Fan Dipole Rebuild - G7SYW's amateur radio bits & bobs...
 

Fan Dipole Rebuild

| Posted in Antennas

Rebuilding the fan dipoles

I purchased some Flex-Weave antenna wire with a view to replacing the thin hook up wire with something a bit more substantial when the weather and station manager would allow.
Flexweave antenna wire

Flex-weave antenna wire has a high number of copper strands (either 168 or 259) and is constructed using a method that makes the finished product similar to rope in appearance. It is sometimes referred to as “wire rope”.

The advantage of this type of wire is that under normal conditions it will not kink and can be tied in knots and untied a number of times.

In the UK this wire is available either “bare wire” or “plastic coated” – I’m using the plastic coated type which helps prevent tarnishing through weathering as I was quite near the coast.

The date is Jan 31 2016, the weather was dry and not too cold and as a bonus, the station manager agreed to the planned works. It was time to go outside and rebuild the complete fan dipole antenna system.

After researching information on common mode currents (see ugly balun) I decided that it may be better to feed the dipole with a balun instead of using the huge unsightly coil of cable hanging from the sky!

Some experimentation constructing different baluns including bifilar and trifilar torroid wound, air cored and coax wound on a torroid core. The core I used is a FT140-43 type.

While the other methods of constructing a balun were perfectly within acceptable limits, the impedance wasn’t even across the frequency range.

The reason for the differences in impedance is due to whether the balun is acting as a voltage transformer or as a current transformer.

In my wisdom (or otherwise) I came to the conclusion that it may be easier to tune the fan dipoles using a balun with a flat impedance.

As my feeder was mini-RG8 coax, I simply wound a torroid core onto the feeder a few inches from the end of the coax that would be at the antenna feed-point.

This type of balun works as a common mode choke in the same way as the ugly balun and should be referred to as a current transformer as opposed to a voltage transformer.

To house the balun for protection against the weather I used two 40mm solvent weld plastic
waste pipe access caps and a 40mm solvent weld connector. The access caps have screw-on caps which
make it easy to assemble and install the balun.
RF common mode current choke

A hole was drilled in each cap for the coax, the bottom hole is 1mm larger to allow any moisture to drip out. The top cap was drilled to the same diameter as the coax and sealed with a small amount of sealant to prevent water ingress when finally assembled.

The coax leading to the dipole feed point is kept short (approx 4 inches) preventing the coax between the antenna and balun from radiating.

The photo was taken before the terminals were sealed against the elements with self amalgamating tape.

Many thanks go to the station manager for her assistance when asked to “hold this for a minute” and for
refilling my tea mug at regular intervals!


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