W0UN notes on the KLM 4L 40M dual-driven Yagi
I had enough requests to post my notes to the reflector so that
they would end up in the archives that I have decided to go that route,
rather than sending copies to the dozen or so who have asked
for a copy.
I had heard about a lot of mechanical issues with the KLM 4L 40M
Yagis--so I decided to review the mechanical design and make any
mods that seemed appropriate before I installed my stack at 85 ft
and 170 ft. Along with a 4L M-squared the stack won CQ WW CW
Single Band 7 MHz for the USA the two years that W0UA operated
it in that category.
Topics to be covered:
Boom Material
Element To Boom Clamps
Elements
Linear Loading Standoffs
Linear Loading Insulators
Phasing Line
Balun
Boom To Mast Clamp
Boom Guy
Hardware
Boom Material:
I own four of these antennas--all of them are virtually identical, but
the fourth one has a QUARTER INCH thick boom, which is total
overkill. I bought this antenna on the used market so I am not sure
if this was an early version or a custom unit. The other three 40M KLMs
that I have all have 3 inch diameter booms of 0.065 inch wall tubing.
This tubing is really way too light weight for this antenna without the
addition of side guys. My two-stack does not have side guys and
one of the booms now has a slight, permanent bend in it. So I highly
recommend side guys if you get significant wind like I did in Colorado.
Light weight Phillystran is probably the best due to its low stretch
factor. The HPTG 2100 should be fine. And I am sure that some of the
smaller Synthetic Textiles rope would work--although it does not shed
ice nearly as well as Phillystran does and might end up as a liability
is a serious ice storm.
Element To Boom Clamps:
Some of the original KLM element insulators have failed on two of
my KLM 6L 20M Yagis. They appear to be a weak link although,
surprisingly, I have not had any failures on the somewhat heavier
elements of the KLM 4L40s. DX Engineering makes a much stronger
version of these insulators (different material) and if your plan is to put
up the 4L40 for 10+ years with no maintenance it would be best to
replace these insulators. When I decide to put these antennas back
up at the new Texas QTH I will probably use new insulators.
I also recommend using THREE hose clamps at the element to boom
bracket. It can accommodate three and this allows the maximum
grip onto the boom. Some hose clamps have a hole in the stainless
steel strap and it is probably a good idea to pin at least one of the
clamps to the boom using this hole. But I did not do this and have
not had any problems with the elements rotating on the boom, probably
due to the use of three hose clamps. Three hose clamps will fit if
you alternate (or stagger) the positions of the screw mechanisms so
that they don't line up with each other.
Elements:
I have had one instance where one section of an element has
slipped out from the next larger size element. I use anti-oxidant
on the telescoped sections and always check to see if the
smaller element section will rotate in the larger section and
continue tightening the hose clamps until this is no longer possible.
But I have still had a problem from time to time on other antennas
as well, and I am beginning to think that pinning the junctions with
an SS pop-rivet (in addition to the hose clamp) is inherently more reliable.
Future installations will be pop riveted ( or SS self-taping screw) as well
as hose-clamped.
Linear Loading Standoffs:
The weakest links in the KLM 4L 40M antenna are the linear loading
rod support insulators. They tend to break and/or slide around the
element. It is my belief that the breaking occurs when the elements
are moving side to side in the wind and the insulators get caught in a
bind with the linear loading rods. So I drilled out the holes for the linear
loading rods (not for the element itself) by a small amount, and I also
chamfered the linear loading rod holes so that the plastic would not "bite"
into the loading rods. This now allows the rods to move freely and not
bind up in the insulators and I have had no failures under high winds at this
historically troublesome spot.
Then I addressed the issue of keeping these insulators in place on the
elements. I always felt that the suggested method of taping them in place
was pretty crude so I devised another way to do this. I found some very
thin stainless steel welding rod -- I don't recall if it is 1/32 or 3/64 inch
diameter--whatever is the smallest diameter filler rod at the welding
shop. I made "hair pins" by folding the rod/wire into a hairpin and
then adding a "Z" bend in the hairpin so that these hairpins could go
through the holes in the insulators and then bend down to lay flat
against the element. With one on each side of the element it was
easy to add a single hose clamp to keep these in place on the element.
This locked the insulators in one position, helping to reduce the chances
of the insulators binding and breaking.
I am sketching a little drawing of the hairpin that will make the idea
a little clearer. But reflectors don't like attachments so if you want
a copy of the sketch you will have to write to me (AGAIN) and I will
forward it to you as soon as it is done.
Linear Loading Insulator:
The exposed fiberglass rods at the two break points in each element
where the linear loading rods are inserted were covered with heavy duty
heat shrink tubing to eliminate any degradation of the fiberglass
rods from UV. Scotch 88 tape would probably provide the same protection
as long as it is installed so that it does not "flag" by wrapping the last
few turns without any stretch in the tape.
Phasing Line:
I did have a failure in one of the phasing lines. The line broke at the
bolt hole at the element. I determined that the boom probably
bounced up and down in the wind and the phasing line was stretched
too tightly. So that when the boom went DOWN it pulled the phasing
line apart. After repairing the phasing line I reinstalled it with a
little more
slack in the two conductors. Of course this made it somewhat slack
in the center with the concern that it would short to the boom under
windy conditions. So I pulled the center of the phasing line upwards
with a short piece of small diameter stretchy Sythetic Textiles rope
attached to the top of the boom-guy mast. No failures since this
change was made (5 years ago).
Balun:
I did not know what was inside of the black box balun that was
supplied with the antenna--so I had no idea of its performance,
nor did I know what the phasing was--and this was a stacked
array with critical phasing requirements--so I replaced the supplied
4-1 baluns with 1/2 wave coaxial cable baluns. No cores equal
no saturation issues to think about. Probably not necessary,
just something I did to be sure.
Boom To Mast Clamp:
I did not use the supplied boom to mast clamps because I mounted
the two antennas across two legs of the rotating tower using
10 inch wide channel. So I don't recall the details of the boom to
mast clamps. But in many antennas of this size the thickness of
this aluminum plate is suspect. I tend to like 3/8 inch and 1/2 inch
material. If this plate is 1/4 inch then I would replace it with something
heavier. Maybe just a sandwich of a second plate identical to the
supplied one or fabricate a new plate from thicker material.
Boom Guy:
I used the tower to top guy the booms on my two beams since they
were side-mounted on the tower. So I don't recall if a separate short
mast is supplied for the top guy (I think it may be). Sometimes the
top guy is attached to the mast and this is a problem during installation,
erection since the mast is already in place on the tower and the beam
NEEDS a top guy for strength during raising. Since this is a fairly
sizeable antenna, I recommend top guying the boom to its separate
own support mast while the antenna is still on the ground. If this
mast is supplied then this issue is already taken care of. Sorry I
don't recall some of the details with the original design since I skipped
some of these steps in my unique installation and it has been en
years since I did this--and the antennas are still on the tower in
Colorado and I am at the Texas QTH for the next few weeks.
I believe that the top guy that was supplied with most of these antennas
was a piece of Phillystran, but the termination was a hokey little
plastic block with 5 holes. I scraped the 5-hole block and used
three wire-rope clips on each end of the top guys--even though this
vintage of Phillystran predates the accepted practice of the more recent
vintage of Phillystran that was designed to be terminated with wire-rope
clips. But I never have had any trouble using the wire-rope clips for
top-guying old-style Philly. But I do NOT recommend doing it this
way for tower guying.
Hardware:
Throw away all SS nuts and lock washers and replace them with
NyLok style aircraft locking nuts with the built in Nylon insert.
I do this on all of my antennas and never have pieces falling off. The
Nylok nuts are CHEAP compared to taking down the antenna
to replace something that vibrated off.
SUMMARY
If you do only ONE thing, then do the treatment to the linear loading
standoffs. Which is the most important modification in my experience.
Do not enlarge the holes for the linear loading TOO MUCH or the linear
loading rods will bounce around inside the large holes and make
a bit of racket. Just to be safe one might want to deliberately
mis-align these standoffs so that under normal conditions the
rods are slightly "spring-loaded" by virtue of their insulators being
offset slightly.
Hope this info is some help to the many people who requested it.
73 ES GL John W0UN
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