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

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.

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.

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.

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

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)

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

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

"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

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