|
|
|
||||||||
 |
||||||||||||||||||||||||||||||||||
|
Part 3: DEPLOYING AND USING
W2IK's E.N.V.I.S. ANTENNA
|
||||||||||||||||||||||||||||||||||
|
Designed and built by Bob Hejl - W2IK- NNN0KSI NAVMC MARS
all rights reserved 2007 - see page one: E.N.V.I.S.
|
||||||||||||||||||||||||||||||||||
|
In the last two parts, you've learned via step-by-step construction how to build an Emergency Near Vertical Incident Skywave antenna for deployment during communications emergencies. You've also learned the importance and basic theory behind NVIS antenna systems. Now you will be introduced to practices on it's deployment. This system is vital when reliable communications is required during a disaster. There should have been more of these systems in use during hurricanes such as Katrina, which wiped out most communications systems including repeater vhf coverage. It is hoped that every ARES, RACES, SATERN and other groups take my advice and develop a cache of E.N.V.I.S. antennas so as to be ready for disasters.
It is also hoped that EmComm groups will practice using this system during drills.
This antenna consists of three in-ground stake supports, three telescoping poles which fit into the supports, the main antenna which has a center connector that locks in place on the center pole (with the antenna's end nylon cords passing through holes on each of the other telescoping poles) and four support stakes with two additional 12 ft. nylon cords.
To deploy this antenna, you need to find an open area which is at least 90 feet long and away from people as the antenna and its supports may pose a potential trip hazard. Try to keep it away from metal structures as this might distort it's RF waveform pattern.
After you've found a good location, divide the length in half and at that point push or hammer in one of the support stakes you've built. Make sure it's in the ground very firmly even if it goes beyond the yellow stake and a portion of the PVC is in the ground as well. (That's why I had you bevel the bottom of the PVC) This will become the center of the antenna. 35 feet in both directions away from the center, but in a straight line, hammer in the other stake/supports so they are firmly in the ground. This will leave you with a total distance between the two outer stake/supports of 70 feet. We make this distance longer than the 66 feet of the antenna so that when we put up the antenna, it's wire ends don't reach the poles, but the nylon cords that you've tied to the ends do so they can be threaded through the hole on the top of each end pole. If you wish to concentrate on frequencies below 7.2 mhz, you may wish to make the total antenna wire length about 100 feet. In any event, you'll need a tuner. You might also want to build a special adapter for lower MARS frequencies and 80 Meters. See webpage addition below.
Place a telescoping pole assembly in each of the stake/supports and by twisting the pole pieces in different directions until they loosen, extend the poles and then lock each one by reversing the twist. They will then be about 6 feet high. Unwind the antenna wire, spreading it out along the length of the pole positions. Take the center of the antenna (where the elbow/connector is) and place it on top of the center pole, match up the elbow/connector hole with the pole holes by rotating the blue mast area and secure them to each other with that last 1/4 x 20 machine screw and nut (or wing nut). Refer to the picture below and to the left. The picture below and to the right shows how each end pole should look with the cord dressed through it. The extra cord beyond the pole should be angled down and away and tied off to one stake just as you would to support a leg on a canopy or tent. An additional 12 foot piece of cord should be looped over the top of each pole, angled down and located about 90 degrees apart from the first stake and attached to another stake. This will create tension in THREE directions ("pull" from the antenna wire, excess antenna poly cord to one stake, looped extra cord to another stake) and will help stablize the end poles and the antenna. When you change the height of the antenna, just release or recover lengths of the cords by unwrapping/wrapping them from around the stakes. When you tighten the antenna up by wrapping the cord around the stake on each side, be careful not to over-tighten the antenna as it could snap. A little "weight-droop" is okay.
|
||||||||||||||||||||||||||||||||||
|
center pole with antenna connector
|
one of the two antenna end poles with poly cord laced through
|
|||||||||||||||||||||||||||||||||
 |
||||||||||||||||||||||||||||||||||
 |
||||||||||||||||||||||||||||||||||
|
to antenna wire
|
||||||||||||||||||||||||||||||||||
|
angled down to one stake
|
||||||||||||||||||||||||||||||||||
 |
||||||||||||||||||||||||||||||||||
|
added green looped cord to additional stake
|
||||||||||||||||||||||||||||||||||
|
When installing the coax to the antenna, do not over-tighten it to the connector. A snug, hand tightening is adequate. Dress the coax straight down to the ground and then direct it away from the antenna for at least 15 feet before dressing it to your tuner and rig. This will prevent RF from being radiated back into the coax shield and back to your rig. It will also prevent "RF bites".
|
||||||||||||||||||||||||||||||||||
|
complete antenna system with storage tube
|
||||||||||||||||||||||||||||||||||
 |
||||||||||||||||||||||||||||||||||
|
You might also wish to include a choke balun (as the coax is attached to the antenna by making 6-8 tightly wound loops in the coax and securing with tape). Remember that you'll need a tuner to use this antenna as different ground conductivity in different locations or when the ground is saturated after a storm will change the resonance of the antenna. It would also be wise to ground both your tuner and transceiver. I have used this antenna on 20, 40, 80 meters and MARS frequencies without problems. You will find the bandwith on 80 to be very narrow, but if re-tuned properly it will work just fine. I haven't included a reflector, but if you feel the need, just lay a straight 70 ft length of wire at ground level just below the antenna line and it will help reflect energy instead of "warming the ground". You'll probably find 40 meters best during the day and then move down to 80 meters at night as the bands "expand". I have found that just by lowering the antenna height you can use 40 meters for a longer time and avoid hearing too many international broadcasters.
You change the height by twisting the blue (lower section) of each pole while holding the white (top) section so it doesn't spin and the antenna remains straight then just raise or lower the white section and twist the blue section to tighten. (This is why I had you make the poles to be snug, but not tight, in the PVC stake assemblies.)
|
||||||||||||||||||||||||||||||||||
|
This entire antenna system when it is broken down into it's component pieces can be stored in a narrow duffel bag or in a length of wide pvc sewer pipe (the thin wall, green or white type) with a capped end and a screw cover (aka clean out cap). You may wish to tie some orange warning tape in several places along the antenna wire run so people won't walk into it.
|
||||||||||||||||||||||||||||||||||
|
This antenna, since it's used with a tuner, is also useful on HF MARS/CAP frequencies.
Use this design. Experiment. Play. Adapt it to suit your needs. HAVE IT READY FOR AN EMERGENCY. This is just one basic E.N.V.I.S. antenna system. Again, this is for temporary emergency deployment and should not be thought of as a permanent antenna system.
|
||||||||||||||||||||||||||||||||||
|
A SPECIAL "MARS-80 METER" ADAPTER TO MAKE THE SYSTEM MORE EFFICIENT ON LOWER FREQUENCIES CAN BE FOUND AT:
|
||||||||||||||||||||||||||||||||||
|
NVIS ANTENNA - 6 FT HEIGHT
|
||||||||||||||||||||||||||||||||||
|
||||||||||||||||||||||||||||||||||
|
click here to visit my other webpages:
|
||||||||||||||||||||||||||||||||||
| page created with Easy Designer |