[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 

<< One thing noticed on my half disassembled antenna though is that ALL OF 

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 

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 

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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