Let us suppose there is a situation for an EMOMM activation requiring HF communication over a short range. I am talking about an area within the WTX Section or within a smaller state. The radio is the same regardless of the size. The antenna is the key factor here. I am referring to a NVIS antenna. NVIS stands for Near-Vertical Incidence Skywave. It is used for short range communications from 200 to as much as 500 miles. It can fill in the skip zone for us.
I am also bringing this up because we have been experiencing some bad band conditions during our Winlink Tuesday Nets. It is my belief that most of the participants have been using their fixed station antennas. These antennas are higher and have a lower beam angle making their signals shoot over top of us. If we direct that beam angle higher then it will return to earth at a shorter distance.
I am not going to get into a lot of technical detail here because there is a ton of information out there on the web. Just Google NVIS and you will get a lot of hits. I encourage you readers to check these sources out for yourself. For those of us in the EMCOMM business be aware that there will come a time when we will be asked to send messages to someone that is outside the range of the UHF or VHF bands. Our other alternative is HF modes. Either voice or digital modes. We must be ready for what comes our way.
For NVIS to work we are talking about frequencies between 2 and 13 MHz. The signal must be bounced off the F-layer back to earth. Frequencies as low as 2 MHz would be used at nighttime and up to 13 MHz would be used in the daytime. Think about this. This only leaves us with the 30, 40, 60- and 80-meter bands. The 30- and 60-meter bands could be used during twilight and dusk when the 40-meter band is shutting down, and the 80-meter band has yet to open and visa-versa. So, the 40-meter band is a daytime band and the 80-meter band is the nighttime band. This arrangement is primarily due to the D layer. In the daytime it is present and absorbs frequencies in the MF and HF range making 80 meters unreliable. At nighttime, the D layer dissipates, and the 80-meter band becomes reliable.
How high do we put the antenna up? The answer is from one quarter to a half wave. This directs our beam angle more upward than outward. Thus, when the signal is bounced back to earth it will be a shorter distance. So, the height will vary depending on the distance to be covered.
If the EMCOMM group has a band plan for the section the NVIS antennas can be tuned to the proper frequency and eliminate the need for the tuner. If the NVIS antenna is a multiband such as a 40/80-meter cross dipole antenna, then a tuner would most likely be required.
National Preparedness Month (NPM)Tips of the Day
There are a few simple steps you and your family can take to become better prepared for an emergency: Get a Kit, Make a Plan, Be Informed and Get Involved. This September, during NPM, please follow these "Tips of the Day" from the American Red Cross and the U.S. Department of Homeland Security to help you and your family get started today. Read the Tips of the Day each day on this Home page. Then help spread the word by adding them in your daily emails and conversations. If you already have an emergency plan and disaster supply kit for you and your family this month would be a perfect time to review them and update them if needed.
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El Paso County ARES® (EPCARES) has immediate openings for communications teams in support for our local EMCOMM needs. The teams consist of two technician or above class operators (ARCT type 4). Must have mobile radios with antennas not mounted in the vehicle. See Amateur Radio Communications Teams (ARCT) below for more details. Then contact me at the email address shown below.
NOTE: The terms ARES® and Amateur Radio Emergency Service® is both registered trademarks of the American Radio Relay League, Inc. and are used by permission.
Questions and comments regarding the content of this article are always welcome and encouraged. Send an email to the address shown below.
Lew Maxwell, KB5HPT
DEC, District 6 – Far West Texas