DIY filter for 1090 Mhz

Richard Lee

I found a small coax filter that I had from my days on 900 Mhz ATV and decided to cut it down for 1090..

Here is a simple 1/4-wave coax filter crudely installed in the middle of the coax feeding a small 1/2 wave dipole.

My Filter is attached to my 1090 coax at the radio, using a Coax 'T' fitting. (SMA).
I don't have access to RF lab equipment anymore, so I had to guess how to 'build' the filter.

There is one interesting feature about this resonate filter, that might help a person without a good lab..
If the end of the filter cable, is left open (instead of shorted), it is what we RF guys call a Suck-Out.
It will pull out RF power, at it's resonate frequency. (Hams install these Suck-Out Traps on TV feed lines when TV owners complain about Ham interference).

So, I connected the 'T' and started trimming the bottom of the coax, moving towards the 45mm mark.
As I cut off small pieces of coax, I could see the signal display (RTL1090) dropping like crazy..
I had the AGC turned off and tweaked the gain for a few 'bars' and kept clipping.

I think the coax must have a velocity factor of .66, since the attenuation at 1090 was extremely high at 45 mm.
So, I soldered a short across the open end and now had my filter..
Taking it in and out of the antenna line, made n0 difference in the signal levels seen from the AC..
So, I left it installed in-line and started plane watching..
After a couple of hours, I could see no reduction in range. Planes were reaching the Maximum Range lines.
And a few were actually exceeding the max range (re-drawing it)..

The nice part about using this kind of filter, is the short circuit provides good static electricity discharge protection for the radio.. (or LNA).
Here's the filter installed. I measured 45mm from the center of the SMA T, to the shorting wire, soldered between the center conductor and shield braid.
Any type of high freq T could be used, but the SMA, BNC & N type should work fine.
I would not use the PL259/SO239 unless it was all I had..

Today, I'm trying out an old LNA (once used for radio astronomy) and decided to dig out my old L-band band-pass filter (1 to 2 Ghz).
Using the BP filter to clean up the LNA output. It might cut down the strong UHF (400-900 MHz) police signals that could be the cause of some front-end overloading..
The LNA is very broad-banded. So far, the max-range coverage is about the same..
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