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[KD9FRQ]

I took part in an exercise last weekend and they relied only on 80m for In State comms unless it was within the local county.

I asked why we don't try 6, 10 or 20m for at least reaching outside our ARES District to the next district as well as jumping over a district should they be "off line" so to speak.

I was told that the wave length is too long and 20 m just hops over to about 300 miles.

I do not understand the difference between 80m and 20m in this case.  The 80m dipole was only 20ft off the ground and broadside the main traffic.  A 20m in the same position should do the same ground wave, correct?

Thank you in advance for your reply.

Ed, KD9FRQ

[K1VSK]

In simple terms, the “ground wave” on 75 meters is effectively longer than that on 20. Twenty meter signals bounce off the ionosphere skipping over long distances.

The subject is something about which books are written and too diverse to describe succinctly here in just a few sentences.

[KD9FRQ]

OK.  That is not how ground wave was explained to me..........so this addes another missing piece tpot he puzzle.

[WB6BYU]

No, this isn't "ground wave" when using a low dipole.

The answer has to do with how radio waves are reflected (or refracted) by the ionosphere.
At any moment there is a factor called the "critical frequency":  below that frequency
signals will reflect straight back down to Earth, allow Near Vertical Incident Skywave ("NVIS")
propagation within a few hundred miles.   Above that frequency the ionosphere doesn't
reflect the waves straight back down, so you can't cover shorter distances, but it may reflect
them at an angle allowing coverage of longer distances.

The "common wisdom" is to use 40m during the day and 80m at night, but here at 45 degrees
North we are using 80m during the day and 160m at night, even during the summer, because
those are the only bands that support local contacts well in the current ionospheric conditions.

For example, here is a current map of the Critical Frequency from this website.
You can see it varies with latitude, season, time of day, etc.



At the moment, the Critical Frequency is between 5 and 6 MHz for most of the continental US,
so 80m (or perhaps 60m) would be the most likely band for local contacts.  But looking at the
red area of the chart (and visualizing it rotation around the world) there are even going to be
times at the higher latitudes when 160m may not be open.

One of my other favorite charts from that site is the Local Area Mobile Prediction (LAMP), which
shows the best bands to use for distances out to about 600 miles from your location for each
hour of of the day.  It looks like this:




Under current conditions, for distances less than 200 miles from my location, I need to
use 80m during the day and 160m at night, except that even 160m might not make the
path between 0300Z and 0600Z.


There are some conditions where the higher HF bands might get though, depending on
the size of your ARES districts, but those aren't as likely when using low dipoles that
are relying on a high angle of radiation.

[KD9FRQ]

Thank you for that explanation.

I now have a better understanding.

[N9FB]

One of my other favorite charts from that site is the Local Area Mobile Prediction (LAMP), which
shows the best bands to use for distances out to about 600 miles from your location for each
hour of of the day.  It looks like this:

thank you for the helpful information and links. 
could you go say something about the challenge of working nearby states (short distances of about 100 to 400 miles) on 14 MHz and up?
i am trying to get WAS on 20m (and maybe 15m and 10m too down the road) and not surprisingly my adjacent states are proving to be the most difficult.  Will these rely on NVIS or can other factors come into play?  Also, is there a most likely time of day when such openings occur? 

Also, if you have the time -- i would love to hear you talk about ground waves.  What distances are possible and is it true that ground wave at lower frequencies can travel farther than w/ higher frequencies?  If that is true, is it due to the longer wavelength? What is the maximum possible groundwave on a band like 80m vs 20m and does day/night have any influence there?

[KM4DYX]

Short skip 20 M and higher propagation can be a problem given the info above about the critical freq. Nevertheless, I recently worked a SOTA operator 250 miles away on 20 M, CW, which must have been due to sporadic E-layer prop.

Ground wave is not impacted by day or night. Longer wavelengths do travel farther. Vertical antennas are required to achieve maximum range. I've managed ground wave, or "surface wave" which follows the curve of the earth, on 80 M out to around 50 miles in favorable terrain.

73,
Al

[WB6BYU]

...could you go say something about the challenge of working nearby states (short distances of about 100 to 400 miles) on 14 MHz and up?

The simplest thing to say about it is, "It's challenging."

Operating from Alaska or Hawaii probably makes it easier, since you don't have to worry about
short distance contacts.

But there are some modes that will cover those distances. 

VOACAP forecasts give you the option to include Sporatic E ("Es") propagation
that can cover shorter distances, though it is more difficult to predict.

Another option is "backscatter", where, for example, you point your antenna at 90 degrees
to the desired path and bounce RF off ocean waves and back to the target area - that
typically requires high power, but can work over some paths on some occasions.

And the "problem distances" will get shorter as we get more sunspots, because the F layer
will support higher angles of radiation.  But that still might not be enough.


Ground wave range does vary with frequency:  at VLF it can cover the whole world.
My 1977 ARRL Antenna Book has a convenient chart of "typical hf ground-wave range
as a function of frequency", though doesn't actually describe the conditions under which
it is calculated (I'm assuming for 1 kW AM, given the vintage).  Approximate distances from
the table:

160m:   100 miles
80m:     60 miles
40m:     45 miles
20m:     30 miles
10m:     10 miles

The limiting factor is the high attenuation of the surface wave, so higher power helps a lot,
and more efficient modes such as CW or FT8 can probaby increase those somewhat.  Note
that the distance also depends a lot on the conductivity:  greater ranges are possible over
salt water, and less over poor soil.

While the ground wave range doesn't technically change with time of day, the usability of
it might.  AM broadcast stations, for example, can find their usable coverage area decreased
at night due to the interference between sky waves and ground waves, which causes fading.
This isn't a problem during the day when the sky waves are absorbed by the D layer.  But
for stations beyond ground wave range, the station will be stronger at night.

[N9FB]

WB6BYU -- thank you for the helpful reply -- i find your contributions here on eham extremely informative and well-articulated  

I have worked IL from here in central IN on 10 meters several times, over distances as short as 94 miles and 152 miles away (these are LoTW-confirmed QSOs too).  Since that would appear to be well-beyond ground wave potential -- especially at that frequency -- was that likely possible due to sporadic E (?) ... both QSOs occurred 2019 July 13 shortly after 1500z and both of those stations have pretty impressive towers and beams while i use just 100 watts and a ZK6BKW modified G5RV up about 42 ft as an inverted vee)...

i also worked the station just 93 miles away (NV9L) Dec 8th and 9th in 2018 on 10 meters around 0100 and 0200z
and a station 192 miles away Dec 10, 2017 at 20:48z 

... all of these were using CW *except* another station in IL (WB9Z) I worked SSB that was 94 miles away on 2011 Dec at 1811z (LoTW confirmed)... this is probably an outlandish question, but: how high would an antenna have to be to improve the line of sight to a distance of 93 miles?   

i imagine another possibility for 20,15 and 10m contacts with stations and states in the 50 to 300 mile range could be long path(?) 
is it also the case that unpredictable propagation influences can come into play, and on rare occasions make the unexpected possible?

[WB6BYU]

Yes, there are other possible modes, and often you just take advantage of the
conditions you find without necessarily understanding exactly how they happen.

E layer propagation is more common on 10m than on 20m, and up there you can
get more into what are generally thought of as VHF propagation modes such as
troposcatter (and possibly meteor scatter or moonbounce).  You can get various
combinations of modes as well.

Long path certainly could be an option under the right conditions on 20m.


I have to admit, however, that I haven't done a lot of research on the topic:  having
lived on the Left Coast all my life, I can make a 1000 mile contact in an adjoining State.

[N9FB]

I have to admit, however, that I haven't done a lot of research on the topic:  having
lived on the Left Coast all my life, I can make a 1000 mile contact in an adjoining State.

   QSL, in terms of 5BWAS i guess should just be happy not to live somewhere like Portland Maine where working the nearby states of VT, MA, RI and CT might seem almost impossible on 20, 15 and 10 meters.  On 20m the only neighbor states I have left to confirm are IL, MI and OH -- and with patience and more BIC (butt-in-chair) time it should be very doable even with my 100 watts and wire antenna ... 

[RFRY]

... I would love to hear you talk about ground waves.  What distances are possible and is it true that ground wave at lower frequencies can travel farther than w/ higher frequencies?  If that is true, is it due to the longer wavelength? ...

Groundwave field intensity at a given distance from the radiator is dependent on...

• Wave Polarization
• Conductivity of the Ground Plane
• Frequency, and
• Radiated Power

Other things equal, higher frequencies and lower conductivity values result in progressively greater attenuation along the propagation path of a groundwave.

Below is an illustration and comparison of this for vertically-polarized waves along a radial path segment from about 5 miles to about 35 miles from the radiator, for the conditions shown in the graphic.

[W1VT]

This year the Geminids meteor shower coincides with the ARRL 10 meter contest.  This can be an excellent opportunity to work those close in states on 10 meters.  In the 2014 10 meter contest I worked all 50 states and the District of Columbia from Connecticut .