Hi Gary & fellow Topbanders...
It seems that Gary's 129 foot wire may be actually 133 to 136 feet long,
depending on radials and ground conditions and whether the wire is bare or
insulated.
I did a quick model on a recent version of EZNEC and found that Gary may
have to add series capacitive reactance of somewhere between -25j to -40j
ohms to get to about 1835 KHz resonance with minimum SWR.
XC = -40j requires about 2200 pfd series capacity.
XC = - 25j is about 3500 pfd series capacity.
The formula is XC = 1/(2 x Pi x F x C)
Where F is in Hertz or Cycles
and C is in Farads
The voltage requirements are not too high but there will be at least 5 or
more amps of RF if you run power, depending on the impedance at your feed
point. For safety sake I would use 2500 volt ratings on fixed caps and
3KV on vacuum variable caps. If you have a favorite center frequency,
combinations of 200 to 500 pfd mica or door knob caps should be
sufficient. If you use door knob style ceramic caps, use a lot of them in
parallel, as they do not like high current.
When done building a bunch of parallel capacitors for the value you
require, try a KW for a few minutes at the desired frequency for minimum
SWR and make sure there is no drifting of the value of XC (detuning of the
series reactance). Then turn off the transmitter and go out and
carefully feel the capacitors to make sure they are not very warm.
I ran 3 - 4 amps through four 1000 pfd, 5 KV caps in series once and darn
near blew them up.
73 & Good Luck
George, K8GG
> I just installed a replacement wire for
> the 160 sloper. I made it 129' as was the
> earlier one that came down in the winds. I
> have to use a spud gun to get fishing line
> over the tree tops and where the spud
> lands, I connect the wire to the eye
> fastener & reel it to the radial plate.
> This time I had to use a different tree as
> the earlier one came down in the last
> hurricane.
>
> Now it is resonant at 1.785 and it is not
> a problem at the lower end of 160 but
> higher up it is. My problem is compounded
> in that I also use this antenna for 10M,
> 12M and the upper part of 15M. I need to
> shorten it for use on these as well.
>
> It is too far away to shorten the wire and
> solder a new eye on it and now it is too
> cold to play with it on the tree (have
> difficulty walking right now). So being
> too long, capacitance at the feed point
> might be my best answer. I have several
> fixed vacuum & doorknob capacitors of
> small pF to choose from that I can put in
> line. Is there a formula that I might plug
> data into to come up with the needed pF?
>
> Thanks & 73,
>
> Gary
> KA1J
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