Friday, November 16, 2012

THE 3 HALF WAVELENGTH HORIZONTAL VEE BEAM!


Editors note: This book appears to be out of print and very difficult to find, even in used condition.Edward M. Noll, W3FQJ, an accomplished consulting engineer, author of many technical books, lessons, articles and antenna designs has contributed an enormous amount of knowledge, hands on experience and skill to Amateur Radio. Ed built, tested and used every one of the 73 antennas in the book. We hope to help his contributions in the Ham antenna field to live on by presenting in edited form, one of his many projects taken from the book!

THE 3 HALF WAVELENGTH HORIZONTAL VEE BEAM!
by Edward M. Nole W3FQJAntennas can be resonated to a specific frequency by making their overall electrical length a whole multiple of a half wavelength. There is a rise in gain with each half-wavelength addition. In the case of a horizontal antenna, the antenna becomes more directive with antenna length. The addition of leg lengths in ODD multiples of a half wavelength ensures a low impedance center feed point because each leg of such an antenna is an odd number of quarter wave-lengths long. Instead of the two lobes of a standard dipole, the 3/2 wavelength standard dipole antenna has four major and two minor lobes if erected in standard dipole fashion, i.e., horizontally parallel with the earth. The Vee configuration of the same antenna can be made more directive by forward tilting of the legs toward the desired direction or station. (See Figure 1 below)
The legs can be tilted forward, like the point of an arrow reversed , from about 90 degrees to around 110 degrees. When they are tilted forward, horizontally, the antenna displays a maximum directivity along a line that bisects the angle. Minor side and back lobes remain; therefore the antenna has omni directional capability as well. (See Figure 2 below)

Gain

The standard 3/4 wavelength per side antenna at 100 degrees "tilt" has 2.5 dB gain according to a chart in the book. 5/4 per side at 86 degrees has 3.3dB gain, 7/4 per side at 76 degrees has 4.0dB gain, 9/4 wavelengths per side at 67 degrees has 4.75dB gain and 11/4 wavelengths per side at 60 degrees tilt has 5.3dB gain.
Editors note
:..."It is assumed these figures are referenced to a standard dipole but not stated in the article."

Antenna resistance rises slowly with leg length and is also influenced by the included angle. These antennas are considered to be "
Short Vee Beams" if at; or under 1 1/4 wavelengths long per side or less than 100 feet per side. Longer vee beams greater than 1 1/4 wavelengths per side require 2 to 1 or 4 to 1 baluns to insure equal currents in each leg for the desired lobes but can achieve much higher gains. An example for 10 meters at 28.6 mhz for a 10 dB gain antenna requires a length of over 283 feet per side! At 40 meters (7.1mhz), you would need only 1143 feet per side!
Man, talk about an antenna farm.....if someone was standing at the center support and another was yelling at you from the other end, it would take over a second for you to hear him at the speed of sound....about 1100 feet per second!


THE ALL IMPORTANT FEED LINE LENGTH

In most cases the horizontal-vee type antennas require some 4 to 6 percent shortening from the standard formulas below. See the examples further down the page for several bands.Transmission line lengths must be cut to an EVEN multiple of an electrical half wave length using the formula:

Length = 492 X vf (velocity factor of line) / frequency:

Example.....492 X .66 / 7.1mhz = 45.73 feet for an electrical half wavelength of .66 velocity feed line like standard RG/58U.

So if you plan to operate a dipole on 7.1 mhz, and the approximate distance between the antenna and transmitter is 100 feet, it is wise to use a length of feed line of approximately  2 or 3 wavelengths long such as 91 or 137 feet corresponding to either 2 or 3 half wavelengths of regular RG/58U line.
These formulas were taken from a chart in the book for calculating the feed line lengths for .66 velocity factor line. There is some very slight difference between using the formula above (492 X .66 / freq) and the formulas in the chart.
The
bold type numbers in the formulas are "magic number constants" arrived at by experimentation.
1 half wavelength     length = 325
/7.1 = 45.77 ft.
2 half wavelengths   length = 650
/7.1 = 91.55 ft.
3 half wavelengths   length = 975
/7.1 = 137.3 ft.

Editors note: NOTICE that the number 325 is added to the "magic number constant" calculation for each half wavelength of feed line added.
So for 4 half wavelengths the formula would be 1300
/ 7.1 = 183 ft. and so on for each half wavelength added..

The other section of the same chart was for .81 velocity factor line such as foam RG/58U and used
400 as the magic number in the formula.

Example: 2 half wavelength line for 7.1mhz is,
800
/ 7.1 = 112.67 ft of feed line.
For 3 half wave lengths of line, just add 400 to 800 = 1200 for the "magic constant"
for each added half wavelength added.As with most antenna projects, some trimming may be needed so cut each leg length a little long and resonate antenna as required.
Leg length examples
for 3/4 wave length per side:
20 METERS LEG LENGTH = 738 / 14.2 = 52 feet less 6 percent shortening = 49 feet15 METERS LEG LENGTH = 738 / 21.3 = 34 feet 6 inches........less 6 percent = 32 feet 6 inches
5/4 wave length formula is 1230/fmhz
Just add the number 492
to the previous "magic number constant" for the next odd higher wavelength addition:
Example:    3/4 = 738/f
                     5/4 = 1230/f
                     7/4 = 1722/f
                     9/4 = 2214/f
                      and so on.

2 BAND OPERATION WITH THE SAME ANTENNA!

If you will notice in the 15 meter version above, the final length comes out to be, 32 feet 6 inches per side,,,,using the standard formula for a regular dipole....468 / freqmhz, 468/7.2mhz = 65 feet for total length of a 40 meter dipole...... each side would be 32 feet 6
inches.....40 meter operation included for free!
What a deal!
This can be applied to two band operation with other designs as well.
MORE GOOD STUFF!
The short vee beam described in the book has a reasonable omnidirectional pattern with a maximum directivity in a line that bisects the angle between the legs. Good low-angle radiation is obtained when a horizontal antenna has a one wavelength height above ground. Below .5 wavelengths give marginal performance.
For lower heights (.5 wavelengths and less), some improvement in low angle propagation can be had by tilting the leg ends below the center feed point. This will help improve DX but at the expense of the omni pattern not being as good and will increase the vertical pattern more skyward at a higher angle.
Try it on higher frequencies too....EXPERIMENT
!
MORE BELOW!
3 HALF WAVE LENGTH HORIZONTAL VEE  PROJECT
Figure 1.

K6LMN added this pattern plot below to amplify or expand on material in the antenna book.



Figure 2.Freq = 28.6 Mhz
50 feet above ground
EZNEC PLOT COURTESY K6LMN,
ROGER
10 METER HORIZONTAL 3/4 WAVE VEE
Project supplied by Roger, K6LMN using EZNEC 2.0


EZNEC 2.0 RESULTS from Roger, K6LMNPlot for this design is same as above
and shows results at Maximum Gain Takeoff Angle.
50 feet above ground
Apex = 100 degrees
Z in = 110 ohms
VSWR = 2.2 : 1

Element lengths 25.8 feet

Good ground
No element losses
#10 wire
Freq 28.6 Mhz
Gain 11.06 dBi
Angle: 0 deg
F/B 2.0dB
Bmwidth: 35.3 deg
-3: 342.3 17.6 deg
Slope 8.98 dBi
Angle: 180 deg
F/S lobe 2.08dB


Max Gain =  11.06dBi
                   -     2.14
                   =    8.92d
                   -     5.50 ground enhancement
Yields aprox  3.42 dBd in free space!
Comments:    Good backyard antenna for 10 Meter band.
                      Apex angle not critical, 95 to 105 degrees OK.
                      Slightly more gain at 105 degrees with stronger
                      sidelobes and higher feed impedance.
More Comments from Roger, K6LMN:
"Now with modern computers and antenna modeling programs we can go back an analyze these old antennas and also create new ones.  I like the EZNEC program since I have used W7EL's programs since the early '80s.  There are a few others of course.  I would like to try YAGIMAX or whatever program that optimizes Yagis
." Roger, K6LMN
If you can locate the book described above, by all means, get it if you have questions. It is loaded with many Dipole and Long Wire antennas by Ed Nolls, W3FQJ that you will enjoy adding to your station for many more years of fun thanks to Ed!
The main article and project above is one of many Ed contributed to Ham Radio, WITHOUT THE AID OF A COMPUTER!......
Many thanks to Roger, K6LMN for providing his time, talents and skill with the additional material!
More fun to come, stay connected with Ham Radio, get on the air!....N4UJW

1 comment:

Jerry Crabill said...

I love the idea!! I do not have 50 feet of height. Will it work if I install at 30 feet high?

Jerry