In 2009 I bought myself 4 antenna's from
I0JXX the 25 elements 70 cm beam
I wanted to build up a antenna group with it to be used on tropo and EME.
After unpacking my first impression was
that the antenna was nice build and looking fine. I started assembling the
antenna.
After my first antenna was assembled I
wanted to measure the return loss but then I found it impossible to get a good
and stable return loss.
After some more measurements I found that
the balun was not working as it should and that there were strong RF currents on
the outer coax cable. This was clearly visible when I moved my hands over the
coax cable, even well outside the hot radiation area of the dipole.
I contacted the distributor were I bought
the antenna and after some more e-mails exchange I became in contact with I0JXX.
Conversation with I0JXX was constructive but very difficult.
After I received some new dipole feeds I
decided to open it up to see how it was constructed. This was a hard job because
its filled with resin which is very strong.
With some brute force and patience to be
sure not to damage the construction I realised to open the dipole feed
construction. After it was opened I found the problem why the balun from the
antenna was not working. It was manufactured like it was an HF antenna. For a 70
cm antenna this construction is absolute no good.
Antenna with original Dipole feed, impossible to get a stable and
good return loss
The dipole feed box opened with resin still inside the box
Some of the resin removed, clearly visible the wires connecting the
cable ground
These wires are already big inductors on 432 MHz
The ground connection from the N-connector is done with a thin strip
of copper
The Balun is not working correctly
The construction of the balun opened, is build
like a construction for low frequencies, not suitable for 432 MHz
Small wires working as inductors coupling to each
other and a ground connection with high inductance coupling to the other
wires.
Total result is an unbalanced dipole and therefore
mantle currents on the coax cable
The dipole with a home made balun construction, worked much better.
No longer unbalanced and less problems to create a good and stable
return loss.
This is my construction to build a waterproof setup with a good
balanced balun.