A half square is a relatively low gain antenna, at 50 feet about 3.6 dB
max gain. The take-off angle is very low, about 18 degrees. Compare
this to a dipole at the same height. At 18 degrees elevation they are
equal. Above 18 degrees the dipole wins big, and below 18 degrees the
half square wins by 3.4 dB at 10 deg, and by 5.6 dB at 5 deg. You are
correct that no radials are required if fed at the top corner. If you
feed it at the bottom, you will need elevated radials to decouple the
feed line. I would not recommend bottom feeding this, feedline
decoupling is a major challenge. Even feeding it at the top you will
need some feedline decoupling, the impedance is different for the two
legs, so you need some chokes to avoid messing up the pattern, and
keeping RF off the feedline. You need to bring the feedline away at
right angles to the antenna as well.
A 2 element wire vertical, ground mounted with an extensive radial
system, including wire and ground losses, elements fed 90 degrees apart
will produce only 2 dB gain at 25 degrees elevation, but with a very
wide lobe in the azimuth direction (182 degrees). Feed them 130 degrees
apart and you can get 2.7 dB gain at 26 degrees elevation, with 136
degrees azimuth lobe width. A vertical with elevated radials should be
similar, maybe just slightly less gain.
Consider a 2 element inverted vee beam, with the center at 50 ft and the
ends at 40 ft, and to make it simple to reverse directions making all
the elements the same length. Now you have a choice. If operated this
way it will very little F/B, but the gain will be good, about 8.9 dB at
35 degrees elevation. At 20 deg 7.1dB, at 10 deg 2.4 dB, and 2.4 dB at
10 degrees.
To get some F/B you could include switched stubs to tune it, or just
operate it 14% above resonant frequency. If operated above resonance you
will have good gain and good F/B and it will make the SWR around 2.4 to
1. Now you can use a matching network to reduce the SWR. For this case,
including wire and ground losses, gain will be 9.7 dB at 36 degree
elevation, with a beamwidth of 79 degrees in the azimuth direction.
Looking at the elevation again, the gain is 7.75 dB at 20 degrees, 6 dB
at 15 degrees, and 3 dB at 10 degrees. If you droop the ends more, this
is equivalent to lowering the antenna. The gain will go down a little,
and the elevation angle will go up some.
Comparison:
El Angle Half Square Dipole 2 Ele Vert 2 El Inv Vee 14% above
Freq.
30 2.1 5.7 1.9 8.7
20 3.6 3.8 1.8 7.1
15 3.5 2 1 5.4
10 2.3 -1 -.5 2.4
And the winner is ....
Jerry, K4SAV
ku8e at bellsouth.net wrote:
>I went out back last night an did some measurements.
>
>I don't have enough room for a 40 meter bobtail so I am counting that out.
>About the highest I can get up into the trees is maybe 55 ft. I presently have
>a wire vertical with 4 elevated radials that works very well... but I want to
>be able to point it in some direction. I was thinking of putting up either the
>Half Square to EU (Ed BTW this doesn't require an elablorate ground system) or
>putting up anothor vertical and phase them. I can get a cardiod pattern
>endfire with 1/4 wl spacing. I could switch this NE/SW manually until I can
>build up a switch. Or I can space them 1/2 wl and get a bidirectional pattern
>NE/SW - endfire NW/SE broadside and switch then..
>
>I have had up a 40 meter Half Square at this QTH before and it worked great.
>It was pointed NW / SE but last spring the county mower guy mowed it down when
>the were cutting the grass in the easement behind us ( where I have my
>antennas) I ended up just replacing it with the wire vertical.
>
>Bill..
>It would be interesting for you to model the 3 el inverted vee wire beam. I am
>thinking the verticals might work better because the beam would be too low for
>40 meters...
>
>BTW does anyone have experience switching directions in a wire beam using the
>stubs ?? There is
>an article in QST this month for an 3 el 80 meter wire beam. He is switching
>directions using stubs..
>
>
>Jeff
>
>
>
>
>
>>From: Bill Coleman <aa4lr at arrl.net>
>>Date: 2005/10/20 Thu PM 10:46:59 EDT
>>To: <ku8e at bellsouth.net> <ku8e at bellsouth.net>
>>CC: <secc at contesting.com>
>>Subject: Re: [SECC] SS Club Competition - Going for the Gavel
>>
>>
>>On Oct 20, 2005, at 5:06 PM, <ku8e at bellsouth.net>
>><ku8e at bellsouth.net> wrote:
>>
>>
>>
>>>The 40 meter vertical might change this weekend as I get ready for
>>>CQWW SSB.
>>>I am thinking of the one following 40 meter antennas depending on
>>>how much room I find out I have :
>>>
>>>1) 2 Half squares - one pointed NE/SW , the other NW/SE
>>>
>>>
>>Do these have to be worked against ground?
>>
>>
>>
>>>2) 3 el wire beam to NE - inverted Vees
>>>
>>>
>>You'll need to get the apex up 50 feet or more for the pattern to
>>become effective.
>>
>>
>>
>>>3) 2 el quad - switchable NE/SW
>>>
>>>
>>Ditto here. Or you might consider delta-loops. If you have high
>>enough supports, these can be easy to tune and very effective.
>>
>>
>>
>>>4) Phased verticals - 1/2 wl spacing - switchable broadside/endfire
>>>
>>>
>>Radials again.
>>
>>
>>
>>>5) Bobtail Curtain to NE/SW, Half square NW/SE
>>>
>>>
>>I've decided that for most effective antennas, you're either looking
>>for higher supports or putting miles of wire on the ground....
>>
>>Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr at arrl.net
>>Quote: "Not within a thousand years will man ever fly!"
>> -- Wilbur Wright, 1901
>>
>>
>>
>>
>
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>
>
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