With a typ tower setup, with HF yagi's on top...... the CM choke should be
installed at the feed point of each yagi. Then the shield / braid of the
coax should also be bonded to the top of the tower. That could be done if
a remote switch box is used. It could also be done by using a flange
plate bonded to the tower / mast..... using a bulkhead coax fitting, with
a PL-259 / 7-16 DIN plugged into each end of the bulkhead connector.
On my last 3 x towers, I then ran the coax down the inside of the tower,
since it was easy to do, since the tower was wide to begin with. I also
bonded the braid / shield to the bottom of the tower. via a HOFFI
lightning arrestor. The lightning arrestor also serves as a demarcation
point. With LMR-600 going up inside the tower......and LMR-1200DB
going towards the basement.
Base of the tower had 3 x 8' copper clad steel grnd rods jack hammered
into the grnd. Cadwelded 2 ga RW-90 stranded cu wire to top of each
grnd rod. The cadwelded potion was just below ground in all 3 of em.
Other end of each 2 ga cable had the big compression crimp lugs on
it....then bolted to the tower legs.
Then a bare 2 ga cu wire from the 3 x grnd rods.... running down the
side of the house ( buried 6" below the grnd) to a 4th grnd rod.... just
outside the basement,...which was then tied into the SPG / 200 amp service
panel etc. Hairpin match used on the yagi's.... so both sides of the DE
were at DC grnd..... via the braid being bonded to the top of the
tower....and also at the base of the tower.
I have seen where a CM choke, located right at the output of the
amplifier, solved a bunch of problems. That was at a buddy's station.
The supplied coaxial solenoid choke on his 2 el 80m (M2) yagi wasn't
cutting it.....RFI. The supplied M2 CM choke on his long boom 6M (M2)
yagi was also semi useless...with RF getting into the mic input. The
quick fix was to slide a ton of beads over the 393 coax that was on the
output of each amp. Problem solved.
That's the case where you can't get to the DE easily...since the DE in
both the 80m yagi and the 6M yagi were both a mile out from the
rotating tower. The real fix was to drop each yagi to the ground... and
replace the 'CM' choke with a real one...and optimized for each yagi.
On really tall towers, like 150-200', ideally the shield / braid should be
bonded to the tower at the top / middle / bottom of the tower, as that is
the procedure used on commercial tower sites.
Jim VE7RF
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