NVIS Info - Near Vertical Incidence Skywave Antennas

[K1JJ]

http://www.armymars.net/ArmyMARS/Gen-Mil-Info/Resources/Dave-Fiedler/Antenna_Performance_for_NVIS_Comms.pdf

I found this article interesting. It’s about high angle antennas for local work.  Amers like to chat locally a lot, so it’s very appropriate for our use.   Notice the Army recommends that the best NVIS antenna is a simple dipole at 0.2 wavelengths high. (50' on 75M)    The difference over salt water or poor Earth is only 3db.  I guess we could say many AMers already use antennas that are optimized for NVIS.

Many of us have experimented with NVIS antennas. I once had up a pair of 75M rotary stacked driven dipoles that were phased 180 degrees out for NVIS work. One was at 190' and the other at 80'.  I was not impressed with its performance over a simple dipole at 50' for LOCAL work, so I built it into something else.  ("Local" meaning <200 miles away on 75M and will change depending upon time of day.)

I also tried a simple 75M dipole at about 40' high with a 5% longer reflector underneath at 5' high. I had a remote relay to break the  center of the reflector. I did not see much difference when I did on the fly.


So, the NEW NVIS project is to try a dipole at 50'.  Underneath it is an extensive radial ground system I put in years ago for three 160M verticals. There is about 15,000' of wire under that tower. I figure this will best simulate the "salt water" ground the Army said would give the maximum 3db of enhancement over poor ground. I have fair to poor ground under the radials, so maybe it will help for local high angle work.

If possible, NVIS antennas should be used with a second antenna available that is designed for lower angles to work >200 mile distances on 75M.

I wanted to post this info to get us thinking about antennas since it's hi hi FB wx out for antenna work.

T

[K1JJ]

I erected the 75M coax-fed dipole at 48' high over the radial field and ran some tests from 4-5PM local time.

After A/B/C switching from the 48' dipole, 65' dipole and high loop array at 190',  I have some results.

With close-in locals, there is very little difference between the dipole at 48' and 65'. (The dipole at 65' has no radials under it)  I see maybe 1 db at best in favor of the low dipole.  The high loops are down maybe 15db as expected.

When listening to some stations in West Virginia (about 500 miles away) I see a big difference and the vertical take-off angles are doing their thing.  On the low 48' dipole, a particular W8 was S7.  On the dipole at 65' the W8 was S8. On the high loops, the W8 was S9+10.    

This series of tests showed that indeed, the lowest dipole has less lower angle than the dipole at 65', but the dipole at 65' STILL has a fair amount of high angle.

The best optimization would be to raise the 65' dipole to about 95' to fill in the gap between the lowest dipole and the high loops.  I'll bet that will show a big difference locally and make the lowest dipole a clear winner for locals less than  200 miles away.  

I will orient the dipole at 95' so that it better favors Washington state to the NW.  There is a lot of AM activity out that way.  Might even add a reflector behind it cuz there is really nothing to the east except for Africa.
 
Vertical take-off angles are the most important aspect of antenna optimization in the real whirl.

More results when I get them.

T

BTW, back to the article - We all know a flat dipole is better than an inverted vee for various reasons. According to their info, it costs 3db to lower the ends into a standard 45 degree angle inv vee.  Some power is absorbed closer to the ground while most is simply not radiated as much in the horizontal plane for NVIS. So, keep the dipole flat for local work.

[K1JJ]

As the night goes on and the optimum angles are getting higher for locals, I'm starting to see the LOWER dipole at 48' much better than the higher one at 65'.

There have been many instances, especially when listening to the VE's in Canada, that there is 6db in favor of the lower dipole.

As the effective ionosphere moves higher after sunset, the lower (higher angle) antenna becomes a better match for local signal angles. However, for stations > 400 miles, I still see the higher (lower angle) antennas better than the lower antennas.

T

[Ed/KB1HYS]

I've found that the low loop (full Wave 80 meter loop) at an average height of 30 feet gets excellent results with stations out to about 200-300 miles, with very occasionally (odd propagation) going further like Chicago or N. Carolina being about the most "Dx". 

For HF Comms (SSB only), the National Guard Armory's here use a Folded Terminated 80 meter dipole on the roof/towers fairly high up (might be 40-50 ft), then there is a big copper-weld wire about 8-10 feet below that tied to ground through the towers.  I suppose they did this to keep the communications 'local'.  They've since pulled the SSB transceivers and have gone to some all digital mess that no one knew how to use.  Come to think of it, no one but me that I knew of actually ever used the SSB gear either. 

[KA0HCP]

To paraphrase Barbara Mandrel:  "I was NVIS when NVIS wasn't cool." 

Between my throwing arm and tree availability I don't think I've ever had a dipole higher than 25 feet.   So what's the big deal?  LOL.

Thanks for doing all the footwork of experimenting we all wish we had the facilities for!

Bill

[kb3ouk]

at night the range of a low dipole can be extended somewhat compared to what the range during the daytime was. my 75 meter antenna is maybe 20 to 25 feet up, and i'm pushing about 190 watts through it. at night, i can hear and be heard by stations it new england that are averaging s9+10 to s9+20 that during the day (if i could hear them at all) would be doing about an s7 or 8. if propogation is REALLY good i have heard stuff from the midwest, i think the furthest was colorado, not very stongly, but fairly readable, i can remember for sure but i think i was heard by someone just barely out in colorado too once (just enough to know that i was also in the qso, but it surprised me that i was heard at alll from that distance)

[Steve - K4HX]

The low loop is a good high angle performer. In the plots below, the loop pattern is shown in red. The dotted line is a dipole. Both are at a height of 50 feet.

[steve_qix]

Hi Tom,

Since I only have one antenna here, I put up a dipole at 90 feet.  It works fairly well at low angles, but I can still talk with the West Coast.

Seems like a very good compromise!

Regards,

Steve

[Steve - K4HX]

Then there's the Jamaica antenna system. It beats a low dipole by a significant margin at the high angles.

[K5UJ]

the problem with the NVIS stuff for hams is they take what the military does as gospel and put up dipoles on 80 m. that are only 20 or 25 feet high, ignoring that the military can run whatever power they need and that they may use a single antenna on a range of frequencies up to 10 MHz.  I've tried explaining to them that cloud burners are FB but they can be 45 or 50 feet high and still have the cloud burner pattern but without all the low dipole ground loss.   Tom you have it right putting it at 40 to 50 feet.  I think below ~50' you start getting into loss due to coupling to ground.  I used to do that low low stuff 25 feet high with a loop.  It tanked badly compared to a dipole 20 feet higher (which you and HX recommended   )

Rob

[K1JJ]

Steve/HX:

On page 10 is a drawing of the Jamacia antenna.  What a robust pattern with gain in the higher angles. Interesting it uses a pair of driven full-wave dipoles spaced 1/2 wavelength apart and fed with openwire in the center. The feedpoint at the center should probably be openwire and brought down to the ground and matched with coax via a good 1:1 coaxial balun.

http://www.vmarsmanuals.co.uk/newsletter_articles/nvis.pdf

I once thought out the layout of that antenna for the site here but realized it is HUGE because of the spacing. Probably the best high angle antenna design going though. I may look into it again. The height requirement is not bad.


Steve/QIX:  Yes, your dipole at 90' works very well for both local and DX there. Good compromise height. I am beginning to think that my QTH is better suited for DX work being on a rocky ledge hill. My coverage to the west coast and Eu is outstanding but I think it's down about 5db locally.  I need to figure something to get a stronger lobe into the higher angles.


Rob: Yes, ~40- 50' is probably the lowest height for a 75M dipole before the ground losses start to eat at it.  I have also found that a dipole at 100' is down too much for the locals but very good for medium range.  A motorized crane type rotary dipole would be the best thang for versatility...

T

[w3jn]

Hi Tom,

Since I only have one antenna here, I put up a dipole at 90 feet.  It works fairly well at low angles, but I can still talk with the West Coast.

Seems like a very good compromise!

Regards,

Steve

And you make it here to Greece regularly with a decent signal.

[Steve - K4HX]

Yep, that thing is big. But 6 dB over a dipole is hard to beat. The plot I posted earlier was with the two full wave elements at a height of 50 feet. The spacing was 0.65 WL.

Steve/HX:

On page 10 is a drawing of the Jamacia antenna.  What a robust pattern with gain in the higher angles. Interesting it uses a pair of driven full-wave dipoles spaced 1/2 wavelength apart and fed with openwire in the center. The feedpoint at the center should probably be openwire and brought down to the ground and matched with coax via a good 1:1 coaxial balun.

http://www.vmarsmanuals.co.uk/newsletter_articles/nvis.pdf

I once thought out the layout of that antenna for the site here but realized it is HUGE because of the spacing. Probably the best high angle antenna design going though. I may look into it again. The height requirement is not bad.


Steve/QIX:  Yes, your dipole at 90' works very well for both local and DX there. Good compromise height. I am beginning to think that my QTH is better suited for DX work being on a rocky ledge hill. My coverage to the west coast and Eu is outstanding but I think it's down about 5db locally.  I need to figure something to get a stronger lobe into the higher angles.


Rob: Yes, ~40- 50' is probably the lowest height for a 75M dipole before the ground losses start to eat at it.  I have also found that a dipole at 100' is down too much for the locals but very good for medium range.  A motorized crane type rotary dipole would be the best thang for versatility...

T

[Steve - K4HX]

Another thing to look at on the Jamaica is the reduced response at the lower angles. I would think this would really reduce the more distant static and other interference, especially at night.

[WA1GFZ]

Just QRO

[KM1H]

Tubes with handles beat glass any day

Im very satisfied with the 80/75 inverted V's at 170', it does low and high angles equally well as Steve modeled here a year or so ago. It works the DX very well and keeps the locals off my back. The few times Ive used it on AM with the LK-500ZC and the TS-950SD has resulted in excellent reports as well as no slopbucket harassment. I listen only on the Beverages which are now up to 10 directions using 5 two wire reversibles. A 160M V is on the same feedline.

The primary 75M AM antenna which is used with the vintage gear on other benches is a sloper starting at 90' on the much closer 100' tower and aiming SW (not that its very directional), it has a 40M inverted V hanging from the center insulator. A 160 vee also starts at the 90' level. They do OK at the KW level on 160-40 and even at 100W when conditions are decent. I still have to make a provision to switch the 170' over to that bench.

Carl

[K1JJ]

Sounds like a good setup, Carl.

Your dipole at 170'.... how high are the ends?   Later today I may model it to see how it does against a dipole at 50' - curious how much high angle it has.

I'm on my way outside now to raise my dipole from 65' to about 100'. It will be an inv vee and will have it more east/ west to fill in the holes.  I'll bet it's down about -8db locally from the low dipole at 48'.  If it works well out towards Buddly, I may add a reflector.  We'll see. 

Cool on your five reversable Bevs. Ten directions will certainly do it.


Steve - OK on the 5/8 wavelength spacing on the Jamaica.  I'm still thinking about it though it wud probably screw up the antenna field here for other patterns. Too frickin big.  I may stick with the four 75M choices and be done wid it.  (dipoles at 48', 100' - and loops at 190' for NE or SW)

T

[Steve - K4HX]

Add a G5RV and you're set.

Sounds like a good setup, Carl.

Your dipole at 170'.... how high are the ends?   Later today I may model it to see how it does against a dipole at 50' - curious how much high angle it has.

I'm on my way outside now to raise my dipole from 65' to about 100'. It will be an inv vee and will have it more east/ west to fill in the holes.  I'll bet it's down about -8db locally from the low dipole at 48'.  If it works well out towards Buddly, I may add a reflector.  We'll see. 

Cool on your five reversable Bevs. Ten directions will certainly do it.


Steve - OK on the 5/8 wavelength spacing on the Jamaica.  I'm still thinking about it though it wud probably screw up the antenna field here for other patterns. Too frickin big.  I may stick with the four 75M choices and be done wid it.  (dipoles at 48', 100' - and loops at 190' for NE or SW)

T

[k4kyv]

I agree that many hams use NVIS antennas on 75, whether they want to or not.  For years, before I put up the tower, my antenna was a dipole about 25' high. I used to get reports within a couple of hundred miles that I had one of the strongest signals on the band.  Some said I had the strongest. Nothing but a single 304-TL, modulated firstly by 211s, later everything from triode-connected 813's and 810s to 833As.

My present antenna is an 80m dipole 119' high in the middle, drooping down to about 100' on each end. I don't get as good reports from locals as I did with the old antenna, but there aren't that many locals on AM any more for me to even be concerned about.  I remember the first QSO I had with the present antenna when I got it up in 1982, with WA4GGL.  First thing he asked me was what I did to my signal because I was so strapping.  Then I pulled a hoax on some of the guys in New England.  I told them I was visiting my wife's parents, and had a temporary dipole set up on Cape Cod, and running 100 watts.  No-one doubted the story until I told them it was all a joke and that I was still down here, running the new antenna with my normal transmitter, the one I still run on 75.

I have often thought that 90' would be a good compromise antenna height for 75, since it still gives low angle lobes, almost a good as a dipole a half wavelength high, but the vertical null is largely filled in so that all that is left is a little dip.  I haven't considered lowering mine, since I'm not sure how it would affect the performance of the tower as a 160m vertical, with the dipole down that low on the radiating element, considering the close coupling due to the proximity of the dipole feedline to the tower and the amount of top-loading that produces with the present configuration.

Of course, for 160m that same height in wavelengths would require a 250' long dipole 180' in the air, which is getting into some serious tower building and expense. So for top band, I think I have the best all-around antenna, the vertical with a radial system comparable to Tom's, although I can load the 119' high 80m dipole as a short dipole on 160 with good results locally.  In Nashville, I am told that the dipole comes in about 30 dB stronger than the vertical, enough to make the difference between full quieting and marginal reception through the urban background hash.

I don't think a radial system makes much difference with a horizontal dipole.  You would need an impractical length of radials, and a lot of them, to simulate a dipole over salt water. Perhaps an elevated ground system, with the wires parallel to the dipole, not laid out in radial fashion, of the proper length and space away from the dipole to serve as a reflector as in a Yagi (with a single perpendicular conductor bonded to and bisecting all the  radials) would make some difference for cloud-burning.  The only way to make sure would be to try it, since computer simulations don't always work as predicted in actual practice.

One example of a computer simulation that didn't work out as predicted was my K6STI 160m receiving loop. See the September, 1995 issue of QST.  It was supposed to be highly effective and omni-directional for receiving skywave signals but respond poorly to ground wave, and thus suppress much of the local electrical hash that comes in as ground wave.  I built one, being very careful to keep everything carefully balanced and all dimensions measured to within a fraction of an inch. The thing turned out to be about as effective a low-noise receiving antenna as a random hunk of wire thrown out the window of the shack with the other end attached to a tree.

Tubes with handles beat glass any day

AKA Antenna by Eimac.

Since my half-wave vertical on 75 didn't work out well enough to be worthwhile, at Timtron's suggestion, I may convert the tuner to work on 40m, making it a full-wave vertical on that band. The main lobe of a full wave vertical with radial ground system is supposed to be omnidirectional with vertical angle somewhere about 50°, with a minor lobe at very low angles. Not truly a cloud-burner, but plenty of high-angle.  It might work well state-side when the skip is short, and the minor lobe might still be good for DX when the skip goes long enough to avoid short-haul QRM.

[Steve - K4HX]

I don't think a radial system makes much difference with a horizontal dipole.  You would need an impractical length of radials, and a lot of them, to simulate a dipole over salt water

Generally true. But in the case of high takeoff angle, the radials could be significant. The far-field pattern at a 90 degree TO angle is formed by ground reflections directly under the antenna. First reflection point at other high angles will be within a few hundred feet - still in the radial field.

[k4kyv]

But would radials be the best layout for the ground screen?  What about laying out parallel ground wires also parallel to the dipole to cover a square footprint, about a half wavelength on a side, and bonding each ground wire at the middle to a perpendicular central "backbone" conductor?

And, how many hams or even commercial HF stations are willing and able to expend enough real estate and copper to lay out a substantial number of radials that would still be in the radial field at a few hundred feet? According to the series of articles in QEX, you need a minimum of about 16 radials when the length of the radials is over about 1/8 wavelength; otherwise the added length actually results in higher ground loss due to resonance effects of the radials.  That appears completely counter intuitive, but the guy who wrote the article claims it proved to be true when he confirmed the computer simulation results with actual experiments.

[K1JJ]

I don't think a radial system makes much difference with a horizontal dipole.  You would need an impractical length of radials, and a lot of them, to simulate a dipole over salt water

Generally true. But in the case of high takeoff angle, the radials could be significant. The far-field pattern at a 90 degree TO angle is formed by ground reflections directly under the antenna. First reflection point at other high angles will be within a few hundred feet - still in the radial field.

Yes, that is my thinking too. For a dipole higher than 1/4 wavelength, the lower angles use ground reflections that can be 1000+' away from the antenna. But the close-in radials may be helpful for the NVIS project.  I don't know how my three sets of 120 radials per vertical (~360 radials total, 125' long, some 250' ) compares to salt water. If it's a good ground plane for a vertical, maybe it's also a decent reflection/groundplane for an NVIS dipole too.

Don: I agree that a screen wud be a better idea, but like you, my radials are already in the ground with no hope of changing them..

I just got in from climbing the tower and raising the dipole to 102' feet at the apex. Feeling like Tarzan, caw mawn. The ends are at 90' and 80'. It should be fun to A/B the lower dipole at 48' against it. This shud show at least 10db in delta. The dipole is broadside directly east and west. Must add a reflector later.

The BEST part of the project is always being back down on the ground and raising the legs with the pulleys. Then walking far away and looking at the antenna...

T

[K1JJ]

With the dipole raised to 102', I ran some more 75M tests at 4:30PM.  I heard a station from Lake Ontario. That is the perfect distance and direction for the 102' dipole. He was about 10db louder on the 102' dipole and down on both the low dipole at 48' and the southwest loops.

I then heard a local K1 who was about 8db louder on the low dipole compared to the others.

There were some W2's that were about the same on both dipoles, so it appears they were in the crossover point of the vertical patterns.

Listening to a distant W8 in WVA, he was loudest on the high loops, followed by -6db on the 102' high dipole and weakest on the 48' dipole.

So looks like the selection of heights are complementing each other.

I will try some contacts out west tonight and see how it does to the west/NW into W8/W9/W0 and Washington State areas.

In general, I think the dipole at 102' is a good compromise for local and farther out work.  The locals are so loud anyway, the small decrease using the higher dipole is not significant, whereas the increase farther out matters more.

T

[KM1H]

Tom, all the end ropes are tied off about 250-275' from the tower, based upon cooperative trees or the hous for one rope. The area behind the tower was mostly clear cut 4 years ago to make it easier to run wires. Then its thick woods again for a few miles which are not all mine.

[Steve - K4HX]

Perfect set up for diversity.

With the dipole raised to 102', I ran some more 75M tests at 4:30PM.  I heard a station from Lake Ontario. That is the perfect distance and direction for the 102' dipole. He was about 10db louder on the 102' dipole and down on both the low dipole at 48' and the southwest loops.

I then heard a local K1 who was about 8db louder on the low dipole compared to the others.

There were some W2's that were about the same on both dipoles, so it appears they were in the crossover point of the vertical patterns.

Listening to a distant W8 in WVA, he was loudest on the high loops, followed by -6db on the 102' high dipole and weakest on the 48' dipole.

So looks like the selection of heights are complimenting each other.

I will try some contacts out west tonight and see how it does to the west/NW into W8/W9/W0 and Washington State areas.

In general, I think the dipole at 102' is a good compromise for local and farther out work.  The locals are so loud anyway, the small decrease using the higher dipole is not significant, whereas the increase farther out matters more.

T