Finally I’ve got round to posting the results that theresjam sent to me a month or so ago. He tested the two filters I’d built on professional equipment. His opinion was that both were OK to put in the RF chain - woo hoo!
The 3 element is currently being used.
This is a long post & it looks like I need to edit some text. This Markdown is difficult …
Many thanks to theresjam for the testing.
I used this https://www.changpuak.ch/electronics/interdigital_bandpass_filter_designer.phpdesigner with the following parameters.
Interdigital Bandpass Filter, based on work of Jerry Hinshaw,
Shahrokh Monemzadeh (1985) and Dale Heatherington (1996).
www.changpuak.ch/electronics/interdigital_bandpass_filter_designer.php
Javascript Version : 09. Jan 2014
Design data for a 3 section interdigital bandpass filter.
Center Frequency : 1090 MHz
Passband Ripple : 0 dB
System Impedance : 50 Ohm
Cutoff Frequency : 1065 MHz and 1115 MHz
Bandwidth (3dB) : 50 MHz
Fractional Bandwidth : 0.0459
Filter Q : 21.8
Estimated Qu : 2712.84
Loss, based on this Qu : 0.14 dB
Passband Delay : 12.732 ns
Quarter Wavelength : 68.76 mm or 2.707 inch
Length interior Element : 61.15 mm or 2.407 inch
Length of end Element : 61.74 mm or 2.431 inch
Ground plane space : 30 mm or 1.181 inch
Rod Diameter : 8 mm or 0.315 inch
End plate to center of Rod : 15 mm or 0.591 inch
Tap to shorted End : 6.26 mm or 0.247 inch
Impedance end Rod : 88.549 Ohm
Impedance inner Rod : 93.734 Ohm
Impedance ext. line : 50.000 Ohm
**** Dimensions, mm (inch) ****
| No | End to centre | Centre to centre | G[k] | Q/coup |
|---|---|---|---|---|
| 0 | 0.00 | (0.000) | ||
| 1 | 15.00 | 37.04 | 1.000 | 0.707 |
| 2 | 52.04 | 37.04 | 2.000 | 0.707 |
| 3 | 89.08 | 0.00 | 1.000 | 1.000 |
| 4 | 104.08 |
**** Box inside dimensions ****
Height : 68.76 mm or 2.707 inch
Length : 104.08 mm or 4.098 inch
Depth : 30.00 mm or 1.181 inch
3 Element Interdigital filter
Around 0.2/0.3 insertion loss and around 88 MHz bandwidth at 6 dB.
Ignore the -6.00 dB marker.
VNA results
My results with a $15 noise source, rtl_power and heatmap.py
Out of tune
After tuning
I also built a 2 element and the results are
Ignore the -3.01dB text
The expensive equipment that theresjam used
My kit
