13cm Interdigital-Filter

Interdigital filter for 13cm

Recently I found a 13cm filter at a fleamarket , probably at Bochum radiotelescope, I don’t remember.

I connected the filter to LibreVNA port 1 and port 2. From the LibreVNA-GUI app I saved a touchstone file. Used frequencies went from 1 to 3000 MHz with 4501 (max) measurement points.

Of course the touchstone file forced me to write some lines of python …

from skrf import Network, Frequency
import matplotlib
import matplotlib.pyplot as plt
import os

filter = Network()
filter.read_touchstone('files/13cmInterdigitalFilterFull.s2p')
filter.name = "Filter 23cm"

# show the properties from the touchstone file
print("filter properties:", filter)

s11 = filter.s11
s11.name = "s11"
s12 = filter.s12
s12.name = "s12"
# lets show what we got
plt.rcParams['figure.figsize'] = [10, 5]
plt.grid(True)
plt.minorticks_on()
plt.title = filter.name
plt.xlabel("Frequency (MHz) ", fontsize=12)
plt.ylabel("Signal (db)", fontsize=12)

plt.title = filter.name
s11.plot_s_db(c='g', lw=1, label=s11.name)
s12.plot_s_db(c='b', lw=1, label=s12.name)
plt.show()

filter properties: 2-Port Network: 'Filter 23cm',  1.0-3000.0 MHz, 4501 pts, z0=[50.+0.j 50.+0.j]

So we see the whole band is rather plain except the small part around the 100 MHz. Lets try to zoom in.

zoom='2200-2500'
regio='2320-2450'
s11 = filter.s11[zoom]
s11.name = "s11"
s12 = filter.s12[zoom]
s12.name = "s12"
plt.rcParams['figure.figsize'] = [10, 5]
plt.grid(True)
plt.minorticks_on()
plt.title = filter.name
plt.xlabel("Frequency (MHz) ", fontsize=12)
plt.ylabel("Signal (db)", fontsize=12)

plt.title = filter.name
s11.plot_s_db(c='g', lw=1, label=s11.name)
s11[regio].plot_s_db(c='r', lw=2, label='s11 in 13cm Amateur band')
s12.plot_s_db(c='b', lw=1, label=s12.name)
plt.show()

Thats all about my old interdigital filter…

Have Fun!