[TowerTalk] How much reactance change on a dipole, from 3500-3900 khz ?

Richard (Rick) Karlquist richard at karlquist.com
Mon Jul 5 08:04:17 PDT 2010


I switch in about 250 pF to QSY from 3550 to 3850 kHz on my inverted vee.

Rick N6RK

Jim Thomson wrote:
> My software is not working to do this calculation at the moment. 
> 
> What I'd like to know is how much reactance change there is, when going from
> say 3500 khz  UP to 3900 khz.  
> 
> IE: 68' LL  rotary dipole resonant on 3500.    I'd like to know how much
> XC there will be on  say  3800,  3850 and 3900 khz.    Knowing that,
> I can calculate how much C  is  required in a motor driven  vac cap.
> 
> As we move up in freq... the value of the vac cap is decreasing. 
> I'm contemplating this scheme  vs   resonating the rotary dipole at say
> 3900 khz... then  switching in various amounts of XL.... with relay switched
> coils, via  a mess of vac relay's.   
> 
> I'm guessing the rotary dipole, if resonant at 3500, will  exhibit  75 ohms of  XL on 3800 khz ?
> That would imply a  560 pf cap would be required.... and even LESS C... on 3900 khz. 
> 
> I was contemplating the same thing for the 40M yagi... resonant at 7000 khz.... and then
> dialing in XXX  amount of pf, to resonate it progressively higher up the band.. to a max of 7300.
> 
> I have a pair of motor driven  10-1050 pf caps [15 kv, ceramic] .    They only take 2 secs to get from
> min to max... so tried slowing em down by reducing vdc... which hardlyslows em at all [3 x sec's] 
> The motor's are 28 vdc.. and stop spinning at 18 vdc.    I went to PWM... and this works very well. 
> Now I can slow em down to a crawl, and maintain almost full torque. 
> 
> If i had some aprx ballpark figures to work with for reactance change VS freq  for both 80+40m
> I can then calculate if this idea is even feasible. 
> 
> Tnx...... Jim  VE7RF  
> 
>      
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