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ARRL Propagation Bulletin ARLP018 (2021)

ARLP018 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 18  ARLP018
From Tad Cook, K7RA
Seattle, WA  April 30, 2021
To all radio amateurs 

ARLP018 Propagation de K7RA

Sunspots continue every day after April 11, the last day with no

Average daily sunspot number rose this week from 35.1 to 47.6, and
average daily solar flux also rose from 78 to 79.2.

Geomagnetic indicators were quieter, with average daily planetary A
index declining from 16.4 to 10.7. The most active day was April 25
with a planetary A index of 20.

Predicted solar flux over the next month is 77, 75 and 72 on April
30 to May 2, 70 on May 3-6, 72 on May 7-9, 73 on May 10-11, 74 on
May 12-13, 77 on May 14, 79 on May 15-23, 78 on May 24-27, and then
75 and 73 on May 28-29.

Predicted planetary A index 8 on April 30 through May 1, then 15,
12, 12, and 8 on May 2-5, 5 on May 6-10, then 8, 12, 20 and 30 on
May 11-14, 15 on May 15-16, 12 on May 17, 5 on May 18-19, 15 and 10
on May 20-21, 5 on May 22-28, then 15, 12 and 10 on May 29-31. After
that, things are quiet over the first week of June.

Geomagnetic activity forecast for the period April 30 to May 25,
2021 from F.K. Janda, OK1HH.

"Geomagnetic field will be:
quiet on: May 18-19, 25
quiet to unsettled on: May 5-11, 17, 24
quiet to active on: April 30, May 12, 20-23
unsettled to active: May (1-4,) 16
active to disturbed: May 13-14, (15)
"Solar wind will intensify on: (April 30-May 1), May 3-5, (6-7,
10-11, 16,) 17-18, (21-25).
- Parenthesis means lower probability of activity enhancement.
- Contradictory indications significantly reduce the accuracy
  of the forecast."

N0JK reports:

"On April 26 at 2100z I copied LU2FFD FF97 on 50.313 MHz FT8 at '-11
dB.' Did not complete a contact.

"This was a sporadic-E to TEP path.

"Jon N0JK EM28 Kansas."

Update from W3LPL:

"Propagation is somewhat better, but at times somewhat worse, as
Solar Cycle 25 somewhat fitfully climbs toward solar maximum, likely
in late 2024. Slowly improving propagation interrupted by more
frequent and intense geomagnetic storms are both encouraging
indicators that Solar Cycle 25 is starting to accelerate to solar
maximum likely in late 2024.

"The recent sustained rise in SFI is a positive indicator that Solar
Cycle 25 is finally starting to accelerate and gradually improve
propagation on the higher bands. So far the improvement is minor,
but the recent sustained increased SFI is good news and an indicator
of likely significant improvement during the night on 20 and 30
meters this Summer and generally improved propagation on 30 meters
and above by the October/November contests.

"Adjusted SFI, the solar flux adjusted for the varying distance
between the Earth and the Sun, has been mostly in the high 70s and
low 80s since mid-April. This slow rising trend is likely to
continue and we're likely to see adjusted SFI sustaining in the low
to mid 80s, or even higher, by this Summer.


"The other good news -- yes, good news -- is that coronal mass
ejections (CMEs) are now occurring much more frequently. More
frequent CMEs are a reliable indicator of accelerating solar cycle
25 progress. Fortunately, CMEs significantly degrade propagation
only when two conditions are met simultaneously:
"- the CME is Earth directed, and
- the orientation of the north-south component of the interplanetary
  magnetic field is southwards (-Bz).

"Hopefully by now May QST has arrived in everyone's mailbox. Because
NASA now forecasts solar maximum in 2024, please revise the four
occurrences of '2025' in my article to '2024.'"

WB8VLC reports from Salem, Oregon on April 24:

"Another 10 meter good day.
"Today 10 is open again on both SSB to South America, Chile, and CW
to New Zealand with great signals even though the solar numbers are
nothing to brag about.
"A quick listing of today's openings up to this time and the band
is still open but I'm just listening now.
"2 contacts is fine for me today as I don't like being a DX Piggy
and I'd rather listen and let others have at it while I do some
radio programming on some new 6 meter FM land mobile radios, and
while I'm listening now the band is still going long to ZL on CW and
Chile on SSB.
"Take care and work some dx.
"73 VLC

"2021-04-24 19:51  CE7VPQ 28.455   SSB FE36fu Chile
2021-04-24 20:54  ZL3IO  28.026    CW  RF80lf New Zealand"

Earlier he reported:

"This is a small report what with me only having a couple of hours
of time to get on the radio these days on weekends and weekday real
work work and these are stations that I worked during my breaks.
"April 22 on 10 meter FT8 was interesting with a nice opening to
Indonesia on 28.074.

"Even though it's FT8 the Indonesians were FT8 audible with tones
actually moving my K3S S-meter to around 55 signals.
"April 20th was a great day with activity on 29.6 FM to New Mexico
and Brazil using my cross coupled 10 FM to 220 remote base while at
work during breaks.
"The weekend of April 18 was typical of what I experience every
weekend, SSB and CW to South America with strong sigs for hours.
"My setup is a K3S with homebrew LDMOS amp at ~300 watts on
SSB/CW into a 4el OWA yagi at 30 feet.  For 29.6 FM I use a Motorola
MAXTRAC with 60 watts out that is cross connected to a 220 MHZ
MAXTRAC set up as a remote base system so I can operate 10 FM from
work during lunch time and I use the same 4el OWA yagi as I use on
SSB/CW when operating FM.
"2021-04-22 01:53 YC9FZ   28.074   FT8  OI71PH    Indonesia            
2021-04-22 01:46 YB2MM   28.074    FT8  OI43tc    Indonesia            
2021-04-20 21:48 PY2HP   29.600    FM   GG66      Brazil               
2021-04-20 19:02 WA6BJH  29.600    FM   DM75am    USA             
2021-04-18 19:50 PY2TMV  28.470    SSB  GG67PT    Brazil               
2021-04-18 19:13 ZV5M    28.020    CW   GG54rl    Brazil"

Forecasting Solar Cycle 25:

Some nice images:

Ted Leaf, K6HI sent this story about a flare in a different solar system:

If you would like to make a comment or have a tip for our readers,
please email the author at, .

For more information concerning shortwave radio propagation, see and the ARRL Technical Information
Service at For an
explanation of numbers used in this bulletin, see

An archive of past propagation bulletins is at More good
information and tutorials on propagation are at

Instructions for starting or ending email distribution of ARRL
bulletins are at .

Sunspot numbers for April 22 through 28, 2021 were 42, 29, 62, 57,
54, 47 and 42, with a mean of 47.6. 10.7 cm flux was 83.5, 77.4,
78.5, 78.8, 80.3, 79.4, and 76.8, with a mean of 79.2. Estimated
planetary A indices were 5, 15, 10, 20, 14, 8, and 3, with a mean of
10.7. Middle latitude A index was 4, 15, 10, 16, 12, 8, and 4, with
a mean of 9.9.


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