ENVIS Dipole Antenna

40 & 80mtr Dipole for emergency comms

Living in Queensland our biggest threat is tropical cyclones during the "wet season", from November to March.
I built an emergency HF antenna that I could quickly erect by myself (and pack away during the dry season)
Bob Hejl W2IK, is an emergency comms guru, and his NVIS antenna ideas resonated with me. He was extremely helpful when we chatted a few years ago and assisted with my design and build. [You can find Bob's envis-envis-envis-envis-info and designs at QRZ.com]


Let's go back to the beginning.........

What is N.V.I.S? Definition: The Near Vertical Incident Skywave (NVIS),an antenna which provides the majority of its radiation at an extremely high angle, so the major lobe is between 75 and 90 degrees to the earth's surface. It provides omni-directional radiation for a distance up to 400 Kilometres, (although I did receive signal reports up to 1600km).
It isn't a long distance antenna, because when you have an emergency you don't want to tell people in Peru! You need to call someone nearby - generally within that 400km range.
The "best average frequencies" are between 3.5 Mhz (80m) and 7.3 Mhz (40m) - exactly the bands that I was looking for.
Based on my research and design we made 3 of these antennas for our club [Bundaberg Amateur Radio Club] for use by the WICEN group.
In 2012 the Queensland Police, who head our disaster management, organised an exercise to prove these antennas - the exercise was a resounding success.
We proved HF communications between Bundaberg and Gayndah, two towns 120km apart with a mountain range in betweeen and normally within the skip zone. We successfully transmitted messages back and forth.

There is a WICEN page about the Exercise, if you have an interest in Emergency Comms.

A few simple components found at any hardware store make up this useful HF Dipole antenna and the following gallery should give you some ideas to follow or adapt.

Technical information

NVIS - a Starting Point if you're interested

If this antenna is of interest to you you can adapt it to suit your needs.
This is some theory, so you can familiarize yourself with the basics and be inspired to build a better one to suit your needs.

The Advantages

  • Usable on several HF ham bands
  • Hears stations just beyond the ground wave range
  • Works into the Skip Zone (an area of no signal)
  • Can reach areas behind obstructions and in dense foliage
  • NVIS stations can establish reliable comms without using repeaters
  • Antenna is low-to-the-ground - quick and easy to erect by solo operator
  • Due to short path to & from ionosphere relatively free from fading & lower path losses due to absorption by the D layer
  • Reduced noise & interference so offer improved signal/noise ratio
  • Works well with low power due to improved signal/noise ratio and low path loss
  • Great addition to field-day or contesting antenna packs
  • Easy antenna to deploy on camping trips to get RF out of valleys or deep canyons

Limitations or Disadvantages

  • Must work frequencies below the Maximum Usable Frequency (MUF).
  • NVIS is not a DX antenna.
  • For best results, both stations should be optimised for NVIS operation
  • NVIS doesn't work on all HF Bands
  • Day and Night propagation differs so a minimum of two different frequencies would be used for 24hr operations

Frequencies for NVIS

Selecting optimum frequency for NVIS operation depends upon many variables e.g. time of day, time of year, sunspot activity, atmospheric noise, and atmospheric absorption.
In Australia we would expect to work the frequencies allocated for emergency services [WICEN].
Propagation prediction software is also available on the web, giving the current critical frequencies.
It's important to prepare a backup plan between operators with alternate frequencies, or agreement to follow a pre-arranged pattern to "find each other" i.e. begin at frequency, then steps of +/- 5kHz.
Whatever works for your emergency comms group!

Practical considerations

1. Remember your amateur radio training and basic antenna propagation theory? An antenna sends out a ground-wave which may travel up to 300km. The antenna is also radiating a sky wave that is refracted by the ionosphere and returns to earth (some greater distance away).
There is a “dead zone” of "no signal" between where the ground wave ends, and where the sky wave comes back to earth. The dead zone is called the Skip Zone.
The NVIS antenna is for "short-distance" HF communication into the Skip Zone. You're attempting to communicate within your region/state/country.
2. Research... the frequencies and performance varies with a number of factors.
Location: NVIS Propagation maps indicate that 40M is not generally available above 30 Degrees North or South of the equator... (Qld = good for me, maybe not as successful in the southern states of Australia).
Height above ground: Technical specs say anywhere from 1.5metres to 5metres, but there are logistical differences in raising an antenna to those greater heights. My final antenna design is kept to a low height, 1.8metres (6foot) above the ground and this is easy for a solo operator to erect and manipulate.
Practical tests & modeling [available on the web] show that better coverage is obtained if the antenna is mounted at about 1/20th wavelength above ground and at this height the background noise level is lower.
This feels counter-intuitive as we're always told... height, height, height... but an NIVS antenna will be less efficient if the height is increased.
Ground: Look at the type of the soil below the antenna? i.e. what's the conductivity of the earth/ground you are operating above?
Poor conductivity will attenuate up to 3db more of your signal than high conductivity soil. If the ground is wet after rain the signals are likely to be greatly increased due to better conductivity.
So better soil conductivity = better performance so soil may be the single most important parameter affecting propagation.
Reflector [counterpoise] Another approach is to lay a wire on the ground under the NVIS antenna wire, to act as a reflector. Simply a wire laying on the ground...the full length of the dipole. It can be connected to a ground rod at one end, or not.
Also consider “watering” your earth stake for a noticeable improvement in both receive and transmit signals.
About Frequencies: Convention says you should use 40M during the day and 80M at night. This is not necessarily correct, especially during sunspot minimums.
Check the propagation maps and notice some areas are actually open for 80M but not for 40M in the morning, so don't discount 3.5Mhz up to about noon local time, and we work 7Mhz into the evening and reach required distances.

That's if folks, as far as I'm interested in taking the theory.
Research and modelling is available on the web will prove that an NVIS Dipole at less than 3 metres is pretty impressive with constant omnidirectional coverage.
As I stated in the introduction, emergency communications was my goal; a solo operator using a 'quick and easy' to erect antenna.
This one keeps the antenna height manageable and utilises the NVIS science to put out effective signals.
It ticked all the boxes for me.

Personal Projects

Never stop learning, sharing knowledge or travelling

Each of these images will link to a dedicated webage where I share my Amateur Radio projects or travel pages.



Building portable emergency cases



Find specifications & technical data

NextG Yagi


850MHz NextG Yagi project

Digital TV Antenna


Facts about improving Digital TV

Motorcycles & Roadtrips


My Bikes & Trips around Australia

To New Zealand


Driving both islands in search of beer

Time Travelling in Wales


Enjoying Wales & remembering the past

Scotland & city of York


Coast and Highlands travelling in 2022

Then to the USA


In search of good craft beers

Learning to make Cheese


Yumbo, delicious homemade cheese

Next up......................


To be sure..

Contact Me

Please email me if I can assist with information about my projects (or if you notice a broken link).