#506 NANOVNA Microstrip Low Pass Filter

Episode 506
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[johnwest]

A suggestion: When you set up your camera to point at a spot on the workbench or desk you might put some tape around that spot so you will know what is in the camera's image window and what isn't when you are working with things on the desk.

[bendunselman]

Oh yes I solved those Maxwell equations for a waveguide also once upon a time way back when I studied physics ... Nice article. Nice experiment.

[rjordans]

I've had some success with a combline bandpass filter for 70cm band. Had some nice steep edges and a somewhat manageable size still, about 4x6 cm. Making this in a microstrip form saved me from having to try and tune quite a few elements. This was three 50mm long strips coupled directly to ground on one side and via a capacitor on the other. Michael Steer had a great video series on his to calculate such a filter which I used for the basis of my version.

[joeteejoetee]

YES to soldering copper adhesive tape to every other conductor. Test it: Measure the resistance between either SMA center conductor, to the 3 rectangular strips (and not with a point probe at the center-line!). Technically: 100K Ohm = "Conductive" in product marketing jargon.
The capacitance of large areas of this tape over a solid conductor may semi-equal RF conductance, and the rough outside edges of the cut tape may actually DC short to metal below, but not reliably connect over time to a thin stripline passing beneath it.
This is a 50 Ohm system, and nil-Ohms are the gold standard at every UHF RF connection.

[markcummings]

I worked on a Rockwell-Collins uWave system back in the mid-late 90's that used strip lines just like these on the PCBs in the RF section. Even then the uWave was 30+ years old. The uWave operated in the 7.5-8GHz band. When we looked at the circuit diagram and started looking for components on the PCBs, which had no component designators to help locate parts, we realised these tracks were actually components and that we were in over our heads .

[amirb]

it's true that microstrip circuits might seem quite large in UHF/VHF and most of the time it's better to use lumped elements but I have faced with many cases (in designing BP and LP and notch filters in UHF) where you end up with ridiculous values of C and/or L which are very hard if not impossible to implement with lumped elements but when you convert the filter to transmission line filter, you end up with very reasonable microstrip dimensions. I think that's the main reason why one would still want to use microstrip at lower frequencies. Another example that pops into my mind, is to implement couplers for example for SWR meter...

[BrendaEM]

Ceramic is supposed to be good for this kind of thing. I have some old alumina end-effectors which might work. If you happen to find yourself in the south bay, I could bring a few to a social distancing coffee shop and carefully slide them across the table with a stick or something.

[rfmonkey]

great video as always, you inspire many.
regards VK3