[450] Problem solved! Balun issue!

George Andrews gandrews at ntplx.net
Mon Feb 3 15:26:41 EST 2014



I found the reason for my antenna problem. A Unadilla balun
came with the antenna, probably typical for a number of
commercial dipoles and wire antenna. The Unadilla baluns are
rated at 2kW. One would think that should not be an issue
when using only 600W sideband. What I did not think about is
that the 2kW rating is for antennae with 3:1 SWR. Once the
antenna SWR is over 3:1, derating is required. On 40 M, my
calculated SWR is 3.4:1. This is apparently enough to cause
heating over a couple of months that resulted in a breakdown
of the insulation in the balun. As the balun was inside the
house, I often checked its temperature by touch and also
with a thermistor attached to the side. I never observed a
significant deviation from ambient. There is air space
around the core and the PVC is probably a good thermal


SSB is about 30% duty cycle, so that is about 200W CW. Not a
lot when considering the unit is rated at 2kW. Information
is not available for a derating curve. 3.4:1 is not what I
would regard as that far from 3:1. Some SWR calculators even
report that as 3:1. I was advised that one should not run
more than 100W into a Unadilla balun. That is likely good
advice. Most baluns are outdoors, so that may prolong the
life. I will either build or buy something like Balun Design
units in the future. Unadilla will only see QRP power levels
from here on out unless I can obtain derating info.


Some may already have an understanding of this, others may
not be aware. 


I am back on the air with 100 W and expect the new balun to
arrive in a day or two at which point I will be back on with
600 W capability.


I have included a photo so you can see the incredible
engineering of Unadilla baluns. 


73, stay warm!




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