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New NVIS Ideas - VA3KOT John - 2020-11-13

If you haven't looked at NVIS operation recently, forget nearly everything you thought you knew about it. As Bob Dylan used to sing: "the times they are a-changing". Actually, what has changed is the solar cycle.

Near Vertical Incidence Skywave (NVIS) is the technique of using a horizonal antenna that is intentionally low to the ground in order to send a signal straight up into space. The signal - if the correct frequency is chosen - will be reflected back down to the ground by the F2 layer of the Earth's ionosphere. NVIS is used where it is necessary to communicate with another station within a radius of about 400km from the transmitting station. A NVIS antenna will usually be quite useless for longer distance communication (an exception to that will be discussed later).

The US military developed a rapid deployment field antenna called the AS2259 for NVIS operation. It comprises a vertical centre support pole with four wire elements stretching down towards the ground at 90 degrees with respect to each other. The four wires form two inverted-V dipoles - one for a daytime band and another for a night-time band. A popular ham version of the AS2259 employs 40 metres as the daytime band and 80 metres as the night-time band. It used to function very well, but not anymore; here is why:

There is a critical frequency above which signals will pass right through the ionosphere and be lost in space. For NVIS operation the critical frequency is referred to as foF2. The critical frequency is constantly changing for every location but it also varies based on the sending station's latitude and the time-of-day. The critical frequency is measured by a device called an ionosonde. To determine the foF2 for your location you must refer to the plot of foF2 from an ionosonde near your station. For GBARC, the nearest ionosonde is located at Alpena, Michigan on the US side of Lake Huron. Here is a sample plot from 13th November 2020.
   
As you can see from the Alpena plot, foF2 is never high enough to use the 40 metre band at our latitude. In fact, the 80 metre band - previously used as a night-time band - is the only usable band during the day. At night it would be necessary to use the "gentlemans' band", 160m, to achieve NVIS propagation. This will change as the solar cycle heads back towards its peak, but for now the old AS2259 ham-special antennas will not do a very good job.

Anybody using an AS2259 style ham antenna may protest that they are still making contacts on 40m with it. That is possibly quite true but the contacts are using regular skywave propagation, not NVIS. The propagation pattern for a NVIS antenna has the strongest signals going straight up. But the pattern also sends weaker signals off at lower angles. The -3dB points in the pattern are typically at about 60 degrees from horizontal. The -6dB points are even lower and will transmit a signal to maybe 1500km away that is only 1 S-unit (-6dB) weaker.

So if it "kinda works" why do we care about the critical frequency anyway? Maybe we don't ... unless we want to get a signal into an adjacent valley on the other side of a mountain. I was camping at Craigleith Provincial Park on the shore of Georgian Bay a couple of years ago. I wanted to check in to a net in Shelburne, Ontario on 80m. In between my station and the net control station was the Blue Mountains and the Dufferin Highlands. I strung an 80m monopole only 8 feet high, worked against ground through a Ground Tuning Unit (GTU) and keyed up with only 5 watts. My signal was copied perfectly by the other stations due to NVIS propagation.

For NVIS to work efficiently BOTH transmitting and receiving stations MUST use a NVIS antenna. In the example just given, my simple NVIS antenna's signal was received on a Beverage antenna (a very long, very low wire used for receive only). If the receiving antenna is for example a vertical, the received signal may be very weak or non-existent. A vertical antenna's "ears" are pointed down, close to the horizon with a huge null in the direction from which a NVIS signal is coming. The same is true even for a dipole mounted high above ground.

How is this relevant to GBARC? Let us imagine a situation in which we have no repeater coverage but we need to communicate anywhere within Grey County. NVIS is the answer. A NVIS net control station in, say, Owen Sound could reliably communicate all the way from Manitoulin down to Toronto, irrespective of the terrain in the region.

NVIS antennas are particularly easy to erect. A simple dipole 8 to 12 feet above ground will suffice. Actually, an antenna laid directly on the ground (a "Grasswire") can also sometimes work. It doesn't matter whether the dipole is fed in the middle, the end or off-centre, it will still radiate towards the sky. A QRP signal is sometimes desirable because the ground wave signal may otherwise interfere with the F2 reflected signal if the transmitted signal is too strong (turn off those amps!). There is some degree of D-layer absorption so it may be necessary to adjust transmitted power accordingly for best receive signal.

If anybody would like to join me in experimenting with NVIS please leave a comment to this post.


RE: New NVIS Ideas - ve3pav - 2020-11-14

that sounds interesting,i'd like to try it out.


RE: New NVIS Ideas - VA3KOT John - 2020-11-14

(2020-11-14, 11:05:16)ve3pav Wrote: that sounds interesting,i'd like to try it out.
Thanks Bobby. It would make a good experiment to try a low-power 80m QSO between Lions Head and Owen Sound. Do you need any help setting up a NVIS antenna?


RE: New NVIS Ideas - ve3pav - 2020-11-14

i found a design for 80/40m nvis,its based on the as-2259 antenna.
i have most of the parts i'll be using fibreglass tent poles that i got at princess,i still need to get some pvc pipe to make the baluns.


RE: New NVIS Ideas - VA3KOT John - 2020-11-14

Bobby, you should maybe just build the 80m part of the antenna for now. The 40m part isn't going to work for NVIS until we reach the peak of the solar cycle starting a couple of years from now.

You can use an "ugly choke balun" just by neatly coiling 10 turns of coax 6 inches in diameter near the feed point of the antenna.


RE: New NVIS Ideas - ve3pav - 2020-11-14

ok thats good to know,what size of pipe for the balun is used


RE: New NVIS Ideas - VA3KOT John - 2020-11-14

No pipe needed at all. Just wind 18-20 feet of coax into a neat coil 6 inches in diameter at the antenna end. It will act as a choke for common mode current that would otherwise flow back to the radio and cause SWR problems.


RE: New NVIS Ideas - VE3WI Dave - 2020-11-15

I've seen designs for a couple of cloud-burner NVIS antennas, basically 2 el. vertical Yagis (dipoles with reflectors directly underneath). 
  • One was a full size 40m dipole at 35' el. with a 5% longer reflector at 7'.  Author claimed good results for nearby contacts during Field Day. 
  • The other was a short centre-loaded 80m dipole with four 10' sections and two 70 uH loading coils.  It was located at 12' el. with a 5% longer reflector at 2'.  Author also claimed good performance.
A balun works better if you wind a single layer on some kind of form (non metallic), rather than just a random coil.  Tom VA3TS used a plastic coffee jar (Folgers I think) as a coil form.


Off-topic:

A crossed dipole inverted-v is a simple, easy to erect antenna for portable ops.  The antennas are also the guys.  They don't interact when they are at 90°, and they can be fed by a single feedline.  80m/40m is pretty big, but an antenna for 40m/20m or higher bands is very manageable. 

The 4' stackable mil surplus fibreglass poles are great.  I bought a bunch from Bob Morton (Maple Leaf Comm) but I don't think he has any more.  Princess Auto used to have some but maybe Bobby bought them all :-)  If we can ever get back to Dayton, there's always lots of both the fibreglass & aluminum versions in the fleamarket. 

(Hint: much less risk of failure at the joints if you glue hardwood dowels into the male ends and tighten hose clamps around the female ends after assembly.  I had 8 of them assembled like this holding up an inverted-v for two years.)

73
Dave, VE3WI


RE: New NVIS Ideas - VA3KOT John - 2020-11-15

Thanks for the idea about strengthening the joints of those fiberglass poles Dave. I have 5 of them holding up one end of my 80m EFHW. It is quite stable but I want to add at least one more. I'll be guying them of course. I'll check out the hardware stores for suitable dowels.


RE: New NVIS Ideas - VE3BYD_Patrick - 2021-06-17

(2020-11-13, 16:39:53)Hi John.Are you still testing out NVIS antennas? I found this thread while searching for 40m NVIS antennas, which I just acquired. I am in Newmarket and would like to see if I can make any contacts. Any advice would be appreciated.PatrickVE3BYDVA3KOT John Wrote: If you haven't looked at NVIS operation recently, forget nearly everything you thought you knew about it. As Bob Dylan used to sing: "the times they are a-changing". Actually, what has changed is the solar cycle.

Near Vertical Incidence Skywave (NVIS) is the technique of using a horizonal antenna that is intentionally low to the ground in order to send a signal straight up into space. The signal - if the correct frequency is chosen - will be reflected back down to the ground by the F2 layer of the Earth's ionosphere. NVIS is used where it is necessary to communicate with another station within a radius of about 400km from the transmitting station. A NVIS antenna will usually be quite useless for longer distance communication (an exception to that will be discussed later).

The US military developed a rapid deployment field antenna called the AS2259 for NVIS operation. It comprises a vertical centre support pole with four wire elements stretching down towards the ground at 90 degrees with respect to each other. The four wires form two inverted-V dipoles - one for a daytime band and another for a night-time band. A popular ham version of the AS2259 employs 40 metres as the daytime band and 80 metres as the night-time band. It used to function very well, but not anymore; here is why:

There is a critical frequency above which signals will pass right through the ionosphere and be lost in space. For NVIS operation the critical frequency is referred to as foF2. The critical frequency is constantly changing for every location but it also varies based on the sending station's latitude and the time-of-day. The critical frequency is measured by a device called an ionosonde. To determine the foF2 for your location you must refer to the plot of foF2 from an ionosonde near your station. For GBARC, the nearest ionosonde is located at Alpena, Michigan on the US side of Lake Huron. Here is a sample plot from 13th November 2020.

As you can see from the Alpena plot, foF2 is never high enough to use the 40 metre band at our latitude. In fact, the 80 metre band - previously used as a night-time band - is the only usable band during the day. At night it would be necessary to use the "gentlemans' band", 160m, to achieve NVIS propagation. This will change as the solar cycle heads back towards its peak, but for now the old AS2259 ham-special antennas will not do a very good job.

Anybody using an AS2259 style ham antenna may protest that they are still making contacts on 40m with it. That is possibly quite true but the contacts are using regular skywave propagation, not NVIS. The propagation pattern for a NVIS antenna has the strongest signals going straight up. But the pattern also sends weaker signals off at lower angles. The -3dB points in the pattern are typically at about 60 degrees from horizontal. The -6dB points are even lower and will transmit a signal to maybe 1500km away that is only 1 S-unit (-6dB) weaker.

So if it "kinda works" why do we care about the critical frequency anyway? Maybe we don't ... unless we want to get a signal into an adjacent valley on the other side of a mountain. I was camping at Craigleith Provincial Park on the shore of Georgian Bay a couple of years ago. I wanted to check in to a net in Shelburne, Ontario on 80m. In between my station and the net control station was the Blue Mountains and the Dufferin Highlands. I strung an 80m monopole only 8 feet high, worked against ground through a Ground Tuning Unit (GTU) and keyed up with only 5 watts. My signal was copied perfectly by the other stations due to NVIS propagation.

For NVIS to work efficiently BOTH transmitting and receiving stations MUST use a NVIS antenna. In the example just given, my simple NVIS antenna's signal was received on a Beverage antenna (a very long, very low wire used for receive only). If the receiving antenna is for example a vertical, the received signal may be very weak or non-existent. A vertical antenna's "ears" are pointed down, close to the horizon with a huge null in the direction from which a NVIS signal is coming. The same is true even for a dipole mounted high above ground.

How is this relevant to GBARC? Let us imagine a situation in which we have no repeater coverage but we need to communicate anywhere within Grey County. NVIS is the answer. A NVIS net control station in, say, Owen Sound could reliably communicate all the way from Manitoulin down to Toronto, irrespective of the terrain in the region.

NVIS antennas are particularly easy to erect. A simple dipole 8 to 12 feet above ground will suffice. Actually, an antenna laid directly on the ground (a "Grasswire") can also sometimes work. It doesn't matter whether the dipole is fed in the middle, the end or off-centre, it will still radiate towards the sky. A QRP signal is sometimes desirable because the ground wave signal may otherwise interfere with the F2 reflected signal if the transmitted signal is too strong (turn off those amps!). There is some degree of D-layer absorption so it may be necessary to adjust transmitted power accordingly for best receive signal.

If anybody would like to join me in experimenting with NVIS please leave a comment to this post.



RE: New NVIS Ideas - VA3KOT John - 2021-06-17

Hi Patrick, congratulations on getting yourself a NVIS antenna. After this weekend I will be out of town for the rest of June but if you are still interested in July send me an email (va3kot@gmail.com). In the meantime, try using WSPR (Weak Signal Propagation Reporter) to measure how far your signal goes. I recommend using QRP signal levels to make sure that only the high angle signals are reported. The lower angle signals from a NVIS antenna propagation pattern will be attenuated enough that they probably won't be picked up by any WSPR station. For example, a 5 watt signal will be attenuated down to 1.25 watts at the -6dB points on the propagation pattern and will not travel very far due to D-layer absorption. If you used 100 watts, your -6dB points will still be at 25 watts and could travel substantial distances masking the NVIS effect. The beauty of NVIS is it's ability to get strong signals into your local region irrespective of the terrain.

John, VA3KOT
Owen Sound.


RE: New NVIS Ideas - VE3BYD_Patrick - 2021-06-17

(2021-06-17, 18:07:45)VA3KOT John Wrote: Hi Patrick, congratulations on getting yourself a NVIS antenna. After this weekend I will be out of town for the rest of June but if you are still interested in July send me an email (va3kot@gmail.com). In the meantime, try using WSPR (Weak Signal Propagation Reporter) to measure how far your signal goes. I recommend using QRP signal levels to make sure that only the high angle signals are reported. The lower angle signals from a NVIS antenna propagation pattern will be attenuated enough that they probably won't be picked up by any WSPR station. For example, a 5 watt signal will be attenuated down to 1.25 watts at the -6dB points on the propagation pattern and will not travel very far due to D-layer absorption. If you used 100 watts, your -6dB points will still be at 25 watts and could travel substantial distances masking the NVIS effect. The beauty of NVIS is it's ability to get strong signals into your local region irrespective of the terrain.

John, VA3KOT
Owen Sound.

Hi John.

Thanks for the reply. I will follow up in July and se what we can achieve. I'll also check out WSPR.

Take care.
Patrick, VE3BYD