|Fig.1 - Withdraw the radiating element carefully to avoid damaging the matching components|
A few years ago I bought a secondhand Diamond X50 dual band (144/430) antenna at a radio car boot sale. From the condition, it looked like it had been installed outdoors for a few years, so I got it at a good price. After cleaning it up and testing it, I installed it on the side of my house on top of a straight pole. It performed well on both the 2m and 70cm bands. A week or two later I started to notice an occasional knocking sound inside the house. It was difficult to work out where it was coming from but I found that it was more noticeable when it was windy outside. Why had I not noticed this before? Could it be something to do with the new vertical aerial I had recently installed? I checked the wall brackets and mast, but all the fixings were nice and tight. After doing some research on the internet I eventually came across something about rattling radiating elements in Diamond vertical antennas. It turns out that after a few years of use, these antennas start to rattle in the wind. The radiating element and its associated phasing coils are made of copper rod. A fibreglass tube protects the radiating element from the weather. When these antennas were manufactured, bits of foam rubber were wrapped around the copper element at several points. This stopped the elements from rattling inside the tube. However, after a few years the foam perishes and the antennas gradually become prone to rattling in the wind. In my case the vibration was being transmitted into the wall of the house via the mast and wall brackets. I managed to dismantle the antenna by withdrawing the radiating element through the bottom of the tube. This was initially very difficult as the parts were corroded and seized-up. Once removed, I could see small traces of the original foam on the radiating element. I cleaned this off and carefully wrapped sections of the radiating element with ‘bubblewrap’ held on with insulating tape. Since bubblewrap in mostly air I figured this would not detune the antenna at all. When the antenna was reassembled and installed, the rattling was no more.
Recently the rattling has come back, so it was time to take it down and do some maintenance.
|Fig.2 - X50 antenna before cleaning and maintenance|
- Give the antenna a good clean if necessary.
- Remove hex head screw and small grub screw. Take hold of the SO239 connector and carefully withdraw the radiating element from the tube. Be careful not to damage the matching components or bend the copper rods and coils. You might need to use pliers to grip the connector if it is tight. Be careful not to distort the connector or damage the threads (you could wrap some cloth around it first).
- Check there are no fractured solder joints on the ceramic disc capacitors.
- Check for foam rubber around the radiating element at strategic places. The chances are that this has perished if the antenna is old. You could replace this with something similar if you have it. As mentioned earlier, I used ‘bubblewrap’ wrapped around and secured with insulating tape. If you pack it out too much you will have difficulty getting it back in the tube. If you do not pack it enough it will be loose in the tube and may be prone to rattling. Figs 3 and 4 show the radiating element before and after the packing has been added.
- Carefully insert the whole thing back into the tube and secure with the grub screw.
- Give the antenna a bit of a shake. Does it still rattle?
- The lower metal tube (with the mounting brackets) can be fitted after the coax has been connected. It is held in place with a hex head screw. I also sealed the join with a little RTV silicone sealant and some self-amalgamating tape.
- After installation check the VSWR across the 2m and 70cm bands.
|Fig.3 - The radiating element|
|Fig.4 - The radiating element with Bubblewrap|
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