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

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

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

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

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

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

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

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.

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

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