I think what a lot of people don't understand about open wire line is how it
accomplishes its "low loss" and doesn't care if the swr is high or not.
It has to do mostly with I squared R losses in the line. A typical 50 ohm
line with 1000 watts of power put into it has a current on it of 4.47 amps
with no reflected power involved.
If the resistance of the wire in the line is say 10 ohms then there will be
about 44 volts drop across that resistance. 44 x 4.47 amps = 210 watts of
lost power in line attenuation. Or you can figure it as I squared R =
4.47x4.47 x 10 = 200 watts.
If open wire line is used with an impedance of 450 ohms and the same 1000
watts applied then the current on the line will be only 1.49 amps.
If the same size wire is used as in the 50 ohm line it will have the same 10
ohms resistance. So there will be a 14.9 volt drop in voltage. 14.9 x 1.49 =
22 watts lost power.
The same thing happens when high swr is present on the line. The current
increases in both cases but the current runs much higher with 50 ohm line
than it does with open wire line because the higher impedance keeps the
current lower and thus the loss lower.
So the higher the impedance line the less power lost to I squared R losses.
73
Gary K4FMX
> -----Original Message-----
> From: towertalk-bounces@contesting.com [mailto:towertalk-
> bounces@contesting.com] On Behalf Of Andy
> Sent: Thursday, April 23, 2009 11:32 AM
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] Ladderline. What Kind ?
>
> > Googling it, I found that you can make your own, but it is not clear
> > what the impedance is, because it depends on the wire gage and spacing.
>
> Also search for open wire lines.
>
> How much does the exact impedance mean to you? Do you have a 450.0 ohm
> antenna on the end of your line? One of the advantages of ladder/open
> wire
> line is it tolerates mismatches very well.
>
> The other day I was looking at a website with plots of impedance vs.
> spacing
> and wire diameter, and it's actually not that critical, the impedance
> doesn't change that much as you vary the spacing once you get past a
> couple
> hundred ohms.
>
> Andy
>
>
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