# [TowerTalk] what size wire for kilowatt TX ??

Sat Jan 3 11:26:34 EST 2009

```Martin Ewing wrote:
> Chas,
>
> A little math shows that 5.5 Amps rms into 50 ohms is 1.5 kW.  My handbook
> says the DC "fusing current" of #26 copper is 20 Amps.  RF skin effects
> increase the resistance (perhaps by 5X at 10 MHz) and heating (25X).  But
> you're not going to have 5.5 Amps continuous.  For SSB, the average power is
> going to be much less than 1.5 kW, so you might squeak by, assuming the SWR
> isn't too bad.  I wouldn't use insulation.  All in all, #26 wire would be a
> poor choice for high power -- but hams have been known to try these things.
> Note that the current is normally greatest at the feed point and decreases
> toward the ends of a half wave dipole.  So you could taper the wire size
> down toward the ends.

Good point (although I don't think the dissipation goes up as the square
of resistance)

Handy skin depth calculator:
http://www.microwaves101.com/encyclopedia/calsdepth.cfm

AWG 30 is 10 mils in diameter, skin depth at 10MHz in copper is 0.8 mil,
so 3.14159*25 (DC) vs 10*3.14159*0.8 (AC) (25 vs 8.. about 3:1
resistance ratio)

>
> It might glow red, melt, or start a fire. If so, use a larger gauge!  Also,
> whatever power comes out as heat is not being radiated as RF.
>
> This all assumes you're talking about a full size dipole.  If your antenna
> is more like a short whip, the current in the wire is much less, since it is
> a high impedance antenna.  Then, your problem is in the antenna
> tuner/coupler and ground system, where there could be high losses.
>
> 73 Martin AA6E
>
> On Fri, Jan 2, 2009 at 11:04 PM, chas <chasm at texas.net> wrote:
>
>> Feeling sorry for those poor souls who moved out of the wrong side of
>> the track neighborhoods and left their towers and antenna farms to go
>> live in gated communities where they are trying to figure out how to
>> put up a 20' piece of wire....  invisible wire is the question here.
>>
>>
>> what is the minimum size or guage of insulated wire which can be used
>> hooked up to a kilowatt amplifier?
>>
>> I live in a barrio (now, 20 yrs ago.. whole different thing) so I have
>> either a king cobra or a dx-cc 12ga with grey insulation, etc.
>>
>> so I have NO problem running my SB-220 flat out thru the things.
>>
>> but what about 26guage wire?  actually, that Cobra is three pieces of
>> 26ga zipped together, 70ft on a leg suspended from a 35' center.
>> it works just fine and seems to have great bandwidth.
>>
>> just curious.  I know that some loops are made from magnetic speaker
>> wire and are used for both RX and TX.
>>
>> anyone have an answer or a source?
>>
>> thanks
>> chas, k5dam
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>
>
>

```