Saturday, February 1, 2014

PE-Xc/Al/PE-Xc aka PEx/Al/PEx the New Copper?

Wanting to build a low profile mono-band antenna, I looked into the old design by J.M. Boyer (U.S. Patent 3,151,328). The DDRR-antenna, as originally designed, i.e. about a quarter wave circumference, allegedly as a rather narrow bandwidth.
There is another variant out there, which is said to got a wider bandwidth. This design asks for a half wave circumference, which the advantage of having a closed loop (less noise!) and no need for an expensive capacitor.

OK, that's a plan, let's think of a DDRR for my flat root top. This antenna would be entirely invisible from the street.
But, what about the B.o.M. (Bill of Materials)? Well, a neat design would call for about 21.34m of soft copper tubing having a 10mm diameter. Well, the stuff exists, however, however, a 25m roll of 12mm soft copper tubing would cost about €185.- presently. Well... that's a bit too stiff or my taste!

The hardware store had also 5m rolls of 12mm copper tubing for about €30.
OK, we are coming closer, I would need to buy 2 such rolls and an additional piece of piping, in order to build a quarter wave DDRR antenna. But, I would need to think of a (variable) capacitor.
Nothing of interest, I decided, and left this particular store.

Next store, maybe their copper prices are lower... no, there were not! However, I noticed some other stuff, which the first store did not sell. The stuff is called PE-Xc/Al/PE-Xc, aka PEx/Al/PEx.
What is that?!
  • PE-Xc or PEx stands for "cross-linked Poly-Ethylene"
Hugh, you might think, is he gone crazy? Building an antenna from PET, the stuff of which fizzy drink bottles are made of?
Well, there is this other material in the and ....
  • Al stands for Aluminium (Aluminum)
The PE-Xc/Al/PE-Xc-stuff is a tubing material made from an Aluminium layer sandwiched between 2 poly-ethylene layers. The interesting bit about this Aluminium layer is that it is supposed to be an Oxygen barrier (cf. wiki) and is therefore sealed by welding (according to many manufacturers).
14mm PE-Xc/Al/PE-Xc
Right, now we know that the PEx/Al/PEx stuff can be interesting for building antennae.

Also good to know, the stuff is very lightweight, compared to Copper tubing. Not only is Aluminium one of the lighter materials, the Poly-Ethylene makes the stuff really tough (10bar of pressure at 95C), such that the metal layer can be much thinner. Mind you, in radio frequency, we are only interested in the most outer diameter of the metal, thanks to the skin-effect.

The price? Alright, time to talk money. I bought a 10m roll of 14mm PEx/Al/PEx for just under €30. Only half the price of Cu-tubing?! Is that worth it? Well, the costs are not hidden in the materials for PEx/Al/PEx, since, a 10m roll of 20mm tubing, the largest diameter this particular store had in stick, is only €35.

Enough about the material itself, you'll find plenty of information about it on the interweb!

A material so light-weight and still stiff calls for designs like hair-needle antennae and alike.

Pretty novel, I thought... NO it is not! There is one design disclosed in the interweb, which makes use of PEx/Al/PEx in a magnetic loop / frame antenna cross-over. Have a look at the "2 TURN LOOP ANTENNA".
What I like about this design is that no electrical connection is made to the Aluminium layer. Why? Aluminium is not exactly a noble metal. Any connection with a metal other than Aluminium may cause problems, one or the other way.

Along those lines, how would one possible connect a capacitor to the Al-layer? Well, that is, considered the dimensions, actually pretty simple. Have a look at the images below, they may look disturbing. And yes, this is pretty ghetto!
RG-213 meats PE-Xc/Al/PE-Xc

close up
And here is where the capacitor resides, between the Al-layer of the PEx-Al tubing and the braid of the RG-213. Of course, the braid of the 213 is made from Copper, however, the is no Galvanic connection between those metals, and hence, no corrosion!
Although the RG-213 fits into the PEx-Al tubing nicely, there is some friction... and that is good!

  1. Building a magnetic loop from the materials shown above.
    A single turn loop made from PEx-Al with a length of RG-213 inserted into both ends of the loop symmetrically. The loop will be tuned by widen or closing it other the inserted length of RG-213, actually acting like a butterfly-capacitor. I will aim for about 14.070MHz. Once tuned, I will tape up the loop endings in order to lock tuning and prevent moisture to enter into the tubing.
  2. Building a quarter wave DDRR, as intended in the first place.
    Similarly to the magnetic loop above, the capacitor will be formed by RG-213 inserted into the PEx/Al tubing. However, there is a problem with the ground connection of the loop! The present idea is to remove the outer PE layer, wrap the Al-layer with Al-foil and clamp this down with a hose clamp. Of course, the Al-foil will corrode, hence, it will have to be replace occasionally.
  3. Building a frame antenna.
    This essentially means to coil up a good part of the PEx/Al to a decent pack, having a coupling toroid in the center winding. This technique, you might have seen at various antennae I published before, e.g. OctaPlumb (different plumbing material though).
  4. Different PEx/Al tubings might fit into one-another, forming capacitive couplings.
  5. I can imagine using PEx/Al with INOX-screws connecting various coils thereby, together with capacitors as disclosed above, forming traps.
  6. The PEx/Al tubing might potentially be usable for shielding a loop made from RG-213. Insert theRG-213 into the tubing, both end peaking out. Solder things down at the "far end" of the loop and slide the far end to the tubing... Follow your imagination for the rest!
First, I had in mind building a DDRR-antenna. However, seen the possible simplicity, I may build a magnetic loop first.

Post Scriptum
It seems that the PE-Xc/Al/PE-Xc, best suitable for amateur radio, is found under "floor heating".

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