Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: Prev 1 [2]   Go Down
  Print  
Author Topic: How Many are Using NVIS Antennas ?  (Read 14061 times)
W8JI
Member

Posts: 9296


WWW

Ignore
« Reply #15 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).
Logged
N3QE
Member

Posts: 2206




Ignore
« Reply #16 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.
Logged
W8JI
Member

Posts: 9296


WWW

Ignore
« Reply #17 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
Logged
WG8Z
Member

Posts: 192




Ignore
« Reply #18 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.
 
Logged
WB6BYU
Member

Posts: 13251




Ignore
« Reply #19 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.
Logged
KF7VXA
Member

Posts: 458




Ignore
« Reply #20 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
Logged
WB6BYU
Member

Posts: 13251




Ignore
« Reply #21 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.
Logged
KF7VXA
Member

Posts: 458




Ignore
« Reply #22 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
Logged
WG8Z
Member

Posts: 192




Ignore
« Reply #23 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
Logged
KF7VXA
Member

Posts: 458




Ignore
« Reply #24 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

The
Logged
Pages: Prev 1 [2]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!