I plan to use a carbon fibre fishing rod as a stealth antenna at my summer QTH . The rod would just be a mechanical support, while the proper antenna would consist of a copper wire replacing the fishing wire (i.e. it would run on one side of the rod, parallel to it)

I am getting worried by the fact that the carbon fibre rod may dissipate significant power because of the RF current induced in it (the rod would be very close to the radiating wire) and of its non-negligible resistance.

I had occasion to broadly measure the rod DC resistance with an ohm x 1000 ohmeter, at about 4 inches of distance on the rod, and the meter reading was about full scale (low resistance). But the resistance range I am worried about is in the order of a few ohms, so my measurement does not tell much. In any case, reading some articles on carbon fibre on the Internet, I learned that the rod must anyway have a few ohms of resistance, quite a dangerous range.

The real question is: shall I expect that the copper wire will induce RF current on the rod that would cause power dissipation in it? Does anyone have experience with such an arrangement?

Please do not suggest me to use a fiberglass rod instead because, for a given length, it would be much thicker and heavier (too much).

73

Tony I0JX - Rome, Italy

On Wed, 2 Aug 2006 19:32:38 +0200, "Antonio Vernucci"
wrote:

I plan to use a carbon fibre fishing rod as a stealth antenna at my =
summer QTH .

Hi Tony,

What frequency?

73's
Richard Clark, KB7QHC

7 to 28 MHz.

Tony

"Richard Clark" ha scritto nel messaggio ...
On Wed, 2 Aug 2006 19:32:38 +0200, "Antonio Vernucci"
wrote:

I plan to use a carbon fibre fishing rod as a stealth antenna at my =
summer QTH .

Hi Tony,

What frequency?

73's
Richard Clark, KB7QHC

On Wed, 2 Aug 2006 20:22:59 +0200, "Antonio Vernucci"
wrote:

7 to 28 MHz.

Hi Tony,

Presuming a length of 3M, and you can tune the antenna; then you may
lose a few dB.

As for heating. The exposure to sun (if the rod is black) will
probably create more heat than the RF. You need to provide a
resistance reading with more resolution than 0 Ohms on the 1000 Ohms
range.

73's
Richard Clark, KB7QHC

Hi Tony,

Presuming a length of 3M

the rod is 8 meter long

, and you can tune the antenna; then you may
lose a few dB.

hard to tell how many dB one would loose, until the power loss mechanism via the rod coupling is understood. I would need to find somebody who had occasion to test that configuration, otherwise I will have to anyway buy the rod (about 100$) and make the test myself

As for heating. The exposure to sun (if the rod is black) will
probably create more heat than the RF.

I am not worried at all of rod heating, but of RF power loss

You need to provide a
resistance reading with more resolution than 0 Ohms on the 1000 Ohms
range.

There are many articles on the Internet that suggest a resistance in the order of one ohm or so. But knowing precisely how many ohms the rod shows woud not help much, as, again, until the loss mechanism is not characterized, noone can stell something sensible

73's

welcome

ORIGINAL MESSAGE:

On Wed, 2 Aug 2006 19:32:38 +0200, "Antonio Vernucci"
wrote:


I am getting worried by the fact that the carbon fibre rod may dissipate significant power because of the RF current induced in it (the rod would be very close to the radiating wire) and of its non-negligible resistance.

------------ REPLY SEPARATOR ------------

The only way to know for sure is to try it. Give it a few watts for a
few seconds and feel for heat. If there is any at all, go to plan B.

If there is no heat, give it 100 watts for a few seconds. Still no
heat? Give it full power for a longer period. Still no heat? You have
your answer.

Bill, W6WRT

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?

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 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.

Dan

On Wed, 2 Aug 2006 21:33:33 +0200, "Antonio Vernucci"
wrote:

hard to tell how many dB one would loose, until the power loss mechanism via the rod coupling is understood. I would need to find somebody who had occasion to test that configuration, otherwise I will have to anyway buy the rod (about 100$) and make the test myself

Hi Tony,

That is the only sure way of knowing. Modeling with EZNEC will
certainly reveal obvious problems.

As for heating. The exposure to sun (if the rod is black) will
probably create more heat than the RF.

I am not worried at all of rod heating, but of RF power loss

Same thing.

There are many articles on the Internet that suggest a resistance in the order of one ohm or so. But knowing precisely how many ohms the rod shows woud not help much, as, again, until the loss mechanism is not characterized, noone can stell something sensible

Total resistance over 8 meters is 1 Ohm? If so, it does not look like
a problem. 1 Ohm every 10cM? Still does not look like a problem.

Download EZNEC and do your own modeling.

73's
Richard Clark, KB7QHC

Richard,

Can you assign different conductivities to different wires in EZNEC? I
don't think you can.

You could distribute the resistance, 1 load per segment each having a
resistance of 1/#segments of the total. It seems important for
coupling to have a realistic diameter (fat) wire for the carbon fiber
pole, but it needs to be lossy.

A load taper from bottom (large tubing, lower resistance) to top (small
tubing, high resistance) would be easy to implement as well. This is
probably only worth it if it seems that the loss will be nonnegligible,
but it could take into account the effect of the element current
distribution on loss.

Dan

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