There is now a part 2 to this article which covers replacing the capacitors found in the antenna, if you’re more interested in that aspect of repair, please have a look here. Otherwise, read on for my solution to a broken Radome.
Background, Specifications & Waffle
At the start of my adventures into Amateur radio, I was lucky enough to be given all kinds of gear by the very friendly VK3HBN as an incentive to getting licensed – after all, who can sit by idly looking at lots of toys that they can’t play with?
One of these items was a lovely (read expensive) Diamond X510MA Dual Band Base Station Vertical Antenna. The specifications are as follows:
- 8.3 dB Gain on 2M (3x 5/8 Elements, Centre Loaded Co-Linear)
- 11.7 dB Gain on 70CM (5x 5/8 Elements, Centre Loaded Co-Linear)
- Max Power 200W FM (and more than likely SSB, remembering that phasing capacitors have Vpeak limits)
- 5.2 Metres tall with 3x 52cm Radials at feed point
- 3 Piece construction
There was however, but one catch
The antenna had suffered a slight mishap. Although details are scarce – perhaps to protect the reputations of third parties – the bottom section of the antenna had suffered a clean break as the consequence of a fall. While the antenna had previously been repaired by placing a section of 32mm PVC pipe over the break combined with some PVC Adhesive and ‘gaffer’ tape for sealing purposes, the repair had come loose and was looking somewhat worse for wear – I decided it was time for a complete overhaul. Details Below.
Dis-assembly & Inspection
Being a modular design, the Diamond X510 is quite suited to servicing. The following procedure is a guide based on my experience:
- If your antenna is assembled, you should break it up into the 3 sections as per the Instruction Manual.
- Taking the utmost care, remove the internal element assembly from the 3 radome sections of the antenna. The top and middle sections can be easily slid out (I recommend from the bottom part as it has a slightly larger diametre due to taper), however the base section is retained by 2 grub screws. The best way to remove the element from the base section is to remove the grub screws, and slide the element out by pulling on the UHF / M Connector. Take care not to damage the foam spaces that keep the elements central in the radome.
- Visually inspect the element sections – check for any kinks, dry joints or signs of water damage.
- Optionally, if your antenna is has been suffering high SWR or other performance issues, test the ‘disc’ capacitors that are scattered along the length of the elements. Many HAMs have reported issues due to failed capacitors (in most cases caused by applying too much RF power). You should probably use an LCR Meter for this, and test at the frequency specified by the manufacturer (its usually 1KHz for disc capacitors if I recall correctly).
My Antenna had a kink near the base of the antenna, just above the LC Tuning network directly above the UHF / M Connector. I can only assume the kink was the result of damage, as the bend did not match the quality of the work displayed in other sections of the antenna. Additionally, all bent sections such as coils are made from copper, not brass, in the X510. Although speaking to the original owner didn’t yield any clues as to how the damage occurred, I straightened the section out as best I could.
As previously mentioned, the bottom section of the antenna’s radome had been cracked in half, and the previous repair attempt had started to fail. For a while, I couldn’t figure out ‘the perfect way’ of repairing the section, so the antenna sat disassembled in the workshop for months, like so much HAM gear does, while I waited for the eureka moment!
Luckily, it did! I had found out previously while storing the antenna that the element would easily fit into standard 20mm conduit. Unfortunately, 20mm PVC conduit is completely useless as a housing for a 5.2m element (it wobbles like crazy), and further investigations lead the conclusion that no PVC Conduit could provide sufficient rigidity. HOWEVER, if I used a small section (~600mm) of PVC conduit to internally sleeve the fibreglass radome, an aesthetically pleasing result could be obtained.
I could only find one type of conduit suitable for this repair – ‘Telstra’ P20. Standard 20mm & 25mm electrical types are too small and two large respectively (plumbing 20mm might be ok). Telstra P20 has an internal diameter of ~23mm, and is available from most Australian Electrical Trade Retailers, I got mine from J&H.
- Clean the inside & outside of the broken radome sections. This is critical to obtain good bonding between the conduit & the radome.
- Test fit the conduit into the bottom broken section of the radome. The bottom section should have a slightly larger diameter (both inside & out), so you should be able to slide it down all the way into the base. To confirm this, look into the hole where the UHF / M Connector mounts up, and you should be able to see the Conduit. Make note of where the conduit exits the broken section, so that you remember the length required.
- Test fit the conduit into the top broken section of the radome. You will not be able to get it to slide all the way up to the join point of the next section, but hopefully you will be able to get at least 200mm worth of overlap. The more the better really – aim for a nice tight interference fit.
- Add the two measurements together, and cut the required conduit length. Clean the cut length!
- Quickly check that both halves of the radome as still clean, and that your cut length fits.
- Apply your adhesive of choice to the inside of the bottom piece of broken radome and a small amount to the outside of the pvc. I used Multipurpose clear silicone, partly because this would allow me to disassemble the antenna in future if needed, and also because I had some. Push the conduit into the base section & clean up any excess adhesive present at the connector mount point (it is fine to leave some on the outside of the conduit for the moment). The adhesive does not have to be especially strong, as long as you have a good tight fit & a decent amount of overlap – remember, a vertical antenna should not ever be under large tension (pulling) loads, the only tension component that should arise is that caused by wind induced horizontal deflection (eg, the antenna bending in the wind).
- If you had previously achieved a nice tight interference fit for the top section of radome – which you should have – you only need to place a small amount of adhesive near the top of the conduit. Slide the top section of radome on, and ensure you push it all the way down to the joint.
- Slide a piece of heatshrink over the radome, preferably double wall glue filled about 150-200mm long, and shrink over the join for a nice, clean, good looking water tight finish.
- After the adhesive dries, reassemble your antenna. TX if licensed!
I hope this is of help to someone else,