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

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

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