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Re: [TowerTalk] How much reactance change on a dipole, from 3500-3900 k

Subject: Re: [TowerTalk] How much reactance change on a dipole, from 3500-3900 khz ?
Date: Mon, 5 Jul 2010 11:32:22 EDT
List-post: <">>
Jim, there was an old DOS program in the ARRL Antenna Book software for  
modeling mobile antennas. A google search will probably turn it up from other  
This program expects coil loading but the output data will be useful for  
your application. The numbers produced are for a quarter wave mode so you 
will  have to double both R and j for a dipole.
I have measured data for the full sized driven element of a 75/80m 3-el  
beam built about 20 years ago somewhere, but the information would not  help 
for a loaded element. My recollection is that it took about 40 to 50 ohms  
reactance to move it from 3700 to 3500 or 3900 KHz.
I never tried a variable C but used relays to switch in +j for cw and -j  
for ssb parts of the band. Now with the wider bands, this might not be a good 
I need to take the beam down and do some ice storm related repairs but my  
poor old knees will not let me climb the 199 ft tower any longer. What is 
needed  is a good ass-jack system to get up there again.
Wonder if anyone else needs/wants an ass-jack for their tower?
Gerald K5GW
In a message dated 7/5/2010 8:06:15 A.M. Central Daylight Time, writes:

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 
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 
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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