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Author Topic: Gauge of feedline, of antenna  (Read 3070 times)
N3II
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« on: October 23, 2017, 07:33:18 PM »

What is the relationship of the gauge of the wire of a feedline and of the antenna itself. Should they be the same? One larger than the other?

Practically: I have some 12-gauge wire for an hf horizontal loop; I have some 18-gauge window line to feed it. Do I lose something by feeding the 12-gauge loop (or doublet) with 18-gauge feedline? If so, enough to warrant getting different feedline? What's the math I should be thinking with?
Thanks, David
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AC5UP
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« Reply #1 on: October 24, 2017, 12:27:06 AM »

At HF, the wire size has little effect on the radiation characteristics of the antenna and feedline...  The choice is based more often on tensile strength, weight, cost and availability than radiation resistance.  Do some research on stealth antennas made from #22 enameled copper 'magnet' wire and you'll find the efficiency and dimensions are very similar to the same antenna made from #12 THNN.  It's counterintuitive at first glance but true that in practical terms the potential for ice or wind loading is a greater consideration than electrical or RF characteristics.

In the case of a parallel wire feedline, 18 gauge copperweld is the usual choice as it's strong and of a practical size for common impedances.  The Z of window wire is determined by the gauge and spacing of the conductors.  Thicker conductors closely spaced have a lower Z than thin conductors at a wider spacing.  18 gauge copperweld will handle legal limit with zero perspiration into a reasonably well matched Doublet with plenty of margin.  You do not need to match wire size between the antenna and feedline.  Pay more attention to the impedance match, mechanical strength and connection integrity of the wires.

BTW:  It's also good practice to add a twist or two per foot to a window wire feedline swinging in the breeze...  Less likely to sail or whip on a windy day and reduces the potential for feedline radiation as each twist is approximately equal and opposite of the twist before and after.
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W9IQ
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« Reply #2 on: October 24, 2017, 01:46:21 AM »

Quote
The choice is based more often on tensile strength, weight, cost and availability than radiation resistance.

In fact it has no appreciable effect on the radiation resistance although it can affect antenna efficiency (resistive losses). Radiation resistance plus resistive losses form the real part (R) of the complex feedpoint impedance.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
G3RZP
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« Reply #3 on: October 24, 2017, 01:50:59 AM »

How thick is the copper in Copperweld? If it isn't substantial - say two skin depths -  then on the LF bands, it could be quite lossy when used as a feedline.
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KF4ZGZ
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WWW

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« Reply #4 on: October 24, 2017, 02:26:27 AM »

Use whatever you have.


Matt
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WG8Z
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« Reply #5 on: October 24, 2017, 03:35:15 AM »

It will be fine.
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G8HQP
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« Reply #6 on: October 24, 2017, 03:40:30 AM »

Thinner wire for an antenna means narrower bandwidth. Thinner wire for a feeder means higher loss. In both cases whether this matters depends on context.
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W9IQ
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« Reply #7 on: October 24, 2017, 04:30:37 AM »

Just to put some numbers to the RF resistance of various gauges of pure copper wire, here is the resistance per 100 feet at 7.2 MHz:

18 gauge = 6.7 ohms
16 gauge = 5.3 ohms
14 gauge = 4.2 ohms
12 gauge = 3.3 ohms
10 gauge = 2.6 ohms

If you then use this to compute the efficiency of a 7.2 MHz, 1/2 wavelength dipole with a 70 ohm radiation resistance, you arrive at the following:

18 gauge = 94% or -0.26 dB
16 gauge = 95% or -0.21 dB
14 gauge = 96% or -0.17 dB
12 gauge = 97% or -0.13 dB
10 gauge = 98% or -0.10 dB

To achieve similar performance at this frequency, aluminum wire must be one even number size larger.

So you can see in this example, you are playing with decimal dust from a loss perspective.  If the antenna is substantially longer or the radiation resistance is significantly lower, then the effect of the gauge of the wire on the efficiency of the antenna will be more pronounced.

For a balanced feedline, the resistive losses will dominate the losses at HF frequencies. The wire gauge will therefore play a similar role, recognizing that twice the length of wire is involved for a given length of feedline.

If you are concerned with complying with the USA National Electric Code (NEC) then the following applies to the antenna and lead-in wire:

    Less than 150 feet in length - minimum of 14 gauge hard drawn copper, copper clad steel, etc.
    Over 150 feet in length - minimum of 10 gauge hard drawn copper, or 12 gauge copper clad steel, etc.

Note that NEC does not allow soft drawn copper (such as ordinary THHN) or aluminum wire to be used for wire antennas.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
KM1H
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« Reply #8 on: October 24, 2017, 08:07:35 AM »

NEC strikes again, just another reason to ignore them.

Dont tell them Im using #8 stranded and insulated submersible pump wire for a 160M sloping vertical, and #12 solid for the same purpose for the initial 4 elevated tuned radials. The remaining 16 radials will be whatever I can splice together.

BTW what is the RF resistance of #14 stranded Copperweld as sold by the usual dealers at 1.8 and 3.5 mHz?

Carl
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N3II
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« Reply #9 on: October 24, 2017, 06:10:12 PM »

thanks for all the helpful advice. David/n3ii.
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AC5UP
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« Reply #10 on: October 25, 2017, 08:37:57 AM »

On a vaguely related note, here's an interesting weekend project for the truly ambitious:   ( meaning every weekend for the next five years )



No doubt capable of radiating a manly signal, I was looking for construction details of the radiating elements.  Specifically the wire type and selection criteria.  Nothing found, but the feedline is described as parallel copper tubing.  I have a hunch the elements are high tensile steel cable as this picture tells me strength was a priority.  Notice how many clamps and insulators are used to break the guys into non-resonant lengths.  That's a fortune in hardware with a near eternity in assembly time.  If a Kevlar or similar non-metallic cable was up to the task I suspect it would have been used.  Considering the largest transmitters are capable of 500 Kay-Dub this is not a case of scrounging for the last nano-watt on the ol' SWR meter.



To Seymour click here:  https://swling.com/blog/2012/12/for-your-holiday-enjoyment-a-tour-of-the-edward-r-murrow-transmitting-station/

"  Each of the antennas is a fascinating work of engineering, but the curtains are especially intricate. The sheer amount of stand-offs, insulators, and the parallel arrangement of elements were something to behold. The average curtain antenna is about 300 feet in height, 240 feet wide, with 20 DB of forward gain.  "

Uhhhhhhhhhhhh...  Mr Wizard...  If we take 500 kW of RF and throw 20 dB of gain on top of it, what's the calculated ERP of the forward lobe ?     ( Hint:  Your S-Meter points East )
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KB4QAA
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« Reply #11 on: October 25, 2017, 09:54:13 AM »

Quote
I have a hunch the elements are high tensile steel cable as this picture tells me strength was a priority.  
Nah.  Too lossy.  Think phosphor-bronze or clad steel.

Phosphor-bronze wire is typically used in naval shipboard and aircraft installations.  Strength is critical for these uses as well.

MIL-W-17211 is a specification for such wire.
Download spec here: http://everyspec.com/MIL-SPECS/MIL-SPECS-MIL-W/MIL-W-17211B_12381/
« Last Edit: October 25, 2017, 10:16:15 AM by KB4QAA » Logged
KB4QAA
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« Reply #12 on: October 25, 2017, 10:17:00 AM »

Quote
I have a hunch the elements are high tensile steel cable as this picture tells me strength was a priority.  
Nah.  Too lossy.  Think phosphor-bronze or clad steel.

Phosphor-bronze wire is typically used in naval shipboard and aircraft installations.  Strength is critical for these uses as well.

MIL-W-17211 is one specification for such wire.
Download spec here: http://everyspec.com/MIL-SPECS/MIL-SPECS-MIL-W/MIL-W-17211B_12381/
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WB6BYU
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« Reply #13 on: October 25, 2017, 10:30:30 AM »

For more information on the design and construction of such antennas, check out Laport's
Radio Antenna Engineering.  Available here.
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WB6BYU
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« Reply #14 on: October 25, 2017, 02:15:04 PM »

Quote from: KM1H

BTW what is the RF resistance of #14 stranded Copperweld as sold by the usual dealers at 1.8 and 3.5 mHz?



This article addresses the loss in transmission lines rather than the RF resistance of the wire
itself, but it shows the measured losses are about 2.5 times higher for stranded CopperWeld
lines than for the same lines with pure copper conductors on 160m.
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