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Author Topic: Affordable NVIS set-up, frequency and time selection  (Read 491 times)

Commsguy

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Affordable NVIS set-up, frequency and time selection
« on: August 28, 2017, 08:09:49 AM »
I have started a new thread on this discussion to keep the Welcome Thread more on-topic for Welcoming new members.
Below I've re-stated some information from the other thread so everyone can easily read what it is I'm trying to achieve here. I've already had some great replies in the other thread for which I am thankful. I've quoted some replies below from the other thread to which have also written a reply - in this thread to keep the other one clean.

A close friend recently started prepping as well and I want to establish a (for HF) rather short-range method of communication. The ranges to think of are 15 or 20 to 75 miles. I'm having the hardest time with finding the right band & time that would provide the highest reliability for daily NVIS communication, all-year and preferably solar cycle round. In the Welcome Thread it was established that the 80m band is the most reliable in a low solar cycle during the day, like we are in now, with 80m being suitable for high solar cycles at midnight. 40 meter would be best during high solar cycles during the day. If I got any of this wrong - please correct me

Commsguy - VOACAP is correct - in winter months during lows in the solar cycle (like now) 80M generally becomes a DX band after dark.

I strongly advise you to regularly check live foF2 readings in your region - it's the magic critical frequency: http://www.digisonde.com/stationlist.html

If I could give one other piece of advice: in the time I've been licensed, I've learned there is no single magic band that's always reliable - so frequency agility is a must.

Whether it was sunspots, or Sporadic E's.... being able to switch bands made communications possible.
As for VOACAP, link probability drops like a brick just after sundown on 80 meters in the winter, only coming back a few hours later at 2300h for the short-distance link. Is that because of 80m opening for DX after sundown? What makes it come back for NVIS at 2300?
I'm well in the know about the critical frequency for NVIS, foF2, it's one of the first things I learned about when starting to looking into NVIS. That's also why I've been looking at the Dourbes Ionogram every day for the last 6 months or so.
I hardly have seen the critical frequency (foF2) above 60 meters, which caused my confusion with so many people and 'research papers' praising 40m for NVIS.

As stated above, opting for frequency agility would be the best solution to most problems, but it would also require more knowledge and equipment to achieve both 40 and 80 meter band capability.

For me personally the more required knowledge and hardware wouldn't be a problem since I am the commsguy in our small prepping group, and I have a nice multi-band rig myself already. I'm looking for a cheap solution to equip someone else, preferably for around 100-150 euros total so that two-way communication can take place. For one-way communications a shortwave receiver like the GP-5/SSB would be a great fit since it covers all bands and then some, but I'm mostly looking for two-way communications in that price range.

As for a transceiver I've been looking at the QRP-Labs 5W CW transceiver kit. The reason that radio has the most interest from me is because of it's low price but much better quality than something like a Rockmite or Pixie. It is however monoband and would require some modding to be able to switch a band pass filter and low pass filter to get both 80 and 40 meter. A ready made multi-band QRP transceiver could also be the chinese X1M Pro QRP transceiver, which is around 250 euros. Such price increase doesn't currently fit the budget, also taking in to account that other accessories needs to be bought such as batteries and coax.

I can't find the context of the original quoted question:

"What would be the most reliable time of day to establish an communication on 80m? Early Morning, Noon, afternoon or at night/midnight?" but the answer depends on the location you are in the world, and the location of the other station! Is it 10,000km away or 10km or 100 or 1000? 80m generally speaking offers only local contacts up to a few hundred km during the day, but thousands of km (or more) depending on the antenna, at night. Near the equator there is a lot more static on 80m than at mid latitudes. Signals during the day are absorbed by the D layer, so don't reflect (refract) well hence short distances, again more so in summer than winter. You may look at 80m as being a band that favors darkness over light. BUT for short distances and NVIS at mid to high latitudes it is good. At mid to high latitudes at night though you especially need to make sure you have a horizontal dipole at a low height compared to 80m wavelength, as a vertical is more likely to give you sky wave reflection of F layer, not blocked by D layer, resulting in large dead "skip zone" before landing back to earth far away, for long distance "DX" contacts.

This plot of how a dipole radiates (low angles to the left and right are for long distance, you want the high straight up lobes for close-in contacts) at various heights, this is for 40m. It would be more helpful if I found one that gives height in relation to wavelength, but to make this valid exactly the same for 80m, simply double the heights given, e.g. the 10m high plot on this 40m graph would be 20m high on 80m, etc. Don't worry about the exact frequency, it will be much the same for 3.5 or 3.7 MHz in this case!



Look at the red line: this is what a dipole up 15m on 80m band would look like, so NVIS even at that height, and few can get an 80m dipole up that high! Look at the green line, if you can get it up 60m in the air, you will have BOTH low angle AND high angle but less intermediate angle radiation: good for local NVIS and long distance. But who can get it up 60m? But on 10m band you can get that green pattern at just 7.5m above normal ground. 10m of course is not much use at this part of the sunspot cycle, we have to wait many years, if ever, to get those good conditions again.

To reiterate, it's about 10 to 100kms that I'm trying to achieve here. From the amateur radio theory I have the understanding that the D-layer hardly absorbs 80m when the wave is perpendicular on the D-layer, so that would have much less impact on short-range NVIS operations.

I have some papers that go into detail about dipole heights and different bands, I'll have to gather all materials I've read so far on NVIS (which is a lot!) and find it again, but I'll upload the lot of it if there is any demand for it.





gil

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Re: Affordable NVIS set-up, frequency and time selection
« Reply #1 on: August 28, 2017, 09:13:56 AM »
Either 2m ssb or 80m NVIS should work. For 75 miles I would go 2m ssb with a Yagi. This way you do not rely on the ionosphere at all. You don't have to worry about your dipole height for 80m. You'll never get it high enough to lower the angle of radiation. For 40m I'd use the super gain NVIS antenna. See the corresponding thread. For your 80m antenna, use a reflector wire under it, 5% longer...

Gil

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RadioRay

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Re: Affordable NVIS set-up, frequency and time selection
« Reply #2 on: August 28, 2017, 09:17:37 AM »
You've done your research - that's clear

80m is my choice here for NVIS with regional friends, at almost 48 degrees north.  We have extreme reliability from 7 a.m. local often past noon and then from 15 Hours to a little after sunset, when the band goes long on 80m. Power levels run from .5 to 50 Watts.  As the thunder storms are becoming less frequent, lower power levels are easier.  This is why I prefer morning skeds - it's the lowest thunderstorm activity, so low noise, low absorption and less chance of receiving 'natural' electro-shock therapy from passing storm clouds.
.




de RadioRay  ..._  ._
"When we cannot do the good we would, we must be ready to do the good we can."  ~ Matthew Henry

swxx

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Re: Affordable NVIS set-up, frequency and time selection
« Reply #3 on: August 28, 2017, 01:44:26 PM »
Research in Europe and Australia has long established that at those mid latitudes the 60m band area (5MHz) is the most reliable short NVIS propagation. It is for this reason that emergency services etc in Australia and elsewhere use this part of HF more than any other, and also the reason why amateur radio was not generally granted much space at 60m as a result of the WARC conference. It is very heavily contested. So in reality, it is not 80m nor 40m, but 60m that is most useful all round. But that does not stop those other bands from being useful for NVIS. But if we are looking for BEST then I think it's 60m.

RadioRay

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Re: Affordable NVIS set-up, frequency and time selection
« Reply #4 on: August 28, 2017, 01:49:41 PM »
Depends upon your latitude.  I'm rather far north, and our critical frequency for NVIS is rarely above 5MHz.


http://www.sws.bom.gov.au/Images/HF%20Systems/Global%20HF/Ionospheric%20Map/WorldIMap0.gif

Otherwise - yes! - I'd use 60m in a heart beat. Before the sun numbers tanked,  5371.5 USB was our community calling frequency very useful in this mountainous State.

de RadioRay  ..._  ._
« Last Edit: August 28, 2017, 01:52:09 PM by RadioRay »
"When we cannot do the good we would, we must be ready to do the good we can."  ~ Matthew Henry

Commsguy

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Re: Affordable NVIS set-up, frequency and time selection
« Reply #5 on: August 28, 2017, 02:26:16 PM »
Either 2m ssb or 80m NVIS should work. For 75 miles I would go 2m ssb with a Yagi. This way you do not rely on the ionosphere at all.
Especially for that last factor, not relying on the ionosphere, 2m SSB would be the most reliable or best all-round solution. I'll keep it 'on board' as for being an option. What kind of power would do the trick at that distance? Obviously the number of elements and TX power can be changed somewhat to achieve the desired performance.

How many elements would you reckon to be suffice for that distance? A simple 3-element measuring tape yagi or an antenna with more elements?
How critical would it be to know someone's bearing? Does it have to be within a few degrees of accuracy?

What about something like a moxon antenna?

You've done your research - that's clear
I'm just trying to NOT reinvent the wheel here - by doing my part on the research I'm just making it much easier on myself later on. I just need someone with much more real-life experience to nudge me in the right directions and verify I'm having the right thoughts in regard to feasibility and such.


80m is my choice here for NVIS with regional friends, at almost 48 degrees north.  We have extreme reliability from 7 a.m. local often past noon and then from 15 Hours to a little after sunset, when the band goes long on 80m. Power levels run from .5 to 50 Watts.  As the thunder storms are becoming less frequent, lower power levels are easier.  This is why I prefer morning skeds - it's the lowest thunderstorm activity, so low noise, low absorption and less chance of receiving 'natural' electro-shock therapy from passing storm clouds.
I guess it'll need some testing to find the sweet spot in terms of the right time frame. Mornings would be great. Especially atmospheric noise is my biggest fear, since both parties would be running QRP power levels at 5W, at best.

Depends upon your latitude.  I'm rather far north, and our critical frequency for NVIS is rarely above 5MHz.


http://www.sws.bom.gov.au/Images/HF%20Systems/Global%20HF/Ionospheric%20Map/WorldIMap0.gif

Otherwise - yes! - I'd use 60m in a heart beat. Before the sun numbers tanked,  5371.5 USB was our community calling frequency very useful in this mountainous State.

I also think 60m is a bit too high around where I am at. Very few times have I seen the foF2 getting higher than 5Mhz in the 6 months I've been monitoring Dourbes.


Thanks everyone for their replies so far!

swxx

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Re: Affordable NVIS set-up, frequency and time selection
« Reply #6 on: August 28, 2017, 02:45:22 PM »
Indeed there is no one magic band at HF. At night even 3.5MHz may be too high and then too low at midday. Gils suggestion for 2m or 6m SSB is thus very good! Not by accident do many militaries use 6m and some wont give it up for amateurs, The SWS site Radio Ray showed above is very useful. I also like VOACAP program by G4ILO for easy quick glance,

gil

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Re: Affordable NVIS set-up, frequency and time selection
« Reply #7 on: August 28, 2017, 03:02:31 PM »
Quote
What kind of power would do the trick at that distance? How many elements would you reckon to be suffice for that distance? A simple 3-element measuring tape yagi or an antenna with more elements?

It isn't so much a power and gain issue but an antenna height issue. Get your antenna high enough and 75 miles should work. I would use a 6 or 7el. Yagi, as any more becomes quickly unwieldy. They are easy and cheap to make. Look at the portable design I made with the elements held by clothespins. You could make a similar fixed design.

Quote
How critical would it be to know someone's bearing? Does it have to be within a few degrees of accuracy?

It depends on the antenna design and the number of elements. The more elements you add the more narrow it gets. That's how the gain is created, there is no magical amplification going on, it's only achieved by directivity.

You really might want to try both, 2m SSB and NVIS, this way you can cover from zero to hundreds of miles.

Gil.