Notch Filter Reverse Harmonic? Real or imaginary...

[devicelab]

Question about my notch filters. I'm using two -- FM & Dale's NOAA filter. I did a recent sweep of Diamond D3000 without filters and it looks 'normal' -- I was comparing it to the Tram 1410 discone. The TRAM actually looked a bit better but it was indoors and not mounted outside, etc.

I did change the stimulus to a narrow range and this attenuation still showed up. There's actually two -- one from the FM (@ 843) and one from the NOAA (@935-950.) Just curious if these are real effects or something the VNA is doing weird. I honestly don't recall seeing this before, so that's what peaked my curiosity.

For grins, I did a sweep of the full setup and I got something unexpected.

Here's what the VNA saw directly:

(If anyone cares to look at the S1P file then here's that file.)

 

[Ubbe]

When you measure a live antenna that are exposed to strong RF signals you can get strange results depending of the quality of the VNA used. The best $5000 analyzers have functions to try and overcome this issue but are still not unaffected that needs to be take into account when evaluating the measurements.

/Ubbe

 

[KevinC]

To add to the above, put a load on the filter and see how it sweeps.