Overhead and it's not there!

[Doodlebug]

I haven't yet been able to confirm this but when an aircraft is directly overhead above one's head does the ADSB signal
from it disappear?   
Due to the reception loss directly above a vertical antenna !

Also the aircraft's antenna,  directly above one, is also badly orientated for you too, adding to the overall signal loss.

So, does the aircraft disappear when overhead I wonder, and slowly return as it recedes from you?  Rather an irony that, if it so.

Albert

[edgy]

...So, does the aircraft disappear when overhead

No, not normally. Mode-S transponders have a lot of power. They are less than 10 miles away at most when overhead (60 kFt).

While the antenna will have 0 dB of gain straight up, the dBm transmitted is large enough, that you want very low gain.

In real radar systems, the receiver has an STC (sensitivity time control) so that lots of attenuation is given to close-in targets and no attenuation as the range increases.

Unfortunately you can't pre-attenuate the receiver on passive systems, so you need a lot of dynamic range to handle 200 W to 200 uW (Watts to micro-Watts). You can see why a lot of dongles with 8-bit dynamic range don't work as well as receivers with a 12-bit A/D.

It may actually take a bit of time for your receiver to recover after a target 5 miles above blasts the receiver, before it can receive a little bitty reply from 150 miles away.

Bottom line, I wouldn't worry about it. There are worse problems...

[Doodlebug]

Thanks for all that, edgy.
It didn't worry me, just rather some of the puzzling thoughts I get from day to day. :-)
Albert