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Author Topic: How Many are Using NVIS Antennas ?  (Read 12788 times)
KF7VXA
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Posts: 448




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« on: January 18, 2014, 04:41:47 PM »

For EMCOMM work, I have found the NVIS antenna a necessity. Of course it may depend on where you live, but where I am, there are some counties that don't get much of any VHF/UHF signal to them and NVIS has proved to be the best way for them to get a good signal out.
The other reason for a NVIS is if the local repeaters go down due to grid outages, the NVIS will allow communication to close in stations. As long as operators have emergency back up power to their stations, they will be able to communicate should VHF/UHF, internet etc. go down.
We have repeaters on mountain tops that have propane back up generators. They will run the repeaters for a while until the propane runs out. Where they are located, I'm not sure even a snow cat could get to them with more propane, so having the NVIS is a very good way of insuring communication in the worst of times.
Even if you choose not to have a NVIS antenna up in your yard, making one up that could be put up is a darn good idea.
I have one for 40 and 80 meters. If I had a choice of just having one, it would be an 80 meter antenna.
Just something to think about.
If you have an 80 meter dipole up, you may want to put a pulley system on it so it can be lowered for NVIS use and raised back up for DX work. Make it do double duty.
The more ways we have to communicate close in as well as a distance, the better.

73's John KF7VXA
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WB6BYU
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Posts: 13005




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« Reply #1 on: January 18, 2014, 05:44:01 PM »

In my experience most ARES groups at least who use HF are familiar with NVIS principles.
Many groups don't use HF extensively, but here in Oregon we have a regular state-wide net
and use HF to link the various counties with the State EOM over distances of 300 miles.

In fact, one of the problems we sometimes encounter is from people who try to be too
strictly in accordance with the military NVIS approaches, in spite of the fact that  (1) they
were initially based on flawed computer modeling using a poor ground model, and
(2) most ham applications aren't worried about the enemy DFing our positions.

Even some of the ham "conventional wisdom" can sometimes lead folks astray.  For
example, over the last 3 sunspot cycles we've had relatively little coverage around
Oregon on 40m, even at mid-day, so we've needed to use 80m during the day and
160m at night.  I've given a couple presentations and distributed antenna designs
to get on 160m because often that's the only band we can use.

The Australian Ionospheric Prediction Service has a handy "Local Area Mobile Prediction"
tool online that shows expected coverage vs. frequency and time of day for distances
out to 600 miles:

http://www.ips.gov.au/HF_Systems/7/1

Also, some of the "serious" NVIS operators make too much of a deal about keeping
the antenna low to the ground - a few feet.  This does make it harder for the enemy
to DF your station, but it also reduces your signal strength unnecessarily.  Higher
antennas (when possible) reduce the trip hazards and are often easy enough to
accomplish in practice.  A good maximum height in feet is the band in meters,
which is easy to remember.  So not higher than 40 feet on 40m (which is rarely a
problem for most hams, unless their primary interest is DX).  Lower is OK, but
especially on 160m you can lose efficiency as you get below about 20' or so.
(The original computer models used a MiniNEC ground equations, which greatly
understated the losses at low heights, making it appear that an antenna 3' off the
ground gave significant gain.)  You can still use those very low antennas, of course -
I put one up at the local hospital that was laying on low bushes beside the parking
lot - but there is no penalty for getting it up 10' to 20' in the air when you can.

I build a couple sets of wire dipoles for 40 / 80 / 160m for the ARES portable trailer,
and we use them on Field Day as well so folks get experience setting them up.  While
both of those sets use loading coils to shorten the 160m elements, in practice I've
never had a problem finding space to run full length wires for a portable exercise
(including, in one case, across the County parking lot, wrapped around a small tree
and tied off to a stop sign in front of the County Commissioners' building.)

But the important thing is getting out and doing it.  Using NVIS requires some extra
skills and knowledge, such as choosing the right band.  I've done workshops where
we practice tossing a rope over a branch and setting up a 28' mast with just one
person (which are equally applicable to improving your VHF/UHF antenna system.)
Getting out and setting up a station a few times a year allows operators to get the
feel of the band conditions throughout the day, for example, and the sort of signal
strengths they are likely to encounter.  And this was in a county where we could
talk to the State EOM on simplex with a line-of-sight path, so HF wasn't something
we were likely to need in many situations.  But there were still some corners of the
county back in the hills where it could be useful.

So, yes, I expect many hams are using NVIS antennas.  The bigger problem is perhaps
to get them to think of using it for emergency communications, and to prepare for
those situations where VHF/UHF aren't sufficient to the task.


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N9AOP
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Posts: 130




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« Reply #2 on: January 21, 2014, 02:10:17 PM »

For in the field use of NVIS antennas, I prefer an easy to set up one.  You can use a version of the teepee antenna or string up TFD's but these take time and take the teepee takes up 60X60 feet.  What I use is either 52.5 or 125 feet of wire and 4 fiberglass 4 ft poles.  Put up the poles and run the wire from the ground to the top of the pole and then out to a support.  Run the same length of wire on the ground under it and attach both to a 9:1 unun.  The SWR us less than 2.0 with this setup.  If you use a coupler (autotuner) at the feed then you can use most any length of wire.  I have used this in the field and covered Illinois on HF during exercises from Chicago and have used it for MARS coverage in the adjacent states.  It is cheap, the wire fits in a glad bag and any support pole would work so long as it is non-metallic.  Things do not have to be complex to work well.
Art, N9AOP
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W5LZ
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Posts: 477




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« Reply #3 on: January 23, 2014, 08:28:04 PM »

Odd.  I've been using so called NVIS antennas for a lot of years and have been able to make as many 'DX' contacts as I've ever tried.  NVIS as in low to the ground, no, or not much 'low angle' radiation at all.  Guess I must be doing it wrong, huh?  Smiley
 - Paul
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W1JKA
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Posts: 1618




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« Reply #4 on: January 24, 2014, 02:01:08 AM »

Re: W5LZ

What a coincidence, I consider all my portable QRP 20/30/40m wire dipoles on top of 4 ft. high bushes/rocks or 15-20 ft. AGL NVIS antennas and also found DX no problem (prop/grey line taken into consideration), I suspect there are more theorist than actual practitioners out there.
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WB6BYU
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Posts: 13005




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« Reply #5 on: January 24, 2014, 07:36:15 AM »

Quote from: W1JKA

 I suspect there are more theorist than actual practitioners out there.



Actually, given most of the 40m / 80m dipole installations I've seen over the years,
I'd say that there are a lot more actual practitioners out there than theorists.  Which,
in a way, is a good thing, because it means we're getting the job done even if our
antennas don't have the theoreticists' official stamp of approval.


While the radiation from your 30m / 20m antennas may be maximum straight up,
that isn't going to help your local coverage much on those bands because the
ionosphere isn't going to reflect it back down to Earth.  To communicate via NVIS
we have to operate below the critical frequency, which is the MUF for the
straight up / straight back down path.  This rarely reaches 10 MHz, and when it
does we'll probably be busy working DX on 10m or 6m instead.  At the bottom of
the last couple sunspot cycles we spent years with it mostly in the 2 to 5 MHz
range - even 40m contacts were unusual for distances less than 300 miles here
in Oregon.

Of course, there will always be some low angle radiation from any antenna, so
you can work DX even though the antenna may not be optimized for it.  But it
will be better for DX if the antenna is higher.


That's another good example that the world isn't binary: we usually don't have
those sort of sharp dividing lines in ham radio (especially antennas) where it
has to be a certain way or it won't work:  a vertical only works
when it has >n radials, or an antenna only has high angle radiation and
not low angle radiation, or a particular type of coax is too lossy when longer
than 73 feet, or when the SWR is > 3.47 : 1.

We're dealing with a continuum, and often in probabilities ("this option will
usually outperform that one").  While it may be good to know the
nominal expected performance of an antenna, propagation, etc., it
is those times on the fringe, when you get the unexpected opening to
Tanzania or the South Pole when the band seems otherwise dead and you
antenna is only a few feet off the ground, that makes ham radio so much
more exciting.
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KF7VXA
Member

Posts: 448




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« Reply #6 on: January 25, 2014, 07:44:49 AM »

Lots of good info.
My primary purpose for the question was to get those who don't have a NVIS antenna up or made for portable use is to try one.
In an emergency, many things can go right or wrong and having a working NVIS will show you just what can be done with one. What areas you can easily work and to fill in those tough to get to VHF/UHF gaps.
Ground, time of year, conditions can all change things somewhat or a lot, so do check at different times.

It's another tool to have in your tool box. In times of emergency, the more ways one has to communicate, the better (within reason).

As I said, we have one county that gets almost no VHF/UHF from any repeaters, so the NVIS is a great thing for them. I'm pretty sure we could get them VHF/UHF if they would put up a repeater, but that's another story.

73's John KF7VXA
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KB4QAA
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Posts: 2259




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« Reply #7 on: January 26, 2014, 02:54:11 PM »

Every 80m/40m antenna I've ever had has been NVIS, since they have never been as high as 30 feet.

NVIS does not work above 40m, nor on 160m. Not even all the time on 80m/40m depending on the Muf.
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AA4PB
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Posts: 12665




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« Reply #8 on: January 26, 2014, 03:01:07 PM »

On 75M, where probably most of the EMCOM activity occurs, most stations are already using an NVIS antenna whether they know it or not.
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WB6BYU
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Posts: 13005




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« Reply #9 on: January 26, 2014, 10:05:06 PM »

Quote from: KB4QAA

NVIS does not work above 40m, nor on 160m. Not even all the time on 80m/40m depending on the Muf.



Why wouldn't it work on 160m?  That's our backup when the Critical Frequency drops
below 4 MHz and we can't work on 80m.  So far it has worked very well - the nets have
had much better signal strengths than on 80m in those conditions and operators have
often been amazed how strong and clear the signals are.

The main limitation is that many hams don't have horizontal antennas up for 160m.
If you have room for an 80m dipole, you can put up a loaded 160m dipole in the same
space (and get 40m coverage as well.)

True, the D-layer absorption can be high during the day in summer, but here at 45N
coverage on 160m is usually improving by the time that 40m is dropping out, and in
winter it often is open most of the day, especially at the bottom of the sunspot cycle.


And don't forget 60m - that was one of the arguments for the allocation was to give
us an intermediate band between 40m and 80m.
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KF7VXA
Member

Posts: 448




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« Reply #10 on: January 27, 2014, 08:02:44 AM »

Quote from: KB4QAA

NVIS does not work above 40m, nor on 160m. Not even all the time on 80m/40m depending on the Muf.



Why wouldn't it work on 160m?  That's our backup when the Critical Frequency drops
below 4 MHz and we can't work on 80m.  So far it has worked very well - the nets have
had much better signal strengths than on 80m in those conditions and operators have
often been amazed how strong and clear the signals are.

The main limitation is that many hams don't have horizontal antennas up for 160m.
If you have room for an 80m dipole, you can put up a loaded 160m dipole in the same
space (and get 40m coverage as well.)

True, the D-layer absorption can be high during the day in summer, but here at 45N
coverage on 160m is usually improving by the time that 40m is dropping out, and in
winter it often is open most of the day, especially at the bottom of the sunspot cycle.


And don't forget 60m - that was one of the arguments for the allocation was to give
us an intermediate band between 40m and 80m.

Good post and I agree. The reason more people don't have 160 meter wire antennas is due to their length, but they will work for NVIS, especially when conditions or other factors call for that band.
I have room for a 160, I'm just growing poor buying coax and it would take a fair amount to reach the center of a 160 dipole and get back to my station hi hi. Maybe I need less antennas....on second thought, NO.

73's John KF7VXA
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N0JL
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« Reply #11 on: February 19, 2014, 05:42:25 AM »

I think all of my antennas are NVIS.  I didn't plan it that way.
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K2GWK
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« Reply #12 on: February 19, 2014, 06:10:43 AM »

I think all of my antennas are NVIS.  I didn't plan it that way.

+1000......Now there is a man that tells the truth!!! I am in the same boat.....
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KF7VXA
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Posts: 448




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« Reply #13 on: February 28, 2014, 12:54:13 PM »

Most of my dipoles are not really high enough, but are more than 10-12 feet off the ground, so I know what you all mean. I use a couple of verticals for DX and plan on a Hex antenna soon for the HF high band.
I've never set up a 160 dipole, but have read some say that at times they need to use 160 for NVIS due to conditions.
I just figure if I can't work things so I can hit the areas I need to with 80 meters, then I'll just be out of luck, try another antenna such as a portable dipole or move locations if necessary.
It's interesting that the military used NVIS so much because it makes you far harder to pin point as to where the signal is coming from. Of course today, I'm sure with the new technology that that is no longer the case. It was during the Viet Nam days and WWII that this was most important.

73's John KF7VXA
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WG8Z
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Posts: 182




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« Reply #14 on: March 07, 2014, 05:38:27 PM »

Why in the world would you want to lower it?  Unless your one of the lucky ones that has an
80m dipole strung up higher than 90ft. My experience is for 80M the sweet spot for NVIS  is 30 to 60ft above the ground. Anything lower than 20ft and your ground loss starts to really kick in.
 For regional coverage (say 0 to 400 miles), MUF cooperating I'll bet ya my dipole at 45 ft running 10 watts will blow the socks off the guy running a dipole < 15ft above the ground running 100 W.   Any takers?
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