MHz, the results are unreliable. It works well enough to tell the difference between two coils, but not the absolute values.
What I would do is adapt a TEM cell design like this one for coil testing:
TEM cells are not suitable for coil testing because the central conductor is a sheet of metal that is far too big to fit through a coil. The sheet metal top and bottom connected to the coax shield are shaped to match the cell to the coax and reduce reflection. Achieving good matching includes the shape of the central conductor. A TEM cell based coil tester does not need to be as large as the one shown in the link.
Replacing the central sheet conductor of the cell with a metal wire or tube should retain a reasonable match to the coax and to the load. It won't be perfect, but it would be better than my fixture or the Pearson version.