As mentioned earlier, it not being easy to precisely predict whether and how much the conductive rod would influence the antenna behavior, I decided to buy a carbon fibre rod measuring 26.2 feet (fully extended). Its diameter varies from 1 1/8 inch at the base to just 1/16 of inch at the top. Its weight is just 0.7 lbs! At it stands very straight when you keep it horizontal.

The first test was to determine its DC ohmic resistance. This is a very difficult test as resistance varies a lot depending on how much you press the ohmeter leads against the rod. Let us say that, on the 1-inch diameter tube, putting the ohmeter leads at a 2-inch distance, and very strongly pressing the leads against the rod, I measured something in the range of 10 ohm. Then, keeping one lead firm, I slided the other lead across the rod: resistance was varying between some 10 and 20 ohms, but there was not a clear correlation between the leads distance and resistance. Anyway, despite no precise data could be obtained, at least I understood that the rod resistance is not all negligible and that it then would probably make little sense to use the rod alone as ground plane radiator (i.e. without a parallel copper wire).

I then laid a bare copper wire (0.1 inch diameter) along the whole rod (reduced in length to about 23 feet, so as to resonate on the 10 MHz band) and tightly taped it to the rod at its top, at its bottom and every about 3 feet. How good were the ohmic contacts between the copper wire and the rod is however hard to tell.

The rod was then erected, standing on an insulator at its bottom. The coaxial cable center conductor was connected to the copper wire (by the very bottom of the rod) and its braid to four radials laid on the ground. In this way the copper wire acts as radiatior, while the rod is just a passive structure put in contact with the wire every 3 feet.

Initial low-power tests at 10.15 MHz showed a very low SWR. Luckily the antenna length was appropriate.

I then applied a carrier at some 1500W and after a couple of minutes or so I saw the reflected power meter oscillating, until it suddently went up a lot.

I immediately went to inspect the antenna and I found that the rod was fairly hot. Moreover there were clear signs of sparking between the copper wire and the rod here and there, and the tape had melted at some points.

It can be concluded that the theory according to which the copper wire simply bypasses the rod due to its much lower resistance does not seem to apply. On the other hand I was feeding the wire, not the rod!

The explanation could be as follows. The RF current only flows in the copper wire due to its much lower resistance, and RF voltage then varies along the wire (maximum at the top, minimum at the base). If we now consider two points where the rod is taped to the wire (3 feet apart), there will be significant RF voltage between those two points. Then the conductive rod, subjected to that high voltage, draws significant RF current, so dissipating power.

Any other comment? An idea would be to spray the rod with a (really) conductive coating, but does such a varnish exist?


73

Tony I0JX