How to get saturation coverage in the skip zone NVIS Antenna How to get saturation coverage in the skip zone Run the PowerPoint in the full screen (Slide show) mode and then come back and view the notes for the individual slides. Tom Sanders, W6QJI Ed Bruette, N7NVP

Problem statement During disasters, WA communicators need to be able to reliably communicate with W7EMD at Camp Murray (State EOC) & other sites around the region via HF on 75 (Pri.), 60 and 40 mtrs (Sec.)

Goal Traffic quality statewide communications using a single transportable antenna and a 100 W transmitter without an external tuner

Desirable attributes Resonant at 7245 kHz, 5373 kHz and 3985 kHz Omni-directional Coverage of WA, OR, ID and BC Portable Easy for one person to erect

What is NVIS? Near Vertical Incident Skywave Cloud Warmer

Propagation Theory When we transmit on a DX antenna there is an area close to our station that receives the signal via the direct path or ground wave. There is a geographic circle, beyond the ground wave, in which our signal can not be heard – the skip zone. Beyond that circle is where we see sky wave propagation. We as emergency communicators using HF need to work the skip zone but not beyond.

NVIS Effect This chart is based on an Ionosphere height of 260 mi.

300 Mile Coverage A half power signal (3db down from effective radiated power [ERP]) will be heard at the edge of the 300 mi. circle if we have a 60 deg. half power take off angle.

Omni Pattern This is the X Y plot for the omni pattern of a NVIS antenna.

The Technical End Of Things Dr. Jelinek’s design Modifying the original design to work on 75, 60 & 40 meters without a tuner Tweaking for optimum performance The first effort to identify a suitable antenna for HF EmComm was to research the internet. A promising design was found but did not meet the criteria of no external tuner.

Drawing Of Original Concept Dr. Jelinek’s design is mechanically weak and requires a tuner to work in the Ham bands.

NEC2 Considerations Original design using EZNEC replaced by NEC2 (Numerical Electromagnetic Code) Derived from original NEC Provides accurate gain data for radiators very close to the ground Gain figures vary with ground conditions Our original computer modeling was done with EZNEC. We discovered it did not accurately portray gain figures for antennas within ¼ wavelength of the ground. Switching to NEC2 solved that problem.

Propagation Considerations “D” layer losses Ionospheric scattering for vertical propagation Importance of critical frequency

Ionosphere Effect

Antenna Skeleton Elements 2 & 3 are 40 meters, elements 4 & 5 are 75 meters and elements 6 & 7 are the 60 meter elements.

Element lengths 75 Mtr legs = 58.32 ft 60 Mtr legs = 43.00 ft Prune these lengths to meet your ground conditions These numbers are straight from the computer modeling and will need to be trimmed for your location.

Omni Pattern

75 Mtr SWR Note the broad bandedness of the curve.

75Mtr Vertical pattern Full power (ERP) is straight up (90 degrees). Half power is at 43 and 136 degrees.

75 Meter Current Distribution All of the current is in the 75M elements.

60 Meter SWR All 5 channels are within reach of the antenna.

60 Meter Vertical Pattern The half power lines are at 41 and 140 degrees.

60 Meter Current Distribution Most of the current is in the 60M elements with a small amount in the 75M elements.

60 Meter Power Considerations 50W ERP limitation Antenna gain Feedline loss Using this design, run a 100W radio at full power QST Feb. 2004 See the Feb. 2004 QST for a good article on 60 meters calculations.

40 Mtr SWR The entire 40 meter band can be covered with this antenna.

40 Mtr Vertical Pattern The half power lines are at 34 and 146 degrees.

40 Meter Current Distribution The majority of the current is in the 40M elements with a small amount in the 75M elements.

How it went together Materials Construction Modifications

There are 3 critical measurements There are 3 critical measurements. 15’ height of the feed point, length of the elements and the 2 1/2 ‘ height of the element ends. The element sag or catenary should be removed as much as possible.

Center Support Coupler This is a compression fitting used to join the 7 ½ ‘ schedule 40 inch and a half PVC together

Center Support Coupler Mating the 2 mast sections together.

Feedpoint Standard 50 ohm feed point.

Wire Connectors The initial effort used a split bolt on each side of the antenna. Ease of assembly and mechanical strength advantages pushed us toward the ground bar pictured on the right.

Parts list 2 1.5 in x 10 ft Schedule 40 PVC pipe – cut to 7.5 ft lengths 1 1.5 in. compression coupler (joint connector) 1 1.5 in. slip coupler 2 1 in x 10 ft Schedule 40 PVC pipe – cut to 2.5 ft lengths (6 ea needed) 6 1 in end caps 6 5/8 in Schedule 20 PVC pipe – Cut to 6 in lengths – Drill hole for wire (6 ea needed) 1 6 ft “T” fence post (fits inside center support)

Parts list (Cont.) 6 18 in metal stakes 1 50 Ohm feed point (Dipole center insulator) 275 ft antenna wire – insulated 14 ga. 2 8.5 in. wire pig tails – transition from feed point to wire elements 2 Short non-conductive strain reliefs 2 Split bolts or 5 hole grounding bars Coax to reach the transmitter

Feed Point 8.5” wire is soldered to the lugs on the feed point, run thru the ground bar and soldered to itself. A shorter piece of non-conductive material is used for strain relief. We used parachute cord.

Feed Point Assembly We ran the feedline down thru the center support pole.

“T” Post The “T” fence post can be driven into the ground using any large hammer.

15’ Center Support Fully erected antenna. It is easy to see the compression coupler joining the 2 mast sections and the slip coupler holding the feed point at the top of the mast. The coax (with a poly bag over the PL259) running part way up the mast is there to protect it from the weather while not in use.

Feed point What appears to be a cap at the top of the mast is really a slip joint. It was left over from a previous effort and serves to hold the feed point since the feed point would not fit inside the 1 ½ “ PVC pipe.

Coax Exit From Center Support A notch was drilled and sawn in the bottom of the mast so the coax would not be crimped.

2.5’ end support and 18” stake The construction stakes are used to keep the PVC end supports verticle.

Slipping a pole over the stake

End pole assembly The smaller (5/8”) PVC is used as an easy adjustment device for lengthening or shortening the element.

Threading the Needle

Capping the End Pole Notch the cap and place it on the end pole to keep the element in place.

40 Mtr End Pole

Backstay for 75 Mtrs 75 meter elements are heavy when using #14 insulated wire. If the antenna is going to stay up for a protracted period or it will be subjected to winds, provide some support to keep the wire from sagging.

Does it work? Ed – Like gang busters! Tom – S meter pin buster! Field Day – Proved the theory. Worked WA, OR, ID, MT and CA as for South as Orange Co. Heard stations outside that radius but couldn’t work them Day to day operations – Not bothered by distant stations as much as those with higher antennas

Beamwidth 75 Mtrs 43 deg. To 137 deg. 60 Mtrs 41 deg To 139 deg. Beam width equates to how far the signal will be heard at half power.

Daytime 75, 60 & 40 Mtr Coverage These are the half power ranges (down 3db from ERP). Assumes the ionosphere is at 200 mi.

Nighttime 75, 60 & 40 Mtr Coverage These are the half power ranges (down 3db from ERP). Assumes the ionosphere is at 300 mi.

Does it work? (cont.) Slight resonant frequency shift with wet vs. dry conditions – lower when wet Meets requirement for single person operation Portability enhanced with use of non-tangling wire Permanent installations need to be reinforced against the wind

Tools required Hammer Screwdriver That’s it! There ain’t no mo!

Hints & tips Tin the feed point end of the wire – better mechanical fit for the ground bar Dipole insulator has an eye bolt in the top for suspension from a tree or skyhook When using suspension method, put 15’ mark on coax to ID the proper height Coax should be perpendicular to the ground

If you are not going to use the 60M portion of the antenna, just substitute guys in place of the wire. An alternate configuration is to install the antenna as shown in this graphic and use one guy line, away from the array, from the top of the center support.

Safety considerations Flag end of wire – RF burn Insulated wire reduces possibility of RF burns Flag lower portions of wire for choking hazard prevention

60 Meter Considerations 5 Channels (Window freq. -1.5 kHz) 2.8 kHz bandwidth limitation USB 50W ERP

Regional 60 Meter Agreement 5405 Nation/International 5373 Washington 5368 Idaho 5348 Montana/Oregon 5332 Regional coordination between states/sections This agreement was made between the Section Emergency Coordinators and State RACES Officers for the states listed. The reason we left 5405 kHz. as National/International is because UK Hams have authority to test on that channel. They do not have permission to use it for general communications yet. Please do not try to DX with the UK while they are still in the testing phase.

Thank you! de Ed & Tom Questions may be submitted to Tom at ww2end@aol.com or Ed at n7nvp@arrl.org.