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Author Topic: Communicating during the Apocalypse  (Read 1187 times)
KA2Z
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Posts: 32




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« on: December 05, 2017, 08:18:33 PM »

I am trying to figure out the best band / freq for communicating with my son (KD2NQP) who lives about fifty miles away from me.
In an emergency I know the VHF / UHF repeaters are good. But suppose the repeaters are not working, cell and telco towers down and power shortages all over.
Using portable generators for our stations, I am thinking that 10 meters would be the best band for simplex communication with a couple hundred watts.
It would be the best line of sight with the least impact from propagation effects.

Any comments? Agree, disagree? Better ideas?

Thanks for any comments....
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WB6BYU
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Posts: 17171




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« Reply #1 on: December 05, 2017, 09:02:37 PM »

There's a really handy tool for that called VOACAP.
It shows a map, you move the transmit and receive locations to where you want them (you can zoom
in the map to see more detail) and it shows you the probability of various HF bands being able to
cover the path at different times of day given the present ionospheric conditions.

You can set the power level and antenna type at each end, along with the mode.  Comes in handy
for getting a sense of how much difference it would make raising the antenna from 35' to 50', for
example.

Under current conditions, when I enter a location that is 50 miles South of me, it shows that 80m
has about a 70% to 90% chance of getting through at night, and about 100% chance during the day.
(this is about the time we need to start moving our nighttime nets to 160m, but VOACAP doesn't
include 160m because it is a MF band instead of HF.)  60m is also good during the day, and 40m
gets about 80% probability for about 6 hours at mid-day.  15m and 12m show up at 2%, but not
10m, 20m or 17m.  Even with 10 watts of SSB and a dipole at 17', 80m is still pretty good most of
the day - better than 40m, though 60m has an edge at times.

The actual results will depend on your latitude:  here at 45 degrees North we find that 40m often
isn't open when it is at lower latitudes.  Thats why having a tool that will calculate it for your
specific location (and the current conditions) is useful.


The "conventional wisdom" for such a short path is 40 during the day and 80m at hight, and relatively
low dipoles should work.  Particularly in the winter at the bottom of the sunspot cycle, we find that
we often need 80m during the day and 160m at night.


If you want a more direct path without worrying about the ionospheric conditions, then, depending on
terrain, 6 m may be your best choice, perhaps with 3-element beams at a reasonable height at each
end.  2m can also cover the path in some cases - again, depending on terrain.
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KA2Z
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Posts: 32




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« Reply #2 on: December 05, 2017, 11:27:40 PM »

Thanks for the info.
I will check out vocap.
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VK5EEE
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« Reply #3 on: December 06, 2017, 02:47:00 AM »

It would depend on the ANTENNA. If you want to use 10m for 50km it will be ground wave. A good vertical high enough on both sides should do it. Or a yagi antenna, but then higher frequencies become even more practical e.g. 6m, VHF, UHF as  you can then build small Yagis with much higher gain. But you will then need more line of sight than at lower frequencies.

On frequencies below 28MHz, you will want sky wave, single hop, off the E layer ideally, and 50km is difficult because that is really within the dead "skip zone" as it is not far enough away for the sky wave to bounce back, and it is too far away for ground wave which would be buried in noise -- on 10m the noise is much less (though if you are talking post TEOTWAWKI then there may be no noise), but, a vertical antenna has to be much bigger, and it's height too, to reach the same result as on 10m.

Therefore for sky wave you would be best off around 5MHz, with an NVIS dipole, simply a dipole that is 5m to 10m above the ground, or an inverted vee with its apex at between 5 and 10m ideally, this means the antenna is only 1/10 or so of a wavelength above ground so the angle of radiation is straight up, and it reflects off the E (day) or F (night) layer back down (but loses power going through the D layer), but signals could be quite strong, likely stronger than the 10m solution, but you may have more noise to content with (and atmospheric static QRN).

Of course if you use Morse Code you'll have up to 20dB gain over SSB so your signal on 10m could be very weak indeed and it'll work. If you had to choose one or the other, for 24 hour reliability, I'd say the 10m or VHF ground wave, but you'll have to test it ahead of TEOTWAWKI to make sure. You can also do NVIS on 80m or 40m, to a degree perhaps also on 30m but that's pushing the upper limit.

If I have any of the above wrong, by all means any propagation expert may correct me.
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W4KYR
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Posts: 1598




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« Reply #4 on: December 06, 2017, 03:55:44 AM »

Both stations 2 meters SSB running 50 watts with a 3 element 2 meter beam antenna

Why? A 2 meter 3 element beam is small, not that expensive, they can be set up anywhere and they are easy to turn by hand

Besides, you want  a directional antenna to minimize being heard by others.

With both stations you will be able to reach 50 miles with no problem

How do I know? I can easily talk 50 miles away (and further) using 2 meter SSB into a 2 meter 3 element beam

What other reasons for using 2 meters SSB? Availability of equipment...
Several radios are already available that have that band (and mode) like the Icom IC 706 MKIIg, IC-7000, IC-7100. Yaesu FT-100, FT-857, FT-897 and other models as well

Of course you aren't stuck with SSB, you can use CW or any of the available digital modes...

There would be no propagation problems like fading

Interference should be minimal to non existent....

You can use other frequencies like 10 or 6 meters, the only problems with using those bands is that directional antennas are much larger and expensive. Besides, you want a directional antenna to minimize being heard by others.



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KA2Z
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Posts: 32




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« Reply #5 on: December 06, 2017, 08:19:17 AM »

Thanks for interesting feedback and comments guys.

Lou, What is TEOTWAWKI?
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KD8IIC
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« Reply #6 on: December 06, 2017, 09:35:46 AM »

  Just what do you think you'll find to talk about, and why?
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WB6BYU
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« Reply #7 on: December 06, 2017, 10:36:44 AM »

Quote from: KA2Z

Lou, What is TEOTWAWKI?



The End Of The World As We Know It.
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WW7KE
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Posts: 603




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« Reply #8 on: December 06, 2017, 12:35:21 PM »

Quote from: KA2Z
Lou, What is TEOTWAWKI?

The End Of The World As We Know It.

And I feel fine. Grin
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W4KPA
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Posts: 74




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« Reply #9 on: December 06, 2017, 07:22:52 PM »

As somebody pointed out, two meter SSB would cover that distance without any problems at all. In fact, depending on terrain, I'd bet that two meter FM simplex would work.
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KA2Z
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Posts: 32




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« Reply #10 on: December 06, 2017, 10:44:29 PM »

2 Meter ssb is not offered in too many radios.

I am starting to think 80 meters might actually be a good chose for day and night communications out to a hundred miles.
An 80 meter dipole with bent up legs is not too hard to put up.
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KA1AL
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Posts: 1




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« Reply #11 on: December 07, 2017, 05:59:45 AM »

For amateur frequencies from 10 MHz on up, VE2DBE generously provides an online tool, Radio Mobile Online -

http://www.ve2dbe.com/rmonline.html

Covers up to 300 km from transmitter. Line of sight calculations, using terrain.  Pretty accurate in my experience.

As WB6BYU said, voacap.com is also very good, but is more for skywave propagation.  I don't think it uses terrain data as does RMO.
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W4KPA
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« Reply #12 on: December 08, 2017, 01:40:01 PM »

Well, 80 meters will certainly cover the distance, although right now, the band is really stretching out at night. You might have a little trouble with a close in station, although if you used an NVIS antenna you'd probably overcome that.  But, I think it would be a mistake to dismiss VHF. Fifty miles isn't really very far. I'm sitting here right now listening to two stations on six meters who are about 75 miles from me. I have no doubt I could break into the conversation even with my dipole antenna. Two meter SSB would cover that with complete reliability and with a relatively small number of eavesdropping stations -- maybe none, given the activity levels on the band. Heck, I suspect you'd find 432 SSB would cover the distance as well with even more privacy.

There are currently five current production radios from the big three that cover 144/432 with SSB capability. The well-tested Yaesu FT-850 and the ICOM IC-7100 can be bought new for $899 and $759 respectively from HRO. Match either one of those up with a yagi that you can mount on your roof, and you'd be trading sledgehammer signal reports over that distance.
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WB6BYU
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« Reply #13 on: December 08, 2017, 02:49:57 PM »

It does, of course, depend on the terrain between the two sites:   you can have a
50 mile path with a 10,000' hill in the middle.

Well, maybe not in New Jersey...
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