On Tue, 27 Jul 2010 06:32:53 -0700, Jim Thomson wrote:
>## ok... this brings up another point. What about your super high Z
>choke baluns, when used on a yagi ? They will also look like a high Z
>.. and probably 45-90 uh.
At the frequencies where we use them, they have a high resistive impedance.
At the lower frequencies where lightning dominates, they look like a lossy
inductance.
>Have you ever measured the uh on them...say from braid of input to the
>choke.. to the braid on the output side ?? What kind of readings do
>you get ?? say with a run of the mill simple LCR meter. Or is the resistive
>component so high, that there is little uh.
That's exactly what I am measuring, except that I'm using a different test
circuit that gives me better accuracy. Rather than measure uH directly, I can
compute it from the resonance curve. You can see values for typical chokes
both in the tutorial and the Power Point slides. These are computed values
from measured impedance curves, and are most accurate near resonance.
Accuracy is not nearly as good at frequencies well below resonance, but it's
"in the ballpark."
>## what happens when lightning hits the DE of the yagi..or will it opt to
hit the boom /other ele's instead ??
Yes, the choke would likely help it choose the lower impedance path of the
boom and tower to earth. Of course, if there's enough of it, it may explode
the choke. :)
>## On the INPUT to my remote switch box on the mast, abt 3' above the top
of the tower, is where I have the
>type-31 choke balun. This is to just supplement the existing bead
baluns at each yagi's DE. The idea here was,
>if RF spillover gets past the existing type 43 bead baluns.... it won't
get past the type-31 balun on the input to the
>remote switch box. The type-31 balun of course, is being used on all
bands..since it's on the input to the remote switch
>box.
Doubling up on the chokes is a good thing. The best place for any choke is as
close as possible to the feedpoint of the antenna itself. That essentially
disconnects the feedline from the antenna as far as common mode current is
concerned. Any feedline between the choke and the antenna can carry common
mode current, coupling noise. Adding the second more robust choke simply
makes it a better disconnection.
>## speaking of lightning arrestor's.... mine was slated to go at the base of
>the new crank up tower.. and tied in to the 3 x cadwelded 8' rods, buss
>system, etc.
I put my arrestors at the coax entry window by the shack. My tower is 100 ft
from the house and 250 ft from the shack, with no connection to either other
than coax and control wiring. Remember that an arrestor attempts to kill the
differential voltage on the coax to protect RX front ends. Grounding the
braid kills common mode, which is the bulk of the strike.
73, Jim K9YC
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