Combined 144MHz Low-Pass
/ 432 MHz
page presents the design and construction of a combined 144MHz Low-Pass
/ 432MHz Notch filter. This filter is to be left permanently on the 2m
antenna coaxial cable to notch out the 70cm RF energy that could damage
the 2m receiver front end.
filter became a requirement for my station
because I co-located a 2m yagi within an EME array of four 70cm
yagi antennas. See the picture on the right. Antenna simulations and
experience have shown that the 2m yagi will cause negligible
deterioration of the 70cm EME array performance as long as it is
positioned exactly in the center of the four 70cm yagis.
Still, as I had expected, the coupled
432MHz RF energy into the 2m
yagi was higher than what my 2m receiver front end could tolerate.
I actually measured 100mW at the 2m antenna when transmitting 300W into
my 70cm EME array. This is a -35dB coupling between antennas. It is
actually better than what I had expected, but it is still
probably too much for the 2m rig to cope with. Maybe there is a low
pass filter at the input of the 2m rig, but I did not want to try it!
Hence the need for a
filter. A low pass filter combined with a notch filter appeared to me
as a good way to do this. A commercial diplexer might have done the
trick, but I did not want to spend, I wanted to build!
the enclosure, I used a similar type filter box recovered from an
AM-6155 FAA RF amplifier. It had compartments which may or may not
improve the 432MHz notch performance. As well, since the enclosure had
more compartments than needed, I connected one end of the filter
circuit to the N type connector using a piece of semi-rigid UT-141
coaxial cable. This is visible on the picture below, in the upper-right
of the enclosure. You may build a box using PCB material or use an
off-the-shelf enclosure. This is not really critical. I would pick a
large enough box so that the inductors do not come too close to the
The main design objectives were:
design activity was a combination of theory, gut feeling and trial-and-error. The resulting filter
consists of a low pass filter stage (L1, C1, L2) followed by a
filter stage (L3,C2). This is shown on the figure to the right.
- At least 30dB of attenuation of
the 432MHz signal.
- Minimal insertion loss (S21) at
144MHz, < 0.15dB
- Low VSWR (S11 or return loss)
at 144MHz, < 1.5:1
- Be able to support at least
100W of 144MHz RF.
Simulations were performed using Ansoft
Serenade SV v8.50. I used a Q (quality factor) of 50 for the inductors
and 500 for the capacitors. The resulting simulation plots are shown on
the right. With these results on hand, it was time to heat up the
on the figure to
on the figure to
|The variable capacitors I used are of
350V ceramic type .
These will work fine with a 144MHz RF power of at least 100W. The suggested capacitance
ranges are written on the circuit schematic.
As for the
inductors, their values and construction details are also
provided on the schematic above. I used enameled copper wire, but bare
copper wire will work the same. Of course, never use stranded wire for
on the figure to
tuning of this
filter consists of first nulling the notch filter by using a 432MHz
signal source (RF generator level below 1mW or 0dBm) and an RF power
meter or 432MHz receiver. Adjusting C2 for minimum signal is the
objective. L3 winding spacing can also be changed and C2 re-adjusted
to make sure that the best null can be achieved. Tuning is quite sharp,
so go gentle on the tuning screwdriver. Using an RF power meter and a
signal generator could be tricky as the power meter will also pick up
the generator's harmonic spurs at 864MHz and 1296MHz. A double check
with a 432MHz receiver or a spectrum analyzer is probably a good idea.
The second step in the tuning process consists of minimizing the VSWR
at 144MHz. This is done by tuning C1 for best VSWR when a 144MHz
transmitter is connected to a dummy load or antenna through the
filter. Use a low enough power to minimize the risk of transmitter
damage. If a good VSWR cannot be achieved, change L1 winding spread and
re-tune C1 for best VSWR. Adjustment of L2 may also help dip the VSWR.
Expect little interaction between the two filter stages. But as a final
pass, the above two steps should be repeated so that any interaction,
even small, between the two filter stages is taken into account.
The resulting performance should resemble what I have measured on my
filter. This is shown on the three figures on the right hand side. The
first figure highlights the very small insertion loss at 144MHz. I
measured less than 0.1dB. The second figure shows the resulting notch
at 432MHz, a depth of greater than 60dB. The third figure shows the S11
reflection loss close to -50dB at 144MHz. This is a 1:1 VSWR.
With such a filter, I don't feel nervous for my TS-2000 if I feed 300W
at 70cm into my EME array. Next, I need to make sure that I don't blow
up my 70cm preamp when I feed 100W into my 2m Yagi!...