I0JXX 70 cm antenna


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.
Return loss of the modified dipole box

26 dB return loss on 432 MHz