[AMRadio] Coax Loss versus SWR


Brett gazdzinski brett.gazdzinski at verizonbusiness.com
Mon Mar 12 18:33:04 EST 2007


What is the balun doing in this case?
What would happen if you removed the balun?

I have never used one, as I think they would just be another
thing to fail or loose power in.

Also, baluns are rated in pep watts and duty cycle, both
are very high on AM.

The 'legal limit' B+W folded dipole was only good for 150 watts
carrier of AM and that was pushing it.

Brett
N2DTS





> 
> Reply by Jim, WD5JKO
> 
>    Donn, everything you say is true. I will take a differing 
> point of view
> for a specific exception. What if the coax to be used is very short?
> By short, I mean like 6-12 feet long like you might have in a 
> mobile, or at
> home where an outside balun resides.
> 
> Consider the case where you have a 100 watt class rig feeding 
> a ATU, and
> then 10' of coax to a legal limit rated balun. The balun can handle
> considerable reactance, and miss match since it is being used 
> at 1/15th the
> power rating. The feedline SWR might go upwards to 10:1, and yet the
> transmitter is happy, and the losses are low, so therefore 
> the radiated
> signal will be fine. The loss from the coax as calculated 
> from this web page
> is:
> 
> http://www.ocarc.ca/coax.htm
> 
> .12 db at 3.880 for Belden 9913 / RG-8 @ 10:1 SWR
> 
> The remaining question is the efficiency of the balun. If the load is
> primarily resistive (= or > balun ratings), then I bet the 
> balun will behave
> pretty well. Now take this scenerio and add a linear 
> amplifier, and all hell
> will break out!!!
> 
> Regards,
> Jim
> JKO
> --
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