Author Topic: Introduction to NVIS  (Read 24483 times)

Offline Alan Georges

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Re: Introduction to NVIS
« Reply #30 on: November 29, 2016, 07:03:13 PM »
New issue of the Ham Radio 360 (aka, the podcast formerly known as 'Fo Time') up, in which Ed Fong discusses NVIS with SCW.  Just downloaded it, now to go listen.  Link:
http://hamradio360.com/index.php/2016/11/29/ham-radio-360-nvis-near-vertical-incidence-skywave-with-ed-fong-wb6iqn/

Offline Alan Georges

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Re: Introduction to NVIS
« Reply #31 on: March 23, 2017, 06:11:38 PM »
A new paper on the topic: https://link.springer.com/article/10.1007/s11235-017-0287-2
Abstract:
Quote
Near Vertical Incidence Skywave (NVIS) propagation can be used for radio communication in a large area (200 km radius) without any intermediate man-made infrastructure. It is therefore especially suited for disaster relief communication, communication in developing regions and applications where independence of local infrastructure is desired, such as military applications. NVIS communication uses frequencies between approximately 3 and 10 MHz. A comprehensive overview of NVIS research is given, covering propagation, antennas, diversity, modulation and coding. Both the bigger picture and the important details are given, as well as the relation between them.

Haven't had more than time to skim it, but it seems to have some interesting new stuff about antennas and digital signals.  Way more than the usual "hang a dipole 10' up and enjoy" ham presentations.

Offline Smurf Hunter

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Re: Introduction to NVIS
« Reply #32 on: March 24, 2017, 02:09:26 PM »
Uploaded to my kindle for poolside reading this week.


Offline Carl

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Re: Introduction to NVIS
« Reply #33 on: March 24, 2017, 05:02:37 PM »
A new paper on the topic: https://link.springer.com/article/10.1007/s11235-017-0287-2
Abstract:
Haven't had more than time to skim it, but it seems to have some interesting new stuff about antennas and digital signals.  Way more than the usual "hang a dipole 10' up and enjoy" ham presentations.

Yep,I plan to read some too. Good article,

Offline Carl

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Re: Introduction to NVIS
« Reply #34 on: March 24, 2017, 05:50:52 PM »
Yep,I plan to read some too. Good article,

OUCH, my head hurts...too many numbers...Great article though.

Offline idelphic

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Re: Introduction to NVIS
« Reply #35 on: March 28, 2017, 10:07:23 AM »
I really need to build a NVIS antenna - just need to decide on which bands I want it for... Guess maybe 40/80m

Offline Carl

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Re: Introduction to NVIS
« Reply #36 on: March 28, 2017, 03:35:16 PM »
I really need to build a NVIS antenna - just need to decide on which bands I want it for... Guess maybe 40/80m

Yes,40,60,80 are the only bands for reliable NVIS work. You might find the AS2259 an interesting antenna as many do who use it.

http://www.marc.on.ca/marc/ares/documents/NVIS_2040-80_20antenna.pdf

Offline idelphic

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Re: Introduction to NVIS
« Reply #37 on: March 29, 2017, 09:49:15 AM »
Yes,40,60,80 are the only bands for reliable NVIS work. You might find the AS2259 an interesting antenna as many do who use it.

http://www.marc.on.ca/marc/ares/documents/NVIS_2040-80_20antenna.pdf

Thanks Carl - Printed for the project board... which is quite long and back logged.... ugh... I need any assistant.  While I work from home,.. work is keeping me busy here of late.

Offline Skispcs

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Re: Introduction to NVIS
« Reply #38 on: March 29, 2017, 11:43:34 AM »
I made a NVIS antenna using this guide from DX Engineering.
I customized it a bit by using PVC tubing cut into pieces that would fit into a pack.
I used a 6 inch long bolt on the bottom as a spike and some tent spikes for securing the lines.

Hiked to the back end of my property, setup the antenna and I was able to send email via HF using the winlink system.

Offline Smurf Hunter

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Re: Introduction to NVIS
« Reply #39 on: March 29, 2017, 01:32:58 PM »
40 meters is the best choice if you only want to try one band.  Unless you have tons of space and can run an 80m length perpendicular like the AS2259.

I will caution that the PVC works in a pinch, but will sag and requires additional support in most cases.


Offline armymars

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Re: Introduction to NVIS
« Reply #40 on: March 29, 2017, 05:46:22 PM »
Smurf Hunter,
  Are you talking about the mast when you say PVC will work?

Offline Alan Georges

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Re: Introduction to NVIS
« Reply #41 on: March 29, 2017, 05:54:29 PM »
40 meters is the best choice if you only want to try one band.  Unless you have tons of space and can run an 80m length perpendicular like the AS2259.
This 80/40 loaded dipole is only about 10' longer than a 40m, it's worth a look: http://thesurvivalpodcast.com/forum/index.php?topic=44578.0  Still giving good service after 4 years in the weather.

Offline Smurf Hunter

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Re: Introduction to NVIS
« Reply #42 on: March 29, 2017, 08:36:01 PM »
Smurf Hunter,
  Are you talking about the mast when you say PVC will work?

Unless you can get PVC to radiate RF waves :)

Yes.  for proof of concept I used PVC, but 15 feet, especially with coupler joints will start to sag.

I hope to find a milsurp aluminum mast/tripod base in the near future.

Offline Carl

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Re: Introduction to NVIS
« Reply #43 on: March 29, 2017, 08:51:18 PM »
Unless you can get PVC to radiate RF waves :)

Yes.  for proof of concept I used PVC, but 15 feet, especially with coupler joints will start to sag.

I hope to find a milsurp aluminum mast/tripod base in the near future.

Actually PVC does conduct RF,as it radiates RF in the form of my BIG STICK VHF/UHF antenna...microwave a bit and see it get hot.
My as2259 is supported with a 18 foot aluminum pool cleaning pole that doubles as a 20 meter vertical in my antenna kit for mobile command post radios.

Offline armymars

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Re: Introduction to NVIS
« Reply #44 on: March 30, 2017, 10:09:01 AM »
I've used 4" drain pipe which works well up to 30 feet, but there is a new type of drain pipe sold at Lowes that is not solid walled and won't work past 20 feet.

Offline Carl

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Re: Introduction to NVIS
« Reply #45 on: March 30, 2017, 12:06:16 PM »
I've used 4" drain pipe which works well up to 30 feet, but there is a new type of drain pipe sold at Lowes that is not solid walled and won't work past 20 feet.

If you have a good farm supply outlet nearby you may find irrigation pipe a good support also though I use electrical conduit in 10 foot lengths and slip smaller and smaller sizes into each other as low cost antenna supports and also the hardware is lower cost than what is sold for antennas.

Offline idelphic

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Re: Introduction to NVIS
« Reply #46 on: April 15, 2017, 11:07:40 AM »
Been looking at the PVC NVIS build and wondering how I could 'hack' it to include a VHF/UHF antenna at the top.    I've also been looking at the idea of the section collars being about a foot long, so you have a six inch socket to slip the sections into.  Right now the hang up is how to replace the SO-239 for the 20/40 dipole so that it will allow for a top socket for the VHF/UHF antenna.

And as that would add to the height,.. sway factor, would think that adding 20' guys would help

Offline Carl

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Re: Introduction to NVIS
« Reply #47 on: April 15, 2017, 11:44:24 AM »
Been looking at the PVC NVIS build and wondering how I could 'hack' it to include a VHF/UHF antenna at the top.    I've also been looking at the idea of the section collars being about a foot long, so you have a six inch socket to slip the sections into.  Right now the hang up is how to replace the SO-239 for the 20/40 dipole so that it will allow for a top socket for the VHF/UHF antenna.

And as that would add to the height,.. sway factor, would think that adding 20' guys would help

Check out the RAPID DEPLOYMENT VHF/UHF /NVIS 2259 and others for ideas...anyone with this link can view and download from my Google Drive.  :

https://drive.google.com/drive/folders/0B5ZIZLZV4AwIY3NoMkthTEVQRE0?usp=sharing

Offline idelphic

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Re: Introduction to NVIS
« Reply #48 on: April 16, 2017, 09:19:36 AM »
Check out the RAPID DEPLOYMENT VHF/UHF /NVIS 2259 and others for ideas...anyone with this link can view and download from my Google Drive.  :

https://drive.google.com/drive/folders/0B5ZIZLZV4AwIY3NoMkthTEVQRE0?usp=sharing

wow - that is pretty spiffy...  I'll have to look at that a bit more and see what I can reverse engineer from it. 

Thanks Carl!

Offline Carl

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Re: Introduction to NVIS
« Reply #49 on: April 16, 2017, 01:51:07 PM »
wow - that is pretty spiffy...  I'll have to look at that a bit more and see what I can reverse engineer from it. 

Thanks Carl!

My drive is open for anyone with that link to view or download files, enjoy.
Here is the link again...

 https://drive.google.com/drive/folders/0B5ZIZLZV4AwIY3NoMkthTEVQRE0

Offline idelphic

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Re: Introduction to NVIS
« Reply #50 on: May 16, 2017, 08:16:33 AM »
Carl -

How well would the NVIS work when placed in an attic, about 25-30 feel (ish) off ground?

Offline Carl

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Re: Introduction to NVIS
« Reply #51 on: May 16, 2017, 11:13:16 AM »
Carl -

How well would the NVIS work when placed in an attic, about 25-30 feel (ish) off ground?

Cut the dipoles a bit longer than math or charts indicate as surrounding materials will detune the antennas and cut to size in place for 40-60-80 meters (these are the bands that work best for NVIS)Be aware that you can have hundreds to thousands of volts at antenna ends so use insulators to prevent carbon bridges from causing fire in your home. Insulators are best though a good taping or plastic cola bottle can be all the protection you need.
  Also,be sure you don't have a metal foil vapor barrier as was used in some construction in the past...you will see it like aluminum foil as part of you insulation package of the home.

  I use 10 feet above ground to prevent passers-by from walking or driving into the antenna,but an attic works well as this is what SMURF HUNTER did successfully ,as I looked on,for his home station.
« Last Edit: May 16, 2017, 11:23:01 AM by Carl »

Offline Smurf Hunter

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Re: Introduction to NVIS
« Reply #52 on: May 16, 2017, 11:55:46 AM »
Yes.  I have 4 band "fan" dipole in the attic.  The only wire that is straight is the 20m half wave.  I also have 40 and 80.  All tune, but the lower bands have a high swr.  It is a complex process to keep such a rats nest to match 50 ohms.  Adding a new wire tends to tweak everything else.  However a decent auo tuner makes it work.

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Re: Introduction to NVIS
« Reply #53 on: May 16, 2017, 12:03:39 PM »
Looking at my elevation again - I don't guess it is really 20-30 feet.  While we are on a 'crest' the attic may only be about 15-20feet.  The house sits about 8-14" above grade and the ceiling height is 8' 6"..


Attic has blown in insulation, so only electrical wires for lights and such are up there.

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Re: Introduction to NVIS
« Reply #54 on: May 16, 2017, 12:05:08 PM »
Yes.  I have 4 band "fan" dipole in the attic.  The only wire that is straight is the 20m half wave.  I also have 40 and 80.  All tune, but the lower bands have a high swr.  It is a complex process to keep such a rats nest to match 50 ohms.  Adding a new wire tends to tweak everything else.  However a decent auo tuner makes it work.

I don't think I would make anything other than a 20 / 40,... I don't spend much time on 40, and less on 80.  Most all of my contacts have been on 20m

Offline Carl

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Re: Introduction to NVIS
« Reply #55 on: May 16, 2017, 12:37:52 PM »
I don't think I would make anything other than a 20 / 40,... I don't spend much time on 40, and less on 80.  Most all of my contacts have been on 20m
(e 17 meters ) sorry,stray test as my voice to text has a fit with the echo in my BOL.

This is OK,though NVIS does not work at 20 meter frequencies. Be sure to tape or insulate as high voltage can develop carbon trails on wood and cause fire..Use end insulators or at least tape well the ends of your antenna wires ,once trimmed to length and remember that a 40 meter antenna can do duty on 15 or maybe 17 meters also and a 4 to 1 current balun can aide you SWR across a wider set of frequencies,with or without a tuner.
« Last Edit: May 16, 2017, 12:45:19 PM by Carl »

Offline LodeRunner

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Re: Introduction to NVIS
« Reply #56 on: November 05, 2017, 02:37:45 PM »
From the days of CW,but just like "Q" codes,it is often given to indicate quality of voice...QOK? (made up 'q' code)

There was an updated standard published a few years ago (sometime between say, 2006 and 2012) for using the "Tone" digit of the RST for digital comms.  It encapsulated two characteristics - stability and quality. 

Stability is important because many digital modes are very sensitive to 'drift' - even a few Hertz of drift during a QSO can cause modes such as WOLF, THOR, THROB, JT4, JT9 and some others to lose decode.  Even BPSK/QPSK and Olivia can lose decode if the transmitter drifts several tens of hertz in a QSO (this may be mitigated/reduced by the software "AFC" control [if available], within reasonable limits). 

Also, the Quality of the modulation envelope is *very* important for any modulation type which has an AM component - this includes all forms of BPSK/QPSK and 'soundcard RTTY/AMTOR', because xPSK uses a 'Raised Cosine Function' form of AM to eliminate spurs during the phase transitions of the modulation envelope.  Most soundcard-created FSK modes i.e. "AFSK", RTTY and AMTOR now implement the same Raised Cosine Function, for the same reason.  So, if you over-modulate these modes you get distortion within the passband, which reduces the decode-ability of the signal, plus "splatter" that interferes with stations outside your transmit passband... sometimes even out of the Amateur Band you're transmitting in, both of which are a big "no no" per FCC regs.

Other modes don't have any AM component of their modulation, so "linearity" is much less of an issue.  However, over-driving the PA can still result in spurs and splatter, so it's important not to overdrive your radio when running any of the digital modes. 

The best rule of thumb is to know what the "AM Transmit power rating" of your rig is, and never drive your digital-mode signal above that power level, with the DRIVE control set for the full PEP power of the radio, i.e. a "100 watt PEP" radio is set for 100 watts PEP on voice.  The manual says the rig does "25 watts" on AM, so use the audio settings of your soundcard (or rig-interface) to reduce the audio drive to the point where you get 25 watts from the output.  Leave the Audio drive setting of your soundcard/rig-interface set as they are from that point out, and you can then adjust the Power Output with the DRIVE control on the radio as normal.

VERY IMPORTANT:  NEVER use 'Compression' when operating digital modes.  Voice Compression intentionally over-drives and then "clips" off the peaks of the audio signal, to raise the average power output.  But, any time you're "compressing" audio this way, you're introducing distortion.  And the distortion increases as a Square Law function - i.e. of you add 3dB of compression, you get (3^2) = 9dB more distortion.  Typical compression settings for HF voice rigs run between 4 dB and 10dB, so this increases the distortion between 16dB and 100dB, which is enough to make your signal totally uncopyable on most digital modes, as well as creating a ton of interference to other users of the band. 
Most radios that have a "Digital" mode setting will automatically disable Compression when put into that mode (read your manual to be sure)...but if you have a rig which doesn't have a dedicated "Digital" mode then YOU have to remember to shut off the Compression yourself.

I'll take a search around later and see if I can find a copy of the RST for Digital Comms table later.  If I do find it, I'll post it.

Cheers

Offline LodeRunner

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Re: Introduction to NVIS
« Reply #57 on: November 05, 2017, 02:55:37 PM »
I really need to build a NVIS antenna - just need to decide on which bands I want it for... Guess maybe 40/80m

NVIS works between 1 and 10 Mhz, so 160/80/60/40 are the bands to choose from.  30M can --sometimes-- support NVIS, but as we head into the bottom of the sunspot cycle it will not be doing so with any reliability for the next five years or so.

Today, 80 and 40 are the most common bands for NVIS - 40 by day, and 80 by night... but probably by next winter, and definitely 2 years from now NVIS on 160M will be a staple of night time communications.  Again, we're entering the lowest portion of the solar cycle.  Plan accordingly.