[TowerTalk] The K0FF 5/8 40M antenna with picture links

[K0FF]

The 40M 5/8 antenna (225 degrees) is arguably one of the best transmitting
antennas ever. I say "arguably" because there is a lot of argument over it.
Some say it's nothing but a 1/2 wave with a little extra (which of course
it is), and some say it's a 3/4 with a little less (also true). Recent
computer analysis points out that the radiation angle can be severely
affected by an inadequate groundplane, and in reality it's no better than a
1/4 wave.
Nonetheless, I've always romantically thought of the 5/8 as the ideal,
mostly because in the AM broadcasting industry it is considered so (and so
did John D. - in his 1st book).

This is a description of the mechanical details of the antenna that I built
and use. The radiating element starts out with a Rohn HDX56 tower. On the
rotor plate I installed a 1-1/2 inch plumbers pipe flange.
On the top plate, where the thrust bearing usually goes, is another pipe
flange, but the threads have been removed on a lathe, and set screws
installed on 3 sides of the lip. Into this is slipped a piece of 1-1/2 inch
waterpipe (threaded on both ends) of about 10 ft in length, and it is
secures by screwing and clamping the flanges. At the top of the
waterpipe I installed a reducer from 1-1/2 to 1"and put a 2 foot
nipple in there. The transition was tightened, drilled and
thru-bolted.
This gave a secure mounting point of the proper diameter for the next
section, which starts out with the insulated base from a defunct Cushcraft
AP8.  The insulator was retained (for future insertion of a motorized
loading coil), but shorted out  with straps for this project. From there on
up, I just added enough tapering aluminum tubing to bring the
total length up to around 85 feet.
The thing needed to be pruned during installation, because there is no
way to determine the effect of that radical a taper beforehand - more on
that later.

Tower picture showing telephone pole "fulcrum" use when tilting the tower :
http://homepages.dstream.net/K0FF/5-8Fourty/etower.jpeg

The rest of the tower sections are assembled, and all the joints are doped
with electrical goo, before tightening.
Now the interesting part - how to hold all that up in the air,
freestanding, and insulated it from the ground? Well I did it by using two
1/4" steel plates, each 8" square, held apart in a sandwich of 4 electrical
insulators .
One sandwich for each of the three legs. A hinge base kit is bolted
to the top plates, and the J-bolts in the concrete go into the bottom plate.
These are the red plastic insulators about 2-1/2 inches in diameter, 2
inches tall, and with a 1/2 inch metal thread at each end, made to hold
giant buss-bars and feeders in electrical distribution panels.

Base detail:
http://homepages.dstream.net/K0FF/5-8Fourty/eInsBaseClose.jpeg

Exploded parts picture depicting a variety of surplus insulators:
http://homepages.dstream.net/K0FF/5-8Fourty/eInsulatorparts.jpeg


The J-bolts have a lot of thread on them, so I first added one nut to each
and tightened it down almost all the way to the concrete. Next is slipped
on an aluminum disc that will receive the radials and groundrod connection.
This disc is thick aluminum and is 1 meter in diameter, with forty quarter
inch holes drilled around the perimeter. Another nut on each J-bolt secures
this in place, and then the leveling and top nut for the insulator sandwich.

Radial plate:
http://homepages.dstream.net/K0FF/5-8Fourty/eRadialplate.jpeg

To facilitate raising the tower, I installed a wooden telephone pole in
concrete, about 2 feet from the tower base, and in the direction opposite
that which the tower lays. A heavy pulley at the 25 foot level, accepts a
5/8 inch rope, and the lawn tractor does the pulling. To protect the
insulators during the pulling up phase, there are some extra holes
(slots)  in the steel plates of the sandwiches. These allow short pieces
of pipe to be thru-bolted in place between the plates, taking any strain
off the insulators. Quite a few up-and-down cycles were anticipated
during the tuning stage, but a little mathematics helped the task be
accomplished in only 2 times.

Also as an added precaution, some guy-points were built into the tower
structure. These are 3 inch long pieces of UNISTRUT, cut so that one of the
running holes is in the middle of the section. They are bolted on
vertically at two levels on each leg. Then they are cross drilled to accept
a 1/2 inch SS bolt, and the Dacron guys with thimbles are slipped in and the
bolt secured with a NYLOK nut.(Nylok nuts have a nylon insert that
effectively
locks the nut onto a bolt, so that no lockwasher is needed).

Guypoint hardware example:
http://homepages.dstream.net/K0FF/5-8Fourty/Guypoint.jpeg

Closeup view of a Nylok nut:
http://homepages.dstream.net/K0FF/5-8Fourty/Nyloknut.jpeg


Now the thing is up, and 40 insulated, tuned 1/4 wave radials lay on the
ground. One third of them are cut for 40, one third for 80 and 1/3 for 160
(always anticipating!)
How do you tune a 5/8? It's not resonant. Well it IS resonant somewhere.
All you have to do is figure out where.
Well a large cross section 1/4 wave (call it 2/8 if you want) vertical for
midband on 40 should be 32.33 ft tall so 5/8 should be about 2-1/2 time
that, or 80.8. A 5/8 works out to be a little longer than that because
there is only one "end effect" to deal with. The math is simple - treat the
antenna as if it were a 1/4 wave on SOME frequency, and resonate it. Once
that's done, all you have to do is match it to the feedline at the
frequency that you intend to use it at.
Follow along: 7.1 MHz is actually 42.25 Meters, freespace. Multiply that
times 5/8 (.625) and that comes to 26.4 Meters. Now what frequency is 26.4
meters a quarter wave for? Multiply 26.4 times 4 and you get 105.6 meters.
That's the wavelength. Divide 300 by 105.6 and you get the frequency which
is 2.846.
Just tune the antenna to 2.846 MHz using your favorite analyzer. Another
shortcut is to first measure the length of what the antenna happens to be,
then check the resonance frequency. Next prune the antenna by a measured
amount, and recheck the resonance frequency. You can then easily figure the
inches per MHz factor, and get the correct tuning point on the second try.
Sure beats all that cut-and-try.

My antenna was resonated using this method, and then matched to the feed
line which happens to be 75 Ohms in this case. A simple series capacitor
made from a sandwich of doorknobs between aluminum plates, and a shunt
inductor brought her right in. The tube amp is set for 75 Ohms so there is
no mismatch anywhere, and the low-loss 75 Ohm TV cable is as good as
hardline on 40, but a lot cheaper.

Matching network:
http://homepages.dstream.net/K0FF/5-8Fourty/eMatch.jpeg

Good building and be safe, Geo>K0FF













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