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.

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:
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

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

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)

"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

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