Question on carbon fibre fishing rod
 

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

Question on carbon fibre fishing rod
 

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

Question on carbon fibre fishing rod
 

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

Question on carbon fibre fishing rod
 

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

Question on carbon fibre fishing rod
 

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

Question on carbon fibre fishing rod
 

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

Question on carbon fibre fishing rod
 

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

Question on carbon fibre fishing rod
 

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

Question on carbon fibre fishing rod
 

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

Question on carbon fibre fishing rod
 

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

Question on carbon fibre fishing rod
 

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

Yes, that is a possibility, although properly interpreting EZNEC results is often not easy. I will try anyway

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.

Well, one ohm is just the order of magnitude. It will be neither one tenth of a ohm, nor 10 ohms. Moreover resistance will vary along the rod due to the very significant diameter change, and RF current is not constant along the rod either. I am beginning to fear that the only way to really understand how things go is to buy the rod, build the antenna, apply high power ( 1kW) to it for 15 minutes, and the touch the rod to determine whether its temperature has raised somewhat.

73

Tony I0JX

Question on carbon fibre fishing rod
 

Tony,

The best thing to do is insulate the wire from the rod at the top and
bottom. Space the wire from the rod by one or two centimetres.

The rod will not be near to 1/2-wave resonance and the antenna will
behave very much as if the rod did not exist.

Remember that loss induced in the rod will be negligible when the
resistivity of the rod is high, or is also very low. I would imagine
the loss would be greatest when the end-to-end rod resistance is in
the order of 150 ohms.

But loss will be induced in the rod only when the rod is near to
1/2-wave resonant.
----
Reg.

Question on carbon fibre fishing rod
 

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.

Question on carbon fibre fishing rod
 

Or if it is 1/4-wave resonant and is in relatively low-resistance
contact with the groundplane.

Dan
But loss will be induced in the rod only when the rod is near to
1/2-wave resonant.
----
Reg.

Question on carbon fibre fishing rod
 

Antonio,

My intuition about the rod and wire in parallel appears to be incorrect
unless the resistance of the rod is quite low, at least on 7MHz. My
EZNEC model shows that the wires in parallel give substantial loss (up
to a couple of dB or so lost in the resistance of the rod) with the
wire connected at the top and bottom and the total rod resistance in
the 1-10 ohm range.

Separated and insulated from the rod seems to work quite well on 7MHz.
However, if the rod is connected directly to the ground radials a large
amount of loss is introduced. (I got 5dB in some cases with fairly high
resistance)

I have found other frequencies where the loss is more than a dB or so
even in this configuration (18MHz is near a halfwave resonance, which
is problematic, as Reg has correctly surmised).

The model shows that the current distribution on the lossy element is
occasionally complicated and bears further investigation. I've posted
the model I put together online. It's in some random configuration of
frequency, load resistance, and connectedness of the lossy rod to
ground, I don't know where it was when I saved it:

http://www.n3ox.net/projects/eznec/carbonpole.ez

It has too many segments for the free version but it probably bears
more investigation ... it is possible for the setup to be quite lossy
on some frequencies. I think Reg's comment about halfwave resonance
and intermediate resistances between very low and very high was quite
accurate, as was mine about quarterwave resonance and connection to the
groundplane.

73,
Dan

Question on carbon fibre fishing rod
 

wrote in message
oups.com...
Or if it is 1/4-wave resonant and is in relatively low-resistance
contact with the groundplane.

Dan
But loss will be induced in the rod only when the rod is near to
1/2-wave resonant.
----
Reg.


Tony, what happens if you load the transmitter into the rod, directly? Upon
reading that the end-to-end resistance of the pole may be about an ohm, that
idea was in my head instantly. Impedance to RF is a different animal, of
course but I would try it on low power, just to measure SWR. And you don't
have to do anything, really, to determine whether you can _hear_ using it!

I hope you keep us informed.

73,
"Sal"
(really KD6VKW)

Question on carbon fibre fishing rod
 

wrote:
My intuition about the rod and wire in parallel appears to be incorrect
unless the resistance of the rod is quite low, at least on 7MHz.
Separated and insulated from the rod seems to work quite well on 7MHz.

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

Question on carbon fibre fishing rod
 

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

Question on carbon fibre fishing rod
 

"Sal M. Onella" wrote in
news:i3eAg.12676$lv.6904@fed1read12:


wrote in message
oups.com...
Or if it is 1/4-wave resonant and is in relatively low-resistance
contact with the groundplane.

Dan
But loss will be induced in the rod only when the rod is near to
1/2-wave resonant.
----
Reg.


Tony, what happens if you load the transmitter into the rod, directly?
Upon reading that the end-to-end resistance of the pole may be about
an ohm, that idea was in my head instantly. Impedance to RF is a
different animal, of course but I would try it on low power, just to
measure SWR. And you don't have to do anything, really, to determine
whether you can _hear_ using it!

I hope you keep us informed.

Right, stick an analyzer on it and see what it actually does with real RF
in it.


--
Dave Oldridge+
ICQ 1800667

Question on carbon fibre fishing rod
 

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

Question on carbon fibre fishing rod
 

Tony, what happens if you load the transmitter into the rod, directly? Upon
reading that the end-to-end resistance of the pole may be about an ohm, that
idea was in my head instantly. Impedance to RF is a different animal, of
course but I would try it on low power, just to measure SWR. And you don't
have to do anything, really, to determine whether you can _hear_ using it!

I hope you keep us informed.

Hi Sal,

what you suggest is certainly a possibility. But until I do not buy the rod, it is difficult for me to precisely measure the rod resistance and then decide whether using the rod alone makes real sense.

I have finally decided to buy the rod, so I will also try to use it as a radiator, with no copper wire taped on it.

73

Tony I0JX

Question on carbon fibre fishing rod
 

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

Question on carbon fibre fishing rod
 

Tony,

It's probably worth mentioning, that all of the lossy situations I
encountered in EZNEC also caused fairly significant modification of the
feedpoint impedance. I suppose it makes sense; this is a necessary
condition for coupling significant power into the rod, and could
potentially give you extra information beyond just measuring the
temperature rise.

73,
Dan

I have finally decided to buy the rod, so I will also try to use it as a radiator, with no copper wire taped on it.

73

Tony I0JX

Question on carbon fibre fishing rod
 

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

Question on carbon fibre fishing rod
 

It's probably worth mentioning, that all of the lossy situations I
encountered in EZNEC also caused fairly significant modification of the
feedpoint impedance. I suppose it makes sense; this is a necessary
condition for coupling significant power into the rod, and could
potentially give you extra information beyond just measuring the
temperature rise.


What I plan to do is to measure SWR in three conditions, i.e. the rod alone (no parallel copper wire), rod + wire connected at the two extremes, and rod insulated from the radiating wire. The measured SWR variation in the three cases will give me an indication of the feedpoint impedance variation, which, as you say, can constitute an indicator of ohmic losses presence.

By means of a tuner, I will be able to deliver the desired amount of RF power into the antenna for quite a wide feedpoint impedance range (and SWR). I can use a short run of coaxial cable to feed the antenna, so the cable loss variation for different SWR values will be negligible on 20 meters.

73

Tony I0JX

Question on carbon fibre fishing rod
 

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

Question on carbon fibre fishing rod
 

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

Question on carbon fibre fishing rod
 

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.

Question on carbon fibre fishing rod
 

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

Question on carbon fibre fishing rod
 

Hi Dan

thanks for your in-depth analysis on EZ-NEC. The only step that surpises me a bit is that, with the top and bottom shorting wires in place, the current through the rod gets quite high, almost 2/3 that of the wire. On the other hand is always difficult to predict reality when electromagnetic phenomena are involved.

The next test you suggested, i.e. with the wire fully insulated from the rod, is just what I had in mind to do. A spacing of about 1-inch would be easy to get, by sliding the copper wire through the rings where the fishing nylon wire is supposed to run. So I'll do that first. I have a good quantity of silver-plated teflon-coated wire, so will try with that as it will stand a fairly voltage. I should be able to make that test by next monday or tuesday afternoon, and I will report results here.

73

Tony I0JX

ha scritto nel messaggio ups.com...
Tony,

From an EZNEC model with essentially the situation you described:
Wire numbers 2 and 5 are both 7.9m high, frequency 10MHz, fed against a
radial system in free space.

Wire number 2 is 25.4mm diameter, 21 segments, a 10 ohm load in each
segment, total "rod resistance" 210 ohms + negligable copper loss. The
wire centers are 7cm apart. Wire number 5 is 2mm diameter copper.
Wires 2 and 5 are shorted together top and bottom (no contact along the
length, but maybe we can see why you had arcing)

As you can see below, the currents in the two "wires" with the top and
bottom shorted are not in phase. Moreover, the current in the rod
(Wire No. 2) is quite appreciable, almost 2/3 that of the wire.


Wire No. 2:
Segment Conn Magnitude (A.) Phase (Deg.)
te1 W4E1 .39744 -167.9
2 .40049 -168.9
3 .39918 -169.3
4 .39463 -169.2
5 .38725 -168.8
6 .3773 -168.1
7 .36499 -166.9
8 .35056 -165.4
9 .33425 -163.5
10 .31635 -161.0
11 .29723 -157.9
12 .27734 -154.0
13 .25727 -149.2
14 .23781 -143.2
15 .22002 -135.8
16 .20528 -126.8
17 .19529 -116.1
18 .19188 -104.1
19 .19661 -91.39
20 .21058 -78.89
21 W3E1 .23611 -66.69

Wire No. 5:
Segment Conn Magnitude (A.) Phase (Deg.)
1 W6E1 .64698 170.23
2 .66204 169.08
3 .67227 168.00
4 .67834 166.94
5 .68045 165.86
6 .67871 164.76
7 .6732 163.63
8 .66403 162.44
9 .6513 161.18
10 .63513 159.83
11 .61567 158.39
12 .59307 156.82
13 .56752 155.11
14 .53922 153.20
15 .50842 151.06
16 .47537 148.63
17 .44036 145.82
18 .40374 142.51
19 .36589 138.50
20 .32725 133.51
21 W3E2 .28776 126.90

So how's the loss? Well, here's the table of load data:


Frequency = 10 MHz

Load 1 Voltage = 14.71 V. at -167.86 deg.
Current = 1.471 A. at -167.86 deg.
Impedance = 10 + J 0 ohms
Power = 21.62 watts

Load 2 Voltage = 14.82 V. at -168.9 deg.
Current = 1.482 A. at -168.9 deg.
Impedance = 10 + J 0 ohms
Power = 21.96 watts

Load 3 Voltage = 14.77 V. at -169.29 deg.
Current = 1.477 A. at -169.29 deg.
Impedance = 10 + J 0 ohms
Power = 21.81 watts

Load 4 Voltage = 14.6 V. at -169.24 deg.
Current = 1.46 A. at -169.24 deg.
Impedance = 10 + J 0 ohms
Power = 21.32 watts

Load 5 Voltage = 14.33 V. at -168.83 deg.
Current = 1.433 A. at -168.83 deg.
Impedance = 10 + J 0 ohms
Power = 20.53 watts

Load 6 Voltage = 13.96 V. at -168.07 deg.
Current = 1.396 A. at -168.07 deg.
Impedance = 10 + J 0 ohms
Power = 19.49 watts

Load 7 Voltage = 13.5 V. at -166.95 deg.
Current = 1.35 A. at -166.95 deg.
Impedance = 10 + J 0 ohms
Power = 18.24 watts

Load 8 Voltage = 12.97 V. at -165.43 deg.
Current = 1.297 A. at -165.43 deg.
Impedance = 10 + J 0 ohms
Power = 16.82 watts

Load 9 Voltage = 12.37 V. at -163.46 deg.
Current = 1.237 A. at -163.46 deg.
Impedance = 10 + J 0 ohms
Power = 15.29 watts

Load 10 Voltage = 11.7 V. at -160.97 deg.
Current = 1.17 A. at -160.97 deg.
Impedance = 10 + J 0 ohms
Power = 13.7 watts

Load 11 Voltage = 11 V. at -157.86 deg.
Current = 1.1 A. at -157.86 deg.
Impedance = 10 + J 0 ohms
Power = 12.09 watts

Load 12 Voltage = 10.26 V. at -153.99 deg.
Current = 1.026 A. at -153.99 deg.
Impedance = 10 + J 0 ohms
Power = 10.53 watts

Load 13 Voltage = 9.519 V. at -149.17 deg.
Current = 0.9519 A. at -149.17 deg.
Impedance = 10 + J 0 ohms
Power = 9.061 watts

Load 14 Voltage = 8.799 V. at -143.19 deg.
Current = 0.8799 A. at -143.19 deg.
Impedance = 10 + J 0 ohms
Power = 7.742 watts

Load 15 Voltage = 8.141 V. at -135.8 deg.
Current = 0.8141 A. at -135.8 deg.
Impedance = 10 + J 0 ohms
Power = 6.627 watts

Load 16 Voltage = 7.595 V. at -126.79 deg.
Current = 0.7595 A. at -126.79 deg.
Impedance = 10 + J 0 ohms
Power = 5.769 watts

Load 17 Voltage = 7.226 V. at -116.11 deg.
Current = 0.7226 A. at -116.11 deg.
Impedance = 10 + J 0 ohms
Power = 5.221 watts

Load 18 Voltage = 7.099 V. at -104.08 deg.
Current = 0.7099 A. at -104.08 deg.
Impedance = 10 + J 0 ohms
Power = 5.04 watts

Load 19 Voltage = 7.274 V. at -91.39 deg.
Current = 0.7274 A. at -91.39 deg.
Impedance = 10 + J 0 ohms
Power = 5.292 watts

Load 20 Voltage = 7.791 V. at -78.88 deg.
Current = 0.7791 A. at -78.88 deg.
Impedance = 10 + J 0 ohms
Power = 6.071 watts

Load 21 Voltage = 8.736 V. at -66.69 deg.
Current = 0.8736 A. at -66.69 deg.
Impedance = 10 + J 0 ohms
Power = 7.632 watts

Total applied power = 1535 watts

Total load power = 271.9 watts
Total load loss = 0.846 dB

The situation is VERY much improved by simply removing the top and
bottom shorting wires (this changes the base impedance very much, by
the way. I adjusted the source to still give ~1500 watts applied)

Total applied power = 1576 watts

Total load power = 30.53 watts
Total load loss = 0.085 dB

The current in the two wires isn't in phase either, but the average
magnitude of the current in the rod is more than a factor of ten below
the average magnitude of current in the wire.

So, space the wire a couple of inches out from the rod with insulators
and you should be fine, except, as Reg said before, where the rod is
1/2 wavelength long.

If you still get heating, I suppose you'll just have to use the rod as
a center support for an inverted-vee antenna or something!

The arcing is coming from the fact that the currents are out-of-phase
by some amount (40 degrees) in the two conductors... and so are the
voltages... you get a potential difference between a point on the wire
and the corresponding point on the rod. I doubt it's sufficient to
jump any distance air gap, it's more that the carbon can't take the
current that wants to flow between the rod and wire and is burning, but
that's just a guess, and you know how good my guess was originally.

Dan

Question on carbon fibre fishing rod
 

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

Question on carbon fibre fishing rod
 

Sorry I will not be able to do the test before some time.

I will report later.

Tony I0JX

"Antonio Vernucci" ha scritto nel messaggio ...
Hi Dan

thanks for your in-depth analysis on EZ-NEC. The only step that surpises me a bit is that, with the top and bottom shorting wires in place, the current through the rod gets quite high, almost 2/3 that of the wire. On the other hand is always difficult to predict reality when electromagnetic phenomena are involved.

The next test you suggested, i.e. with the wire fully insulated from the rod, is just what I had in mind to do. A spacing of about 1-inch would be easy to get, by sliding the copper wire through the rings where the fishing nylon wire is supposed to run. So I'll do that first. I have a good quantity of silver-plated teflon-coated wire, so will try with that as it will stand a fairly voltage. I should be able to make that test by next monday or tuesday afternoon, and I will report results here.

73

Tony I0JX

ha scritto nel messaggio ups.com...
Tony,

From an EZNEC model with essentially the situation you described:
Wire numbers 2 and 5 are both 7.9m high, frequency 10MHz, fed against a
radial system in free space.

Wire number 2 is 25.4mm diameter, 21 segments, a 10 ohm load in each
segment, total "rod resistance" 210 ohms + negligable copper loss. The
wire centers are 7cm apart. Wire number 5 is 2mm diameter copper.
Wires 2 and 5 are shorted together top and bottom (no contact along the
length, but maybe we can see why you had arcing)

As you can see below, the currents in the two "wires" with the top and
bottom shorted are not in phase. Moreover, the current in the rod
(Wire No. 2) is quite appreciable, almost 2/3 that of the wire.


Wire No. 2:
Segment Conn Magnitude (A.) Phase (Deg.)
te1 W4E1 .39744 -167.9
2 .40049 -168.9
3 .39918 -169.3
4 .39463 -169.2
5 .38725 -168.8
6 .3773 -168.1
7 .36499 -166.9
8 .35056 -165.4
9 .33425 -163.5
10 .31635 -161.0
11 .29723 -157.9
12 .27734 -154.0
13 .25727 -149.2
14 .23781 -143.2
15 .22002 -135.8
16 .20528 -126.8
17 .19529 -116.1
18 .19188 -104.1
19 .19661 -91.39
20 .21058 -78.89
21 W3E1 .23611 -66.69

Wire No. 5:
Segment Conn Magnitude (A.) Phase (Deg.)
1 W6E1 .64698 170.23
2 .66204 169.08
3 .67227 168.00
4 .67834 166.94
5 .68045 165.86
6 .67871 164.76
7 .6732 163.63
8 .66403 162.44
9 .6513 161.18
10 .63513 159.83
11 .61567 158.39
12 .59307 156.82
13 .56752 155.11
14 .53922 153.20
15 .50842 151.06
16 .47537 148.63
17 .44036 145.82
18 .40374 142.51
19 .36589 138.50
20 .32725 133.51
21 W3E2 .28776 126.90

So how's the loss? Well, here's the table of load data:


Frequency = 10 MHz

Load 1 Voltage = 14.71 V. at -167.86 deg.
Current = 1.471 A. at -167.86 deg.
Impedance = 10 + J 0 ohms
Power = 21.62 watts

Load 2 Voltage = 14.82 V. at -168.9 deg.
Current = 1.482 A. at -168.9 deg.
Impedance = 10 + J 0 ohms
Power = 21.96 watts

Load 3 Voltage = 14.77 V. at -169.29 deg.
Current = 1.477 A. at -169.29 deg.
Impedance = 10 + J 0 ohms
Power = 21.81 watts

Load 4 Voltage = 14.6 V. at -169.24 deg.
Current = 1.46 A. at -169.24 deg.
Impedance = 10 + J 0 ohms
Power = 21.32 watts

Load 5 Voltage = 14.33 V. at -168.83 deg.
Current = 1.433 A. at -168.83 deg.
Impedance = 10 + J 0 ohms
Power = 20.53 watts

Load 6 Voltage = 13.96 V. at -168.07 deg.
Current = 1.396 A. at -168.07 deg.
Impedance = 10 + J 0 ohms
Power = 19.49 watts

Load 7 Voltage = 13.5 V. at -166.95 deg.
Current = 1.35 A. at -166.95 deg.
Impedance = 10 + J 0 ohms
Power = 18.24 watts

Load 8 Voltage = 12.97 V. at -165.43 deg.
Current = 1.297 A. at -165.43 deg.
Impedance = 10 + J 0 ohms
Power = 16.82 watts

Load 9 Voltage = 12.37 V. at -163.46 deg.
Current = 1.237 A. at -163.46 deg.
Impedance = 10 + J 0 ohms
Power = 15.29 watts

Load 10 Voltage = 11.7 V. at -160.97 deg.
Current = 1.17 A. at -160.97 deg.
Impedance = 10 + J 0 ohms
Power = 13.7 watts

Load 11 Voltage = 11 V. at -157.86 deg.
Current = 1.1 A. at -157.86 deg.
Impedance = 10 + J 0 ohms
Power = 12.09 watts

Load 12 Voltage = 10.26 V. at -153.99 deg.
Current = 1.026 A. at -153.99 deg.
Impedance = 10 + J 0 ohms
Power = 10.53 watts

Load 13 Voltage = 9.519 V. at -149.17 deg.
Current = 0.9519 A. at -149.17 deg.
Impedance = 10 + J 0 ohms
Power = 9.061 watts

Load 14 Voltage = 8.799 V. at -143.19 deg.
Current = 0.8799 A. at -143.19 deg.
Impedance = 10 + J 0 ohms
Power = 7.742 watts

Load 15 Voltage = 8.141 V. at -135.8 deg.
Current = 0.8141 A. at -135.8 deg.
Impedance = 10 + J 0 ohms
Power = 6.627 watts

Load 16 Voltage = 7.595 V. at -126.79 deg.
Current = 0.7595 A. at -126.79 deg.
Impedance = 10 + J 0 ohms
Power = 5.769 watts

Load 17 Voltage = 7.226 V. at -116.11 deg.
Current = 0.7226 A. at -116.11 deg.
Impedance = 10 + J 0 ohms
Power = 5.221 watts

Load 18 Voltage = 7.099 V. at -104.08 deg.
Current = 0.7099 A. at -104.08 deg.
Impedance = 10 + J 0 ohms
Power = 5.04 watts

Load 19 Voltage = 7.274 V. at -91.39 deg.
Current = 0.7274 A. at -91.39 deg.
Impedance = 10 + J 0 ohms
Power = 5.292 watts

Load 20 Voltage = 7.791 V. at -78.88 deg.
Current = 0.7791 A. at -78.88 deg.
Impedance = 10 + J 0 ohms
Power = 6.071 watts

Load 21 Voltage = 8.736 V. at -66.69 deg.
Current = 0.8736 A. at -66.69 deg.
Impedance = 10 + J 0 ohms
Power = 7.632 watts

Total applied power = 1535 watts

Total load power = 271.9 watts
Total load loss = 0.846 dB

The situation is VERY much improved by simply removing the top and
bottom shorting wires (this changes the base impedance very much, by
the way. I adjusted the source to still give ~1500 watts applied)

Total applied power = 1576 watts

Total load power = 30.53 watts
Total load loss = 0.085 dB

The current in the two wires isn't in phase either, but the average
magnitude of the current in the rod is more than a factor of ten below
the average magnitude of current in the wire.

So, space the wire a couple of inches out from the rod with insulators
and you should be fine, except, as Reg said before, where the rod is
1/2 wavelength long.

If you still get heating, I suppose you'll just have to use the rod as
a center support for an inverted-vee antenna or something!

The arcing is coming from the fact that the currents are out-of-phase
by some amount (40 degrees) in the two conductors... and so are the
voltages... you get a potential difference between a point on the wire
and the corresponding point on the rod. I doubt it's sufficient to
jump any distance air gap, it's more that the carbon can't take the
current that wants to flow between the rod and wire and is burning, but
that's just a guess, and you know how good my guess was originally.

Dan