That's much, much more difference than can be explained by the different
conductivities of the metals. Either some very resistive alloys are
involved, or there are differences between the antennas other than the
type of metal.

Roy Lewallen, W7EL

H. Adam Stevens, NQ5H wrote:
One place where I can clearly see the difference between copper and aluminum
is comparing my (copper) Nott screwdriver with my (aluminum) Tarheel
screwdriver or my (aluminum) Hi-Q mobile antenna on 20 meters.
Mounted on my Durango, at resonance the Nott's impedance is 9 ohms, while
the Tarheel is 20 ohms and the (smaller tube) Hi-Q is 30 ohms; Measured with
the same whip in all cases.
This is a case of a short antenna.
The difference is significant and easily measured.
73
H.
NQ5H

Roy
The measurements are reproducible. That's for sure.
And you can bet the aluminum in the Tarheel and Hi-Q are alloys.
Using the Nott (just copper, no plating) I get a 9 ohm load at resonance on
20, 10 ohms on 40 and 80.
With a simple toroidal autoformer the mobile rig sees 50 ohms resistive at
resonance on all three bands.
73, NQ5H
H.

"Roy Lewallen" wrote in message
...
That's much, much more difference than can be explained by the different
conductivities of the metals. Either some very resistive alloys are
involved, or there are differences between the antennas other than the
type of metal.

Roy Lewallen, W7EL

H. Adam Stevens, NQ5H wrote:
One place where I can clearly see the difference between copper and
aluminum
is comparing my (copper) Nott screwdriver with my (aluminum) Tarheel
screwdriver or my (aluminum) Hi-Q mobile antenna on 20 meters.
Mounted on my Durango, at resonance the Nott's impedance is 9 ohms,
while
the Tarheel is 20 ohms and the (smaller tube) Hi-Q is 30 ohms; Measured
with
the same whip in all cases.
This is a case of a short antenna.
The difference is significant and easily measured.
73
H.
NQ5H

H. Adam Stevens wrote:
With a simple toroidal autoformer the mobile rig sees 50 ohms resistive at
resonance on all three bands.

Got any idea what the efficiency of your autoformer is?
I'm assuming a single coil for a primary, tapped down for
the load. Would a 4:1 transmission line transformer be more
efficient over all the bands?
--
73, Cecil, W5DXP



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It's measurably better than a tuner in terms of near-field strength;
14 turns of #12 insulated copper wire on a ferrite toroid ripped from an R7.
How much coax is needed for that transmission line transformer?
Mobiles are so inefficient, I'm always looking for that last 0.1 db.
73
H.
NQ5H

"Cecil Moore" wrote in message
...
H. Adam Stevens wrote:
With a simple toroidal autoformer the mobile rig sees 50 ohms resistive
at
resonance on all three bands.

Got any idea what the efficiency of your autoformer is?
I'm assuming a single coil for a primary, tapped down for
the load. Would a 4:1 transmission line transformer be more
efficient over all the bands?
--
73, Cecil, W5DXP



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Roy
I looked up the numbers.
The Nott is 2" diameter bare copper 3' long.
The Tarheel is 2" diameter painted aluminum 4' long.
The Hi-Q is 1" diameter powder-coated aluminum 3' long.
I used the same whip for all measurements.
If we take the conductivity of copper to be 100, aluminum is then 60 and
aluminum alloys are as low as 30.
That's a factor of two between the Nott and Tarheel and the reduced surface
area of the Hi-Q explains the additional resistance there. Considering the
difference in surface area and resistivity among the antennas, the measured
impedances seem quite reasonable to me.
73
H.
NQ5H


"Roy Lewallen" wrote in message
...
That's much, much more difference than can be explained by the different
conductivities of the metals. Either some very resistive alloys are
involved, or there are differences between the antennas other than the
type of metal.

Roy Lewallen, W7EL

H. Adam Stevens, NQ5H wrote:
One place where I can clearly see the difference between copper and
aluminum
is comparing my (copper) Nott screwdriver with my (aluminum) Tarheel
screwdriver or my (aluminum) Hi-Q mobile antenna on 20 meters.
Mounted on my Durango, at resonance the Nott's impedance is 9 ohms,
while
the Tarheel is 20 ohms and the (smaller tube) Hi-Q is 30 ohms; Measured
with
the same whip in all cases.
This is a case of a short antenna.
The difference is significant and easily measured.
73
H.
NQ5H

There are several variables here other than the conductivity of the
primary metal. Again, what you're seeing is not "the difference between
copper and aluminum" as initially stated, but a number of other factors
-- diameter, length, coating, and alloy --, combined.

If the DC conductivity of two materials differs by a factor of two,
their RF conductivity differs only by a factor of the square root of
two, or about 1.4. This is because the skin depth is greater in the less
conductive material, which partially offsets the conductivity
difference. The relatively small difference in DC conductivity between
pure copper and aluminum is further reduced by this effect, so you'll
very seldom be able to see any difference.

Roy Lewallen, W7EL

H. Adam Stevens, NQ5H wrote:
Roy
I looked up the numbers.
The Nott is 2" diameter bare copper 3' long.
The Tarheel is 2" diameter painted aluminum 4' long.
The Hi-Q is 1" diameter powder-coated aluminum 3' long.
I used the same whip for all measurements.
If we take the conductivity of copper to be 100, aluminum is then 60 and
aluminum alloys are as low as 30.
That's a factor of two between the Nott and Tarheel and the reduced surface
area of the Hi-Q explains the additional resistance there. Considering the
difference in surface area and resistivity among the antennas, the measured
impedances seem quite reasonable to me.
73
H.
NQ5H


"Roy Lewallen" wrote in message
...

That's much, much more difference than can be explained by the different
conductivities of the metals. Either some very resistive alloys are
involved, or there are differences between the antennas other than the
type of metal.

Roy Lewallen, W7EL

H. Adam Stevens, NQ5H wrote:

One place where I can clearly see the difference between copper and

aluminum
. . .

"H. Adam Stevens, NQ5H" wrote in message ...

The Nott is 2" diameter bare copper 3' long.
The Tarheel is 2" diameter painted aluminum 4' long.
The Hi-Q is 1" diameter powder-coated aluminum 3' long.

A 2" diameter bare copper rod or cylinder at 14MHz, 3' long, should
have an RF resistance about 6 milliohms. The worst aluminum alloy
you're likely to see should be about 12 milliohms; 24 for the 1"
diameter. What am I missing here? How does that translate to a
change from 9 ohms to 20 ohms to 30 ohms at the feedpoint? If the
cause is resistance heating of the copper or aluminum tube, what's
doing the impedance transformation, and how is it so efficient? That
much loss should result in measurable temperature rise in the tube (or
wherever the loss is), at 100-W power levels.

Or perhaps my image of what you're measuring is all out of whack.
Same loading coil in each case? I'd kind of expect the loading coil
to be the main loss mechanism, if all the connections are tight.

Puzzled and seeking enlightenment,
Tom

Well Tom, I'm just looking to explain the data: I'm puzzled as well.
Same truck, Durango with a ball mount right rear.
Same MFJ analyzer. Same very short coax from inside the truck.
Everything grounded with 2" copper strap.
Same 14.300 MHz; Same whip. The measurements repeat reliably.
Three motor-tuned antennas, Nott, Tarheel, Hi-Q.
Three different impedances at resonance (ie purely resistive load.)
9, 20 and 30 ohms. Now I can see the radiation resistance being slightly
higher with the Tarheel, it's a foot longer, but that hardly explains a
factor of two. So it must be the loss resistance. Part of the difference is
the base tube, and the copper/aluminum/diameter issue obeys the appropriate
scaling laws. (BTW I think the Hi-Q is 1.5" diameter, but I'm not sure and I
am not at the ranch where the antenna is stored.)
When I adjust the tap on the transformer to give a 50 ohm load to the MFJ
for each antenna, the Nott gives the greatest near-field signal strength.
Perhaps a further investigation of the remaining sources of resistance is in
order. The Hi-Q should have the least leakage, it's a beautiful piece of
work. The Tarheel appears to be built of better materials than the Nott.
(Lexan vs PVC for example.) Go down to 80 meters and they're all 10 ohms;
coil losses clearly dominate there where radiation resistance is tiny.

I think a complete solution to Maxwell's Equations would be helpful, but I'm
busy at the moment.

For all I know the paint or powder coating on the aluminum antennas is the
real culprit.
The Nott's just bare copper.
What is the radiation resistance of an 8 foot whip antenna resonant in a
16.5 foot world?
Just a bit less than 10 ohms, right?
Maybe the comparison should be to BARE aluminum.
Just my morning thought on a puzzle I've been looking at for several months.

73 es tnx fer qso
de nq5h
k


"Tom Bruhns" wrote in message
m...
"H. Adam Stevens, NQ5H" wrote in message
...

The Nott is 2" diameter bare copper 3' long.
The Tarheel is 2" diameter painted aluminum 4' long.
The Hi-Q is 1" diameter powder-coated aluminum 3' long.

A 2" diameter bare copper rod or cylinder at 14MHz, 3' long, should
have an RF resistance about 6 milliohms. The worst aluminum alloy
you're likely to see should be about 12 milliohms; 24 for the 1"
diameter. What am I missing here? How does that translate to a
change from 9 ohms to 20 ohms to 30 ohms at the feedpoint? If the
cause is resistance heating of the copper or aluminum tube, what's
doing the impedance transformation, and how is it so efficient? That
much loss should result in measurable temperature rise in the tube (or
wherever the loss is), at 100-W power levels.

Or perhaps my image of what you're measuring is all out of whack.
Same loading coil in each case? I'd kind of expect the loading coil
to be the main loss mechanism, if all the connections are tight.

Puzzled and seeking enlightenment,
Tom

Roy Lewallen wrote in message ...
That's much, much more difference than can be explained by the different
conductivities of the metals. Either some very resistive alloys are
involved, or there are differences between the antennas other than the
type of metal.

Roy Lewallen, W7EL

By the way, I found a very nice and complete listing of conductivities
of aluminum alloys at
http://www.ndt-ed.org/GeneralResourc...ctivity_Al.pdf
just after I posted last night. Others may find this useful. Even
the worst of them is not as much as three times the DC resistivity of
the best. So at RF, the worst aluminum alloy will have about twice
the loss of the same diameter copper conductor.

Cheers,
Tom