[TowerTalk] Motor Driven vac used in 40M DE?

[Jim Thomson]

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