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Author Topic: How to Determine the Maximum Amount of Watts a Dipole Antenna Can Handle  (Read 2806 times)
N1IRF
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Posts: 37




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« on: April 25, 2013, 11:57:24 AM »

I am interested in building a dipole antenna. I was thinking of using either 18awg or 26awg stranded copper wire. The highest power I plan to transmit is up to 100 watts. I know that the amps generated is 1.4 amps.  How can I determine what the maximum amount of watts can a dipole antenna handle based on what wire I am using?
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WA3SKN
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Posts: 5437




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« Reply #1 on: April 25, 2013, 12:02:43 PM »

1.4 amps is what 100 watts is at 50 ohms.  The impedance will vary along the antenna!
BOTH the current and voltage will vary along the antenna.  But you should be OK with either wire gauge for your purposes.
Good luck with the project!
73s.

-Mike
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N1IRF
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Posts: 37




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« Reply #2 on: April 25, 2013, 12:04:42 PM »

I should also add that I will be using cw, ssb, and various data modes.  I think this may be a factor.
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WH7DX
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Posts: 1029




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« Reply #3 on: April 25, 2013, 12:13:36 PM »

If it doesn't glow or stink.. you're OK...  Grin

I would use 14 or 18awg.   DX Engineering sells some good black antenna wire.  Hard to see etc.  I think that is 14awg.   I use that on 40, 80, 160 dipole and 600ft beverage running up to 700 watts or so.

The DX Engineering dipoles look to be solid? 12 perhaps.   Thick stuff.   I think that was rated up to 3000 or something.    

I personally wouldn't use anything over 18awg (I'd stick to 14).   The cost difference isn't a big deal.  It's not radials etc.   I don't know the scientific aspect of it.. but I'm thinking a nice thick piece of copper is better than a tiny one like 26awg.

http://www.dxengineering.com/search/product-line/dx-engineering-premium-antenna-wire?autoview=SKU&keyword=antenna%20wire

Have fun!

Bryan
WH7DX
« Last Edit: April 25, 2013, 12:16:23 PM by WH7DX » Logged
WB6BYU
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Posts: 13028




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« Reply #4 on: April 25, 2013, 12:19:12 PM »

I can tell you from experience that even #32 wire will handle 100 watts - the limiting
factor is the mechanical strength.

There are several different factors that limit the power handling of an antenna:  
current is one: too much current causes the wire to melt (due to the power dissipation
in a short length of wire.)  The ends of a dipole are at a high voltage, and you can
get arcing and corona effects depending on how the end of the wire is shaped.
(A sharp point will arc at a lower voltage than a large smooth surface - one reason
for the ball on the ends of some antennas.)  Usually the voltage isn't a problem at
100 watts, except for very short antennas.

Yes, you have 1.4 amps at the feed point.  You can tables for the current carrying
capacity of various wire sizes:  that is a continuous rating, typically for cables in
conduits where there is little air flow, but it is a reasonable starting point.  An antenna
in free air has much better cooling than a wire in a conduit, however, so it can handle
more than the nominal current without overheating.  Also, modes such as SSB have
a low average power level, so the total heating is less at 100W than for a continuous
carrier mode.  (Even CW is about 50% duty cycle while transmitting, and half that if
you allow the other station a chance to say something.)  So unless you are very long
winded, the actual heating effect from a 100W transmitter will be less because of
the duty cycle, since the wire needs to heat up over time before it will melt.

For most ham use, you're safe with a wire size rated for half the calculated current,
and usually lower than that, especially on SSB.

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N1IRF
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Posts: 37




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« Reply #5 on: April 25, 2013, 12:20:29 PM »

The only reason I am considering 26awg is for a random wire antenna.  I live in a apartment and I would like to keep my landlord and neighbors from seeing a wire antenna.
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W8JX
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« Reply #6 on: April 25, 2013, 12:27:33 PM »

The only reason I am considering 26awg is for a random wire antenna.  I live in a apartment and I would like to keep my landlord and neighbors from seeing a wire antenna.

There is not a problem with 26 ga power wise though mechanical strength could be a issue. You might tie it off with fishing line that is clear to hide it too.
« Last Edit: April 25, 2013, 12:50:07 PM by W8JX » Logged

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AA4PB
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Posts: 12685




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« Reply #7 on: April 25, 2013, 01:02:14 PM »

I did that one time. It worked fine handling 100W. It tended not to survive wind storms too well. It also looked like a big pipe when the sun glistened off the shinny wire at just the right angle so it wasn't always as invisible as I would have liked.
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G3TXQ
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Posts: 1464




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« Reply #8 on: April 25, 2013, 02:51:48 PM »

I didn't see any mention of frequencies - the resistance of wire increases with increasing frequency, and the power handling reduces.

By way of example, the resistance of #26 wire is 9 times greater at 30MHz than it is at DC. So 1.4A at 30MHz would have the same heating effect as 4.2A at DC.

73,
Steve G3TXQ
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VA7CPC
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Posts: 2357




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« Reply #9 on: April 25, 2013, 05:43:07 PM »

Here's a bet:

. . . If you transmit 100 watts from a long-wire (random-length) antenna inside your apartment,
. . . both you and your neighbors are going to get RFI.

Alarm systems, especially, tend to pick up RF.  So do remote TV loudspeakers-- e.g. "5+1" surround-sound gear.

Now, if they don't know you've got a rig (A GOOD IDEA!), they won't know whom to blame.  But the effects will be there.

Using an unbalanced antenna close to the rig almost guarantees "RF in the shack".   And much of that RF will appear on the AC power lines.

I would urge you to use a dipole, instead.  If you have wood siding, string it up at the edges of the ceiling.  Don't be afraid to bend the wires.

.             Charles

PS -- with an attic antenna (Alpha-Delta DX-CC or DX-EE).
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W5LZ
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Posts: 477




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« Reply #10 on: April 26, 2013, 04:50:04 AM »

I don't think the power handling ability is going to be a huge factor, but the conductor's mechanical strength will be.  If it can't hold up it's own weight (and a little more) then the antenna made from that conductor isn't going to stay up very long.  'Stealth' is a different thingy altogether.  I'm sure there are various ways of 'hiding' it, but nothing is ever completely 'stealthy'.
Good luck.
 - Paul
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K7KBN
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Posts: 2765




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« Reply #11 on: April 28, 2013, 06:25:34 PM »

Make two identical antennas.

Put one of them up, connect it, and feed power to it.  Raise the power level until the antenna or feedline melts.  Measure the power constantly until meltage occurs.

Take down the remains of the first antenna/feedline and put the other one up.  Never apply as much power to this one as to the first one.

(HINT:  If you reach 1.5KW and nothing's going wrong, you should stop at that point). Does not apply on some bands.  Void where prohibited by law.  Some restrictions apply.  See store for details.
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73
Pat K7KBN
CWO4 USNR Ret.
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