[TowerTalk] 80m rotary dipole

[Jim Lux]

----- Original Message -----
From: "Joern DK5TT" <dk5tt at opretzka.de>
To: "towertalk" <towertalk at contesting.com>
Sent: Saturday, February 12, 2005 3:48 PM
Subject: [TowerTalk] 80m rotary dipole


> Hallo towertalkers!
>
> I´d like to build a shortened rotary dipole for the 80m band. The dipole
should have a length not longer than abt 25m/82ft and should stand wind
speeds of at least 100kph/62mph. The biggest problem on my side is the
mechanical construction of the element (tube diameters and lenghts) and the
loading device.

Think of it as two monopoles or mobile antennas back to back...
So, there's several loading approaches:
base loaded (this is the "shorty 40" approach).. the coil at the base also
serves as part of the matching network.
center loaded (you could use one of the standard 80m mobile antenna
equations, although it would be the rare mobile op who gets to run a 12
meter long whip!)

A capacity hat (on the ends) will also help a lot.  For a horizontal dipole,
you might be able just to hang wires from the end.

Tom Rauch,  W8JI, has a website with lots of information on this whole
thing.

Structurally, if you can extend the mast up above the dipole, even a few
meters, then you can use a truss cable from the top down to each end, which
will at least support the mass of the horizontal boom (and make center
loading much more practical). A triangular structure (a strut from the
bottom) would also help, because you're basically trying to support a 12-13
m long canteliever.

Does it need to hold shape in 100 km/hr winds, or just not fail?  Do you
have icing or snow accumulation issues?

> As I didn´t find many information about that on the internet I would be
thankful about any tip!
>
> Thanks in advance and 73
> Joern DK5TT