1090 Mhz BP filters on sale

[Richard Lee]

An $80USD filter for $20USD on Amazon..
The reviews look okay. http://www.amazon.com/dp/B010GBQXK8
This should be good of blocking out signals from nearby cell towers,
that might be 'blocking' your 1090 receiver, making it less sensitive.

If your system already has long range, you don't need this kind of hardware.
Since it does insert some loss into your antenna cable.
That's the cost of getting rid of adjacent channel interference.

Looks like a pretty good deal. Might improve SDR rigs, since
it's unlikely they have any build-in filtering.
But, if you live in an interference free area, there's no need for a BP filter.

 

[Lee Armstrong]

I'd only recommend these for RTL devices as a decent ADS-B receiver will have the front end filtered out.

 

[ab cd]

Even with RTL device DVB-T usb dongle (RTL2832U+R820T/R820T2), not every one gets improvement. It depends on location & system setup.

http://discussions.flightaware.com/...-b-filters-now-for-sale-worldwide-t35842.html

 

[Richard Lee]

The Amazon BPF helps some installations, but hurts others. One Amazon review said it helped him, but only when using a high-gain antenna. But, it didn't help with a 1/4 wave ground plane antenna.

Nice part about the USB dongle radio, is you can search the bands above and below 1090 and see if there are super strong signals, that might be de-sensing the dongle.
I can see all that 900 mHz stuff on my ICOM R-8500 receiver. In this area, there is all kinds of strong signals on the 900 mHz band..
But, the filtering in the 1090 Puck radio seems to work pretty well. Adding on extra filtering didn't improve performance.

 

[ab cd]

What do you think, will this make a simple 1090 MHz filter?

http://uk.rs-online.com/web/p/wire-wound-surface-mount-inductors/8988201/

 

[Richard Lee]

It should work okay. But the Q-Factor is a consideration https://en.wikipedia.org/wiki/Inductor#Q_factor
and you have to keep the leads very-very short.

I would cut the cable shields flush with the foam insulation.
I would place the two cables side-by-side, and solder the shields together.
Than install the L&C across the two center-pins, keeping that center conductor as low as possible.

But, the only way to find out is to test it.. When I worked in a lab, I had access to a Q tester..
It really helped when I was building filters..

 

[ab cd]

It should work okay. But the Q-Factor is a consideration.....

Sure.
Higher circuit losses (resistance) = Lower Q = Wider Bandwidth = Poorer filtering.
Lower circuit losses (resistance) = Higher Q = Narrower Bandwidth = Better filtering.

 

[ab cd]

How about SMD components on a small piece of pcb?

 

[Richard Lee]

If it's close to being at resonate frequency, it's going to work, to some degree.
Your connections on each end are going to be sensitive.
Any length of wire soldered on the ends is going interact and pull the component off resonate freq a bit.
Every connection has to be kept very short when you are working at 1090 MHz.

How is the Q factor of those parts?
I've tested a lot of capacitors that had a different capacitance at higher frequencies.
At 100 MHz, it was 20 pF but it changed at 1,000 MHz.. It became some other value. Just enough to mess up the project..
I had to sort caps and inductors, to get the right values for my filters..
Some of the SMD stuff is made for low freqs and do poorly above 100 MHz.

Here's some common caps.. Far from perfect.

 

[ab cd]

So the LC series resonant circuit, which is excellent tuner/filter up to HF, gradually becomes poor as frequency goes up, and finally becomes unreliable/unpredictable at Ghz. All this mainly due to lead inductance!

So the option left is SAW (surface acoustic wave) or Cavity filter, or 1/4 coax stub filter. I understand Flightaware filter is a cavity filter. They have chosen cavity over saw mainly due to cost considerations.

 

[Richard Lee]

That Flightaware filter looks too small to be a cavity filter. I once made a filter for 1296 MHz and it was pretty large..
One like this type.. http://www.changpuak.ch/electronics/Coaxial_Tank_VHF_Filter_Designer.php
It worked pretty well. I think it's still around here. I wonder if I can re-tune it to 1090?

 

[ab cd]

Have you seen my last year's "brain wave", the "Double Whip Filter"?

http://forum.planefinder.net/posts/5650/

 

[Richard Lee]

I just took a look.. That is an interesting way to make a filter using to 1/4 wave resonate elements.
It seems like the impedance matching would be difficult to get done.
If you did the same thing with a pair of 1/4 wave ground planes, inside a larger can (or box)
coupling them together, without wrecking the input/output Z match..

I just googled interdigital filter and looked at the images.
There are only a few ways to I/O, and yours looks a bit different.

If I was building that kind of rig, I would start off with extra long elements,
and then start cutting them shorter and shorter, until I got the best match to my cable..
Is that how you did it?

 

[ab cd]

I started with 70mm whips, with idea to gradually trim to 65mm. Unfortunately my noise source failed immediately after the first measurement (too hot a chip). The experiment halted, and no further progress. I am now considering to purchase a low cost signal generator, sweep it around 1090 mhz, and a software to plot the spectrum. I will restart the experiment when I get some reliable equipment. The $22 noise source seems to be a waste of money. Others also complained of very high chip temprature, but they were lucky their unit did not fry like mine.