[TowerTalk] (no subject)
K7GCO@aol.com
K7GCO@aol.com
Thu, 16 Mar 2000 16:51:01 EST
In a message dated 15.03.00 17:30:27 Pacific Standard Time, dx@sparkster.com
writes:
<< One thing noticed on my half disassembled antenna though is that ALL OF
THE RIVETS ARE LOOSE! >>
Great Advice: <Also, upon reading the neat ideas about assembling joints at
cooler temps, it occurred to me that a pitcher of ice water would serve
nicely to cool the
joint (and yourself) in hot weather prior to popping the rivets. Nothing
like a cool pop in summer, eh? I guess you could use beer, too, but if would
be a waste on the aluminum, although still effective! :MARK_N1LO>
Best advice of all: <Mark, excellent advice. I will cool the elements, draw
slowly on the rivets and quickly on the beer. --Dave K0QE> Right on!
This clearly points out that new designs need to be checked regularly.
I used to overhaul well made beams before a test or a contest. I cleaned the
joints inside and out with special steel and brass shot gun cleaning brushes
on a drill. The above illustrates that aluminum rivets just do not have the
strength to support the stresses of aluminum tubing in the wind 24 hrs a day.
Was the joint .049" wall outer thickness with more wiggle room than a .058"
wall. Rivets are a compression fit with a stretched aluminum wire that
breaks in installation. The weakness of the rivet allows it to work. When
the pull wire breaks, the compression is slightly less which is a step in the
wrong direction. The hollow walls of the compression pop rivet are thin--so
the will compress. Steel rivets have a steal stretch pin that can apply more
compression force and they leave a steel pin to rust in side it. Too much
Band-Aiding.
The mention that rivets are used in airplanes is NOT APPLICABLE in any way as
there is no comparison in the construction or the way they are hammered in
place. At
Boeing I was a Quality Assurance Engineer and did all kinds of checks on the
rivets fit. They vary from a solid 1/8-3/8" of a special alloy. They are
jack hammered with a big bucking bar behind it That expands it tightly in
the hole and with compression that holds and there are far more than are
needed with no RF conduction requirement. You can't expand a rivet like that
in tubing.
Although I still use SS clamps coated with Anti Seize with aluminum particles
and or silicone grease, one must realize each clamp creates 2 more
connections for the RF going over it. RF travels only the skin of the
aluminum, over each clamp and back down on to the aluminum. If the clamp is
not greased, Aluminum Oxide forms under it which tends to increase the RF
resistance and I have some dandy pictures of bad cases showing heavy Aluminum
Oxide and deep pits under it.
It's never been pointed out that there is a very thin hair line "Conductivity
Ring of RF Contact" right at the diameter change all the way around if it's
clamped very tight. It's fairly low resistance only when it is new or just
cleaned. The further the clamp is from the joint the less the pressure at
the highest conductivity point. I haven't seen the riveted joints and I do
not know how far it is from the diameter change and if the outer tubing is
even split. If not there is eventual wiggle room. It's only a "Localized
Pressure" that gets lose and no way compares to the tight SS clamp which
stays the same all the way around unless there is differences in expansion
with temperature. Drilling burrs inside the hole as N1LO mentioned will
further compromise the rivets pressure in particular with constant wind
stresses. The riveted joint will eventually turn into a capacitive joint as
the Aluminum Oxide builds up which it can with lose joints fairly fast with
moisture. I've measured many joints with no DC contact. With a tight joint
there is no RF conduction in side the joint unless it gets by the thin ring
of contact at the diameter change due to a lose clamp or Aluminum Oxide build
up. It turns into a "Ring Of Resistance and Capacitive Reactance." Even the
tightest clean joint unprotected with the lowest RF resistance all the way
around is only temporary.
In parasitic yagi arrays, the Q of the director is a factor of the gain which
is lowered by any resistance and any eventual capacitive reactance of any
joints, changes the resonant frequency. These affects in the director affect
the pattern the most. I once wrapped .005" copper foil like a cigarette
paper around a director coated with contact cement of a 3 element beam and
then wrapped tape around it to keep it shiney. The Q increased enough along
with the .01" diameter increase to decrease the DE Z 5 ohms. I always wanted
to silver plate that foil. I could get 3' wide copper sheet that I would cut
long strips from. 3M had rolls of different widths of copper foil with
adhesive back that was great stuff.
I got some joints apart with great effort that have been together 45 years
and the Aluminum Oxide diameter increase was .015" on the inside tubing. It
tested capacitive. Electrolytic capacitors were made of Aluminum Oxide and
water--yagi joint material when uncoated. Without Anti-Corrosion Conductive
Grease and regardless how tight it is--there will be corrosion--the bitter
enemy of unprotected yagi joints.
Ropes in element works. It takes at least 4 diameter changes in a 20M
element to prevent the "Dreaded Mechanical Vibes" without the rope. It's a
good mechanical design but not electrical. There are 8 press joints of
potential loss and with 8 clamps, 16 more press RF conduction joints (1 on
each side of the clamp) or 24 total--72 for a 3 element beam. NOTE! There
are ZERO press wire electrical joints in a quad except the feedpoint and I
solder that joint also. How many grease the SS Clamp contact and the worm
gear?
I solved the problem totally with a flea market aluminum solder. My solder
joints are 6 years old and still look new--Zero Ohms, Maintenance and no
grease either which is the desired goal we seldom get. I wish I could show
the pictures.
Corrosion will always occur with dissimilar metals (even variations in
aluminum), moisture and air. Even the rivet aluminum is dissimilar to tubing
aluminum--and much softer. Every telescoping joint not properly protected is
another resistor and eventual capacitor that develops with time. Howsomever,
"Properly Designed Quads" have 1 soldered twisted joint per element that is
always zero ohms wiggling in the wind and no pop rivets. The RF efficiency
of quads due to joint contact stays at 100%. The contact resistance of .062"
aluminum welding wire quads will change unless coated (non included) as they
are bolted--I had one to test. That could be soldered with flea market
aluminum solder. The DE with the 5 wires tied together on each side and
soldered would actually improve the connections.
A Hints & Kinks soldering tip 40 years ago showed how bad Aluminum Oxide or
as I call it the "Dreaded Alumitoid Yagititus" or "Aluminum Cancer" really
is. It described how to solder aluminum--with conventional solder. It said
to make a solder pool over the joint (which takes a lot of heat) and scrap
the contact areas--under the solder. The solder pool keeps the air from the
joint. Enough Aluminum Oxide occurs immediately after scraping in air that
it is the reason conventional solder won't stick to aluminum (it's real oxide
fussy) but it will if air is kept off the freshly scraped joint under the
solder pool. So even just cleaned joints aren't all that great. The
aluminum solder sold at the flea market will indeed stick without the solder
pool. It's great stuff and strong.
Penatrox has one problem--it has ZINC in it I'm told. That's a dissimilar
metal and exactly what you don't want at that ring of electrical contact. It
prevent Aluminum Oxide inside the joint but that's only half the problem. I
noticed starting 45 years ago I had SWR changes in a year in beams coated
with it 5 miles from salt water. Clean and recoated it returned to normal.
Band-Aids usually fail. I got damn tired of cleaning yagi's. Use a design
that has Longevity built in--not Constant Maintenance, Mechanical &
Electrical Deterioration and Band-Aids. Antennas are mounted high off the
ground, expensive to erect and to take down unless you have tower skills and
equipment which few have. All kinds of weather takes it's shot at it 24
hours a day and winter areas can prohibit tower work or adjustments until the
spring. It can be a very expensive and high maintenance item you can't use
for months at a time. Even Force 12 has replaced rivets with bolts in the
mount. The License Manual didn't tell you that. Now you know. Buy and
protect accordingly. Rig problems can be fixed immediately and not dependent
on weather and height. K7GCO
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