Applying Heat Transfer Paste or Araldite to a Transmitter Board

Thermal paste in contact with any of the circuit trace lines will change the capacitance of those traces. To know how much of a change requires circuit layout knowledge.

Thermal paste does not provide any additional cooling to a system. It maintains the cooling of a system by filling voids between warping heat source and the cooling heat sink. Thermal paste works best when it is as thin as possible between source and sink.

RF power amplifiers are notoriously inefficient devices for valid reasons far beyond the scope of an engineering blog.

My only suggestion is to look for any poor thermal connections between components and heat sinks. (Loose hardware, frozen fan, clogged vents, etc.) It is common to find tiny amounts of thermal compound between source and sink. You might have too little compound between source and sink but too much can be just as bad.

I see that araldite is a collection of encapsulating adhesives. I would not use this anywhere that is getting too hot.

Regardless of what might happen to power, coating the circuit board inside with araldite or thermal paste would cause it to run hotter. Common practice to cool a circuit is to increase surface area and air flow.

Currently there is no contact between the circuit board and the metal casing apart from a solder joint at each corner to ground the board to the casing. I was under the impression that using some form of non conductive heat transfer compound to bring the board into better contact with the casing would result in better heat dissipation because I would also attach a large aluminium heat sink to the casing. I once had a small 10 mw transmitter which I completely encapsulated with epoxy to make it waterproof and it had no negative effect on performance that I could see. The 10mw transmitter only got slightly warm. This transmitter I have is much more complex and seems to have a crystal controlling the frequency.

I greatly agree with Joshi and his/her comment about forward and reverse power. Most importantly about this comment is that the RF energy has to go somewhere. Have you tested the Voltage Standing Wave Ratio (VSWR) between amplifier and antenna. VSWR is an important aspect of antenna theory.

My point here, providing more cooling may hide a problem instead of fixing it.

What is quite a bit??

General comments already given are correct.

I would consider a fan to be fitted [allowing for the possibility of recent problems causing heating] to reduce heating.

It is hot to the touch after a while of use but not hot enough to be painful.