Sunday, 25 January 2015

Cellflex/Heliax DF Loop - Hmmm, Probably Not

Whilst sound in theory, the physical construction of a VHF RDF loop antenna using entirely 'hardline' coax has a number of drawbacks, which mean I probably won't pursue the design any further,

Firstly, these types of coax use a solid, continuous outer conductor, which is corrugated.  This makes it very difficult to remove just a little, theres no trimming it off with a pair of side cutters like with braid!

 The next problem, is that the inner conductor is just copper plated. This means it has a huge thermal mass, but very little of the heat you put into it goes towards warming up the copper so you can solder to it!

As one side of this antenna requires a 1/4wave of outer to remain, one end of the 1/2wave length of Heliax was bared back and the outer copper cut and soldered to the inner

Heres the cut in the middle. At VHF, this gap may be too small, and increasing it isnt easy. Likewise, adjusting the 1/4wave section to make a good match but cutting away a bit of the outer at a time, is going to be very tedious. 

But the big problem came here, with the Cellflex handle! Just look at the mass of copper! Even cutting it was difficult, and required the employment of a full size hacksaw

Soldering all this together was a nightmare. None of my soldering irons is capable of providing anywhere near enough heat for this job. Ultimately, the task fell to a butane torch. The whole thing was much closer to brazing than soldering.

And, with the loop now attached, a major structural problem came to view. The inner is taking all the bend strain at the 1/4wave cut.

 Although the Heliax is rigid, it will deform easily if bashed about. With the loop being some foot or so wide, I dont think its got the necessary strength to survive real life use. I will test it, and see if it performs as a loop antenna, and if it does indeed exhibit a single deep null, as to the design. But, I really think this is a dead end project. Indeed, im not yet even sure how the heck i'll connect a feedline to it!

 I think perhaps the 3-element 'Tape Measure' beam will be more successful. If, that is, I can find anywhere that stocks the T and cross pieces!

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