Hi Dan

If you can measure the resistance at more than one point on the surface
of the rod, does this imply that the rod surface is conductive
everywhere along its length?

I am not sure having fully understood your remark. I presume that, the materal being homogeneous, the rod surface is conductive everywhere the same way. The various elements of the rod touch each other, so there is electrical continuity along the whole rod. Clearly, toward the rod top, resistance will be higher due to the thinner diameter

If you could attach the wire to the rod so that it touches everywhere
along the length, then the current would divide between the rod and the
wire according to their respective resistance per length (most of the
current would flow in the wire) and the currents induced in the pole
would be in phase with the currents induced in the wire.

I agree that currents would divide between the rod and the wire according to their respective resistance, but this is not my main worry as the very low-resistance copper wire would nearly fully bypass the rod resistance. What I am instead worried about is that, the rod being thick, the RF current may not be the same along the rod circumference. In other words, at the point of contact between the rod and the copper wire, the rod current could be lower than that at its opposite side. Such extra current could develop due to the rod electromagnetic coupling with the radiating wire. However, I am not sure whether my reasoning makes real sense


I think it would help to have the wire attached at least at the bottom
of the pole and the top of the pole.
If you attach the wire only at the top and ground the bottom of the
pole, you make a rather lossy folded monopole. If you attach the wire
at both the top and bottom of the pole and insulate the whole structure
from ground, it's more like a cage monopople with one lossy wire and
one good wire.

My idea is to have the wire attached at both the bottom and the top of the rod as you suggest. The bottom copper lead would also be connected to the center conductor of the coaxial feed cable (the cable braid would instead be connected to the radial system).

Now an interesting case. Suppose that the copper wire is instead kept fully insulated from the rod (though very close to it). Would you expect power loss to occur in the conductive rod due to RF currents flowing through the rod due to its electromagnetic coupling with the radiating wire?

73

Tony I0JX

Antonio,

The carbon fiber rod is very thin with respect to a wavelength. There
won't be any significant of the RF current variation around the
circumference of the rod.

If you have the wire insulated from, but closely spaced to the carbon
rod. Then the rod becomes a parasitic element ONLY coupled to the wire
via the fields. If the rod is insulated from ground by a base
insulator there is probably no problem. I think the current will still
divide approximately the same way. There may be some subtle
differences.

The situation where you allow the rod to touch the ground (or worse,
the ground radial system) is drastically different. Now you have,
instead of a monopole, a weird parallel transmission line. A closed
top gives a folded monopole, but opening the top (insulating the wire
from the pole everywhere) doesn't change the fact that it's a
transmission line.

Imagine the following: the top is open, you're operating on 30m where
the pole and wire are very close to 1/4 wavelength long. This gives a
very *low* impedance at the bottom of your open transmission line stub.
I think a great deal of differential mode current could flow in this
case, and I think it would cause a good amount of loss.

I think your plan of having the wire connected to the pole at top and
bottom is sufficient if you have a good base insulator.

Dan

P.S. I wondered about the surface of the rod because I thought perhaps
there would be an outer nonconductive layer of thin plastic film to
protect from carbon fiber splinters. If there's no such film, then
spiralling the wire around the pole or clamping it periodically to the
pole will make for good contact everywhere.

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

Perhaps a fiberglass rod would be a better choice?

"Antonio Vernucci" wrote in message
...
Hi Dan

If you can measure the resistance at more than one point on the surface
of the rod, does this imply that the rod surface is conductive
everywhere along its length?

I am not sure having fully understood your remark. I presume that, the
materal being homogeneous, the rod surface is conductive everywhere the same
way. The various elements of the rod touch each other, so there is
electrical continuity along the whole rod. Clearly, toward the rod top,
resistance will be higher due to the thinner diameter

If you could attach the wire to the rod so that it touches everywhere
along the length, then the current would divide between the rod and the
wire according to their respective resistance per length (most of the
current would flow in the wire) and the currents induced in the pole
would be in phase with the currents induced in the wire.

I agree that currents would divide between the rod and the wire according to
their respective resistance, but this is not my main worry as the very
low-resistance copper wire would nearly fully bypass the rod resistance.
What I am instead worried about is that, the rod being thick, the RF current
may not be the same along the rod circumference. In other words, at the
point of contact between the rod and the copper wire, the rod current could
be lower than that at its opposite side. Such extra current could develop
due to the rod electromagnetic coupling with the radiating wire. However, I
am not sure whether my reasoning makes real sense


I think it would help to have the wire attached at least at the bottom
of the pole and the top of the pole.
If you attach the wire only at the top and ground the bottom of the
pole, you make a rather lossy folded monopole. If you attach the wire
at both the top and bottom of the pole and insulate the whole structure
from ground, it's more like a cage monopople with one lossy wire and
one good wire.

My idea is to have the wire attached at both the bottom and the top of the
rod as you suggest. The bottom copper lead would also be connected to the
center conductor of the coaxial feed cable (the cable braid would instead be
connected to the radial system).

Now an interesting case. Suppose that the copper wire is instead kept fully
insulated from the rod (though very close to it). Would you expect power
loss to occur in the conductive rod due to RF currents flowing through the
rod due to its electromagnetic coupling with the radiating wire?

73

Tony I0JX