Antenna Standoff at Tower

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William E. Reid:
I would like to "fly" an inverted-V from a thirty-foot metal tower. I'm concerned about interaction between the grounded, metal tower and the antenna wire. How far out from the tower should I mount the stand-off holding the apex of the antenna? Thanks in advance.
WB6FTI Bill in Kuna, Idaho

Bill:
I am sure that "experts" and W8JI will chime in here, but what I do, more out a matter of expediency, I made a bracket from a 6 foot long piece of "angle iron". I put a pulley on both ends and I drilled holes so I could attach the angle iron to the tower legs using "U" bolts.

This was placed as high up on the tower as I could get it, I ran a line thru both pulleys and back down to the ground.

I tie my dipole to the free end on the side of the angle with the longest leg.

I can now run the dipole up or down as desired.

Steve Katz:
Far enough so the inverted vee wires can't touch the tower.

It really doesn't matter, since the tower bisects the inverted vee, where there's a lot of cancellation so almost nothing at this location will impact performance.  Same reason the feedline doesn't change performance or pattern of the inverted vee, as long as it drops vertically from the feedpoint, bisecting the angle of the apex opening.

I usually use a 24"-36" standoff arm, so that in strong winds, no matter what happens, the inverted vee wires can't come in contact with the tower.  But shorter ought to work in most cases.  

WB2WIK/6

Robert S. Greenstein, Esq.:
Don at Alpha-Delta recommended my 82' long Alpha-Delta DX-CC be about 18" from the tower.

You did not mention if your tower was fixed, guyed, or is a self supporting multi-section crank-up.

In my scenario, I've got a Quad planned to go on a 3 section lattice-style crank-up tower.  The bottom of the Quad will be about 2~3 feet below the top of the tower.  I want my dipole as high as possible, but no closer than 10 feet below the bottom of the Quad, and the tower must be free to be fully cranked down nesting all three sections.  

Here's how I'm going to do it:

A stand-off bracket will be u-bolted just under the top of the tower to the center vertical tube through which the mast runs. This mast pass through tube is about 6" long at the top center of the tower and is welded at each end to tower plates also welded to the tower.  The tube is part of the tower and certainly stong enough.  Your tower may not have this but many do.

In this position the stand-off will not impede nesting of the tower sections.

The stand-off bracket is comprised of round pipe with two angle iron sections running vertically
at each end and is galvanized.  The exterior end u-bolts to a 10' long section of 1 1/2" UV tolerant gray PVC electrical conduit tubing with a swivel pulley (see: radioworks.com) connected to the bottom.  A dacron antenna rope acts as a halyard line to raise and lower the dipole as desired.  Should your QTH be in a high wind area you may wish to substitute a 10' section of TV mast for the plastic conduit.

Also, as mentioned in the ARRL Antenna Book and the ARRL Handbook, a guy wire or other support (i.e. stand-off bracket or vertical) if less than about 14 feet will not resonate on the HF bands.

Hope this helps.

'73
Rob
K6RSG

Michael K. Wingfield:
Bill, it's more a mechanical concern than an electrical one.  Use a standoff long enough to prevent physical contact between the antenna legs and the tower.  Take into account any movement the antenna may have in windy conditions. Make sure the standoff is solidly attached and can easily support any load you put on it.

No longer the tower climber I used to be, I abandoned the previous approach of affixing the antenna feedpoint directly to the standoff.  Instead I use a large eye-bolt at the standoff and run good quality rope thru it.  This makes it easy to raise and lower the antenna for modifications and repairs.

73 Mike

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