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eHam Forums => Emergency Communications => Topic started by: KF7VXA on January 18, 2014, 04:41:47 PM



Title: How Many are Using NVIS Antennas ?
Post by: KF7VXA 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


Title: RE: How Many are Using NVIS Antennas ?
Post by: WB6BYU 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.




Title: RE: How Many are Using NVIS Antennas ?
Post by: N9AOP 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


Title: RE: How Many are Using NVIS Antennas ?
Post by: W5LZ 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?  :)
 - Paul


Title: RE: How Many are Using NVIS Antennas ?
Post by: W1JKA 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.


Title: RE: How Many are Using NVIS Antennas ?
Post by: WB6BYU 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.


Title: RE: How Many are Using NVIS Antennas ?
Post by: KF7VXA 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


Title: RE: How Many are Using NVIS Antennas ?
Post by: KB4QAA 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.


Title: RE: How Many are Using NVIS Antennas ?
Post by: AA4PB 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.


Title: RE: How Many are Using NVIS Antennas ?
Post by: WB6BYU 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.


Title: RE: How Many are Using NVIS Antennas ?
Post by: KF7VXA 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


Title: RE: How Many are Using NVIS Antennas ?
Post by: N0JL on February 19, 2014, 05:42:25 AM
I think all of my antennas are NVIS.  I didn't plan it that way.


Title: RE: How Many are Using NVIS Antennas ?
Post by: K2GWK 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.....


Title: RE: How Many are Using NVIS Antennas ?
Post by: KF7VXA 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


Title: RE: How Many are Using NVIS Antennas ?
Post by: WG8Z 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?


Title: RE: How Many are Using NVIS Antennas ?
Post by: W8JI on March 10, 2014, 05:32:38 AM


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.) 

It is interesting to read that.

I don't know where the nonsense that NVIS antennas have to be a few feet high originated, but it is pretty difficult to stop that silly idea. Despite QST articles by Dean Straw and efforts by others, that "keep it really low" myth keeps coming back.

Losses increase dramatically when a dipole is significantly less than .1 wavelengths above earth, which is 24 feet on 80 meters.

As a long time 160 meter operator, 160 is open with useful NVIS most of the time, day or night. There are exceptions when it is not useful are the solar minimum at night when critical frequency falls below 160 meters, and during high solar activity a few hours plus or minus solar noon in summer.

Except for antenna size, power line noise, and antenna SWR bandwidth, the 2 MHz end of 160 would be a great choice.  A local daytime only BC station that operates in the top end of the BC band lost their tower, and they did Ok for a few weeks with a low dipole I installed (except for fading).


Title: RE: How Many are Using NVIS Antennas ?
Post by: N3QE on March 10, 2014, 07:00:33 AM
My main antenna is up 80 feet. I'm on the east coast. On 15M and especially 10M, it's easy to work California and Oregon and Washington State over and over again but hard to work the states in the middle.

I once bought into the "NVIS theory" that a lower antenna would have more high-angle, short distance radiaiton. I strung up a couple of low slopers, based on modeling to optimize high angle radiation. I did extensive testing. Uniformly... the low NVIS antennas were 10dB or so weaker at ALL distances.

I think NVIS is just a keyword for "poor antenna". You can work lots of guys on 40M with a poor antenna and 100W, You can probably even earn DXCC with a poor antenna. I did that for years when I was a kid. It can be fun. But it's not as much fun as a good antenna.


Title: RE: How Many are Using NVIS Antennas ?
Post by: W8JI on March 11, 2014, 07:08:42 AM
My main antenna is up 80 feet. I'm on the east coast. On 15M and especially 10M, it's easy to work California and Oregon and Washington State over and over again but hard to work the states in the middle.

I once bought into the "NVIS theory" that a lower antenna would have more high-angle, short distance radiaiton. I strung up a couple of low slopers, based on modeling to optimize high angle radiation. I did extensive testing. Uniformly... the low NVIS antennas were 10dB or so weaker at ALL distances.

I think NVIS is just a keyword for "poor antenna". You can work lots of guys on 40M with a poor antenna and 100W, You can probably even earn DXCC with a poor antenna. I did that for years when I was a kid. It can be fun. But it's not as much fun as a good antenna.

NVIS antennas are old things, and can work quite well for short or medium distances. We used them in the 1960's on 160 meters for regional communications, like from Toledo to Detroit or Cleveland, or even Chicago.

On 75 meters, in the infamous 3830 and 3895 kHz regional battles between alligators and quirmers, NVIS was the only thing competitive.

Efficient antennas required:

1.) Somewhere between 1/10th and 1/4 wave height above ground

2.) A ground counterpoise or wire grid system to mitigate earth losses if the antenna was at lower height ranges

3.) Propagation conducive to very high angles


Somehow some really whacked-out stuff got published and repeated. I'm not sure where it came from, but once something wrong is in print, no matter how ridiculous or wrong, it takes on a life of its own. Somewhere the absolutely silly idea that a NVIS antenna works best at a few inches or feet above ground, or works well on bands that do not support near vertical angle reflections, crept into commonly distributed NVIS knowledge.

There are many "total nonsense" ideas in old papers and articles that can be dredged up, and many are from military sources. There are also some valid ideas that can be misapplied to different applications where they become incorrect.

NVIS is only applicable on frequencies that are at or below the critical frequency of the ionosphere. We are lucky if that frequency ever reaches 8-10 MHz during summer noon, and it is often well below 3.5 MHz on winter nights in solar minimums.

The worse thing of all, however, is the silly idea that an antenna less than .05 wavelengths above dirt is an efficient radiator.

When I listen to the Georgia ARES net on 75 meters, I'm amazed at how poor many of the signals are. I have about 50-80 acres of receiving antennas with hundreds of thousands of feet of cables and wires, and an S1 local noise floor, and I can barely hear some of the signals. The weakest signals, without fail, come from really short (whip) verticals, all-band no-tuner antennas, or really low dipole antennas. Two watt signals from Europe are sometimes louder than regional 100 watt emergency communications stations that use pathetic antennas, and no one bothers to tell the people they have a problem.  I hear people trying to check in that no one else anywhere in the region hears, and without fail when I contact them, they are using some magical antenna contraption.

73 Tom


Title: RE: How Many are Using NVIS Antennas ?
Post by: WG8Z on March 11, 2014, 02:07:54 PM
Kinda takes the Communications part out of Emergency Communications doesn't it Tom?
For the most part the ragchewers and traffic handlers got it together.
IMHO the ARES guys read too many bogus articles.
And ya can't tell em any different.
 


Title: RE: How Many are Using NVIS Antennas ?
Post by: WB6BYU on March 11, 2014, 05:35:36 PM
Besides the use of the misleading MiniNEC ground model at low heights, military
use (where the phrase "NVIS" was coined) had some other considerations that
don't apply as often in ham communications:

1) reduction in ground wave to avoid DF by hostile forces
2) reduced height profile to avoid exposure to hostile fire
3) complete coverage with no nulls for tactical operation at short distances
4) potential operation over a wide range of frequencies (typically 2 - 12 MHz)
5) reduction in interference from more distant stations
6) simplicity to allow non-technical personnel to install antennas.

How much difference does the ground model make?  Lots!  A model of an 80m
dipole mounted 3' off the ground shows -6dBi radiation level using the high
accuracy Summerfield-Norton equations (currently one of the better choices)
compared to +19dBi using the MiniNEC model.  That's a difference of 25dB, or
1 watt compared to 300 watts!  While the Summerfield-Norton model might not
always be perfectly accurate, it certainly is a better approximation of reality.  So
the original studies showed no downside to low antennas due to ground losses.

Of the other factors, #1 is rarely a problem for most hams, unless they are
causing intentional QRM on 75m and don't want to be caught.  A low antenna
radiates less ground wave (especially off the feedline if it doesn't use a balun)
and that is what most amateur HF DF systems use to take bearings.  (The FCC,
on the other hand, can calculate the relative distances from multiple locations,
including corrections for ionospheric effects, and can get quite accurate
triangulations even on high angle paths.)

#3 is also improved by reducing the ground wave radiation:  the concern is that,
at certain distances, the ground wave signal would have the same received strength
as the NVIS signal, causing fading that reduced the reliability of the path.  (Remember
that, in some cases, the stations communicating might be less than half a mile apart.)
However, hams typically have VHF as a backup for the very short distances over which
this effect may appear.

#2 may be an issue in some CC&R communities, or when storms have taken down
all available supports.  In those cases, you can still operate at low heights if needed,
but otherwise raising the antenna usually improves performance.  How much?  In the
case of the 3' high antenna I cited previously, raising the antenna by 10' gives a 6dB
improvement, equivalent to running 4 times the power.  That's worthwhile, especially
if it allows for longer operation on limited battery power, but not so much that a very
low antenna can't still make contacts.

#4 is why some of the military antenna solutions are less effective for ham use.  In
order to avoid pattern break-up at 12 MHz, the elements are made shorter than optimum
on the lower bands.  Since hams would rarely be using NVIS above 40m, we would
typically make our antennas longer, thereby increasing efficiency (especially on 160m.)
The wide frequency range also requires the use of autotuners, while on the ham bands
we can just cut multiple dipoles and use those instead.

#5 has been the subject of a lot of anecdote and argument.  My EZNEC models don't show
a significant difference in the ratio of signals arriving at low angles vs. those overhead over
the typical range of heights, and I've never observed such an effect in practice.  Others will
recite specific instances where it happened to them.  On the other hand, the presence of
such interfering signals from longer distances often indicates that operation should be moved
to the next lower band, where the D-layer absorption will provide additional attenuation for
low angle signals.  In other cases, proper use of the RF gain control may help.  But if you
are struggling to pull relatively local signals through 80m interference from half way across
the country, that probably means you should be on 160m instead.

#6 leads to simplified antenna selection criteria - many operators don't have the knowledge
(or, often, the conditions) to choose an optimum antenna height.  They just want to throw
up some sort of antenna and have it work, often relying on a base station with a stronger
signal to keep the frequency clear and/or choose the frequency. 


You may also find some references to studies that show a dipole in an upright V (with the
ends higher than the center) can give better performance than one in an inverted V (with
the center higher than the ends.)  This might be true under certain conditions, but isn't
universal.  What does seem to be the case is that, at least at relatively low heights, raising
either the ends or the center will improve efficiency, and the inverted V configuration
with the ends tied off by ropes usually provides the best performance for a given total
mast length (because it can raise the ends only half as much as the center after dividing
it for each end.)

That's not to say that hams can't follow the military recommendations if they want to, but
they don't always give optimum performance in ham usage.


Title: RE: How Many are Using NVIS Antennas ?
Post by: KF7VXA on March 12, 2014, 07:02:40 PM
I sure understand what is being said, my question, and I know it depends on many factors is...If one wants a HF antenna to cover as much of the close in local area without picking up stations for many hundreds of miles, what would be the best antenna and height to use.
We have a county that due to mountains, gets no VHF/UHF so NVIS is needed as the last ditch way to make coms with them.
What looks like the best compromise for this? Or is there one?

73's John


Title: RE: How Many are Using NVIS Antennas ?
Post by: WB6BYU on March 13, 2014, 10:08:14 PM
There are a number of effective NVIS designs, and they tend to be fairly
simple.

A dipole is a good start.  Assuming you want to cover 160m through 40m,
then don't install it any higher than about 40'.    It will work at lower
heights, but the efficiency will drop somewhat (especially on 160m.)  But
signals are often good enough on 160m that heights of 10' to 20' are
quite serviceable.   (If you can get it up at, say, 60', that will be fine for
80m and 160m, and you can add a reflector wire under the antenna for
40m.)

Here's an example:  http://www.hamuniverse.com/ae5jufielddayantenna.html
The original was for 40m, 80m and 160m, and you don't have to use the same
construction technique, but the method of using one element on 2 non-adjacent
bands (such as 160m + 40m, or 80m + 20m) makes adjustment easier than
trying to do the same thing on two bands such as 80m + 40m.  I built the
original for our local ARES group, and it works well, with no tuner needed.

The other ARES HF portable dipole kit uses colored Anderson connectors for
manual band switching:  basically an 80m dipole that can be opened in the
center of each leg to work on 40m, or a loading coil can be added at the same
point for 160m.  It can be adapted for other frequencies as well.


If you want to use a doublet with open wire feedline, you need to keep it under
1.25 wavelengths (EDZ) on 40m or the pattern breaks up, while longer wires
are more efficient on 160m.  Somewhere around the 150' range is probably
about the best trade-off.

One problem with many types of antennas, including horizontal loops and
OCFD types, is that when the pattern is designed for one band (say 80m) there
is a null overhead on twice the frequency (40m in this case.)  You can get around
this problem with an OCFD by bending it in the middle.

I solved the problem for a loop antenna on two bands (160m and 80m in the
prototype) by folding the antenna into a figure-8.  It worked well as an all-band
antenna, but the pattern was most suitable for NVIS on the two lower bands.

A simple end-fed wire that is mostly horizontal runs into the same problem
with trying to maintain an optimum overhead pattern as the frequency is
changed.  One of the better designs that I've come up with uses 90' of wire
strung around 3 sides of a square.  That would be a good choice with a remote
auto-tuner.


There are more complicated types that try to achieve higher gain, but trying
to achieve performance over a 4 : 1 frequency range is still difficult.  Basically
these involve covering a larger area with some sort of horizontal broadside /
colinear array.  I do have a design fed with open wire line that gives some
vertical gain on 80m and 40m, and works basically as a slightly extended
dipole on 160m.  I can dig out the details of the design, but I've never had
a situation where a dipole wasn't adequate when the band was open.


Title: RE: How Many are Using NVIS Antennas ?
Post by: KF7VXA on March 15, 2014, 02:03:54 PM
Thanks for the great info. In our case, beside having comms with in 500-600 miles, we need to be able to communicate with this one county that cannot be reached with VHF/UHF or ground wave HF due to mountains all around it. It is about 75 miles from us, so we need to insure than we have a decent radio wave that will return down as close to the point or origin as possible.
In this one case, even with a 6db or more loss in signal, all we need is a readable phone signal to that county, digital should be easier still. I think they are still on regular VHF/UHF for all their emergency services.
It is no problem to build a special antenna just for that county and have our other NVIS antenna(s) up higher.
I have so much snow in my yard now, I'm waiting for it to go down some and then we can experiment to see what will and will not work. I think that is going to be the best way to insure good comms the majority of the time. I was just looking for a starting point to save time and materials. I enjoy experimenting, so we will come up with something that will work.
I'm still trying to get the local group in that one county to see if they can get a repeater up (maybe with a MESH system), one that has solar back up so we have a better chance with VHF/UHF and high gain directional antennas if needed. Money seems to be the biggest problem. It's a poor county and getting the one's in charge to realize the importance of a repeater may be tough. Maybe an individual who has the means can do so. It's not my county, so I'm not involved in their affairs, I just want to insure we can communicate with them if it become necessary. Us rural folks tend to look out for others the best we can.
Worst case is to find someone at a distance that can relay between the two counties on HF. We will make it work somehow, someway, we always do.

Thanks and 73's John KF7VXA


Title: RE: How Many are Using NVIS Antennas ?
Post by: WG8Z on March 16, 2014, 07:44:11 AM
John
Did a statewide exercise a couple of years ago. Midsummer On 75 meters. I was running my flat top dipole at 45
feet.... Another station 80 miles away with a flattop dipole at 35'.  Morning and evening we had great coms with just 5 watts S9 or better.  High noon (lots of D-layer absorption)  we had to bump the power to 100 w. to maintain the s-9.
 The guys running the low antennas < 15 feet were very poor performers. I could copy all morning and eve but they were weak. They all had difficulty with each other. Noon time NONE were even in the game.
One guy had a dipole at 90 feet and his signal was on par with those running 30 to 60 feet. His complained of high noise floor from storms 600 miles away.
Best performing station (best signal reports and ability to copy others) was running a flat-top at 55 ft.
The flat tops or drooping dipoles consistently out performed the inverted V's.
Tested all distances from 0-200 miles.

In addition we checked regional coms, 0-600 miles.
As you can guess the higher antennas did best.
None of the < 20ft were even in the game.

No modeling, no theory, just real world results.
YMMV
73
Greg


Title: RE: How Many are Using NVIS Antennas ?
Post by: KF7VXA on March 19, 2014, 03:32:59 PM
John
Did a statewide exercise a couple of years ago. Midsummer On 75 meters. I was running my flat top dipole at 45
feet.... Another station 80 miles away with a flattop dipole at 35'.  Morning and evening we had great coms with just 5 watts S9 or better.  High noon (lots of D-layer absorption)  we had to bump the power to 100 w. to maintain the s-9.
 The guys running the low antennas < 15 feet were very poor performers. I could copy all morning and eve but they were weak. They all had difficulty with each other. Noon time NONE were even in the game.
One guy had a dipole at 90 feet and his signal was on par with those running 30 to 60 feet. His complained of high noise floor from storms 600 miles away.
Best performing station (best signal reports and ability to copy others) was running a flat-top at 55 ft.
The flat tops or drooping dipoles consistently out performed the inverted V's.
Tested all distances from 0-200 miles.

In addition we checked regional coms, 0-600 miles.
As you can guess the higher antennas did best.
None of the < 20ft were even in the game.

No modeling, no theory, just real world results.
YMMV
73
Greg

Great reply Greg,

You can't always have your cake and eat it too.
My real concern was to be able to get reliable comms a short distance away (within 75 miles) using HF. I am just looking for the best set up as not to have the signal touchdown further than we need it to, so for that, getting your signal as close to a 90 degree angle is of course going to be what would be ideal. In the real world, there are many variables. I was just looking for some ideas for this one situation. I also wanted to avoid too much distance as the one high antenna that you pointed out that was receiving all the lightning crashes. I like the idea of putting a reflector wire under the horizontal dipole that was also mentioned. That should give a little bit of directionability upward.
I will try the height you suggested for the 75 meters, I think it may be the best compromise.
The responses one gets are many times different and one has to go with what they know and compare it to what replies are received. The real world one's are almost always the best, not a guess or just theory. I read another post about a guy who would bury his NVIS antenna a few inches under the ground, kinda just passed over that one.
Were it flat land or rolling hills, it would be far easier that having the station you are trying to reach with mountains all around it. The joys of rural living. Sometimes it's hard to visualize what someone is dealing with. Thanks for the reply's

73's John KF7VXA

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