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Nvis the 4x4 outback gang

Nvis the 4x4 outback gang

Postby zs2cx » Wed Apr 27, 2011 12:47 pm

NVIS
(No i dont have a Phd ,just a plain antenna homebrewer) :geek:
Often I am asked:
What is best for “NVIS”?
What is NVIS?
The answer can be one of many with emphasis on: antennas not higher than 10m, for the lower bands. NVIS to me fit the criteria of an antenna working straight up giving about 600km coverage and up to 1400km when good conditions exist. Now before u run of to build that NVIS antenna give a short vertical a try. Say what! Yeah… a normal mobile can perform on the short haul at times better than a dipole up high… I have seen it with my own eyes. Some reckon u have to let the mobile antenna hang back like the old 108 inch whips, the one’s that knocks the neighbors windows out :mrgreen:

I tested it both ways and found it did not really matter. What I found is on the longer haul, 800km, the antenna mounted in the normal vertical fashion was better (in the mobile antenna study). U don’t need a special sort of antenna for NVIS chaps, it is just the height that is low compared to the freq of operation, the effect, high angle of radiation, meaning straight up. Loop antennas have been known to perform excellent for NVIS. Maximum useable frequency (MUF) is just as important as a low antenna for given frequency of operation, meaning: Choose your band of operation for the given time of the day, as well as season. In winter we use 80m band all day long, so for emergency ops in winter use your 80m gear.

Image
that will be an old fashioned setup :mrgreen:


Feed it with coax or the old fashion way with open feeders. The 9:1 balun is a transformer, matching 50 to 600 ohm.

We have 600 ohm at the feed point, if u used a FT200 core, u may want to use a FT43 for low power, turn 30 turns for the primary and 10 turns for the secondary, I used 2,5mm wire silicon coated, use what u have for the job, or power level. This will work for any open feed line spaced 100mm apart.
I can not except that u know what I am talking about so here is a sketch.

Image

Agh ja u have seen that befor haven't u ?
but they do not tel u how to build it, Hmmm.

So we turn the primary, neatly spaced, and when we let go the wires look all "croocket"
well are u using that dreadfull plastic coated wires, yeah... use ordinary copper with the varnish man :oops:
, Use PTFE tape, plumbers stuff to wind the core....or dip the core in 2k paint with hardner added, then we wont have any surprizes later.
u knew that.. yip good.
ok so what next, we wind the primary and secondry test it and dip it for a final in the 2k paint... What EPOXY glue:!:

Nope stay away form epoxy, it will interact with your balun, epoxy glue dissipate RF pretty good.
Setting UP
I used the MFJ269B and connected it to the radio terminal and ground, to the ground and antenna I connected a 1k variable resistor,
1. Select your freq of operation on the MFJ
2. Turn the variable resistor from 1k downwards to where u find the best match.
3. Now remove the resistor (from the circuit) and measure it with your ohm meter.
4. The value of the resistor will be close to 600 ohm.
5. If u find the ohm value to be too far from your 600 ohm target, remove or add wire to the secondary turns.

PS: Some “FUNDI’S” reckon this is rather a Un-Un and not a balun. Whatever chaps, all we need is something to get the power transformation from one side to the other.
So there u have it....
for Nvis we use what we have given the time of day for freq determination and the max MUF ( max usable freq). :roll:
Now go0 ahead en NVIS me. :twisted:
CROC
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zs2cx
 
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