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

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Subject: [TowerTalk] How much reactance change on a dipole, from 3500-3900 khz ?
From: "Jim Thomson" <>
Date: Mon, 5 Jul 2010 06:05:42 -0700
List-post: <">>
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 

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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