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
sources.
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
approach.
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?
73,
Gerald K5GW
In a message dated 7/5/2010 8:06:15 A.M. Central Daylight Time,
jim.thom@telus.net 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
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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