Has anyone tried this b4 ? I have a F-12 340N, used, of recent vintage. I
have done a few mods to it already, including replacing the 4" x 12" long
flat plates with upside down 4" channel AL [.281" thick] I also
installed TWO SS DX engineering U bolts and solid AL saddles at each end of
the new 4" channel. This was done on all 3 x els. The stock boom to
mast plate was replaced with a new one. The new one consist of a 8" x 17"
wide, 3/8" thick, 6061 T6 plate.. with 3 x 3" dx eng SS U bolts and solid al
saddles on each end. The mating vert plate is also 3/8" thick... but
only 8" wide x 14" tall. There I used those DX eng jumbo clamps... made for
a 3" mast. They look like oversized AL saddles on each side of the mast..
aprx 2.5" bearing down on the mast..from each side.
OK... here's the deal. I'm seriously thinking of installing a Jennings
ceramic 10-1050 pf @ 15 kv motor driven vac cap in series with the hot leg
of the DE. The normal [rebuilt with large diam tubing] helical hair pin
would be installed, in the normal fashion, across the bottom of the cap.. over
to the other side... which is just strap, down from other side of DE. The
Cal-AV EB-3 bead balun is tied directly in parallel with the helical hair
pin.
On paper, after doing the math's... this should work... and result in a flat
swr across the entire band. The stock swr curve is ok.... but I want to be
able to operate at the opposite ends of the band. F-12 measures the swr
curve in the shack, using the FT-1000MP's internal SWR meter.... and 400'
of LMR-400 to the yagi. The real swr curve, measured at the DE, is of
course, higher. [worse]
Have any of you tried this b4.. or seen it done ? The idea was to tune the
DE to the bottom of the band... or even a little lower. Then with 1050 pf
dialed in.. [ 21.7 ohms Xc]..... swr should be flat at 7003 khz. As I move
up the band, less pf required. .. [ more XC]
Of course with less pf... more XC results... and the peak V across the cap's
input and output will increase. Also.... the peak V across the DE center
insulator will be higher. That has been improved.. via Teflon sheeting beneath
the 4 x u bolt clamp assy.
On a similar note, this motor driven vac cap spins VERY fast.. abt 2.5 seconds
from min to max, using rated 28 vdc. I tried dropping the Vdc down to as
low as 16-18 vdc.. but it only reduces the speed a tiny bit. I came to the
conclusion that PWM may well work. I ordered a unit from my local emporium,
rated for 9-30 vdc @ 50A.. and also a 24 vdc @ 12.5 A CCS switching type
supply, to run it. Per their notes, I should be able to reduce the speed
to a crawl.. and still maintain most of the torque. Apparently my PWM unit
and also the 24 vdc regulated supply has arrived. I will be picking both up
tomorrow.
Any thought's on this experiment ?
later........... Jim VE7RF
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