Fixed Installation Tri-Band 18 June 2005 NVIS, Another Look Fixed Installation Tri-Band Contact Tom at tjsand@wavecable.com Contact Ed at n7nvp@arrl.org Tom Sanders, W6QJI Ed Bruette, N7NVP 18 June 2005

What is NVIS? Near Vertical Incident Cloud Warmer Skywave 18 June 2005

Propagation Theory 18 June 2005 18 June 2005 Low take off angle antennas produce propagation as depicted in the graphic. An NVIS antenna uses high take off angles (straight up) and eliminates the skip zone created by low take off angle antennas. 18 June 2005

18 June 2005 NVIS Effect This chart is based on an Ionosphere height of 260 mi. If your antenna has high take off angles like shown above, communications can be established within the radii shown. 18 June 2005

300 Mile Coverage 18 June 2005 18 June 2005 From Silverdale, WA 300 Mi. coverage includes the entire state plus portions of neighboring areas. 18 June 2005

18 June 2005 The “distant stations” in NVIS are fairly close, <1000 Mi. 18 June 2005

18 June 2005 Overcoming physical obstacles is one of the strengths of NVIS antennas. 18 June 2005

18 June 2005 Think about the radiation pattern of the beam antenna. It’s focused in front of the antenna and is low to the horizon. 18 June 2005

18 June 2005 Now turn the beam so that it points straight up. The radiation pattern is straight up too. 18 June 2005

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18 June 2005 The fundamental difference between this and a wire NVIS antenna is the materials used for construction. 18 June 2005

5/8 Wave 75 Meter Vertical Radiation Pattern 18 June 2005 EZNEC 4.0 plot shows this is a DX antenna. The low take off angle supports both DX transmissions and listening. This is not a good antenna for “Local EmComm” 18 June 2005

Propagation Considerations 18 June 2005 Propagation Considerations “D” layer losses Ionospheric scattering for vertical propagation Importance of critical frequency The maximum useable frequency (MUF) is the highest frequency that returns to earth. The critical frequency is the highest frequency at which a vertically propagated signal (NVIS) will be returned to earth. For a given ionization density as the propagation angle approaches vertical. The MUF declines to match the critical frequency. 18 June 2005

18 June 2005 Tri-band NVIS antenna installed at 17’ at N7NVP’s QTH. 18 June 2005

18 June 2005 75 Meter SWR Chart generated by EZNEC ver 4.0. Excellent standing wave ratio and 50 ohm match. 18 June 2005

Bandwidth 75 Meters 4005 – 1.5:1 3950 – dip 3875 – 1.5:1 18 June 2005 Bandwidth as determined by an MFJ-259B. 18 June 2005

75 Meter Vertical Radiation Pattern 18 June 2005 75 Meter Vertical Radiation Pattern Vertical pattern generated by EZNEC. The arrows indicate the half power angles of radiation. In other words, the full ERP (Effective Radiated Power) of the signal is at 90 deg. but there is still significant power being radiated as we come down the pattern. If we assume a 100w signal, you would still radiate 50w at the half power take off angles. 18 June 2005

75 Meter Current Distribution 18 June 2005 75 Meter Current Distribution Nearly all the current flow is in the 75M elements. When pruning the antenna for your ground conditions tune the 75M then 60M and finally 40M elements. 18 June 2005

18 June 2005 60 Meter SWR Chart generated by EZNEC ver 4.0. Good SWR an 50 ohm match. 18 June 2005

Bandwidth 60 Meters 5390 – 1.5:1 5360 – dip 5317 – 1.5:1 18 June 2005 Bandwidth as determined by an MFJ-259B. 18 June 2005

60 Meter Vertical Radiation Pattern 18 June 2005 60 Meter Vertical Radiation Pattern See 75M vertical slide (17) for comments. 18 June 2005

60 Meter Current Distribution 18 June 2005 60 Meter Current Distribution Current flow in the 60M element. There is some current in the 75 meter elements, very little in the 40M element. 18 June 2005

40 Meter SWR 18 June 2005 18 June 2005 Chart generated by EZNEC 4.0. Excellent SWR and 50 ohm match. 18 June 2005

Bandwidth 40 Meters 7295 – 1.5:1 7245 – dip 7225 – 1.5:1 18 June 2005 Bandwidth as determined by an MFJ-259B. 18 June 2005

40 Meter Vertical Radiation Pattern 18 June 2005 40 Meter Vertical Radiation Pattern See 75M vertical slide (17) for comments. 18 June 2005

40 Meter Current Distribution 18 June 2005 40 Meter Current Distribution Most of the current is in the 40M element with some in 60M and very little in 75M. There is interaction between 40M and 60M. 60M must be tuned before the 40M elements. 18 June 2005

Dual Band Yes; you can remove the 60 Meter elements! 18 June 2005 For those who can not or will not operate on 60M, you can remove those elements from the design and still make it work. We have only modeled this in the computer, we have not done a field test of this configuration. 18 June 2005

18 June 2005 When you remove the 60 M elements the 40 meter spacing must be maintained at 48” at the end of the element. 18 June 2005

18 June 2005 75 Meter SWR Chart generated by EZNEC 4.0. 18 June 2005

75 Meter Vertical Radiation Pattern 18 June 2005 75 Meter Vertical Radiation Pattern Chart generated by EZNEC 4.0. 18 June 2005

40 Meter Dual Band SWR 18 June 2005 18 June 2005 Chart generated by EZNEC 4.0. 18 June 2005

40 Meter Vertical Radiation Pattern 18 June 2005 40 Meter Vertical Radiation Pattern Chart generated by EZNEC 4.0. 18 June 2005

18 June 2005 Omni Pattern It is easy to see in this graphic that NVIS antennas generate an omni-directional pattern. This pattern represents the radiation pattern on all 3 bands. 18 June 2005

Feed Point Impedance vs. Height 18 June 2005 As a half wave dipole approaches the ground, current is induced in the dipole by the signal reflected by the ground. As can be seen from the chart the radiation resistance goes to zero over a perfect ground as the separation between the dipole and the ground approaches zero. However, over a real ground, the radiation resistance declines to around 50 ohms and then increases to very large values as the separation between the dipole and ground approaches zero. In our design we selected a height which would optimize the 50 ohm match. Chart is from the ARRL Antenna Book. 18 June 2005

Depth of Current Penetration 18 June 2005 The ground is not a plane reflective surface. Current penetration in the ground is a function of the dielectric constant and conductivity. These vary with location. For WWA ground conditions vary between average and poor with significant penetration between 15’ and 30’. Chart is from the ARRL Antenna Book. 18 June 2005

Where is your RF ground? 18 June 2005 18 June 2005 Ground is not at the surface of the earth in most cases (blue line in the graphic). Finding where ground really is an expensive proposition and not worth the effort. The easiest way to overcome the problem is to use reflectors under the antenna (orange lines in the graphic). In our case we use one 75M, two 60M and two 40M reflectors all cut to 105% of the radiators. 18 June 2005

How It Went Together Materials Construction Modifications 18 June 2005

Parts List 1 - Feed point - 50 Ohm #14 insulated stranded wire – 280’ 3 - ½” x 10‘ PVC cut to length 2 – Insulators Tie wraps 3/16” rope cut to length Coax to the shack 18 June 2005

18 June 2005 The coax choke can be replaced by a 1:1 balun. 18 June 2005

Spreader Lengths 2 – 17” Next to center insulator 18 June 2005 Spreader Lengths 2 – 17” Next to center insulator 2 – 34” 2nd from center insulator 2 – 50.5” End of 40 M element 2 – 25.25” End of 60M element Another spreader could be used between the center insulator and the end of the 40M element Black (or gray or sky-blue-pink or ???) Krylon paint can be used to make the PVC fade into the background. 18 June 2005

18 June 2005 Spreader placement was determined by the catenary that needed to be supported. We started at the 60 and 40M element ends where we had known element spacing to work with (2’). Then we divided the remaining space in thirds to place the other spreaders. The spacing of the holes in the spreaders was determined by installing the 2 spreaders at the 60 and 40M element ends and pulling all the elements taught. We then measured from the center insulator to the 1/3 and 2/3 points of the 40M end spreader and drilled holes in the PVC to match where the wire should pass thru the spreader. The idea is to maintain uniform spacing all along the element length. 18 June 2005

Element Lengths 75 Mtr legs = 57.33 ft 60 Mtr legs = 45.4 ft 18 June 2005 Element Lengths 75 Mtr legs = 57.33 ft 60 Mtr legs = 45.4 ft 40 Mtr legs = 34.25 ft Prune these lengths to meet your ground conditions This does not include the wraps to secure the wire at either end. Since my ground conditions will differ from your ground, cut the wire legs long and trim to tune to the desired resonant frequencies. Tune the antenna when the ground is dry. 18 June 2005

18 June 2005 No rope required. 40 Meter Tension 18 June 2005

Tensioning method for 40M 18 June 2005 Tensioning method for 40M Tension on the 40M wire (bottom element in the photo) is maintained by putting a tie wrap on the 75M element at the point where the spreader is perpendicular to the wire and the 40M wire is taught. A tie wrap is placed on the 60M wire so that it slightly bows the spreader maintaining the tension required to minimize the catenary in the 40M wire. 18 June 2005

Spreader keepers 18 June 2005 18 June 2005 Put tie wraps on both sides of the PVC to prevent the spreaders from moving out of place. We recommend you tighten the tie wraps using pliers to make them tight enough so they don’t slide along the wire. Additional friction can be created by hot gluing both sides of the wire where it penetrates the spreader. 18 June 2005

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60 Meter tension UV resistant line 18 June 2005 18 June 2005 60M tension is maintained using a piece of small line that is UV resistant. We opted to tie it off to the egg insulator. 60 Meter tension 18 June 2005

60 Meter Tension 18 June 2005 18 June 2005 Ensure the spreader end of the tension line is tied off both above and below the wire so that it does not slide up the PVC. 18 June 2005

Beamwidth 75 Mtrs 38 deg. To 142 deg. 60 Mtrs 36.5 deg To 143.5 deg. 18 June 2005 Beamwidth 75 Mtrs 38 deg. To 142 deg. 60 Mtrs 36.5 deg To 143.5 deg. 40 Mtrs 34 deg. To 146 deg. This is the half power beam width as depicted in the vertical pattern graphics. 18 June 2005

Night Time Ionosphere (300 mi.) 18 June 2005 Night Time Ionosphere (300 mi.) These distances represent the half power distances. You can talk beyond these circles but your signal will drop off rather sharply the farther you go. An added benefit is that you will not hear DX or it will be sharply reduced in signal strength as compared to a low angle radiation antenna. 18 June 2005

Hints & tips Solder wires at the feed point 18 June 2005 Hints & tips Solder wires at the feed point Solder feed point pigtail to all other wires Dipole insulator has an eye bolt in the top for suspension from a tree or skyhook Coax should be perpendicular to the antenna 18 June 2005

18 June 2005 Each wire is attached to the center insulator, wrapped back on itself and soldered. Once all the elements on one side of the center insulator are attached, draw the center insulator pig tail thru the eyebolt, wrap it around all 3 wires and solder. Weather proof the coax connection as needed for your environment. 18 June 2005

Choke balun – 18’ coiled 9-10 turns 18 June 2005 Choke length is for Multi-band antennas 3.5 to 10 MHz. (ARRL Antenna Book) Tape and or tie wrap the coax at 3 or 4 places around the coil to keep it together. Choke balun – 18’ coiled 9-10 turns 18 June 2005

Hints & tips (cont.) Ground conditions will drive element lengths 18 June 2005 Hints & tips (cont.) Ground conditions will drive element lengths Wet vs.. dry Use an antenna analyzer!!! Tune 75M first, then 60 then 40M There is interaction between the 60 & 40 meter elements When the conditions are wet the resonance of the antenna will drop about 50kHz on 75M and 100kHz or so on 40 and 60M. When conditions are dry, 5 or more days since the last rain, tune the antenna for the middle of the band segment you intend to operate in. That way you’ll be able to use the antenna without a tuner most of the time. 18 June 2005

Reflectors Ground wires laid directly under the antenna 5 Total! 18 June 2005 Reflectors Ground wires laid directly under the antenna 5 Total! 1 on 75M and 2 each on 60 & 40M Spacing is important – 2.5” 60M on either side of 75M & 40M on the outside of 60M Reflectors do work! They add gain to the antenna and also improve the feed point match and make the beam width more narrow. 18 June 2005

18 June 2005 The 75M reflector is laid on the ground directly under the antenna. The rest of the reflectors are placed in reference to the 75M reflector. 18 June 2005

Tri-band Reflector Lengths 18 June 2005 Tri-band Reflector Lengths 75M – 65’ 60M – 47.9’ 40M - 35.6’ These lengths are for the element lengths we used. If your lengths are different, make your reflectors 105% of your element lengths. 18 June 2005

18 June 2005 This design can be placed in a 90 deg. or greater configuration. Closing it up less than 90 deg. impacts performance significantly. Any angle between 90 and 180 is fine. 18 June 2005

18 June 2005 On a RF non-reflecting roof, reflectors are required. The antenna will be influenced by other objects on or below the roof. An antenna analyzer will greatly aid the tuning effort. 18 June 2005

18 June 2005 17’ is not a magic number. The antenna will work anywhere between 15 and 35’. 20’ is the sweet spot based on a compromise of gain and ease of installation. 18 June 2005

Dual Band Element Lengths 18 June 2005 Dual Band Element Lengths 75 Mtr legs = 59.7 ft 40 Mtr legs = 35.25 ft Prune these lengths to meet your ground conditions This does not include the wraps to secure the wire at either end. These numbers are computer generated and have not been tested in the field. 18 June 2005

Dual Band Reflector Lengths 18 June 2005 Dual Band Reflector Lengths 75M – 62.5’ 40M - 37.25’ This measurement should equal 105% of your element lengths. 18 June 2005

18 June 2005 This is another example of an NVIS antenna. It has been used to support the Iditarod race in Alaska several times. 18 June 2005

60 Meter Considerations USB 2.8 kHz bandwidth limitation 5 Channels (Window freq. -1.5 kHz) 50W ERP limitation Antenna gain Feedline loss QST Feb. 2004 or ARRL FAQ 18 June 2005

Regional 60 Meter Agreement 18 June 2005 Regional 60 Meter Agreement 5405 Nation/International 5373 Washington 5368 Idaho 5348 Montana/Oregon 5332 Regional coordination between states/sections This agreement was reached by the ARES® SECs and State RACES Officers of the states listed. It only applies during disasters and EmComm exercises. 18 June 2005

Thanks to Bill Balzarini, KL7BB for all the drawings! 18 June 2005

Questions are welcome Contact Tom at tjsand@wavecable.com Contact Ed at n7nvp@arrl.org 18 June 2005

Thank you! de Tom & Ed 18 June 2005