FlightAware Discussions

FlightAware ADS-B filter VNA measurements

I thought some of you might find this interesting.

I swept a FlightAware 1090MHz mode S filter across my VNA at work

NOTE:
S-parameters describe the input-output relationship between ports (or terminals) in an electrical system.
In general, Snm represents the power transferred from Port m to Port n in a multi-port network.

If we have 2 ports (or terminals), like a filter with two terminals, or a coax with two ends between antenna & receiver, generally called Port 1 and Port 2, then:
S12 represents the power transferred from Port 2 to Port 1.
S21 represents the power transferred from Port 1 to Port 2.

S11 represents how much power delivered to port 1 is reflected back to power source, and hence is known as the reflection coefficient or return loss (sometimes written as gamma Γ). If S11=0 dB, then all the power is reflected from the port 1 and nothing passes to other ports. If S11=-10 dB, this implies that if 3 dB of power is delivered to the port 1, -7 dB is the reflected power. The remainder of the power was “accepted by” or delivered to the port 2.

If anyone is interested in digging into the data, I am including a link to the s2p files

Enjoy

dropbox.com/sh/c6zgwzyqonfo … b_4Ca?dl=0

No image showing :frowning:

Sorry about that. Try now.

Yes, it is showing now.
Originally you have linked image from Dropbox, but dropbox does not allow hot-linking, and that is why it was not showing.
Now you have linked from Imgur, which allows hot-linking, and that is why it is showing now.

Thanks for uploading VNA output.
Very informative, specially the image, as very few persons have access to a VNA, and can only view the image.

Quite welcome. I am happy to help where I can. I have a new LNA4All coming on it’s way, I was planning on sweeping it the same way on the VNA
Fortunate enough to have access to a 4-port 13.5Ghz Agilent N5221A PNA. Kind of nice to be able to play with $100k toys every now and then.

My plan was to sweep the LNA on it’s own, with the filter on the input, and with the filter on the output. No idea if there will be a giant difference, but it should be mildly interesting anyhow. My LNA will be bias-T powered, so hopefully I can still feed external power.

Are there any other measurements that would be useful to see on the image? Not everyone intuitively understands what S11 etc means.

Waiting for results.

Lucky! I am planning to purchase one when I win a lottery :mrgreen:

Adding the note below should be enough.

NOTE:
S-parameters describe the input-output relationship between ports (or terminals) in an electrical system.
In general, Snm represents the power transferred from Port m to Port n in a multi-port network.

If we have 2 ports (or terminals), like a filter with two terminals, or a coax with two ends between antenna & receiver, generally called Port 1 and Port 2, then:
S12 represents the power transferred from Port 2 to Port 1.
S21 represents the power transferred from Port 1 to Port 2.

S11 represents how much power delivered to port 1 is reflected back to power source, and hence is known as the reflection coefficient or return loss (sometimes written as gamma Γ). If S11=0 dB, then all the power is reflected from the port 1 and nothing passes to other ports. If S11=-10 dB, this implies that if 3 dB of power is delivered to the port 1, -7 dB is the reflected power. The remainder of the power was “accepted by” or delivered to the port 2.

That is a good explanation for what S parameters are. Are you suggesting that I put that note with the shared photo?

When I get the LNA, are there any specific measurements that you, or anyone else, would like to see? I don’t know that I have the ability to measure noise figure, but smith charts, impedance, gain, SWR etc are all easy enough to graph. Let me know what you would like to see, and I will do my best to accommodate.

Yes, you can put that note with the photo if you like.

I will appreciate if you make and test the concept I have posted few days ago:
post205572.html#p205572

Try different horizontal spacing between two internal whips from 1 inch (25 mm) to 1/2 inch (12mm)

If you want to send the o e you made, I would be happy to test it.

I discarded it long ago after my only test equipment (Noise Source) fried itself while I was trying to test it in June 2015. :frowning: :cry: :angry:

post171174.html#p171174

post177222.html#p177222

.

looking at the plot, for a cheap filter its not too bad.

Ins Loss of 1.9dB at Cf, 3dB points at roughly 950 and 1200 MHz rolling off to >40dB down at 900 and 1250 MHz approx.

For blocking insidious things like 148 and 450MHz POCSAG pagers, DTV, FM radio and some cell phone stuff, its good “bang for your bucks” material. The only way that you are going to get better specs is as what people have mentioned before - a lot more expensive cavity filters.

Thanks FA for a cheaper alternative!

I agree. 1.9db of loss isn’t great, but considering how much this filter costs, its not all that bad.

Now the best question will come when I try to determine if it’s better to put the filter ahead of the preamp or behind it. My initial guess would be to put the filter ahead of the preamp, to prevent the preamp from being overloaded and desensed.

I am thinking of using my RTL-SDR to “sweep” like a spectrum analyzer, so that I can see how much local interference I actually have.

rtl_power is an existing tool that will do this

Thank’s for the heads up! I am doing a sweep now. It’s really cool how communities like this get together to help each other out.

The factory specs on the FlightAware ADSB filters are 2.5dB or less insertion loss but QA testing on our side shows that 95% are below 2dB. With the majority of units (a bit over half) closer to 1.6dB .

Here are some pictures of the insides of the FA ADSB filter.
post202357.html

We did spec out a cavity filter that we can retail for $40-50 but we weren’t sure if people will be interested.
Specs on the cavity filter are 0.3 to 0.4dB insertion loss and >80dB of rejection.

The feeling was that about 1dB isn’t a lot for the large price increase. Also it is fairly easy to lose 1dB in many other places when installing a site.
We had a few post on this forums about users that have tried cavity filters and it is mixed if they got an improvement or not.

BTW, we did pass on a price drop on the FA ADSB filters.
camelcamelcamel.com/ADS-B-1090M … ext=search