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
In a message dated 7/5/2010 8:06:15 A.M. Central Daylight Time,
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
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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