[Top] [All Lists]

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 11:46:57 EDT
List-post: <">>
Jim, a further thought on the motor driven tuner:
As you know, the loss in the variable C will be negligible. The loss in a  
loading coil is related to the Xl and Q. The more Xl, the greater the loss. 
With  this in mind, you might consider setting the C to a maximum value at 
3500 KHz  and adding just enough inductance to resonate the system at that 
frequency.  Also, part of the loading can be the network that transforms the 
feed impedance  (around 30 ohms or so?) to 50 ohms. 
With a bit of luck, the matching network will be usable over the range of  
frequencies you operate. I had a setup like this about 35 years ago to tune 
a  75m ground plane made of the upper 60ft of a tower/shunt feed and lower 
guys as  radials. The matching network in that case did cover the entire band 
from 3.5 to  4MHz by only running the variable C for resonance. The 
bandwidth was very  narrow, only about 25 KHz at the 3 to 1 vswr points but the 
motor driven C would  reduce that to a very low value.
In short, I think your plan will work just fine and would personally go for 
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  


TowerTalk  mailing  list


TowerTalk mailing list

<Prev in Thread] Current Thread [Next in Thread>