Scenario:
I have a DC-DC charger in my car and it causes RFI in my HF radio.
The DC-DC charger is fed via 12volts from my caravan batteries which are charged by solar panels on the roof.
How can I eliminate/reduce this noise?
Consider the following:
The interference is usually coming from the switch-mode voltage controller on the output of the solar panel. These can be extremely noisy.
Sometimes the noise is RF energy being fed back into the panels themselves and they act as antennas for the noise source. Another possibility is the output ripple from the solar controller is extending into the vehicle electrics through the DC power rail.
On the solar panel side of the regulator, feed the panel cables through some large ferrite rings. If you can get about 3-4 turns of the wire through the rings it should help quite a lot. This will provide a high-impedance path to the noise, while not effecting the panel output current.
On the output of the solar panel controller, you may resolve the issue with a commercially made filter like this one from Jaycar:
https://www.jaycar.com.au/10a-dc-noise-filter-improved/p/AA3074
Another approach if you wish to build something for yourself, is to create an Active Filter to go in series with the inverter output. There is an excellent discussion on this method in this You Tube video:
https://www.youtube.com/watch?v=wopmEyZKnYo
Lastly, don't forget to consider adding ferrite rings to both ends of your coax feedline, as what is known as 'common mode noise' can be picked up by your feedline.