Sunday, June 7, 2020

A Ham Radio Series 3 - HF Propagation Practicalities

As I start writing this, I’m going back and forth between typing and working across the southern tier of the US by Sporadic E skip on 6 meters.  East Central Florida to eastern California and Arizona.  While not impossible this time of year, it’s also not to be dismissed as trivially common. 

Of course, I don’t know more than a few of you well enough to know your interests in radio, but let’s resume where we were last time. 

The practical question comes down to this: how do you know that the band you’ve set your radio to is going to go to the F2 layer and come back down far away?  It's common for the ionosphere to allow, for example, 21 MHz signals to propagate over a path, but not 24 MHz, and certainly not 28 or 29.  Which frequency escapes into space and which refracts back down depends how densely ionized the ionosphere is where the signal hits it.
The density of the ionization depends on how much high energy radiation is coming from the sun and that varies with the (roughly) 11 year long solar cycles.  Around World War II, a relationship between propagation and observable radio emissions from the sun was realized and the world standardized on monitoring solar radio emissions, called the Solar Flux, at a wavelength of 10.7 cm, or 2.800 GHz.  That’s just above the 2.4 GHz WiFi band.  Note to people who think being exposed to microwave or WiFi frequencies is new to humanity; 2.8 GHz radio has been coming along with sunlight for as long as we’ve been on the planet! 

The 10.7cm Solar Flux Index (SFI), is one of the longest running records of solar activity but only goes back to 1946, while sunspot observations go back hundreds of years.  The measured solar flux correlates well with the sunspot number as well as a number of UltraViolet (UV) and visible solar irradiance records.

The strongest solar cycle on record occurred in the 1950s.  Cycle 19, as it’s called, is still remembered fondly as the source of 24 hour openings on 10meters and openings on 6 meters.  Observations of sunspot numbers exist back to the time of Galileo, and those cycles show that the 20th century was remarkably active.  We appear to be settling back to more typical amounts of activity.  The consensus of groups trying to predict the next cycle, 25, are saying it’s going to be much like the past cycle, which was said to be the weakest in over a hundred years.  That’s going to put a premium on operating with denser crowds of signals in the bands, receivers that handle big (nearby) signals well, and better ways to extract hard to copy signals.

(Source) Two things about this plot; first, note the highest peak, that's 19 in the late 50s/early 60s; second, note it doesn't include the cycle just completing; that one is very similar to the peak around 1910. 

You might say, “that's cool but it still doesn’t answer how you can tell if your signal will be propagated to that location you want to contact.”  A simple answer is you can listen.  If you’re trying to hear a country with a large ham population, if you can hear them, chances are someone can hear you.  But what if you want to contact a country with a small population?  What if everyone is listening? 

There are propagation prediction programs widely available to the amateur.  Some, like VOACAP (Voice of America Coverage Analysis Program) are the result of big government studies and expenditures.  Others are the result of different analyses and there are many websites and programs available.  As mentioned in the preface, the ARRL produces graphics like this one, which shows predicted propagation from the East Coast of the US to the areas described and a graph with a few colored lines. This plot is for last month, May, 2020 and assumes an average Solar Flux of 66 which is Quiet Sun or no solar enhancement. 

The different colors represent the frequencies expected to be open to those locations to different probability levels.  The red curve is always highest; it's the Highest Possible Frequency that will be available on 10% of the days in the month (that's 3 days out of the month; which 3?  good question).  The green curve represents the 50% of days curve or that half the time, the best frequency will be that high.  That used to be called the Maximum Usable Frequency or MUF.  Finally, the light blue curve represents the Lowest Usable Frequency for a 1500 Watt transmitter using CW (Morse code).  For lower powers or other modes, that frequency may be higher; between the shown light blue curve and the green curve.  The horizontal lines seem to represent amateur HF bands.

The "East Coast" is a big area and it would help to know where this data is centered, but assume for the moment it applies to us in Florida.  Let me pick the East Coast to Australia chart.  0000 hours UTC is on the left, which is 8PM EDT.   Three days of the month, the 12 meter (24.9 MHz) band should be open to Australia, but 50% of the time, the 15 meter (21 MHz) band will be open.  By 0400 UTC, or midnight local that has closed down to 20 meters (14 MHz) and it's going away fast.  From then until dawn (around 1000 UTC), the most reliable band looks to be 30 meters, even going down to 40m.  After sunrise, the curves show the opening returning to 20 meters, and finally to 15 then possibly 12m again from 2000 to midnight UTC, or 5 to 8PM EDT. 

Should you go for the 50% chance, green curve or try for those 3 days a month that it goes to the red?  The advantage of going to the Highest Possible Frequency is that absorption in the ionosphere is lower closer to it, so a weak signal might do better.  The drawback is the other 90% of the month you sit around listening to nothing.

One of the issues of charts like these is that they don't address any of the real world effects; a guy with an 80 foot tall tower and high gain antennas experiences this differently than someone operating portable with backpack antenna and QRP power.  The good propagation programs attempt to use that sort of information to make predictions more accurate and reliable.

No matter what the propagation forecasts or predictions say, sometimes you've just got to listen.  There is no substitute for listening.  Even if you're not listening, but a piece of software is. 


  1. And most bands have beacons scattered around the world that you can use to see if there's any propagation between you and the beacon.

    Sounds like you lucked into another really good opening on 6!

    I only have one radio that covers 6 Meters, and I suppose I should throw up a vertical or dipole Real Soon Now because I'm planning on selling that radio.

    Cycle 19 happened about 3 years before I got interested in radio, and 6 years before I was first licensed. I still remember how vividly the OT's talked about it. Chicago to Japan with a Heathkit "Sixer" and a vertical or a dipole! S9+30 signals on 20 Meters, from all over the world, 24/7!

    Must have been a hoot to have been a Ham then.....

    1. I became an SWL around 1967 and people were talking about 19 and how the current cycle wasn't as good.

      If you're going to put up a 6m dipole, do it this week. This weekend is the ARRL June VHF contest. Signal report and grid square are the report. I've had better results in that contest than any other VHF/UHF contest; I've worked all of the lower 48 on 6m in that contest, though not in one year.

  2. You did it, SiG! Just soldered together a 6M dipole out in the garage. I can get the center about 25' up, and the ends will be about 15' above the ground.

    I'll string it up tomorrow and get the analyzer out to prune it.

    Thanks for the kick in the rear!

  3. Excellent stuff, thank you. Is the black art of Antenna coming up in the series? I'm fiddling about with a Nooelec SmartXTR SDR dongle (Elonics E4000 chipset), an Alinco DJ-X11 receiver and a nifty Albrecht AE6110 "CB" (one solder dab and it goes 25-30MHz)...and living in a 7th floor apartment in a non-permissive area Maidenhead JN59sw. The 6m magic band is restricted to registered static shacks only. I'm working on Ham license, but something global happened recently to delay that; so olny listening and CB for now.. I've cut a dipole for CB that I can also use as a sloping v with plugin resistors to earth, I've got materials for longwire, jungle, L, and a discone...ideally I'd like a magloop, adcock and yagi too. Any suggestions? Next on the shopping list is a digital/twinband like Yaesu FT2, then ideally a FT817 or FT897. Hope the big yellow thing picks up again. All the best for you and yours, may The Lord be with you.

    1. Thanks for the kind words. Yes, there is going to be some antenna coverage in the next few weeks, but this is a topic with dozens of books written about it and I can't cover much more than a few basics.

      When you say you're in a 7th floor apartment, do you have a balcony or porch or someplace outside to put an antenna? Even if you put it on the porch while operating and then bring it back inside. Do you have to point it through a window? Porches or something outside are better in the directions that are open, but you can get contacts through a window. If to point in a desired direction you have to point through the building, walls and other rooms, you've got a tough situation on your hands!

      I had the first version FT-817 back in the early '00s; got it in '02 and had it for maybe 5 years. Great little rig, but QRP, after all. If you can swing the FT-897 over the lower power 817, I'd recommend that.

      More so than the antenna, I'm concerned about a ground where you are. Do you have an automatic antenna tuner? So that if you want to use a CB dipole on 14 MHz (or 18 or...) you press buttons and let it transform the antenna to the load the transmitter wants? Something like an LDG AT-100PROII Auto Tuner? (not recommending it, but I have (and had) others in their products line and they all worked well.

    2. Hello again!

      Yes, I have a balcony, it's oriented east. It is all reinforced concrete so I'm worried about coupling as well as stray currents from the other 95 apartments. I chose a dipole to avoid grounding, but there are central heating pipes I guess I could use and drop the longwire straight down. Hmmm, must get some white wire.

      Hopefully I can keep everything stealthy. The dipole frames the roof and sidewalls and no complaints yet...maybe need some plants and one of those cat safety nets for camo.

      No autotuner, though I have a SWR/Power meter that goes from 1.8 to 528 MHz, and a just arrived NanoVna that I've yet to explore that covers 50kHz to 900 MHz. Also a lovely Hameg MH203-7 oscilloscope that's also in the queue for work on a dead channel, though I've no idea if that's useful for antenna work. Too much to do, never enough time! Thanks for the reply!

    3. The thing about ground is unless it's closer than 1/20 wavelength away it isn't ground and it can behave very oddly. That means if you're on 10 meters, a ground lead can't be longer than a half meter. Don't worry - most hams don't have that on 10 meters and nobody has it on 2 meters or higher.

      You probably have either wire mesh or steel bars (rebar) in the concrete which will behave like a ground. Almost like a metal screen. Which can help minimize your station causing a problem for people in adjacent apartments in any direction.

      As for antenna tuning, you don't need one if you operate a resonant antenna (dipole, Yagi, cubical quad) but if you put up one wire for all bands you need one. The NanoVNA will tell you what the impedance of the antenna is; the tuner will transform that impedance to the 50 ohms the radio wants. The Hameg is a good for general troubleshooting, but I've never tried to come up with ways to use one on an antenna.

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