Saturday, January 4, 2014

6m omni-directional antenna

6m omni-directional antenna
(horizontally polarised)


Concept

The design follows the idea of the so called "CobWebb-Antenna" for the high frequent shortwave bands. The basic concept of this idea appears to be a folded dipole made of twin-lead  which is then bend in a circle of rectangle. Basically the rectangle seems to be the simpler option.
The folded dipole itself and the way to calculate it's dimentions is well described in the literature, i.e. Rothammel. The idea is to make this dipole from standart twin-lead used for electrical home installations. The remaining problem: What is the velocity-factor of you twin-lead.... I guess you will have to measure it (should not be a big deal when following one of the many articles on that topic).


Dimensions used for 50.15 MHz

Material used:
2 x 0.75 mm²
Total length of twin-lead:
293.2 cm
Length of one dipole leg:
146.6 cm
Distance feed to short:
88 cm

There certainly is some room for optimisations of the dimensions given above... any volunteers?


Photographs

... say more than 1000 words ...





Feeding the aerial using 50Ohms coaxial cable. One of the two leads of the aerial is cut open, the other remains as is (if you are working carefully enough).


Cable ties to close the loop mechanically, leaving the possibility to apply a little more tension at the very final stage.


For the short-circuit bridges just scratch of the insulation, do not hurt the leads below. I used a little bit of solid copper conductor to join both wires electrically (one twist and a little bit of solder will do too).


Cable ties again... Here they are forming the all so important cross structure, together with some bamboo poles.
The feeding RG58 and the fibre-glass mast will go through the large looped cable tie.

Next to cable ties and luster terminals I really do like to use duck tape. This way the edges easily attach to the bamboo poles of the supporting cross.


Experience

Smooth SWR about 1:1.1 measured by my IC703. The beacon PI7SIX (JO22nc to JO22eb) tuned in nicely...


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