Date: Tue, 30 Jul 2013 21:46:28 -0500
From: "Jim in Waco wb5oxq" <wb5oxq_1@grandecom.net>
To: <TowerTalk@contesting.com>
Subject: [TowerTalk] How high is enough
When choosing a height for a beam is 1/4 wave above ground at the lowest
frequency enough? 1/2 wave? I see Force 12 rating the gain on their beams at
74'1".
Why that height? At 20 meters is 100' a lot better than say 60'? I am talking
about relatively flat terrain. I see Stepp-ir makes a beam that even has a 80
meter dipole in it. Would that workmuch better on 80 than a wire dipole at an
equal height? I have never had enough land to put upmore than 50' bit nest
year I might have over an acre and be able to go much higher. I just wonder
how high when you spend more to go higher than it is worth? If I had the room
for a wire dipole to run any direction, what would be the best choice from
central Texas considering being in the center of the country east/west wise? I
want to get the best bang for my buck! wb5oxq
## F12 uses 74 feet simply cuz they used a 72 foot tall crank up tower +
short mast. Like others have noted use the info in the ARRL ant book to draw
some conclusions.
Here in the pacific northwest, higher is better, end of story...at least for
dx..and on any band. My problem with the arrl info is its averaged out over
the entire 11 year cycle.
Typ, when we are in the lower part of the cycle, the angles are lower, and
vice – versa. You also have to factor in what bands you use. Throw 40m
into the mix, and you
will soon see that 100 feet eats 50 feet hands down. In the pacific north
west, 180 feet on 40 m blows away 90 feet every time for dx.
Now add 80m into the equation. 120 feet is barely a half wave up. And yes, a
80m rotary dipole will easily outperform a 80m fixed wire dipole. Either 80m
ant has a 12-14 db
F-S ratio....at least on DX signals.....and 4-8 db on local stuff. With a
single, fixed wire dipole, you can easily lose 4-14 db just from the dipole
being oriented wrong.
Some folks will use 2 x 80m dipoles...at right angles.... like a T, not an
X...to get around this problem. That works, provides you have 4 x supports.
Your typ inverted vee on 80m will radiate just as well off the sides as the
ends, and is omni directional at typ 50-100 foot apex heights. The shortened
80m rotary dipole
at 70-100 heights is a big step above the inverted vee with its apex at the
same 70-100 feet.
You may also be involved with 50, 144, 220, 440 mhz bands..... line of sight
stuff. Here again, higher is better. Factoring all of this in, IF you have
a choice, and only one tower
to be installed, opt for the higher one. You can also do more stuff with a
taller tower, like install a lower tribander, on a side mount for high angle
signals...and then point
the low and high ants in different directions too. Some will opt to just
rotate the entire tower..in which case everything points in the same direction.
Some will use tic-ring or
side mounted rotors. Some will opt for a tall mast above the top of the
tower, to handle more ants. Some will opt for a 2nd or 3rd tower. The sky
is the limit.
Of course folks only have so much room, and so much $$. Do you do most of
the operating during the day...or at night ? A 40m yagi can be used night or
day..and
for the entire 11 year cycle. A 17-15-12-10 m ant is typ a daytime
operation..and towards the top of the cycle. If you work day shift...and are
busy on weekends,
with family and yard work etc, 17-10m might be a waste vs a 40m yagi.
It’s a lot of trade offs to consider. I operate at night, so a 40m yagi + 80m
rotary dipole
is my choice. If you intend on using 160m, and want to shunt feed the tower,
or hang a qtr wave sloper from the top, or an inverted vee, or use an inverted
L , the
taller tower will be the ticket.
Later...... Jim VE7RF
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