Author Topic: Propagation Modeling for Weekly Roundtable  (Read 8701 times)

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Quiet1

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Propagation Modeling for Weekly Roundtable
« on: December 21, 2013, 08:29:37 AM »
OK this isn’t a propagation report, it’s a propagation prediction.  But it probably best belongs here.  Hip boots on, we're going in deep!

Summary:  There is interest on this forum for establishing a weekly inter-regional (i.e., larger than a few states) SSB phone roundtable radio discussion.  Thus far these efforts have been only minimally successful.  Web-based propagation modeling using voacap.com was done for likely emergency communication practice scenarios on the commonly used 20, 40, and 80 meter bands in December, March, June, and September (the solstice and equinox months).  Based on this modeling and with common bug-out-able radio gear (100 watt barefoot rigs & portable wire dipoles at 10 meters height), the 40 meter band is the best all-round for our weekly evening roundtable attempts.  80 meters is nearly as good, but fewer people have that capability.  20 meters is not good for early evening propagation.  If we want daytime roundtables, 20 meters is the only viable band.  Winter is the best season, spring and fall are still good, and summer is marginal for both day and night.  Overall coverage is roughly half of CONUS under best winter conditions, shrinking to roughly one quarter of CONUS under best summer conditions.  A few selected cases were also run using quarter-wave vertical antennas at both TX and RX locations.  Other than a slight reduction in overall signal strength, little difference was seen in propagation features for the two antenna types.  As the sun shifts from solar max conditions, expect a mild weakening of propagation over the next year.

-----

OK, y’all got all that?  These wednesday night roundtables have become a goat-rope of band hopping and web-chat contacts, and last wednesday’s event was a 98% bust.  Face it, we’re mostly working with minimal equipment, so we’ve got to work smarter not harder.  Since we’re all about emergency preparedness in this forum, most of us have bug-out-able SSB phone radio equipment (BoR) in mind, something along the lines of a compact 100 watt radio, a low-strung wire dipole, and maybe some kind of Buddipole or screwdriver antennas.  But we know that so much more is possible!  (or at least I heard it on the internet...)  OK then, let’s compare coverage areas for this BoR equipment and for a relatively modest 3-element beam atop a 50 foot tower at 1000 watts for last wednesday’s 20 meter scenario:

dinky packable dipole:


modest base beam:


These semi-portable antennas plus barefoot rigs do make the situation slightly challenging.  We’re not completely attempting the impossible to try and have inter-regional comms, but we are having to work within some heavy constraints.

I don’t want to load down this post with too many modeling results, and I didn’t want things to get out of hand, so I stuck with 20, 40, & 80 meters.  There are other bands, true, but... really, these are the three big ones.  Power was set to 100 watts (the model automatically knocks it down to 80), and antennas are rigged at 10 meters height.  For our current mid-winter conditions here are the results for 40 & 80 meters (20 was just shown above):

40 meters:


80 meters:


OK, so 40 meters gives good coverage of about half the CONUS in the evenings.  80 meters is similar but not quite as good (and the antennas are much bigger).  20 meters has a huge skip zone, maybe +/-400 miles E-W and +/-200 miles N-S.  So based on this 40 is the way to go for an evening roundtable.

Looking out to spring & summer (fall is similar to spring) on 40 meters we have:

March, 40 meters evening:


June, 40 meters evening:


Spring & fall are still pretty good, but summer gets pinched down by about half.  I’ve moved all of the times up to be about equally far into the evening as our current 01 UTC time.  This puts the terminator over the West Coast, so for spring/fall & summer the respective times to similarly place the terminator are 02 & 04 UTC.  (Yeah, I know, midnight kind of sucks, but I’m trying to do apples-to-apples comparisons here.)


Now what if we want to try a daytime roundtable? (I’ve gotta work weekdays, but maybe on weekends):

40 meters, December noon:


20 meters, December noon:


Clearly, 20 (and above) is the only viable band for daytime inter-regional comms with our power and antenna limitations.  How’s it look in summer?

20 meters, June noon:


Kind of patchy, but it’s about the best we’re gonna get right then.

OK, I could keep going and overload everybody with a bunch of graphics, but I think the main points are made.  I did model out everything – all 3 bands in all 4 seasons, both day & night – but the bottom line is 40 meters is best for the evening and 20 is best for daytime.

Two more questions I had were about the antennas, and about the yearly variation.  About the only other packable HF antenna other than than a flattop dipole is a dipole strung as an inverted V, or maybe some kind of quarter-wave vertical.  Maybe a screwdriver antenna...  Using voacap.com’s quarter-wave vertical whip option gave a slightly reduced coverage area, but it wasn’t a huge difference.  Another consideration is that we’re right on top of solar max right now.  How’s that going to change things in a year?  Modeling this and next december’s scenarios showed that propagation will be a little weaker, but it’s just noticeable.  None of this is worth bulking out this post with any more plots.

OK, to sum it up: For inter-regional roundtables using packable radios & antennas, 40 meters evening and 20 meters day contacts are more likely.  Things are pretty good now, will get worse near mid-summer, will improve again each winter.  As we move off of solar max, things will get slightly worse overall.

A LOT of assumptions have gone into this modeling, and these are for broadly averaged seasonal conditions.  I modeled everything centered on my QTH, but if you're in the CONUS you can move the coverage patch around in your mind's eye and be close enough.  For the most part, 100 watts into a flattop dipole at 10 meters was assumed.  Probably nobody runs exactly that combo, but I had to bound the problem somehow.  Most importantly, day-to-day solar flare-ups are not included, only broad monthly trends.  Actual results WILL vary!!!

I didn’t include CW or any digital modes here.  That’s another topic for another day.

At least now we have some rules-of-thumb as to what to try and what to expect.
« Last Edit: December 21, 2013, 08:39:36 AM by Quiet1 »
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Re: Propagation Modeling for Weekly Roundtable
« Reply #1 on: December 21, 2013, 09:59:57 AM »
Q

Great analysis.  I have forgoten about your other post on this subject cuz life got in the way.  Thanks for taking the time to do this.
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spacecase0

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Re: Propagation Modeling for Weekly Roundtable
« Reply #2 on: December 21, 2013, 11:06:28 AM »
that is amazing,
looks like I need to stick to 40M if I want to make it all the way over there.
I like day nets so my batteries will get to recharge before the sun sets,
good to know what I would like to do will not match reality,

and this antenna is likely not the thing to expect to work
http://www.mfjenterprises.com/Product.php?productid=MFJ-1840T
or the 80M version I have as well
http://www.mfjenterprises.com/Product.php?productid=MFJ-1880T

ghrit

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Re: Propagation Modeling for Weekly Roundtable
« Reply #3 on: December 21, 2013, 12:04:12 PM »
Really nice work.  Going from those plots, is it fair to assume that if you can "see" me, I can "see" you?

This is what I have these days for an antenna, right at 20 feet above ground.
http://www.mfjenterprises.com/Product.php?productid=MFJ-1775 

Of course it doesn't match the modeling too well, but based on the experiments over the last three weeks, I can hear (but not pull anything intelligible out in among the other QRM&N.)  Come spring melting, I expect to put up something suitable for 80, but that is in the VERY early planning stages.  Also might make an effort to raise the 1775 another couple feet, as well as do a bit more trimming of the cap whiskers to raise the actual resonance a bit.

In the meantime, I'll be on board for another try on Jan 8.  I could be persuaded to try 20M sometime in the afternoon as well.  (I'm retired, so I have less time restrictions to work around than you probably have.)
« Last Edit: December 21, 2013, 12:06:28 PM by ghrit »
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Quiet1

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Re: Propagation Modeling for Weekly Roundtable
« Reply #4 on: December 21, 2013, 01:21:58 PM »
Really nice work.  Going from those plots, is it fair to assume that if you can "see" me, I can "see" you?
Thanks.  And you're exactly right, if I can see you, you can see me.

BTW, I'm running one of these 40/80 trap antennas: http://www.mfjenterprises.com/Product.php?productid=MFJ-17758  I've got it rigged as a low inverted V (~20' at the top, ~130 degrees), mostly to do NVIS.  It can claw its way to the Carolinas, central Texas, and central FL, but that's about it.

MFJ also makes the same thing in 20/40 meters: http://www.mfjenterprises.com/Product.php?productid=MFJ-17754
At only 42', it wouldn't be hard to hang as an inverted V.  I'm seriously thinking about getting one of those for a go-box antenna.  The lower band is always seriously compromised in these trap antennas, with the bandwidth squeezed down to about 0.1 MHz, but man would that thing be handy.
« Last Edit: December 21, 2013, 01:33:01 PM by Quiet1 »
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ghrit

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Re: Propagation Modeling for Weekly Roundtable
« Reply #5 on: December 21, 2013, 01:44:40 PM »

MFJ also makes the same thing in 20/40 meters: http://www.mfjenterprises.com/Product.php?productid=MFJ-17754
At only 42', it wouldn't be hard to hang as an inverted V.  I'm seriously thinking about getting one of those for a go-box antenna.  The lower band is always seriously compromised in these trap antennas, with the bandwidth squeezed down to about 0.1 MHz, but man would that thing be handy.
0.1 is pretty narrow.  For sure, previous tuning to an agreed freq would be necessary.

Rather than a trap dipole, I wonder if a two or three band fan dipole wouldn't be a better bet, especially if on the move in forested areas.  (Not so easy, I suppose, in the deserts ---)  Might be less tuning troubles with a fan.  Dunno.
« Last Edit: December 21, 2013, 08:21:26 PM by ghrit »
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spacecase0

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Re: Propagation Modeling for Weekly Roundtable
« Reply #6 on: December 21, 2013, 05:09:06 PM »
here is my favorite go box antenna,
but you have to be in a windy area,
HF kite antenna
my friend got all the numbers just a bit off by the way (wire gauge number, and how long everything is).

Quiet1

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Re: Propagation Modeling for Weekly Roundtable
« Reply #7 on: December 21, 2013, 06:05:21 PM »
0.1 is pretty narrow.  For sure, previous tuning to an agreed freq would be necessary.
It is, but the 40 meter General phone band is only 0.125 MHz wide.  Get the antenna well-tuned and centered on the band, and you don't miss much.  It's what I'm doing now on 80 meters, and my FT-450D's internal tuner can clean up the band ends enough where I can operate over the entire 0.2 MHz – just barely.

But whatever happens on this front, it's happening in the new year!
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Re: Propagation Modeling for Weekly Roundtable
« Reply #8 on: December 28, 2013, 12:28:17 PM »
I just raised my antenna about 20 feet. It will be interesting what the improvements will be.
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Quiet1

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Re: Propagation Modeling for Weekly Roundtable
« Reply #9 on: January 21, 2014, 06:52:22 PM »
For what it's worth, here're the voacap predictions for tomorrow's roundtable (in fact, for the entire month).  I dialed back on the power to match the just-makes-it propagation I'm getting to Tempstar's qth.  Had to use 10 watts for the model setting, though I'm (along with most of us) running 100 watts; it's just a normalization thing.  The results are only a little more pessimistic than the 100 watt predictions posted earlier.

Remember, these predictions are for average monthly conditions, so if the sun does something whacky tomorrow all bets are off.  Also remember, this is centered on my qth but you can slide the coverage around in your mind's eye and get the general idea.


80 meters:


40 meters:


20 meters:
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