Min range for ADSB

If reducing gain does not solve the problem, then most likely it is the antenna’s radiation pattern which is causing this.

Normally a high gain antenna has two horizontal lobes like infinity symbol ∞ . However if antenna is not properly tuned, the two lobes may either get pulled up or pulled down. In your case the lobes seem to be pulled up, causing nearby planes to disappear.

This can be tested by installing the antenna upside down, making upwards lobes downwards.

If it is hard to install antenna upside down, then other easy alternative to test is to make a 1/4 wave antenna like “Quick Spider” or “Cantenna”. Both have horizontal lobes and dont need any tuning/trimming.

Most likely you will be able to see planes on runway/landing/take off with these 1/4 wave groundplane antennas. These are very easy to make and cost almost nothing. No harm in trying.

How to Make CANTENNA