During Hurricane Ike(2008) in Houston, TX I was part of the communications. I used a 40m dipole NVIS AT 3feet off the ground due to the high winds. It worked well for communicating with everyone from Galveston to Houston and all surrounding areas. Less than 100 miles worked well that night and into the next day. Like you said NVIS is not for DX but is great for local area communicating 📻🎧
The one and only Dr. NVIS, Ben A Witvliet, says, approx 0.18wl for Tx and 0.16wl for Rcv with variation for ground conductivity. At 0.02wl the Tx antenna will see about -12dBv. The Rcv antenna will see about -3dBv at 0.02wl. Also, a 100W NVIS station using a dipole at right height and freq will provide 50duBv signal over 150km radius. On 2m FM that would be about +20 on most S-meters. HF there is a lot more variation.
hi David, you help me a lot for to have the motivation and finally pass the exam (and receive my radio amateur certificate/license this week).. thanks a lot, love your way to "transmit" your knowledge.. now i'm va2azb!! ❤
Hi Dave, I have been told that placing a wire that is 5% longer than the antenna, directly under the wire (on the ground or 1/10th WL) will help focus the signal in a more vertical direction. I have not tried this yet, but intend to if our Idaho snow ever melts. It would tend to make sense as it would act as a reflector. Thank You for your informative videos....73
Being in VK we speak metric (these days). I'd say NVIS is good for 900km radius. Totally agree with the fact that it doesn't all go straight up. But I would take you to task on the best hieght above the ground. Any one that has modeling software can see that a dipole is 70 ohms when resonate (in freespace). When you lower this to the ground you see it interacts with the impedance. This "interaction" happens given the fact energy is reflected from the ground back up into the feilds. Ground as we all know isn't a perfect conductor so at some point how close do you come to the ground? When the ground it self starts to swallow up all your power. I'd say its 10% under the free space 1/4 wave for the operating frequency. Just enough to make the ground drop the radiation resistance to 50 ohms (if you want to feed it with coax). Then that's it. Obviously this doesnt work for multi band. But for single band or fixed frequency its the go. From what I can see in the modeling software you get a nice party balloon shape. 73's from VK2
Hi Dave, I'd be interested of you can point me to the theory behind the optimum height which you quote. If you assume that the ground, directly under the antenna, provides the conventional phase-reversal, of the wave, by reflection, that would dictate that the maximum NVIS signal would occur when the horizontal dipole is a quarter-wave above ground. I think I have confirmed this using NEC modelling software, but perhaps it does not accurately model the effect of the ground-reflection for NVIS.
During Hurricane Ike(2008) in Houston, TX I was part of the communications. I used a 40m dipole NVIS AT 3feet off the ground due to the high winds. It worked well for communicating with everyone from Galveston to Houston and all surrounding areas. Less than 100 miles worked well that night and into the next day. Like you said NVIS is not for DX but is great for local area communicating 📻🎧
The one and only Dr. NVIS, Ben A Witvliet, says, approx 0.18wl for Tx and 0.16wl for Rcv with variation for ground conductivity. At 0.02wl the Tx antenna will see about -12dBv. The Rcv antenna will see about -3dBv at 0.02wl.
Also, a 100W NVIS station using a dipole at right height and freq will provide 50duBv signal over 150km radius. On 2m FM that would be about +20 on most S-meters. HF there is a lot more variation.
hi David, you help me a lot for to have the motivation and finally pass the exam (and receive my radio amateur certificate/license this week).. thanks a lot, love your way to "transmit" your knowledge.. now i'm va2azb!! ❤
Congrats VA2AZB📻🎧
@@david_W5QDF ❤️❤️❤️
Thank you Dave for the educational and fun video. Always interesting subject material. 73, Dave, AC3HT
Thank you, Dave. Have a great weekend. N0QFT, Glen
Hi Dave, I have been told that placing a wire that is 5% longer than the antenna, directly under the wire (on the ground or 1/10th WL) will help focus the signal in a more vertical direction. I have not tried this yet, but intend to if our Idaho snow ever melts. It would tend to make sense as it would act as a reflector. Thank You for your informative videos....73
I found it helpful when you are working with less than ideal soil conditions...
Being in VK we speak metric (these days). I'd say NVIS is good for 900km radius. Totally agree with the fact that it doesn't all go straight up.
But I would take you to task on the best hieght above the ground. Any one that has modeling software can see that a dipole is 70 ohms when resonate (in freespace). When you lower this to the ground you see it interacts with the impedance. This "interaction" happens given the fact energy is reflected from the ground back up into the feilds. Ground as we all know isn't a perfect conductor so at some point how close do you come to the ground? When the ground it self starts to swallow up all your power.
I'd say its 10% under the free space 1/4 wave for the operating frequency. Just enough to make the ground drop the radiation resistance to 50 ohms (if you want to feed it with coax). Then that's it. Obviously this doesnt work for multi band. But for single band or fixed frequency its the go. From what I can see in the modeling software you get a nice party balloon shape.
73's from VK2
My end fed half wave 40m is about 6 feet off the ground. 3000 miles on 20m, 1000 on 40m.
Definitely, definitely 20
Hi Dave, I'd be interested of you can point me to the theory behind the optimum height which you quote. If you assume that the ground, directly under the antenna, provides the conventional phase-reversal, of the wave, by reflection, that would dictate that the maximum NVIS signal would occur when the horizontal dipole is a quarter-wave above ground. I think I have confirmed this using NEC modelling software, but perhaps it does not accurately model the effect of the ground-reflection for NVIS.
Great video Dave, as usual!! 73/K6SDW
At about 3:35...isn't the optimal height for 40M going to be 4M in height?
i would be very careful with that electrolytic corrosion, it can be extremely destructive. It obviously depends on the moisture and PH of the soil.