Weather is not kind to aluminium antenna parts, often giving rise to corrosion that may result in high resistance joints that then reduce radiation efficiency.
It is good practice to document antenna behavior at installation, and through life, measurements can be compared to that benchmark to possibly reveal changes resulting from corrosion. See Diagnosing a possible antenna problem by comparison with a baseline for more discussion.
WARNING: wear eye protection when using any of the appliances mentioned in this article.
Corrosion preventative compounds
Above is the kind of structure that often develops corrosion in the overlapping / clamped tube sections. They can be recovered by cleaning the corrosion products out, and reassembling them with purpose specific conductive waterproof grease containing irregularly shaped shavings of zinc. The way in which it works is that the sharp particles of zinc penetrate the insulating oxide layer on clean aluminium providing a low R conducting path, and the grease prevents ingress of water and oxygen, so preventing corrosion.
Some forms use a silicon grease base for applications where mineral grease is incompatible with wire insulation, but they are more expensive.
Whilst these pastes are very suited to low pressure joints between clean aluminium parts (such as in the assembly above), a good waterproof grease is quite suited to high pressure joints (like stainless fasteners in aluminium as shown below). The grease serves to prevent ingress of water and oxygen. Eliminate the electrolyte, and you eliminate electrolytic corrosion… even for metals well spaced on the galvanic table (like aluminium and metals like stainless steel, copper, brass).
Online experts often opine that grease is an insulator, surely this cannot work. It is naive to think that even a polished surface does not have some surface irregularities (asperities), brushed surfaces more so.
The diagram above illustrates the nature of surface to surface contact at asperities even if the surface roughness is just microns. As pressure is applied, these asperities will deform somewhat, contact area increasing with pressure and contact resistance decreasing. With moderate contact pressure if grease is used, it is squeezed out and metal to metal contact occurs.
The advantage of the compounds with irregular zinc particles is more effective ‘punching’ through the very thin (if recently brushed clean) oxide layer. Waterproof grease alone will work better than nothing, and works quite well with high bearing pressure joints.
Note that corrosion preventative compounds whether for joining conductors or fastener anti-sieze application may be suited to only certain metals joined, eg copper bearing compounds are not usually recommended for aluminium to aluminium joints electrical joints.
Fasteners are often frozen from corrosion, especially threads in aluminium, and especially plated steel fasteners.
For decades I have used this DIY penetrating fluid… mix half ATF (auto transmission fluid) and half acetone, shake it up real well and apply that to the frozen threads.
Above is the penetrating fluid in a fully sealed dispenser (a few dollars on Aliexpress). If the container is not sealed, the acetone will vaporise and escape, even with this sealed container and cap, the small molecular size permits slow escape through the plastic bottle. You could equally use a small pipette or hypodermic syringe, only a very tiny amount needs to be applied.
Apply the penetrating fluid a couple of times a day for a few days, then try to work the frozen parts out. You might find you can tighten them a few degrees and then loosen by the same amount or more… that is REALLY good progress, apply more fluid and it works in as you work the fastener back and forth.
Heat can help… but don’t do it near plastic, and it needs to be concentrated heat. Best leave this one alone (yes, this IS the reason I mentioned it).
BTW, acetone melts some polymers (eg ABS)… try not to get the fluid on plastic… or wash it off real soon.
If you use stainless fasteners, be aware that they are prone to galling… especially on nyloc nuts… so you should coat the threads with (marine) grease before assembly.
For female threads that are damaged, one option may be to Helicoil them, the repaired thread is usually stronger than the original. Search Aliexpress for “wire thread insert” for generic insert kits (drill, tap, tools and inserts).
Cleaning corrosion products
Sometimes chemical agents may work well, most times my experience is that mechanical methods like stainless steel scratch brushes and small stainless wire wheels work well. Wet/dry abrasive paper may be useful, but clean the residue off.
I never use steel wire brushes, it embeds in the aluminium surface and will discolour and rust. (If you have ever seen aluminium MIG welds with serious black discolouration, the probable cause is that the aluminium surfaces were contaminated with steel from wire brushing.)
I never use Scotch-Brite or equivalent as final polish on contacting surfaces as again, it can embed in the soft aluminium surface and may make a good electrical connection difficult.
Above is an inexpensive flex shaft to suit a Dremel rotary tool. This shows a stainless wheel wire brush, but end brushes and cup brushes may also be useful. (Note that the chuck on this tool uses the common Chinese M8x0.75 thread rather than Dremel’s 0.28″ 40tpi, and so the nuts and collets are not interchangeable).
The handpiece is just under 20mm diameter, so with brush mounted it can be used to clean the inside 100mm+ of a tube with ID greater than 25mm.
This one cost under $20 many years ago, I see them on eBay today for $16 inc shipping from an Australian seller.
There are lots of types of rotary abrasive and polishing pads, brushes, flap wheels etc that may be quite effective for surface conditioning… but pay attention to whether they embed foreign material in the surface. Search for ROLOC pads on Aliexpress for some examples, I have used some of these for good outcome polishing aluminium cylinder heads and pistons on small engines.
All that said, small stainless wire wheels in a suitable flex drive are probably the most useful and relatively low cost.
Another really useful device for scratching corrosion on a small scale is a Bergeon 2834-C Glass Fibre Scratch Brush used by watchmakers for removing rust from train bridges etc. Clones are available for under $10 on Aliexpress. They are very effective, but take care of the glass fibre dust hazard.