Topband: Low band antenna project questions
grants2 at pacbell.net
Mon Mar 7 14:56:56 EST 2016
Good recommendations. I did a couple of things differently for my 160 T
hung to 3 trees,
I use Lewmar or Ronstan or Harken small swivel blocks, cheaper, more
durable in tough environments and the swivel helps keep lines from
tangling or twist from messing things up. Don't use 3 strand rope.
Whichever I can find cheapest in 30mm or so.
For any serious load the ball bearing blocks are terrific. I loop my
hoist line thru the block to the tie off (or weight or bungee), so if
the antenna wire breaks, I still have a line through the pulley. I use
a separate line over the tree branch for the pulley and after a while
the tree grows around it and it won't come down. The wind oscillation
should be in the antenna hoist line, not the pulley line to minimize
chafe. Let the block sheave do the moving. With a loop the hoist line
can be replaced with a new one with a careful splice/tape to go thru the
block. 1/4" braid dacron from Synthetic is awesome and plenty strong
enough, and ok on the hands with gloves to what I can pull (100# +).
For the top T wire, I use Davis RF 13ga copperweld stranded with the
poly cover, no stretch, less weight, high strength, and less corrosion
I always use a bowline knot and Scotch 33 tape the bitter end after
another half hitch. It's a great knot but I've seen them shake out
under cyclic loads.
I use a 50::25 ohm TLT which is a very good match for my antenna, easy,
broad band and zilch loss. My T is tuned low in the band and then use
series caps to cover the band to 1980 or so.
On 3/7/2016 9:48 AM, Jim Brown wrote:
> Hi Gary,
> The antenna you describe should work quite well on both bands, but
> I'll suggest a couple of tweaks to make the matching easier. First,
> make it a Tee -- if you have a catenary, you can support a Tee as
> easily as an L. Second, make the top section a fan (like a fan
> dipole) with short elements to resonate it on 80 and longer ones for
> 160. Third, don't worry about remoting the tuner unless you feedline
> is very long. Unless the match is really bad, feedline loss on 80 and
> 160 is pretty low, especially if you use RG8. Also, you can make the
> tuning more broadband (and electrically lengthen the vertical section
> by 1-2 percent) by using two parallel runs spaced 12-18 inches, tied
> together top and bottom. Do a simple NEC model to get dimensions.
> Finally, use as many radials as you can, don't worry a lot about
> length, just think more is better. :) BTW -- 50 ft on the ground will
> be close to a quarter wave on 80, 100 ft on 160.
> As to physical details -- get a good pulley at each end, tie one end
> down, put a weight on the other end, and use some sort of "mechanical
> fuse" at the feedpoint so that wind doesn't break it. I use a mating
> pair of Pomona connectors -- when the wind blows, they simply
> un-mate. For the fan spreaders, cut short lengths (12-18 inches is
> great) of 1/2-in PVC conduit, drill holes about 3/4-in from each end
> to pass the wires through. Make this antenna as physically robust as
> possible to withstand the wind. At a minimum, #10 THHN for the long
> top sections that carry the stress. #12 or #14 is fine for the shorter
> top sections. Don't make any soldered connections -- they don't
> weather well, and wire tends to break at a soldered joint. Instead,
> use split-bolt copper clamp connectors sized to fit the wire you're
> using. For support rope, use 5/16-in rope from
> http://www.synthetictextilesinc.com/supportham.html It's resold by
> lots of ham vendors, but Synthetic Textiles is a bit cheaper. Smaller
> rope is sufficient for strength, but you'll appreciate the larger size
> when you're trying to pull on it to maximize tension, which pulls it
> higher. :) Don't use hardware store pulleys -- instead, use marine
> pulleys (good) or this excellent "rescue" pulley, which is also easy
> to rig.
> Out here in CA, the West Marine is the place to buy marine pulleys.
> 73, Jim K9YC
> On Mon,3/7/2016 5:50 AM, NC3Z Gary wrote:
>> Now I want to be able to use this antenna for 80M as well as the non-DX
>> portion of 160M. I can house a autotuner at the base (or make my own
>> network but that would require control lines). My thinking is to make
>> the 160M a 5/16 WL vs 1/4 to be more beneficial to 80M tuning without
>> loosing anything on 160M.
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