Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WD0EJA, Jan 11, 2020.

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  1. WD0EJA

    WD0EJA Ham Member QRZ Page

    What are those things up there?

    60 foot tilt over tower at our location.

    160, 80, 40, 20, 17, 15 and 10 meter Isotron Antennas.

    Works well for NVIS


    Have you heard of this? It stands for NEAR VERTICAL INCIDENT SKYWAVE.

    Why would knowing about this be useful?

    Many radio operators are interested in DX or long distant contacts, but not all. There are many applications where contact is desired a short distance away, under 400 miles. This is where NVIS plays a part.

    The radio waves travel near-vertically upwards into the ionosphere, where they are refracted back down and can be received within a circular region up to 650 km (400 miles) from the transmitter. If the frequency is too high (that is, above the critical frequency of the ionospheric F layer), refraction fails to occur and if it is too low, absorption in the ionospheric D layer may reduce the signal strength.

    There is no fundamental difference between NVIS and conventional skywave propagation; the practical distinction arises solely from different desirable radiation patterns of the antennas (near vertical for NVIS, near horizontal for conventional long-range skywave propagation).

    The dipole therefore, is the more practical for NVIS over a vertical antenna.

    The most reliable frequencies for NVIS communications are between 1.8 MHz and 8 MHz. Above 8 MHz, the probability of success begins to decrease, dropping to near zero at 30 MHz. Usable frequencies are dictated by local ionospheric conditions, which have a strong systematic dependence on geographical location. Common bands used

    in amateur radio at mid-latitudes are 3.5 MHz at night and 7 MHz during daylight, with experimental use of 5 MHz (60 meters) frequencies. During winter nights at the bottom of the sunspot cycle, the 1.8 MHz band may be required.

    So, how high should your antenna be for NVIS. Right where you have it. The low frequencies are very reliable on NVIS even if your antenna is quite high. It is more of a function of wavelength above ground rather than feet or meters. At 60 feet you are only 1/4 wave on 40M. This height is fine for NVIS.

    How do you use it?

    NVIS is dependent on time of day, season, your latitude and the band you select. As you listen to the various bands you will start to see a pattern of which bands work at different times and seasons.

    This phenomenon provides very reliable HF communications. It defies obstacles such mountainous or rough terrain. It is excellent communications for emergency work and is used regularly for this.

    While the old sun seems to be on sabbatical, you can enjoy HF using NVIS.


    Ralph WD0EJA
    Last edited by a moderator: Jan 11, 2020

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