On 1/15/2011 11:22 AM, Jim Lux wrote:
> Having a structural "fuse" is a good idea, assuming the consequences of the
> fuse blowing are acceptable.
Flexibility and elasticity should also be taken into account.
There are things that will work and things that will destroy the
antenna, particularly a vertical.
Verticals as well as all structures have mechanical resonances.
Unfortunately the verticals tend to have them in rather inconvenient
places and frequencies.
Most verticals (trap and otherwise) are advertised as not needing guys
and that isn't necessarily a lie. It's not exactly an all encompassing
Mounting a vertical on a flexible mast (all masts are flexible to some
extent) can change things drastically and amplify the ability of the
vertical to oscillate while adding its own modes to the verticals.
The point is when mounting a vertical on most anything that elevates it,
guying of the vertical at one or two points becomes more than advisable.
Mounting the vertical on a pipe mast makes it pretty much mandatory.
Over the years some of the installations I've had raised more than a few
eyebrows. The system that was on top of the 45G was one. This
installation was up for nearly 10 years and the failures were not due to
http://www.rogerhalstead.com/ham_files/Tower29.htm What you see is a
TH-5 at 100', a 7L C3i 6-meter yagi on a 29' plus boom at 115', and a
14' cross boom at 130' holding a pair of 11L C3I 440's and a pair of 12L
C3i 144 Yagis. All of this was mounted on a 2" structural steel tube
that extended about 20' down into the tower.
However a vertical would not have survived on that mast. Even my duoband
Diamond 144/440 repeater antenna would not have lasted. With winds
gusting to 70 the thing looked like a Bluegill fly rod that had just
tied into a record keeping Bass. The amount of movement was
astonishing. That the 144 and 440 antennas survived is almost
unbelievable after seeing how much they were whipping around up there.
Even 30 to 40 MPH winds put on quite a show.
I had an AV640 at 32' on an Aluminum tower for a while, then added a
section to take it to 40 feet. I cut a ring out of 1/4" Lexan using hole
saws. I first cut the od, then using the pilot hole from that hold saw
cut out the center to fit the AV-640 at roughly 2/3rds it's height. I
had marked out the 120 degree points and drilled 1/4" holes there. I
used 3/16ths Dacron rope for guys.
The AV-640 is going back up on a 24' length of 2" 6061-T6 with 3/16"
wall. (I do have some 1/4" wall if necessary). Due to the flexibility
of the mast AND the antenna this will require guying the mast at the top
and the antenna at the 2/3rds level again. BTW the mast is pinned and
pivots at the base and is anchored to the top of the NW guy post for the
If and I emphasize the "IF" the antenna is properly supported (guyed) as
well as the mast I see no reason for the mechanical "fuse" approach.
Guyed towers are "more or less" naturally rigid although they too have
mechanical resonant points and the manufacturers guying recommendations
take that into account along with the proper guy tension and size.
Masts OTOH tend to have resonances that are much closer to points that
lest them oscillate in typical winds. The 43 foot vertical will tend
to "whip" around with a very low frequency oscillation from the base.
Raising the base with a mast turns the base into a pivot point and
amplifies the oscillation. With a longer mast you can actually see the
antenna ends (top and bottom) moving in opposite directions putting the
null or bending point some where near the middle.Guying at this point
can actually make the oscillation even worse. The longer a specific
mast the more the null or pivot point will move up the vertical away
from the base.
Guying (at the proper locations) centered on the nodes will dampen or
eliminate the oscillations, but given enough length and strong enough
winds these oscillations can contain far more energy than would be
indicated by the specific wind load at those velocities for any specific
Think of the antenna structure as a giant mechanical resonant circuit.
With every oscillation cycle the antenna gains energy, just as in a tank
coil in an amp. Every cycle is additive. the energy will build until
EITHER the lost energy equals the input energy OR the point of
destruction is reached. That destruction can be spectacular and
*dangerous* although it rarely reaches that point.
Me? I'd not hesitate to experiment knowing the limitations of the
materials and structure and I'm willing to risk the thing self
destructing in many instances. BUT there is a reason for doing things
the right and informed way.
73 and good luck,
> I've thought about the pin/socket scheme, but always worried about it
> getting stuck.
> On Jan 15, 2011, at 8:01 AM, "Clint Talmadge W5CPT"<email@example.com>
>> There has been some discussion here on the reflector about the strength of
>> the pipe/tube to mount a vertical. It occurs to me that I may not want a
>> very strong pipe/tubes as a mount.
>> Years ago I dug a hole about 3 feet deep and buried a piece of 2" thick
>> walled PVC and cemented it in so that the top is just flush with the lawn.
>> Over the year in this I have a used: a thin walled aluminum pipe, a piece of
>> aluminum 2" X .25" wall mast (from Texas Towers) as well as a piece of thin
>> walled galvanized steel tube (from a support on a store display).
>> The first failure I had was the thin wall steel tube. But I think it became
>> a sacrificial element, as when it failed the HF6V did not even reach the
>> ground and had no damage at all. The second failure was the thin walled
>> aluminum tube. It allowed my KU4AB 6M loop to touch the ground but other
>> than being a little tweaked, was not damaged.
>> So I think I will defy conventional wisdom and stay with thin walled tube to
>> mount my various experiments.
>> Clint Talmadge - W5CPT
>> TowerTalk mailing list
> TowerTalk mailing list
TowerTalk mailing list