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Author Topic: 40 meter sloper info.?  (Read 2533 times)
HS0ZJU
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Posts: 163




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« on: December 26, 2011, 07:30:12 PM »

If someone could help with the following question.

1. The length of a sloper for 40 meter phone

2. Is any type of balum required

3. How critical is the angle?

4. What is the best angle?

73s  hs0zju
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WB6BYU
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Posts: 13019




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« Reply #1 on: December 26, 2011, 08:12:35 PM »

A quarter wave sloper MUST be fed against a tower, water tank, or other large object for
a ground plane, which becomes part of the resonant structure and affects the wire length.
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HS0ZJU
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Posts: 163




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« Reply #2 on: December 26, 2011, 08:56:50 PM »

It will attach to my tower which is 21 meters...I believe i can use a 1/2 wave. Can you help with any of the questions  i asked above?

A quarter wave sloper MUST be fed against a tower, water tank, or other large object for
a ground plane, which becomes part of the resonant structure and affects the wire length.
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WB6BYU
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Posts: 13019




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« Reply #3 on: December 26, 2011, 09:36:16 PM »

It's a trade-off between making it as steep as possible to maximize the vertically polarized radiation
and keeping it spaced away from the tower which will affect the resonant frequency.

But if you can space it out 1m from the top of the tower (or at least use 1m of rope on the top
end of the tower) then 20m of 3mm wire should put resonance between 7.1 and 7.2 MHz with
the wire sloping 30 degrees from vertical (which puts the far end rather close to the ground out
14m from the tower).  The only problem is that the feedpoint impedance is around 100 ohms: 
using 1/4 wavelength of 75 ohm coax would give a better match.  In practice you'll probably
adjust the slope either to fit the available space, or to keep the end of the wire out of the
way at least 2 to 3m above the ground.

There is some directivity to the pattern, with maximum signal away from the tower. F/B my be
15dB or more at around 35 degrees elevation angle, with perhaps 10dB at 20 degrees.  But the
front lobe is quite broad, with more than 180 degrees half power beamwidth, so basically equal
radiation over a half circle of azimuth.

Going to a 45 degree slope lowers the gain slightly and increases the depth of the rear null at
20 degrees vertical angle.  But it also makes maximum gain more broadside too the antenna
rather than away from the tower, though not by much - the dip away from the tower is only
2dB or so, so the beamwidth is slightly wider but the total gain perhaps 1dB less.  The
resonant frequency increases about 100kHz for the same wire length and the feedpoint
impedance drops to about 90 ohms.

In practice the angle isn't particular critical - the flatter the angle, the more radiation is
horizontally polarized rather than vertical, especially to the sides.

As with any dipole, you don't have to use a balun:  whether you get common
mode currents will depend on the length of the feedline and what is connected to the
shield in the shack.  A balun just makes performance more predictable.  If you don't use one,
you might try reversing the ends (or the balun connections) to see if the common mode
currents contribute more to the radiation one way or the other.  (I would guess that having
the coax center connected to the upper wire would be best.)
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KY6R
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Posts: 3133


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« Reply #4 on: December 27, 2011, 06:35:48 AM »

If you could hang 4 or 5 1/2 wl dipoles, and feed each with 36' feet of coax that has a velocity factor of .66, then you will have a K1WA Array. When using a coax switch - the element that is switched in is the driver, and the length of the coax feeders (3/8 wl), make the others look like reflectors. You would get a directional system that approximates a 4 square.

Now - you should be able to use shortened half dipoles. I have used this system on 20M - because I did not have a tall enough tower. I did not try a shortened 40M version - but am thinking about doing this since I now am putting up a 55' tower.

I might need to fold the ends of these dipoles back - maybe 5 feet on each end.

This system was documented in the ARRL Antenna Book - but not the latest edition - it was in many of the previous editions - the 21st I believe has it.
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