Wednesday, November 11, 2015

I0QM Balun for 70MHz antenna

Another design of real balanced-unbalanced unit, this time for 4m - 70MHz band.

Previous post gave us some general idea about I0QM balun, this time some pics and "making of"
4m version.

Starting with accurate cable measuring, we need to have 1/4L and 3/4L coax lenght.
This time I have spare parts of Italian RG213 C.T.E International cable.

Slightly smaller at diameter than ordinary RG213 MIL-C type, but more flexible.
Of course another cable will have different velocity factor.
Using this cable lenght of 690mm were cut for 1/4L at 70.200MHz, giving X=0,5 Ohms at given frequency.
The other part is 3* 690mm. For those unfamiliar with coax baluns, "lenght" is a distance measured at shield of a cable,
not his inner conductor.

Before assembling the cable I did some test, here's analyzer output of SWR and RL.
Balun was terminated with non inductive 50Ohm.

Additionally small ABS box, N-F connector and ~80g of expoxy resin will be suitable for our project.

The box has needed holes drilled, one for N connector, another end two parallel holes for coax cables.
This will be output end of balun.

After accurate cutting 1/4L and 3/4L of coax cables shield were soldered each end
and slipped into box. Hot silicon glue was added to seal cable before applying resin.

Next, small piece of soldered metal pad was added to give shield connections with N connector.
Next step was adding epoxy resin to seal all the connections and make them waterproof. And the other end with soldered M4 joints sealed with self amalging tape :

Here's assembled balun, ready to use.

73 & GL!

No comments:

Post a Comment