A few weeks ago I posed a question about the power handling capability
of the balun supplied with the Cushcraft X7 triband yagi, since it
appeared rather small in size but was rated at full legal power.
Having burned up a Hy-Gain BN-86 balun a few years ago running 1500
watts RTTY during a contest, I was trying to avoid a repeat with the
darn thing 80 feet in the air. :-)
Good news.
Several hams replied to my post saying not to worry, the Cushcraft
balun would handle the power with no problem, and last night I
confirmed it.
The antenna is still on the ground (on three-foot sawhorses) so I
thought if it was close to resonance, I'd try running full power
through it and measure the temperature rise. Turns out on 15 and 10
meters the resonance and SWR were just about perfect, while on 20
meters the resonance was a little below 14.000 but the SWR was right
on, so I powered it up. I used a digital remote sensing thermometer to
measure the balun temp before and after. I removed the balun cover to
make the temp measurements, but left the cover on while running power
through it to allow normal heat build up. I ran it for 90 seconds on
each band. I did this at 1am so the bands would be completely dead,
and they were.
On 20 and 15 the starting temperature was 60 degrees F and the temp
after 90 seconds was 70-72 degrees F, depending on what part of the
balun I focused the thermometer on. On 10 meters, the temp was a bit
higher, around 80 degrees. A rise of even 20 degrees is negligible,
IMO, since even at 80 degrees the balun feels barely warm to the
touch. BTW, 90 seconds seems like an eternity when you're sitting
there waiting for something to smoke. :-) Bless you, Alpha!
I was impressed. The Cushcraft engineers have done their homework on
this one and I have full confidence it will run nice and cool even in
a contest situation. I hope to have the antenna up later this week. If
it works as good as it looks, I'll be very happy. It is replacing a
four-element quad which is probably a better performer, but has broken
too many wires too many times. If the quad was on a crank-up or
tilt-over where I could get at easily to repair it, I might keep it,
but at 80 feet in the air on a freestanding tower, repairs are a major
project. The X7 should be a lot less trouble, and has a single feed
line to boot. No more switching.
FWIW. Comments welcome.
Bill W6WRT
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