TL;DR

Advice on improvements to the installation of a Diamond D777, connected to a Uniden Bearcat UBC360CLT scanner.

In the photo below, ignore the white antenna for now (and the focus) - I’ve staged it to show the the experimental ‘broomstick’ antenna I cobbled together using an old roll of 75 ohm RG6 coax, stripped to form a crude dipole, an ugly balun for good measure, them led that down to the Uniden’s 50 ohm input. Not ideal, but this was able to receive many local aircraft towers, and aircraft transmissions from 100 miles away. Also, the occasional marine communication.

Encouraged by this, I replaced the wooden effort with a purpose-built Diamond D777 antenna, on the advice of an independent radio shop. Another pic:

This is connected to the Bearcat with 20m of the correct 50 ohm mini 8 coax.

The performance is not that much better than the Heath-Robinson affair. Low aircraft come in a little clearer, but it’s otherwise much the same, and doesn’t seem to get any marine traffic. So, all I can think of doing is lofting it a little higher, to get a better line of sight and perhaps take those ground radials away from the metal chimney cap. Is this worth doing, or are there better solutions such as a different antenna?

TL;DR: Advice on improvements to the installation of a Diamond D777, connected to a Uniden Bearcat UBC360CLT scanner.

frequencies between 25-960 MHz

It looks like the Diamond D777 covers the 118 - 136 & 225-400 MHz aircraft bands:
https://www.hamradio.com/detail.cfm...9kMgbEYPnzZaVB7kC8mYPP0HKccDE2eTxAmtX3ybzfzhH

That's a pretty wide band to be using a single antenna. Have you looked at multi-band antenna options? Or maybe using an RF switch to switch in different antennas?

It looks like the Diamond D777 covers the 118 - 136 & 225-400 MHz aircraft bands:
https://www.hamradio.com/detail.cfm...9kMgbEYPnzZaVB7kC8mYPP0HKccDE2eTxAmtX3ybzfzhH

Although the Bearcat will do 25-960 MHz, it misses out the 174-476 MHz range, for some reason.

Inverness Airport Air Traffic Control, ATC Frequencies. AM.
Tower: 118.400 MHz. Approach: 122.600 MHz (You will hear these most often.)
ATIS: 109.200 MHz. Fire: 121.600 MHz
To identify local activity use https://www.flightradar24.com
or https://www.flightaware.com/live/
or https://globe.adsbexchange.com/

Most marine voice traffic will be on VHF Channel 12: 156.600 MHz, FM.
Listen for phase modulated digital traffic on the two AIS channels.
Each moving vessel will transmit every 2 to 10 seconds.
Channel AIS 1 : 161.975 MHz
Channel AIS 2 : 162.025 MHz
Use https://www.marinetraffic.com to identify local traffic.
https://en.wikipedia.org/wiki/Automatic_identification_system

Thanks for the suggestions for frequencies. I have a list of those I can already receive:

118.400 Inverness tower
122.600 Inv approach
126.925 Aberdeen radar
119.875 Low-altitude airspace info
127.275 Scottish control
122.750 RAF Tain
134.850 Air info
119.575 Lossiemouth
119.480 Stornoway tower
119.050 Aberdeen Dyce radar

I can’t get 109.200 Inv ATIS weather on either antenna. Marine came in intermittently on the dipole, but nothing on the D777.

Horizon in nautical miles = 1.25*√(Antenna height in feet)

Which for your setup gives:

11.1nm = 1.25*√(79ft)

So what you're receiving makes sense regardless of transmission power or gain. I'm not convinced the metal chimney top will have much (if any) effect either.

Also, if you're within 3-4 miles of the coast, any maritime traffic is going to have to be within ~10nm of the coast which for a lot of non-leisure traffic could be quite close depending on exactly where you are. What sort of maritime comms are you receiving? It may just be that there isn't anything there to receive? Have you cross checked with AIS?