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Author Topic: Fiberglass or Aluminum mast  (Read 9568 times)

Posts: 188


« on: February 03, 2012, 08:42:04 AM »

  I'm finding that I need to get my dipole up higher.   There are military surplus, fiberglass or aluminum pole masts on ebay for fair prices.   I want to raise one of these along side the house.  The base will be on the ground.  Support mid way at the roof, then either guy wire (or not) the upper 25' of mast.

  Are the fiberglass or aluminum versions more durable?   I've seen more damaged aluminum masts around.  I'm guessing that the glass masts are better.

  For a 45' ? mast, are there other options beside the military surplus on Ebay.

  Thanks for your help in advance.

Posts: 17476

« Reply #1 on: February 03, 2012, 09:25:23 AM »

Aluminum holds up better than fiberglass in my experience.  I have several sections of
fiberglass mast where the bottom has cracked open due to sideways force, typically
from the wind.  This happened at heigths of 24' or 28' while guyed at the top and
supporting a wire antenna.  (I now use these as the bottom sections.)

If you do use the fiberglass, I recommend guying it every 3 sections to avoid having a
joint in the middle between the guys.  The bottom section can be 5 sections.  Avoid
any sideways force on the masts - they certainly won't withstand being tilted up into
place very well.  You can get by with fewer guys for a temporary installation.

The aluminum is probably more expensive, but I would recommend that for anything above
about 30', or for supporting any sort of load.  We've often used it for 3-element yagis on
Field Day up to 28' or so with a single set of guys.

Rather than trying to assemble the mast and tilt it up, stacking them vertically is much
easier and puts less stress on the joints.  I attach the guys (estimated lengths) to the
top section, put a second section on the bottom, then walk out until two of the guys are
tight and lean the mast against them so they hold it up.  Then grab the next section, pick
up the mast while keeping it leaning against the two guys, slip the next section on the
bottom, and set it back down.  This requires moving a bit closer to the guy anchors each
time to keep the mast leaning slightly against the guy ropes.  If you are going to put a
beam or other large object on top of the mast, set it up first without the antenna, get it
positioned where you want it and the guy ropes adjusted, then lower it back down the
same way, put the antenna on, and reinstall it.  It actually goes fairly easily once you get
the hang of it.  I've put up 40' of the fiberglass mast sections this way by myself.

Posts: 188


« Reply #2 on: February 03, 2012, 09:47:22 AM »

  Okay.  Thanks Dale for the setup tips.

  I would support the lower 20' of the mast with stand offs from the house.   The sections above the roof will have guy wires.

   The Ebay Fiberglass masts have a refinforcing collar on the female end. I wonder if they're any better than those without the colar?.

Posts: 17476

« Reply #3 on: February 03, 2012, 10:27:53 AM »

Quote from: KF6BJW

   The Ebay Fiberglass masts have a refinforcing collar on the female end. I wonder if they're any better than those without the colar?.

In that photo, some masts have the reinforcement, some don't.  The ones I have that are broken are
all reinforced.  Once the fiberglass cracks there really is no convenient way to fit it back together and
glue it, other than perhaps sawing out the shards then wrapping several more layers of fiberglass around

But the masts weren't designed for that sort of height - they were used to hold up camo netting,
generally in 8' to 16' lengths. Yes, I've put it up to 40' to see how it would work, but I wouldn't
do that on a regular basis.  It's great for temporary masts though - I keep several bundles of
7 sections tied up with bungee cords ready for ARES or emergency use.

Oh, and the fiberglass coating degrades in the sunlight:  after a few years outdoors you have to
be careful how you handle them or you may get a handful of fiberglass splinters.

Posts: 157

« Reply #4 on: February 03, 2012, 10:32:27 AM »

Take this with a grain of salt, because I don't have a whole lot of experience erecting masts.  However, I'm in much the same position as yourself at the moment and I've decided to get a Rohn H50 mast and top it off with one of the milsurp 4 ft. fiberglass sections (to get the balun away from other metal as much as possible).  The main structure will be about as strong as I can practically make it, and I'll be stabilizing it the same as you--bracketing to the house and guying above the roof level.

I felt that even the milsurp aluminum poles, while great for a temporary installation, probably aren't good enough for a permanent (well, at least long-term) installation.  Of course I tend to overbuild things when possible.

Posts: 17476

« Reply #5 on: February 03, 2012, 11:37:54 AM »

The fiberglass mast sections are much heavier than you need for that application.
Especially if you try to tilt up the mast, the top section will bend from the weight.
(The push-up masts should be pushed up vertically rather than tilted up anyway.)

I'd use a length of PVC pipe, usually the 3/4" thinwall stuff that is quite light.  You
don't need a full 4' to keep the balun away from metal (at least not with ordinary
HF baluns) and a foot or two should be adequate, though even that may not
be necessary.  Another option to look at is ABS pipe, which is available in larger
diameters and is lighter than PVC.

I have a 40' push-up mast extended to about 30' in the back field, secured to a
sturdy post at 1' and 7' off the ground but with no guys.  It wobbles around a lot,
especially when the sparrow hawk lands on it, but works well enough for holding
up wire antennas.  A section of fiberglass mast on the top probably would bend
it over more than the hawk, but probably is acceptable with sufficient guying.

Posts: 7718

« Reply #6 on: February 03, 2012, 12:25:45 PM »

With aluminum you have the information to calculate the stresses and design accordingly. The book Physical Design of Yagi Antennas is enough to do this.

With fiberglass we do not have this data.

PVC is out as it is quite weak. Wood can work and there are published table of strength or take a design from the ARRL Antenna Handbook and use it guyed, as shown.

Posts: 17476

« Reply #7 on: February 03, 2012, 01:00:53 PM »

The mention of PVC was only for a top insulator to keep the balun away from the metal mast
if deemed necessary.  As long as the guys are attached to the push-up mast and not the
PVC pipe it should be fine for that purpose. 

Posts: 1279

« Reply #8 on: February 03, 2012, 08:33:05 PM »

Hello, I do not understand that the fiberglass is week. The S9 antennas up to 41 feet are fiberglass and hold up quite well. DX engineering sells a 50 foot fiberglass telescoping mast for $138.00. I would say if the antenna is properly guide and uses a UV coating it would should last just as long as the aluminium type. It is also non conductive and less likely to be hit by lightning. I do not know how good the surplus masts on EBay are, but I would guess the the commercial ones I mentioned are quite good. One other thing the S9 antennas are made to be walked up at 41 feet with out a problem.

Just another point of view.



Posts: 17476

« Reply #9 on: February 03, 2012, 09:10:46 PM »

There is a big difference between a sectional mast designed for short lengths and telescoping
or straight whips designed to be self-supporting.

The mast sections are much heavier for the same length, and the forces inside the joints put
more stress on the socket where they join.  The masts can take quite a bit of downward force
on a mast, but won't tolerate much sideways force - either due to the wind blowing against
the side of the mast, or from trying to tip one up into place.

Think of it this way:  if you are trying to tilt up a 28' mast then at the lowest joint there is
a 24' lever acting on a 3" overlap where the bottom section fits into the top.  That's a
100 : 1 lever.  The sections weigh something over 2 pounds each, so even allowing for
the weight distributed over the length of the mast you've got at least 600 pounds of force
acting in a small area.

I have several such broken sections with which to demonstrate the result.

That's not to say that other types of fiberglass whips can't work at such heights, but that
these specific military surplus mast sections aren't a good choice to install that way past
a relatively low height.

But stacking them is relatively easy - I've put up 20' to 24' by hand with no guys, just
slipping it over the top of a T-post for temporary testing.  You can also do it by leaning the
mast against the side of a building or sticking it up through the branches of a tree.

Posts: 188


« Reply #10 on: February 03, 2012, 09:51:57 PM »

  The Rhon and DX Engineering products give me options.   In my dreams, I would have a crank up mast.

Thanks for all of your guidance.


Posts: 5

« Reply #11 on: February 04, 2012, 04:07:24 PM »

aluminum 4 ft sections are stronger thn fiberglas. The fiberglass tends to crack at the joint, in the center of the  mast assembly.  ie half way up or so.

Several guys around here use the 1.5" OD x 4 ft surplus mast sections bought used at mil surplus stores, up to about 40 ft. 44 ft is pushing it a bit.  If freestanding, they use three rope guys at about 25-30 ft up, and again at the top.  If the tension isnt too heavy, you can get by with  just one level of guys. 

They use  aluminum sections  rather than fiberglas to give a stronger mast that resists bowing  while you hoist it up.  The bowing at the midpoint will cause failure if you are not careful.

I just put  a 40 ft mast up.  My wife and I that is.  Aluminum sections taped at each joint and a hoist rope at the 25 ft point thrown over a tree.  Once up, I tied the 25 high rope back to the tree, and tied another section at about 8 ft off the ground to the same tree.  Then with a pully up top, I hoisted a couple of inverted L's ( one qtr wave wire for 160m and one for 80m, fed with a single coax at the bottom.)   Looks nice and straight with just a slight bowing towards the 160m  L far end.  Nice. Easy and effective.
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