Bigpond
NextG Yagi Project

With the Australian call sign of

Here is an item of interest

The quest to find and build Emergency Antennae

E.N.V.I.S.

Emergency Near Vertical Incident Skywave
***This page last update 8-Aug-2012 **

Background:

Being a relative newcomer to Amateur Radio I am keen to use my skills in times of natural disaster. Australia has a long and savage history of bushfires fuelled by years of drought and widespread flooding of some major river systems and with only 20million people living here, the emergency services can be stretched to capacity. That's why I joined WICEN. [Wireless Institute Civil Emergency Network] My local group is sponsored by the Bundaberg Amateur Radio Club.

Whilst Bundaberg WICEN increases its skills and equipment inventory to be able to respond where needed it seemed incumbent on me to also have a personal action plan. Bundaberg is considered sub-tropical so Tropical Cyclones are our biggest direct threat [between November & March each year] but there is also a Bushfire risk as we have some rugged national parks just to the West.

In 2006 Cyclone Larry devastated a town just to the north called Innisfail. Massive damage, No electricity or telephones and the need for Evacuation Centres gave our State a wake up call. As a result the authorities began a systematic rewriting of outdated disaster management plans. For us that means Bundaberg Regional Council, SES [State Emergency Services], EMQ [Emergency Management Queensland] and Welfare groups will need to draw all the resources together develop the plan. Whilst we hope it will never be needed, WICEN Bundaberg will be part of the planning and exercises - our aim is to be prepared.

So what do you need to be able to sustain communications in the event of no telephones (landline or mobile) and probably no electricity for several days?

I began to make a list; First get my amateur radio gear in order so an emergency station could be picked up and taken where it was needed.

My emergency radio set up- Timber cradle
holds everything securely for transportation & field work

Second; Build antennas that I could erect by myself.

Whilst training is great, we often have a group which includes some strong young men to do the heavy lifting - in the real world if I need to respond either at my own QTH or elsewhere, I need to be self sufficient yet still produce efficient radio signals if I am to be a viable emergency operator.

If you're reading this and you have pump-up masts, mechanical power and/or muscle power - this site probably isn't for you. But if you're not so endowed then read on because here's what I learned about building antennas that one person can erect without the use of tools [well OK you will need a hammer - but a rock or tyre lever would suffice]

The E.N.V.I.S. DIPOLE

I fell in love with this antenna when I first read about Bob Hejl W2IK and his dedication to emergency comms.

But wait... this is supposed to be a resource page, so let's go back to the beginning.........

What is N.V.I.S?

Definition: The Near Vertical Incident Skywave (NVIS) antenna provides the majority of its radiation at an extremely high angle. This means the major lobe is between 75 and 90 degrees to the earth's surface and will provide omni-directional communication for a distance of ??? - and here the experts disagree. Some say 400 Kilometres but some testing shows up to 1600km. Suffice to say we're not talking about long distances or DX however.
The frequencies involved can be 1.8 Mhz under very poor conditions up to 14 Mhz under excellent conditions, with the average being between 3.5 Mhz (80M) and 7.3 Mhz (40M).

Why use an NVIS antenna?

The advantages

·  Usable on all HF ham bands
·  Used to hear the stations just beyond the ground wave range
·  NVIS works into the Skip Zone - the area too far away to receive groundwaves but not far enough to receive skywaves reflected from the ionosphere
·  NVIS signals can reach areas behind obstructions and in dense foliage.
·  Two NVIS stations can quickly establish reliable comms without using repeaters
·  NVIS propagation is relatively free from fading
·  NVIS antennas are usually low and can be erected quickly and easily by one or two people
·  Simple dipoles work very well
·  Valleys & low areas are no problem for NVIS propagation
·  With a short path to and from the ionosphere means lower path losses due to absorption by the D layer
·  NVIS techniques reduce noise & interference and offer improved signal/noise ratio
·  Works well with low power due to improved signal/noise ratio and low path loss
·  Great additional antenna for Field Days or contests and fills in the gap
·  Easy antenna for HF mountain topping and camping trips to get RF out of deep canyons.

Limitations or Disadvantages

·  Must work frequencies below the Maximum Usable Frequency (MUF).
·  NVIS is not a DX antenna.
·  An Antenna Tuner is required for good match
·  For best results, both stations should be optimised for NVIS operation
·  NVIS doesn't work on all HF frequencies and work best where atmospheric noise is a problem
·  Day and Night propagation differs so a minimum of two different frequencies would be used for 24hr operations

Antennae that work well for NVIS

Dipole
An effective half-wave dipole for NVIS is positioned low to the ground – i.e. approx. 3M or lower. When a dipole is brought very close two ground the angle of radiation goes up. This dipole can be used at very low heights, resulting in some loss of vertical gain, but often, a more substantial reduction in noise and interference from distant regions. Heights of 1.5M to 3M above ground are not unusual for NVIS setups, and some people use dipoles as low as 0.5M high with good results (relatively weak signals, but a very low noise floor).

Feedpoint impedance also goes down with very low dipoles and an acceptable SWR with 50 ohm coax is likely. A tuner is a must however to get the best from this dipole - but try it without and see how you go.

This brings us to ease of erection. Low dipoles are easily erected, light tubing can be used and in an extreme emergency very low dipoles can be supported by traffic cones with a notch cut in the top.

Most dipoles will gain an extra 2 db or so of vertical gain if you allow the centre to droop a little (300 – 500mm). Allowing the centre to droop means that the end supports don't have to be as sturdy, which makes installing a good NVIS dipole that much easier.

Inverted Vee
The inverted vee, is another good NVIS antenna, which can be simple to support. An inverted vee will work almost as well as a dipole so long as the apex angle is kept gentle--about 120 degrees or greater. An inverted vee is often easier to erect than a dipole, since it requires only one support above ground level, in the centre. But gaining the height required (around 4-5Metres) can put it out of the reach of a simple centre pole/one person installation.

Adding a Counterpoise
The high angle radiation of a dipole (or inverted vee) can be enhanced by adding a counterpoise wire below it, about 5% longer than the main radiating element, to act as a reflector. The optimum height for such a counterpoise is about .15 wavelengths below the main radiating element, but when the antenna is too low to allow for that, a counterpoise laid on the ground below the antenna is still effective.

Frequencies for NVIS
Selecting optimum frequency for NVIS operation depends upon many variables e.g. time of day, time of year, sunspot activity, type of antenna used, atmospheric noise, and atmospheric absorption. There are no hard and fast rules – except if being used on the frequencies allocated for emergency services [WICEN] - you would naturally try those first.
Recent on-air experiences may allow you an educated guess or at times trial and error is the solution to find a signal that works. Propagation prediction software is also available on the internet which shows the current critical frequencies.
 Whatever the strategy you use, be prepared with a backup plan between your communication partners which involve alternates, or follow a pre-arranged scheme for trying all available frequency choices in a scheduled pattern. i.e. x.xxx Mhz +/5Mhz.

Practical Considerations
1. Get the theory straight in your head: Amateur Radio training taught you that an antenna sends out a Ground Wave - and that signal travels 0-300km. At the same time the antenna is also radiating a sky wave that is reflected off the ionosphere and is sent back 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 came back to earth. This dead zone is called the Skip Zone.

The NVIS antenna is designed to send a signal into this Skip Zone – the previously dead zone. It is therefore designed for ‘short’ distance HF communications – the point being that during an emergency you do not want to communicate worldwide – you need to send your emergency messages within your own region/state/country.

2. Learn what factors influence your NVIS antenna performance: The frequencies and performance varies with a number of factors.

• Location: NVIS Propagation maps usually show that 40M is not available above 30 Degrees North or South of the equator

• Height above ground: The antenna height above ground for NVIS antennas is subject to much discussion. Some say anything 1.5M to 5M others 10M but you'll agree there are huge logistical differences in raising an antenna to those greater heights - that's why this antenna a low height - easy for man or woman to erect and manipulate - wins my vote.

Practical tests & modeling [available on the internet] 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.

It may take some time for this to sink in because we're always told... height, height, height... but an NIVS antenna will be less efficient if you increase the height. The aim is to get as much of your signal going UP as possible and antennas at lower heights work extremely well on 40M & 80M. 

• Ground: What is the quality of the ground below the antenna? i.e. what’s the conductivity of the 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 or damp after rain the signals are likely to be greatly increased due to better conductivity.

NVIS above excellent Ground out performs an antenna above good ground at optimal height! So Ground may well be the single most important parameter in NVIS?

• Reflector [counterpoise]: Yet another approach is to run a Ground wire at the surface where the antenna is mounted which acts as a reflector. One or two ground wires under the length of wire (the full length of the dipole) which may or may not be connected to a ground rod at one end.

If running two wires keep them about 300mm apart.
Considered “watering” your earth stake – an improvement will be noticeable in both receive and transmit signals.

• A word about frequencies: They always tell you that you should use 40M during the day and 80M at night. This is not necessarily so, particularly during sunspot minimums. Once again, check on those propagation maps - you'll see some areas are actually open for 80M but not for 40M in the morning - so don't discount 3.6Mhz up to about noon local time and we work 7.1Mhz into the evening and reach good distances out into the pacific from East Coast Australia.

Well that's if folks, that's the theory - well as far as I'm interested in taking it.

Do some web research and check the modeling if you're so inclined. You'll find the proof that an NVIS Dipole at <3M is pretty impressive with constant omnidirectional coverage. See below for links to sites that gave me inspiration to build and test my own.

As I stated in the introduction, emergency communications was my goal - with antennas that were ‘quick & easy’ to erect – preferably single handed. Keeping the antenna height low, utilising the NVIS idea to put out effective signals ticked all the boxes for me.

Update: August 2012.

Based on the above research and design we made 3 of these antennas for our club [Bundaberg Amateur Radio Club] for use by the WICEN group. [That's "Wireless Institute Civil Emergency Network"]. In June 2012 the Queensland Police who head our disaster management organised an exercise "Contact 2012" and the group and these antennas were a resounding success. They proved HF communications between Bundaberg and Gayndah [120km] normall within the skip zone area of no signal.

Below you will find a pictorial diary of building the ENVIS Dipole, enjoy

Building an E.N.V.I.S. Dipole for 40 & 80M

40/80M Dipole components laid out. 3 pole set to left is 40M dipole.
The 2 pole set (at right) is the 80M extension kit.
Pegs are driven into ground and blue poles slide over pegs.
Pole are self supporting - no need for guy ropes until tensioning the Antenna wire

What's with al the labels?

This is one antenna which might lay dormant for a time and memories fade - Clear labels speed up laying out your equipment

Also in emergency situations volunteers may help - Clear labeling reduces time and the frustration of running about a field wondering where things might fit

Time to think about storing and carrying the antenna - it's off to the sewing room

Here's a rugged duffel bag idea to store antennas - made from old jeans

2010 WIA National Field Day

In October 2010 the WIA called for a National Field day to field test emergency radio gear for the upcoming storm and bushfire season and to heighten awareness of Amateur Radio around the country.

This Field Day set for 12 hours 8am to 8pm, seemed a good time to test the radio gear & antennas in the emergency kit.

We have a good relationship with Century batteries so they loaned us 4 x 450ah truck batteries... yes it was overkill to power two Yaesu FT-857D transceivers but the fact is... we didn't know how many batteries we'd need - nobody has done this kind of testing in our club so what better way to learn than sit out in the sunshine all day and play radio.

We took Amp readings during the course of the day so now we have data and so do Century batteries. When we hold the next exercise, due in Late November we'll all know how many of the batteries to take out on site.

The batteries weighed 60kg each so the poor old farm vehicle became central to the Field station.

The E.N.V.I.S. Dipole was the star performer on the day surpassing the signals on the 40/80M Vertical.

There was nothing wrong with the vertical - in fact it was so well matched that it didn't need an antenna tuner but the Dipole was continually 2-3 "S" points above the Vertical.

VK4ION and VK4MAX worked the 12 hours and had a great time. We're lining up for next year for sure. Below are some photos.

The full story is at BARC FIELD Day webpage.

LINKS

I wish to acknowledge the information from the following sources. Some helped me with antenna design, others with the background theory and modeling.

Bob Hejl W2IK - this man is a hero of mine. He loves emergency comms and just reading his pages will inspire you to want to build better emergency antennas. I've built several of his designs over the years and they all work! If you have difficulty seeing some photos on his home pages, check out his pages at the ArmyMARS Link

Pat Lampert W0IPL - A valuable N.V.I.S. resource with links to test results, modeling & antenna design

Dr. Carl O. Jelinek N6VNH [SK] - Designs for Inverted Vee N.V.I.S antenna

W2BLC Antenna Listing - Here you'll find a quick reference to most amateur radio antenna

Contact VK4ION

If you have any feedback on these pages or would like to contact me email

Goodbye and thanks for calling