Bandpass filters - a really important tool for fieldday and multi /P
As i am involved in Flora Fauna activation, and i enjoy operating /P
with the other HAM's in my area, i also know the problem it can be
when multiple stations transmit from the same small area.
I would really love to be able to operate the bands i like without
having to make all kinds of agreements and timetables with my
fellow operators. and THATS where bandpass filters fit in!.
Another area that fits the bill perfectly is a contest enviroment.
with several operators and radios transmitting from a small area
on multiple bands at the same time. That cant be done at all
without some form of filter arrangement.
Im not an avid contester so i just needed basic filtration from the neighboring bands
thats why i settled on the 3-pole butterworth configuration shown above.
click here to download PDF on the filters
These filters, when done carefully, will give you 45-50dB attenuation
and thats plenty enough for my operating style.
The normal way to do this is to put a single filter in a tin box and change
the box everytime you change bands. I wanted something easier and i
also wanted a way to supply 13.8v DC to my ATU sitting at the bottom
of my antenna. So i needed a way to incorporate a Bias-T in the design.
The Bias-T is a chapter on it's own, but my design ended up looking like this
Rotate the switch and select the filter you need. the <> is straight through.
The switch at the top is for switching the Bias-T on/off so i can use the filters
without blowing the fuse when running on a antenna with a DC short, or when
using the filters on a resonant antenna without tuner.
Heres a couple of images of the box internals.
First one with filter section in the top of the image, 80m to the left
and the lid with the rotary switch and band indicating LED's.
You can see the realys too for switching the filters in and out of the circuit.
Second one is seen from the end with the coax connectors visible.
All relays are interconnected with RG-316
heres a simple Bias-T circuit you can use in your own projects.
it works very well from 160m - 2m
