Is there a general rule as to the optimum way copper should be
distributed on a longwire antenna for maximum propagation? E.g.,
should the first half be heavier gauge than the second half? How much
heavier?

Ken KC2JDY


Ken
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In article ,
says...
Is there a general rule as to the optimum way copper should be
distributed on a longwire antenna for maximum propagation? E.g.,
should the first half be heavier gauge than the second half? How much
heavier?

Ken KC2JDY

I looked into this a couple of years ago. There are two factors;
DC ohmic resistance and RF skin effect. I found three different
huristic formulas for RF skin effect in ARRL handbook, ITT Radio
Engineers Hndbk and a book I found in the Cornell Engineering Library
stacks. They agreed within a factor of 2^-2. The numbers I ran
indicated that for power 1 kw and Freq 30 mhz 16 guage AWG harddrawn
stranded would outperform 14 gauge solid or copperweld except for
tensile strength and was all the copper it would be economic to use.
Between 10 Mhz and 50 Mhz the use of tubing gives mechanical design
advantages but doesn't gain you a lot of electrical performance. The
higher the freq the more significant skin effect becomes. For long
antenna wires at low freq the tensile strength issue could be more
important than skin effect indicating it would be better to use quality
copperweld or solid harddrawn. 17 guage AWG copperweld electric fence
wire performs badly. It's very cheap, but the copper plating depth is
shallower than the skin effect depth and the steel core is grainy,
brittle, and rusts thru quickly at any stress fracture.
In theory the gauge of the wire could be heavier at current nodes
on the antenna, but in practice I doubt it would be worth the trouble to
join sections of different gauges. I certainly wouldn't want to bother
with a tapered wire. ;-] 73 KC2LVQ

Is there a general rule as to the optimum way copper should be
distributed on a longwire antenna for maximum propagation? E.g.,
should the first half be heavier gauge than the second half? How much
heavier?

============================

There is only one general rule. The wire MUST be thick enough to withstand
the longitudinal tension due to its own weight and the amount of sag in the
middle. Otherwise propagation will suddenly cease.


Tapered copper wire, to match diameter to the current distribution along
antenna wires (one band only), is available only by special order for space
exploration and defence purposes. If it WAS available, for a given amount
of copper, the improvement in signal strength would amount to less than
1/140th of an S-unit. The same improvement could be achieved with an
enormous cost saving just by changing wire gauge from 18 to 14 awg.
---
Reg, G4FGQ

Reg, fantastic, you've said it as well as it could be said. But do you
really need 14 awg? Wouldn't 16 awg do?

Malcolm

"Reg Edwards" wrote in message
...
Is there a general rule as to the optimum way copper should be
distributed on a longwire antenna for maximum propagation? E.g.,
should the first half be heavier gauge than the second half? How much
heavier?

============================

There is only one general rule. The wire MUST be thick enough to withstand
the longitudinal tension due to its own weight and the amount of sag in
the
middle. Otherwise propagation will suddenly cease.


Tapered copper wire, to match diameter to the current distribution along
antenna wires (one band only), is available only by special order for
space
exploration and defence purposes. If it WAS available, for a given amount
of copper, the improvement in signal strength would amount to less than
1/140th of an S-unit. The same improvement could be achieved with an
enormous cost saving just by changing wire gauge from 18 to 14 awg.
---
Reg, G4FGQ

Malcolm said -
But do you
really need 14 awg?
Wouldn't 16 awg do?

===================

Yes, but you wouldn't be saving so much by NOT using tapered copper wire.
It's a mind-boggling matter of economics. If tapered wire WAS manufacturable
I'm sure there would be a mad rush by suckers with more money than sense to
buy Specially Tapered Low-loss G5RV's including the transmission line.


Actually, for a complete professsional design, the distribution of current
along the antenna wire should be taken into account when considering wire
diameter. It affects radiating efficiency. Only hot-spots need be
considered.


Working backwards, take an 80m 1/2-wave dipole using 20 awg enamelled copper
magnet wire (my favourite stuff). At 3.75 MHz the resistance of 20 awg
copper wire is 0.206 ohms per metre. Overall end-to-end dipole resistance =
8.24 ohms.


Now take a 1 KW transmitter. The current AT THE DIPOLE CENTER =
Sqrt(1000/70) = 3.74 amps.


Radiating efficiency = 140/(140+8.24)*100 = 94.4 percent, where 140 = 2*70
is end-to-end distributed radiation resistance.


At the dipole centre the power dispated in the middle 1-metre length of wire
= 3.74^2 * 0.206 = 2.88 watts corresponding to 73 milliwatts per inch of
wire. If you have a brain-surgeon's sensitive finger tips this will feel
slightly warm and nothing to worry about.


A radiating efficiency of 94.4 percent corresponds to an undetectable loss
of 1/25th of an S-unit relative to perfection and no sleep need be lost on
this account either.


The foregoing implies that antenna wire diameter depends more on wind
speeds, ice formation, and the cost of the pair of masts needed to support
it. Recently educated Ph.D's having difficulty with arithmetic may leave
the hard bits for later reading.
----
Reg, G4FGQ

Very good. Furthermore, following your ideas, the diameter of the wire at
a current anti-node could be infinitely thin!

Malcolm

"Reg Edwards" wrote in message
...
Malcolm said -
But do you
really need 14 awg?
Wouldn't 16 awg do?

===================

Yes, but you wouldn't be saving so much by NOT using tapered copper wire.
It's a mind-boggling matter of economics. If tapered wire WAS
manufacturable
I'm sure there would be a mad rush by suckers with more money than sense
to
buy Specially Tapered Low-loss G5RV's including the transmission line.


Actually, for a complete professsional design, the distribution of current
along the antenna wire should be taken into account when considering wire
diameter. It affects radiating efficiency. Only hot-spots need be
considered.


Working backwards, take an 80m 1/2-wave dipole using 20 awg enamelled
copper
magnet wire (my favourite stuff). At 3.75 MHz the resistance of 20 awg
copper wire is 0.206 ohms per metre. Overall end-to-end dipole resistance
=
8.24 ohms.


Now take a 1 KW transmitter. The current AT THE DIPOLE CENTER =
Sqrt(1000/70) = 3.74 amps.


Radiating efficiency = 140/(140+8.24)*100 = 94.4 percent, where 140 = 2*70
is end-to-end distributed radiation resistance.


At the dipole centre the power dispated in the middle 1-metre length of
wire
= 3.74^2 * 0.206 = 2.88 watts corresponding to 73 milliwatts per inch of
wire. If you have a brain-surgeon's sensitive finger tips this will feel
slightly warm and nothing to worry about.


A radiating efficiency of 94.4 percent corresponds to an undetectable loss
of 1/25th of an S-unit relative to perfection and no sleep need be lost on
this account either.


The foregoing implies that antenna wire diameter depends more on wind
speeds, ice formation, and the cost of the pair of masts needed to support
it. Recently educated Ph.D's having difficulty with arithmetic may leave
the hard bits for later reading.
----
Reg, G4FGQ