Towertalk
[Top] [All Lists]

[TowerTalk] Feedline (choke) question

To: towertalk@contesting.com
Subject: [TowerTalk] Feedline (choke) question
From: "jim.thom jim.thom@telus.net" <jim.thom@telus.net>
Date: Sat, 27 Sep 2025 09:52:36 -0700
List-post: <mailto:towertalk@contesting.com>
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
_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk

<Prev in Thread] Current Thread [Next in Thread>