Joint conductivity is a bit of thread drift. The OP (Me) was talking
about mechanically repairing antennas. The C19XR to be more specific, so
no, I did not address the conductivity issue, but stayed with the
mechanical. I appreciate the thread drift as it exposed weaknesses in
many designs, or certainly things to be aware of when making changes to
the original manufacturer's designs.
The rivet close to the end of the joint is a weak spot mechanically, but
I suspect it was directly related to age and electrical length. All
elements I have disassembled had clean Aluminum right up to the end of
the outer tube in line with the rivets. To me this points out the
validity of the design able to maintain the electrical length after many
years. I've never found a loose, blind pop river
It was a C3i design that had the solder fail, not F12. Again, this is a
mechanical issue and was addressed in the assembly instructions by using
multiple coats of clear Krylon spray. Unfortunately, clear Krylon does
not weather well and is gone within a year. Although not as pretty,
several coats liquid electrical tape works far better and lasts for
years. I'd guess that Silastic RTV, of either type would also work.
Being a relatively thin coat, out in the air, the Acetic Acid soon
evaporates although a purist would use the non acetic type.
Why did the lead/tin solder dissolve? Either it was acid rain, or more
likely being the tallest object around the corona with the presence of
clean/pure water leached atoms from the solder
The ability to survive rough handling, again is Germain to the original
post. Antennas are relatively large and fragile. How many have had
elements bent or broken during installation?
Swedging tubing on site also has problems other than the lack of
accessibility. Like welding it changes the temper, but unlike welding it
does not remove the temper, it hardens the tubing.
Often, tubing used in antennas is already tempered through drawing, so
swedging makes it even harder, often producing wrinkles.
At first, additional hardness makes the tube more brittle and the
swedged section is the only mechanical connection between elements.
Good, or bad? It depends on the specific installation, but is one method
for a rapid taper OTOH there are few chances for "do overs" when a tube
is swedged down on site.
Very often the split tube with the SS hose clamp is less than optimal as
too large a clamp is often used, while the smaller clamps are easy to
strip. The smaller hose clamps fit around the tube, but the larger ones
only contact a third to a half of tube circumference. Often there is
room for two of the smaller clamps.
The mechanical challenges for large antennas are more demanding than for
typical tribanders, VHF antennas and up.
Many hams are only interested in the electronics or digital
communications, rather than mechanical construction and many of these
are not what most of us would call mechanically inclined.
73
Roger (K8RI)
On 3/10/2016 Thursday 5:57 AM, ve4xt@mymts.net wrote:
Hi Roger,
I'd suggest all the failures you cite aren't actually related to the issue of
joint conductivity.
Solder deteriorates when exposed to weather. We know that, so why F12 didn't
account for it, I can't say.
Rough handling damages joints. Still not related to the hand-wringing over
whether a joint between two parts of an element is capacitive or resistive or
some combination both. My point was simply history suggests, absent damage or
poorly chosen fastening methods (such as plain solder or clamping unslitted
tubes with hose clamps), whatever electrical characteristics may exist at a
joint of two parts of a tubular element are of little consequence. At least at
the time of assembly.
A poorly placed rivet that allows for slop to develop is also unrelated to the
question of whether noalox serves to introduce a fatal amount of reactance.
The right choice of fastener is important for the longevity of the connection,
no question. But at the time of assembly, all of the current fastening methods
(rivets, hose clamps, compression rings, screws, etc.) should all render
differences in joint conductivity to below our ability to discern on the air.
I guess if one really wanted to be picky, the ultimate solution would be to
invent a device that could extrude tubing on site with a built-in taper. No
joints!
You could build your yagis the same way eavestroughers manufacture seamless
gutters on site. With enough adjustability to the extruder, you could even
manufacture one-piece booms.
73, kelly, ve4xt
ps: the on-site extruder was a joke. Please, no comments on whether the
metallurgy is sound!
Sent from my iPad
On Mar 9, 2016, at 10:59 PM, Roger (K8RI) on TT <K8RI-on-TowerTalk@tm.net>
wrote:
It would certainly seem like it, but I'm not so sure. Most of the better antennas work
well once tuned up and may work great for some years. The C3i antennas were great
performers for some years. Yet when taken down, the coax match on all had the solder
dissolved to the point where the matching section was no longer connected. They would
have stayed up for 4 or 5 more years before they failed, but a flock of Cormorants
decided "THAT C3i 7L 6-meter antenna" was going to be their roost. So I think
failures are often attributed to the weather or age rather than design.
I found a broken element tight at the first rivet on the C19XR. Admittedly it
was from rough handling. My wife had some neighbors move the antenna (on saw
horses) when I wasn't available to help. Holes in elements weaken them at that
point, so I'd want that first rivet a little farther back so the larger tube
would support the smaller at the weakest point.
Loose rivets? I wonder if that was an installation problem? With pop rivets
even cleaning the hole out with even a 1/64th over size drill would likely be a
source for problems. Pop rivets should be snug. If installed one-at-a-time the
following rivets may not want to go in, resulting in a hole being redrilled
73
Roger (K8RI)
On 3/9/2016 Wednesday 8:21 PM, Kelly Taylor wrote:
Isn’t a lot of this talk a bit of picking the pepper?
If joint conductivity of aluminum elements was a serious issue, wouldn’t we have heard
something about it in the, what, 60 years we’ve been using aluminum tubing for antenna
elements?
73, kelly, ve4xt
On Mar 9, 2016, at 6:36 PM, George Dubovsky <n4ua.va@gmail.com> wrote:
If you weld the aluminum tubing, you destroy the heat treatment in the Heat
Affected Zone, resulting in a much weaker element.
73,
geo - n4ua
On Wed, Mar 9, 2016 at 6:43 PM, Tom_N2SR via TowerTalk <
towertalk@contesting.com> wrote:
If you weld the tubing together, what is the wind rating then? Kind of
difficult to get apart, but very little risk of losing electrical contact,
rivets failing, rusted screws, etc.
Someone should try it and report their results.
Tom, N2SR
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Roger (K8RI)
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