Probably not carbon fiber tho, right?

I doubt it's possible to make a carbon fiber pole without end-to-end DC
continuity. It could be high resistance, but I think you're always
going to have fiber-to-fiber contact along the whole length of the
pole.

Carbon fiber, just as a point of general information in this thread,
seems to have about 800 times the resistivity of copper according to
matweb.com, and about 20 times the resisivity of 304 stainless steel.
Not something that will make for a great radiator.

If you assume fairly low frequency such that the skin depth in carbon
fiber is deep enough that current will flow uniformly in a 1.6mm (1/16
inch) tube wall, and uniformly in a 1mm wire, you need a 10cm (4 inch)
carbon fiber tube to match the 1mm wire in conductor loss!

Interestingly enough, I read recently (I don't remember if it was QST
or on the web) about someone who was using a fiberglass fishing pole
for a vertical antenna and had significant detuning problems until they
moved the wire far away from the pole. I, on the other hand, built a
center loaded 40m antenna on a pair of fiberglass poles and it tuned
exactly as one of Reg's programs and EZNEC said it would, suggesting
that my poles are fairly good insulators.

I still think I'd like a carbon fiber fishing rod... sounds nice and
strong. Maybe it's best used as center supports for inverted vees and
the like, and for carefully considered vertical applications.

Dan

My South Bend Sunny Day SD-20 pole doesn't show any DC
resistance even when the paint is scraped off.
--
73, Cecil http://www.qsl.net/w5dxp

I wish to thank all people contributing to understanding the issue.

The discussion confirms me that precisely predicting what happens when using a carbon fibre rod is not easy. On the other hand that rod is so appealing for realizing a stealth antenna leaning on the balcony railing.... Imagine a 27-foot rod, coming down to just 4 feet, weighing just 2 pounds or so, having a diameter of less than 1 inch at the base and about 0.08 inch at the top.... and standing well straight!

So, what I plan to do is the following:
- buy the rod @ about 100$ (it will so also be possible to make more precise resistance measurements than those I can take at the store)
- quickly build a classic 20-meter ground plane test antenna, by extending the rod just as much as needed and taping an insulated copper wire on the rod, parallel to it. I will connect the copper wire only at the rod top and at its base which will be insulated from ground and connected to the coaxial cable center conductor. I will then put four radials on the ground, connected to the coaxial cable braid
- I will apply 1500W RF for some ten minutes. Assuming that the rod causes a loss of just 0.5 dB, this would mean dissipating 163 W on the rod that, considering its low mass, should become hot enough to detect it! Then, if the rod does not get hot at all, I can conclude that no virtually no power gets dissipated in it.
- I will repeat the experiment by keeping the copper wire fully insulated from the rod, though still taped on it
- finally, I will report the test results here.

73 to all

Tony I0JX

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

Whether the current flows in the wire or in the rod depends on
inductance and inductive reactance.

A thin wire has greater inductance and impedance per unit length than
a thick rod.
----
Reg.

Whether the current flows in the wire or in the rod depends on
inductance and inductive reactance.

A thin wire has greater inductance and impedance per unit length than
a thick rod.

True, the rod having a resistance by far higher than that of the copper wire, I would believe that current will anyway pass through the wire, despite its lower diameter.

73

Tony

Antonio Vernucci wrote:
Whether the current flows in the wire or in the rod depends on
inductance and inductive reactance.

A thin wire has greater inductance and impedance per unit length than
a thick rod.


True, the rod having a resistance by far higher than that of the copper wire, I would believe that current will anyway pass through the wire, despite its lower diameter.

73

Tony


Actually if the rod conducts it will share the current flow according to
the law or parallel resistance.

Dave WD9BDZ