Jaycar have some RS have them. I prefer 0.0047 to ground, not all have them. The last lot I got had 300V~ (AC) rather than 275VAC. These are normally decorated with UL & various other standards associations stamps, on them.

If you are getting the changes in signal strength noted then you have an inferior aerial feed. I would go so far as to get one of those little plug in testers & check all of the power points, especially earth. The antenna needs to be around 25 feet: But! It needs to ground.

If the set has no earth wire then it is reliant on the mains, or a separate one (some sets have an isolated antenna coil primary). The Americans relied on using line caps on non grounded / non groundable sets (hot chassis). But that meant that the cap could be on Active, bleeding charge onto the chassis. This relied on Neutral being at earth potential & often is tied to it. No so now with RCD's : Load side.

A transformer set can be mains earthed.

As I get a lot of Lightning & other spikes I have MOV's on some stuff across the mains & one active to ground. Some power boards now do this. That is designed to cause the MOV to trip & that's not uncommon here. I am waiting for the AusNet transformer (22kV) to go up in smoke as its leaking its toxic oil out (they have looked at it).

Hi, mine is mains earthed and I have a long wire antenna. The radio seems to "shift gear" for want of a better analogy, where the volume and sensitivity jumps up an appreciable notch, after half an hour. Today I pulled it apart and removed the screws holding the shield can base and sanded the contact to chassis on both can base and chassis to get continuity, I did this also on the inside of the base and outside of the can where they meet and also the top. I put it all together and was listening to it thinking it was cured, then after half an hour, Wheeeeeeee! off it went like a frog in a sock, but as soon as I touched the wire from the coil canister to the valve top it stopped, to resume as soon as I let go. Just off the top of my head at a wild guess, I got a piece of aluminium foil from the kitchen and scrunched it around the wire and it hasn't played up yet, seems to be sorted, touch wood! The wire must be crook, but the aluminium foil seems to be shielding it and has stopped the oscillation, so hopefully it'll behave now!