After few years of sitting on top of my apartment building I decided to bring my Dunax GPN-4B antenna down for repair as top capacitive hat was broken. I found out that there actually no technical details about fixing it. So I went through some work of gathering such information.
First I found out that manufacturer is out of business. Antenna is not in production any more and manufacturer site is down. I asked help from fellow hams, some sent me some info they had, some sent me contact info for manufacturer.I also used my own antenna to take measurements, so finally I got quite complete details that I believe could help other fellow hams trying to repair their own antennas but also ones that want to build similar antenna.
Dunax GPN-4B is quite interesting vertical antenna, covering four bands: 7, 14, 21 and 28 MHz. Radiating element is constructed traps and for each band there are two wire radials. Antenna was meant to be used both as stationary and portable (when collapsed it is just 1.1 meter long).
It is constructed using aluminum tubes that fit each other. Length of bottom and top segments are meant to be adjusted by sliding tubes into each other. Antenna is supposed to be fixed at console at the bottom and it can keep up with quite heavy winds.
Damage it suffered was actually not surprise. Top capacitive hat was made of aluminum sheet 1 cm wide and 80 cm long, attached to top element at the middle. It did not like as it could last long in harsh conditions. However, I believe it were not conditions that broke it, but birds. They simply like hanging on anything and horizontal element on antenna seems ideal standing point for them.
When I brought antenna down I found damage on one of elements. It seems water found way to get in one of the tubes, then iced and caused crackles on tube from inside out. Everything else on antenna was as good as new.
While I had antenna down I measured traps using antenna analyzer to get resonating frequencies. I guess it might help someone. I could not take other technical info on traps as that would require me to break them open, which I did not want to do.