NVIS operation to the next valley over? - Page 1
Forum section: electronics / rf-microwave

author: cdev
author_rank: Super Contributor
timestamp: February 14, 2022, 04:09:52 am
content: I have never used HF VLF radio and am just wondering if I am ham licensed if I could use very low bands to reliably communicate with neighbors whose homes may be only a few miles away but over a hill. So VHF and UHF wont work (except via a repeater) But maybe NVIS will, if I can make an antenna that resonates.. Maybe a magnetic loop? Just for the next town over? Is it reliable?Thanks!

author: DaJMasta
author_rank: Super Contributor
timestamp: February 15, 2022, 06:26:09 am
content: Ham bands vary with country, but yes, generally some low MHz bands are available to transmit on and provided you stay within power limits and stay out of other bands, you can develop and utilize pretty much any antenna that fits the bill. In the US, the basic technician license doesn't have access to those lower frequency bands, but the higher license classes do. It's worth mentioning that the channel width decreases with the frequency, so while single sideband voice is still usable there, most of the lowest frequency bands are generally reserved for morse.There's also CB radio which doesn't really permit you to use your own custom antennas/amplifiers (to my knowledge), but which doesn't require a license to run and at 27MHz or so, may fit the bill for communication off the layers of the atmosphere.

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: February 15, 2022, 10:28:19 am
content: Very short range NVIS I would target 80m or 160m. With a low dipole (5-10m off the ground) your pattern will be mostly straight up. Lower frequencies are better for NVIS, even 40m will not be particularly reliable you need to go lower.27MHz CB is too high in frequency for NVIS.Reliability still isn't guaranteed.

author: 1design
author_rank: Regular Contributor
timestamp: February 17, 2022, 12:26:06 pm
content: Here you can check real-time worldwide fof2 and other parameters useful for NVIS operation. Beware of signal rotation and degradation when using NVIS propagation.https://prop.kc2g.com/fof2/

author: cdev
author_rank: Super Contributor
timestamp: March 02, 2022, 10:53:54 pm
content: Quote from: Geoff-AU on February 15, 2022, 10:28:19 amIs 1.8 Mhz likely to work for (QRP) NVIS, when the two stations are separated by a hill? I'm assuming the antenna would be constrained by size to being fairly inefficient, maybe a magnetic loop made with several turns of thick wire? I have a bunch of "monster cable" for example.

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: March 03, 2022, 01:09:18 am
content: Your best bet is not pinning your hopes on one band, it's having a couple of options. Broadly speaking, when ionisation increases the F2 layer becomes a good reflector of radio waves at higher and higher frequencies. But at the same time, the D layer starts absorbing more and more of the low frequencies. So your usable frequency range is a window that shifts up and down.The problem with making a 160m antenna is that as it gets closer to the ground the impedance drops, so you end up with SWR issues and your transmitter folds back its power output.Magnetic loops are fun to build but very difficult to make well, and multiple turns is not normally the way they're done (although I have seen designs). I've found they are generally quite miserable in terms of efficiency but you might not need a lot of power to make your contact. It's worth experimenting with, I have been heard interstate on one of mine. Just... stay a few metres away from the magloop while transmitting. They have a very high circulating current.Ultimately you're going to have to experiment and find out what works for you. Also accepting that it might not work 100% of the time (which is why having the flexibility to try different frequency bands and/or antennas is useful).

author: cdev
author_rank: Super Contributor
timestamp: March 03, 2022, 01:25:21 am
content: Multiple turns is not my usual practice with mag loops but then again Ive never made one that reached down to 160 meters (for receive of course) I am not land rich so I see a mag loop as the only practical antenna I see myself making for 160 M. Luckily I have copper tubing handy. It may not be very efficient but I see it likely as being affordable. Its just a loop and a capacitor and a coupling loop (or toroid) I am expecting my first rig, whatever it is to probably be QRP. Can't afford more power. So the signal however it is will just need to bounce right back to earth.. for me to reach hams in nearby towns.that are over hills. .

author: cdev
author_rank: Super Contributor
timestamp: March 03, 2022, 01:52:50 am
content: Quote from: Geoff-AU on March 03, 2022, 01:09:18 amI know how to make the magloop as efficient as possible. I have a desktop DMM that can measure very low resistance with high precision.. I see that as the biggest challenge. As far as variable capacior, I hope to get a large vacuum variable and have add on high voltage caps if necessary to reach the lower frequencies. It would be better than attempting to use an antenna tuner with a wire antenna or similar. I will put it on a sturdy photo tripod I often use for such things. It will get a loop up a bit but not that much. There wont be much power involved, either . If a magloop is tuned to the right point, the SWR can be quite good. Perfect.. for a few KHz... The bandwidth is narrow. Thats how I like it.They make great antennas.. There is nothing else quite like it. They sizzle..

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: March 03, 2022, 02:41:42 am
content: You'll need a doorknob style capacitor, and needs to be very low loss. I found them to be a less than satisfactory experience trying to multiband my loop but you might do better with more determination. The main downside to this approach is that as you push a loop lower in frequency, the bandwidth gets narrower and the efficiency drops. Tuning a loop with such narrow bandwidth is a pain, because your presence nearby starts to detune the loop.On the plus side, magloops don't need to be high off the ground.They're great fun, but I'm more of a fan of the "get as much wire in the air as high as you can and use a tuner" mentality now

author: El Rubio
author_rank: Regular Contributor
timestamp: March 27, 2022, 02:48:41 pm
content: First, are the neighbors you are trying to communicate with licensed as well? I would look into a passive repeater consisting of two yagi’s back to back on the hill. One would be directed to your location and the other directed to the neighbors. I live near New Orleans and we have no hills nearby. So, I have no experience with this, but have read about it. Are there any active repeaters that can reach both sides of the hill?

author: cdev
author_rank: Super Contributor
timestamp: March 28, 2022, 12:29:04 am
content: Quote from: El Rubio on March 27, 2022, 02:48:41 pmI'm mostly looking for a reliable means of communicating with nearby hams which does not require a repeater. Or high power.I'm quite familiar with magnetic loops for receiving.Seems that magnetic loops are a popular military antenna for vehicles.. Ive read that they can offer a reliable means of communication over the immediate horizon (including into nearby valleys) that is not subject to the unpredictibility of the ionosphere quite as much as HF F2 skip.

author: A.Z.
author_rank: Super Contributor
timestamp: March 28, 2022, 08:28:48 am
content: Quote from: cdev on March 28, 2022, 12:29:04 amSo you're seeking for NVIS since that will allow you to "bounce" your signal to the other side of a hill or other obstacles, and the choice of bands from 40m to 160m will be appropriate, depending from season, time and other factors. The problem, if we want to call it this way, is that for NVIS to work, all the stations should have NVIS antenna, the reason is easily explained; let's say your station (call it A) has an NVIS antenna, then on the other side of a hill/mountain another station (call it B) has a "non NVIS" (say a vertical) antenna; now, station B may be able to receive signals from stations A, even if probably somewhat attenuated, but station A will not receive signals from station B since those will be radiated at low anglesThe above being said, I think that willing to be able to cover 160 to 40 and use NVIS, you may consider the idea of putting up aT2FD(or a so-called "T3FD") antenna, a good plan for such an antenna can be found herehttp://hflink.com/antenna/Build_a_T3FD_Antenna_by_Tony_Rycko_KA2UFO.pdfHTHanother idea for an NVIS antenna is shown herehttps://static.dxengineering.com/pdf/wp-nvis-rev2.pdf

author: cdev
author_rank: Super Contributor
timestamp: March 28, 2022, 08:28:57 pm
content: For a tunable loop, a vacuum variable is the best. But they can be expensive. Too expensive.I suspect strongly that a big rotatable magnetic loop can outperform most traditional HF antennas on the low bands in a practical setup because of the size for the traditional antennas can make them difficult to build and manage unless you have a large parcel of land to site them on. Thats why they are popular antennas for navies and diplomats.. The longest dimension that I have is not very large. Its a bit larger than most British hams have but not by much.Quote from: Geoff-AU on March 03, 2022, 02:41:42 amThey really are "fun" I think, especially if tuned to a very high Q and rotatable.. this makes them good direction finding antennas.. One can make good use of their sharp null to tune out sources of noise.. They can be really good for shortwave listening for exotic foreign stations.

author: A.Z.
author_rank: Super Contributor
timestamp: March 29, 2022, 07:15:18 am
content: Quote from: cdev on March 28, 2022, 08:28:57 pmIf you're going for a STL (Small Transmitting Loop), check the designs found herehttp://www.kk5jy.net/magloop/but remember that to "skip a hill" BOTH stations will need an NVIS capable antenna

author: dropkick
author_rank: Contributor
timestamp: March 29, 2022, 09:32:50 pm
content: How big of a hill are we talking?What bands, power, modes, and antennas have been tried? Considered 2m SSB or 6m as well?

author: cdev
author_rank: Super Contributor
timestamp: April 03, 2022, 08:34:49 pm
content: Low hills, maybe 1000 meters tops. Not mountains. Kind of like the hills in the UK.

author: AG6QR
author_rank: Frequent Contributor
timestamp: April 03, 2022, 09:16:39 pm
content: I'm a licensed ham (my callsign is my forum ID). I live in a hilly area. My local radio club does a weekly net, testing the ability of NVIS HF radio to communicate around this hilly county in the event a natural disaster makes conventional communications unavailable. They try three bands each week, but the bands chosen vary, depending on the season. In the winter, the lower bands are most likely to work, 80m or perhaps even 160m. In the summer, 60m and 40m are more likely to work, and if the sun is really active, 20m might even work. Some weeks, no band works well.Since longer wavelengths diffract more, I would expect that 160m or longer would have a reasonable chance of working over a 1000 m hill for a short distance, even without bouncing off the ionosphere. Ground wave plus diffraction may carry the signal. While I don't have experience with 630m or 2200m, those bands would be interesting to experiment on for this kind of scenario.Regardless of the band, more power and more efficient antennas will always improve the chances of working. Morse code penetrates through noise better than SSB voice. There are some modern computer-based modes (JS8call is the current hot topic) that can work as well as, or sometimes better than, Morse.