NVIS June 2008 Near Vertical Incident

NVIS June 2008

Near Vertical Incident Skywave ◦ A radio propagation mode ◦ An alternate method to obtain reliable communication at distances less than 200 - 300 mi ◦ Uses an antenna with a high radiation angle ◦ Requires use of frequency below “critical” cutoff What is NVIS?

A ground wave signal Useful on all HF frequencies Insensitive to propagation conditions Absolutely reliable What NVIS is Not

◦ NVIS can be a good emergency solution for coverage at distances and circumstances not favorable to VHF ◦ Effective use requires some understanding and planning Why learn about NVIS

NVIS Propagation

Formation of a Skip Zone

Frequency <= fo. F 2(MUF)

Selection of appropriate frequency Use of an antenna supporting high radiation angle Sufficient Power Both ends operating with NVIS antenna Experienced Operators What helps NVIS to work?

Ionosphere ◦ Sun Induced Ionization F - Layer ◦ F 2 (Reflects at Night) ◦ F 1 (Reflects during the Day) E Layer (Specialized) D Layer (Daylight) Blocks below 5 -6 MHz Frequency Selection is based on Propagation Considerations

Critical Frequency ◦ The Critical Frequency is the highest frequency that the ionosphere will reflect vertically ◦ The Critical Frequency is the MUF for NVIS ◦ LUF will be dependent on D layer absorption which is greatest during mid Day The generality of using 40 meters during the day and 80 meters at night does not always work. Propagation Conditions establish the Critical Frequency

This Map can be Used as guide for NVIS frequency Support The map shows that 5 MHz is the highest frequency that will support NVIS in our area at the time the map was created. 20 UT = (20 -6)= 14 = 2 pm Ionospheric Map http: //www. ips. gov. au/Main. php? Cat. ID =6&Sec. ID=4&Sec. Name=North%20 Am erica&Sub. Sec. ID=3&Sub. Sec. Name=Ion ospheric%20 Map

Ionospheric Map June 20 10 pm

Frequency Selection Criteria ◦ Critical Frequency sets an upper limit Use Ionospheric Map to determine MUF During periods of low/no sun spot activity the critical frequency will be higher Amount of daylight and distance from the equator is a factor ◦ D-Layer considerations set lower limit Frequency Selection

D-Layer ◦ The D-layer blocks frequencies below about 5 Mhz ◦ The D-layer is less effective at blocking during low/no sun spot activity Current Conditions ◦ 75/80 meters is the best alternative ◦ 40 meters will be a viable day time alternative as the sun spot cycle progresses Frequency Considerations Cont

75 m vs. 40 m at 75 miles

What factors are important to antenna selection ◦ Horizontal polarization ◦ Enhanced vertical radiation ◦ Ground wires needed ◦ Band(s) supported ◦ Space available Antenna Considerations

Optimal Vertical Radiation

Antenna height vs. radiation angle

Low elevation of antenna provides: ◦ Attenuation of low angle dx Less interference ◦ Attenuation of low angle noise Lighting and other noise sources ◦ Attenuation of desired signal Object is greater attenuation of undesired signals than desired signal Reception Considerations

Radiation resistance vs. height

Low elevation will drop the radiation resistance of the antenna Steps must be taken to match the lower radiation resistance of the antenna Employ matching methods or consider something like a folded dipole Antenna matching

At least 10 to 15 feet of elevation is desirable (0. 05 to 0. 1 wavelength) Matching considerations (possible low Rx) Good ground conductivity is an asset Selection of operating frequency (Band) is critical and not always determined by a simple rule of thumb Points to remember

What’s the Deal About “NVIS”, Dec. 2005 QST Near Vertical Incident Skywave (NVIS) Antenna, Pat Lambert – W 0 IPL Near-Vertical Incidence Sky-Wave Propagation Concept, Excerpt for US Army Field Manual 24 -18 Appendix M http: //www. ips. gov. au/Main. php? Cat. ID=6&Sec. ID =5&Sec. Name=North% References

Store House 75 meter antenna. Height has now been doubled. Rx low due to proximity to roof

Store House VHF/UHF Antenna TRAM 1480 Gain VHF 144 -148 MHz: 6 d. B UHF 435 -450 MHz: 8 d. B