-----Original Message-----
From: Ian White
Sent: Monday, December 01, 2014 1:47 AM
To: 'Jim Thomson' ; towertalk@contesting.com ; 'Steve Hunt'
Subject: RE: [TowerTalk] Baluns...just one more time.
VE7RF wrote:
### Im assuming that the top of the tower..or a qtr wave back from the
feedpoint of a yagi, say on the boom itself on a 10 or 6m yagi, would
be a
low Z point ?
I may well be wrong, depending the height of the tower, and how well
bonded it is to the ground....say via 3 x 8 foot copper clad grnd
rods,
and cadwelded 2 ga cu wire to each tower leg.
It isn't clear what you're aiming to achieve here, Jim. If you connect
the coax shield to the boom at a point that has a low impedance down to
the ground at the base of the tower, that will *maximize* unwanted
radiation and noise pickup by the boom, mast and tower. Surely our aim
should be the exact opposite?
1. Minimize unwanted common-mode current on the outside of the coax
shield
.AND.
2. Minimize crossover of that CM current onto the boom, mast and tower
(to prevent them becoming an unwanted part of the transmitting antenna,
and also a receiving antenna for noise).
#1 is best achieved by a high impedance common-mode choke (aka "feedline
choke", "line isolator" or "current balun") at the feedpoint. I think
most of us here on TT understand that... I hope...
#2 is the problem area, if we also wish to connect the shield to the
metalwork of the boom and tower for protection against lightning. To
minimize the crossover of CM current at the bonding connection, the
optimum location would be at a minimum in the CM current on the outside
of the coax shield and also a maximum in the impedance of the boom, mast
and tower down to the ground below. But where is that optimum point?
Does it even exist? And when we change bands, won't it move to somewhere
else?
It seems to me that the best strategy is to focus intensively on #1, and
aim to reduce the CM current on the feedline to a very low level. Issue
#2 - the location for bonding - then becomes far less critical.
But there are also some horrible examples of badly installed CM chokes
that may make the whole situation even worse than before, so perhaps we
should also add a third requirement:
#3. Don't make mistakes!
One of the most common mistakes when installing a CM choke is to tape it
to the boom. There are many criticisms of reactive air-cored chokes but
worst of all is the way they are so often just taped to the boom.
Remember that the shield is connected only to one side of the driven
element, so the capacitance between the shield and the boom will
UNbalance the feedpoint! CM chokes of all descriptions need to be spaced
away from the boom by a block of insulating material, or suspended
inside a plastic enclosure.
There are serious doubts about CM chokes in metal enclosures. As well as
unnecessary cost and weight, a metal enclosure raises a whole series of
unnecessary questions about unbalanced stray capacitances, and how or
where to connect the box to the shield and/or the boom. Those issues can
be completely avoided by using a plastic enclosure; or in many climates,
no enclosure at all (neither coax nor ferrite require weatherproofing).
Another bad practice is to pass the coax through the boom at the
feedpoint, which seems to have become fashionable for 6m antennas
recently. Once again, think about the unbalanced capacitance.
73 from Ian GM3SEK
## what I was trying to achieve was an optimum place to bond the shield
to (a) either the boom...or (b) at the top of the tower, or close to the
top of the
tower. I was under the assumption that bonding the braid within a physical
qtr wave
would be best. IE: somewhere between the xcvr side of the CM choke..and
tower.
## This is on my to do list, but I will measure the C, in pf, between
braid and boom,
on a 3 foot length of 213-U..taped to a 2 inch OD boom. I think we are all
going to be in for
a surprise here..as the only thing between the braid and the boom is..the
vinal jacket of the
213-U. I will test it 1st taped to the boom, then again with a sheet of
micarta or scrap plexiglas,
say .25 inch thick, between coax and boom. Then try it again with .5 inch
thick material.
C should drop by a factor of 4.
## Another potential issue is.... if a hairpin is used, BOTH sides of the
DE are tied
together, through the low xl path of the hairpin. IF a beta match is
used, aka Hy-gain 204BA style,
the mid point of the hairpin is bonded to the boom. A beta match is just a
hairpin that has had its
mid point bonded to the boom. The mid point is suppose to be an electrical
neutral point, so you can
either bond it to the boom, or float it like a true hair pin. So the UN
balance you are talking abt, when coax
taped to the boom, on the xcvr side of any CM choke, may well not happen.
IMO, if the CM Z is high
enough on the balun, bonding the braid back towards the tower may be a
moot point. A lightning arrestor
may not be required either, since the center conductor of the coax + braid
are bonded..via the hairpin, plus
braid is bonded to top + base of tower.
## IF a multiband yagi is used, like say a 40-10m yagi..which uses one
feedline, a qtr wave on 40m would be
.75 wavelengths on 15m. Buts that a long way to go, aprx 35 feet...till
the braid gets bonded to the tower.
## The winding technique used by K9YC uses huge loops of coax on the
cores....and these loops have to be kept well
away from the boom, the stray C between them and the boom will upset their
performance. If a bead balun is used,
the beads, or the PVC tube they are in...can be right against a boom.
## some multiband yagis, like the f12 5bA use 3 x feedlines, one for 20m,
2nd one for 17+15m, and the 3rd one
for 10+12m. 3 x chokes are used.
## If a rotary dipole is used, it will be very tough to keep any CM choke
balun away from the mast, unless some insulating
material is used.
## OK, if CM choking Z is not high enough..and braid is bonded to both top
of tower...and base of tower...and tower base is earth
grounded, will any residual CM current continue flowing along the
braid..from base of tower to shack, or will it get diverted
to the towers grnd system ? Same question..if the coax braid is also earth
grnded at the shack end of the coax... just b4 it enters the
home.
Jim VE7RF
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