[TowerTalk] balun design summary and data

[Tom Rauch]

> If choke impedance isn't the correct measure of
> worth, what is?

While Z is important, the value of resistance and reactance is what 
you need to know if you run any power. If anyone measures the 
impedance, also measure or calculate Rp. From Rp and voltage, 
we can get dissipation. From that heat. Ignoring that could be a 
balun-fatal mistake.

Example:
Rp of a "beam balun" I measured is 740 ohms on 7 MHz. With a 
50 ohm feedpoint, 1.5 kilowatts is 274 volts. Normally half that 
voltage appears across the balun (137 V).

E^2 / R is 25.37. Dissipation is about 25 watts. Imagine a twenty 
five watt lamp enclosed in a sealed PVC housing less than a foot 
long in the hot sun, or even a winter night without Arctic winds.

That seemingly low amount is far beyond the safe limit of the 
beads used even in open air! The curie temperature (only 118 deg 
C) would easily be reached in sustained contest operating. Once 
that happens the Rp resistance of the beads drops, the balun quits 
being a balun, and heating generally increases more. The beads 
may not recover and go 3 Mile Island.

Just an example why resistance is important in transmitting 
applications. A tiny fraction of overall power can be disaster in a 
closed space.

Looking at impedance alone is a small part of the problem. This 
balun example was of a "40-10 meter 1500 watt rated balun".
That's why it was in my junk box, and a coil of coax is on my 
antenna.

Unless you know Rp, you have no idea if the balun will work for 
anything more than QRP.


73, Tom W8JI
w8ji@contesting.com

--
FAQ on WWW:               http://www.contesting.com/towertalkfaq.html
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm