Tweaks

[suffolk3000]

Heres my output it dont look good to me

[caius]

@suffolk3000

i have a join in loft to living room where rtl lna is
sooo cav up top and lna in loft

Ideally you’d put the cavity filter before the LNA because it is very low loss and protects the input of the LNA from strong out of band signals, and have them both as close to the antenna as you can - directly under the antenna if possible.

If you already have a join in the loft, then it would make sense to put both the cavity filter and the LNA there. The overall signal strength at the receiver will be similar but you will get a lower noise factor.

@suffolk3000

its my aerial man comes £70 + each time so i don’t call him often

Personally I wouldn’t spend the money just to make that change - if you end up having to call him to repair damage or something like that in future, then maybe do it then. If you are using decent low loss coax to the loft already though, it won’t make a huge difference. The advantage of being able to readily access the LNA should it fail from lightning or static or something might outweigh the small performance benefit.

@suffolk3000

just a last note output is same with splitter 1 into 2 as without splitter

So this does at least demonstrate something - you are putting a 3dB loss (which halves the power reaching the receiver) in line and it makes little to no difference to the results you are getting. The nature of mode-S/ADS-B is such that you aren’t really trying to pull signals out from the noise floor most of the time.

@suffolk3000

Heres my output it dont look good to me

Most of that stuff is not going to be an issue. As long as the pre-amp is not getting overloaded by any of it then the airspy will cope fine with that.

Here’s a wide scan I did of my receiver, which has the LNA on the mast:

I have these annoying images every 24MHz across the whole band:

I’m pretty sure the culprit is actually here:

It’s the control and data channels for TfL’s digital trunked network for the buses. They seem to be using a nuclear powered transmitter as it’s by far the strongest signal I can see here - way stronger than any of the broadcast FM stations. That screen shot is from an rtl-sdr with the gain set to 0, using a very small rubber duck antenna lying flat on the floor inside. I’m fairly sure the images are coming from the LNA input being overloaded rather than the airspy, despite the built in input filter, but it’s not having much noticeable effect on adsb decoding.