[TowerTalk] Antenna Question

[Steve Hunt]

Bob,

The answer is a little more complicated than might at first appear.

Let's assume the feedpoint impedance of your dipole is 64+j0 at 
resonance. That will indeed result in a 7:1 SWR on the open-wire line,  
referenced to 450 ohms.

But, depending on its length, the open-wire line will transform that 
impedance to something between 64 ohms - if it's an even multiple of a 
quarter-wave - and 3164 ohms if its an odd number of quarter waves. So 
if you then transition to coax, the SWR (referenced to 50 ohms) will be 
anywhere from 64/50=1.3:1 to 3164/50=63.3:1. Do you see that it's the 
IMPEDANCE at the transition which is the fixed parameter, not the SWR.

I'm not sure what your planned overall system comprises, but we would 
typically place a current balun at the transition between the open-wire 
and the coax. Whether or not an impedance transformation like 4:1 is 
useful will depend on the design of the rest of the system. Unless 
you've done the calculations and know that a 4:1 will help, a 1:1 is the 
better "default" choice.

Finally, I hope you can see that you can't simply take the SWR and 
divide it by the balun ratio - you have to be dealing in impedances, not 
SWR. Here's why:

Suppose I take a 200 ohm and place it across the end of a length of 50 
ohm coax - I'll have an SWR of  200/50=4:1.
If I now place a 4:1 balun between the 200 ohm load and the coax, I'll 
reduce the load impedance by a factor of 4 and the SWR on the coax will 
be 1:1
In that case the SWR has indeed been reduced by an amount equivalent to 
the balun ratio.

But now repeat the experiment with a 20 ohm load.
Without the balun the SWR on the coax will be 50/20=2.5:1
Now introduce the balun and the load impedance will drop to 5 ohms, 
giving an SWR of 10:1
In that case we've INCREASED the SWR by the balun ratio.
If we'd connected the balun the other way round (1:4) the load impedance 
would rise to 80 ohms giving us an SWR of 80/50=1.6:1

Do you see that there is no simple relationship between the SWRs and the 
balun ratio. And the link between the two is even more tenuous if the 
balun is positioned between feedlines having different characteristic 
impedances - for example 400 ohm ladderline and 50 ohm coax.

Hope that helps some,
Steve G3TXQ

Robert Kirkland wrote:
> When I put up my low frequency dipole from the tower to a tree it will be 
> center fed with 450 Ohm ( home made ) open wire line. Eznec says I will have 
> a 7:1 SWR. A 1:1 Balun would convert that to 50 Ohms with the 7:1 SWR. My 
> question is what happens if I use a heavy duty 4:1 Balun. Do I divide the 
> 7:1 ratio by 4, or what?. And a why, would be helpful. Also throw in any 
> math. Thanks.
> Bob, w5pvr
>  
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