Topband: Caged Inv-L - Pros and Cons ?

[Bill Wichers]

My vertical (without toploading) is a 6-wire cage. I used the 18awg insulated copperweld and spreaders I made form aluminum sheet and extrusions. It tunes well, although I can't compare it to a single wire since I've only ever used the cage.

The big downsides I have with my setup:
1 - there is much more weight involved so it sags down from the tree branches more. This limits the max attainable vertical height (I have about 85 feet of radiator as I was going for a 5/8 wave on 40m and use it on 80 and 160 as well).
2 - there is a lot more wind and ice load to worry about. The ice loading makes #1 more of an issue. Ice/snow sometimes makes it heavy enough that the feedpoint lays on the ground and the antenna is unuseable until the snow/ice melts off. Windload mostly just bows the antenna into a bannana shape and maybe affects tuning a little but that's about it.
3 - the design of the supports becomes more of an issue and you need to provide a means to limit the amount of twisting the vertical can do in the wind. I did this in my setup by machining an aluminum center piece for the top with two stainless eyebolts on the sides for a horizontal support rope and one for the vertical radiator. The antenna is suspended between two large oak trees. The horizontal rope's tension going through the top bracket makes a sort of spring that keeps the antenna in one position axially. This has worked well for me and the antenna does not get twisted and tangled as it blows around in the wind.
4 - grape vine loves to climb the wire spreaders! Be warned! I swear it reaches out from nearby low trees longing to climb into the clear sky on my innocent antenna system :-)

The cage is a *lot* more work to erect than a single wire would be. I'm not sure I'd do it again. I am able to tune over a large amount of the band (a little mfj autotuner can easily tune the entire band), but the structural issues are much more involved than they'd be for a single wire. 

I also had an issue with keeping the wire tension balanced between the spreaders. The top and bottom spreaders are made of aluminum bar stock in a hexagonal spoke configuration. They are *strong*, but they took some effort to build. The top assembly takes all of the weight and the top and the bottom together maintain the general shape of the wires and their individual tensions.

The spreaders along the vertical at 10' intervals are open hexagons made of .080" thick aluminum sheet cut into 1/2" wide strips and bent into shape. The hexagon is "closed" by a small aluminum piece reveted to the ends of the hexagonal hoop. The strips present little area for ice to accumulate which was a concern for me (there is some buildup though). I put small brackets at the corners of the hexagon using pop rivits and then three wires in an "every other wire" pattern around the spreader are laced to the bracket with small stainless wire. The remaining three wires slide in polyethylene tubing (raindrip vertical pipe which has about a 1/8" center bore and maybe 3/32" thick walls) pieces that are laced to the remaining three brackets. The three sliding wires are fixed only at the top and bottom of the entire antenna while the three fixed wires are attached to every spreader. This allows the spreaders to be set in the horizontal plane without any tensioning issues. Originally I fixed every wire to every spreader but I ended up with some tight and some slack wires as it proved impossible to balance the tension between each wire segment and each spreader. In a 4-wire setup you may have a similar issue.

You can get good results with the cage but I'd be most concerned with the additonal weight (and resulting sag of your supports) and complexity of construction of the cage over anything else. I've had mine up for two years now and most of my maintenance issues have been clearing off grape vines and installing additional radials (I have around 33 right now and enough more wire to get up around 70-80).

-Bill KB8WYP
[Sent using Blackberry Messaging]

----- Original Message -----
From: topband-bounces at contesting.com <topband-bounces at contesting.com>
To: topband at contesting.com <topband at contesting.com>
Sent: Tue Jan 24 21:29:57 2012
Subject: Topband: Caged Inv-L - Pros and Cons ?

I am considering modifying my Inverted L such that the vertical part (~ 70 ft) is a cage of say 4 wires , purely for bandwidth. Besides BW any Pro's and or Con's to this idea ? my current configuration allows me to use from about 1800-1860

Dan N8DCJ
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UR RST IS ... ... ..9 QSB QSB - hw? BK