“You are the lead station operator in the Resistance receiving station
mentioned in the first question. You have received the message sent by
the operator in the capital at 1 pm local time in the first example on
160 meters and must now forward the vital information received to
Resistance HQ via HF radio. Once you transmit this message, you will
immediately relocate to another predetermined location you have selected.Assume the following:
-your station is located at approximately 65 degrees West and 10 degrees
North;
-Resistance HQ is located somewhere in the Intermountain Western united
States, New Mexico to Montana, Eastern Oregon to the Western Dakotas;
-Resistance HQ has receive capability 24/7/365 and will be waiting to
copy your message during whatever window you have told them to listen on
whatever frequency segment(s) you have specified;
-The message from the capital of Venezuela consists of 25 each 5 letter
encrypted groups. You will re-encrypt the message prior to
retransmission using a OTP, but there will still be a minimum of 125
random letters to transmit;
-You are required to use any of the ITU region 1 authorized amateur
radio frequencies and modes from 1.8 to 29.7 mHz;
-You will have been onsite for at least a week prior to receiving the
message from the capital of Venezuela, and will have access to a small
house nearby the station site, but are forbidden to set up equipment at
the house;
-You are required to complete the transmission to HQ in less than 20
seconds, and to evacuate the transmit site in less than 15 minutes after
completing the transmission leaving no material behind. You have 4
dedicated helpers with no electronics or radio training available;
-You have a compact 4wd crew cab pickup truck for transport, and
everything, your crew included, must fit into the truck. No radio
equipment may be visible from outside the vehicle;
-Assume the ground is level farmland with very rich loamy soil planted
in low-growing crops or grass, with tall trees (>50′ high) at the field
boundaries with steel t-post electric fences around each field, and that
the field lines run north-south/east-west. Further assume that each
field section is 8 hectares in area square. The surrounding general area
is agricultural, both crops and stock.”Questions-
What frequency segment and time will you select to minimize DF
likelihood and maximize the chance that HQ will acknowledge it? What
will your alternate(s) frequencies be, and under what circumstance will
you use them?What antenna(s) systems will you use for transmitting this message?
How high will they be placed? How will you orient and erect them and
take them down to minimize possibility of observation? Explain in
detail, including specifics of antenna and transmission line.What mode will you use for transmitting the message? If digital,
which specific mode and why?Before you leave for Venezuela, you will be given an opportunity to
study data available through NOAA on radio propagation. Which ionosonde
stations will you study, and why?What will your cover story be if you are stopped by Venezuelan security
forces?
Thoughts?
What frequency segment and time will you select to minimize DF
likelihood and maximize the chance that HQ will acknowledge it? 10 meters low-below 28.150-28.300 Daylight probably noon.
What
will your alternate(s) frequencies be, and under what circumstance will
you use them? 15 meters 21.0–21.200. During a contest to blend in with other digital traffic.
What antenna(s) systems will you use for transmitting this message? String a low wire dipole the same height as the fence top wire (fence top wire used as a director/reflector. Use a corner of a field pointing se to nw generally and have the dipole about a wavelength from the fence at the same corner angle to produce directive pattern.
How high will they be placed? fence top wire height assume to be 5-6 ‘.
How will you orient and erect them and take them down to minimize possibility of observation? Reflector would be the fence the wire dipole can be supported easily with ropes from the fence and assisted with branches from trees if needed.
Explain in detail, including specifics of antenna and transmission line. Antenna could be small diameter wire 14 gauge. the cobra head could be any non conductive material like a piece of wood. short section of window line or even cable coax could be utilized.
What mode will you use for transmitting the message? If digital,
which specific mode and why? Contestia or dominoEX can be used but Oliva 1000/32 may be best shot due to current poor conditions.
Which ionosonde stations will you study, and why? Belize. Lies in nearly the direct path of the receiving station from the transmit area.
What will your cover story be if you are stopped by Venezuelan security forces? Working with local farmers and my team is with the UN food and agriculture organization. Paint the truck all white as well.
What is the MUF between Venezuela and the western US today?
By the time I saw your reply it was 2130 Central Time and maximum usable frequency was approximately 3 megahertz
I chose 10 and 12 meters because it would be very difficult being inside the skip Zone locally to DF my position. Or so I would hope
Being DFed is certainly something to avoid, but that is not the first problem to solve. The first problem to solve is to pick a frequency that is most likely to get from A to B.
The old rule of thumb, which is still valid, for long haul HF communication is that you want to use the highest frequency you can ( to maximize signal strength- shorter wavelength have less loss) that will be reliably propagated over the path you have selected.
You have to first take a look at what the ionosphere is doing where your signal will bounce off it, and to do that you need to get an idea of where those reflections will occur. 3000 km is about what the average maximum hop is on HF, give or take. How many hops will it take to get from Venezuela to, say, Colorado?
Where will these hops happen?
Anyone?
Keypounder
2 hops (complete up, down, up down).
Short path propagation would put it over the Caribbean over and between Jamaica and the DR/Haiti, (down) somewhere between New Orleans and Havana Cuba in the gulf of Mexico, Over southern Oklahoma and into the area of Colorado.
Excellent. Correct answer, sir!
So what is the maximum MUF in the Carribean and over OK during any 24 hour period with the present low sunspot cycle?
So what is the maximum MUF in the Carribean and over OK during any 24 hour period with the present low sunspot cycle?
between 29 and 31 mhz if i can read the contour lines with these bad eyes
Anon-
Not sure where you got your information, but the models I look at show an MUF around 18 mHz or a bit higher. Curious what model you are using?
The old rule of thumb is that the MUF for a 3000 km hop is 3 times the FoF2. This is NOT highly accurate; the MUF is affected by a number other factors not measured by the FoF2, and is known to vary from that value, above and below. If you are iinterested there are a number of technical papers available online on this topic. Search ” FoF2 MUF correlation” and you’ll have many hours of technogeek pleasure reading ahead.
It is possible that 10 meters may be open between Idaho and Venzuela right now, but it is not likely. The FoF2 for Puerto Rico, a good substitute for the western Carribean, has been running around 7 mHz, give or take, during the afternoon. Austin Texas, a reasonable proxy for Oklahoma, is a bit lower, around 6 to 7 mHz, peak. While a 21 mhz signal may make it to Oklahoma, if the MUF isn’t above 21 mHz there, your signal will go sailing into space, never to return.
As stated above, I would pick the 20 meter ham band as a primary, and I’d make my transmission around 3 or 4 pm local time, to balance the drop in MUF over the Carribean, where I have some margin, with the need to time things so that my MUF over Oklahoma was as high as possible. My secondary frequencies would probably be 10.1 mHz if 20 meters wasn’t good from lack of solar flux, and 18.068 mHz if there was geomagnetic storming from excess solar wind ( coronal hole or the like).
As stated below, a long Vee beam fed with ladder line will handle any ofmthese frequencies with ease.
Keypounder.
Once you know how many hops, and where the reflection points are, then you can figure use the data available to you to determine what a reasonable minimum MUF is for that specific path. These days, computers can give you the answer almost before you ask the question, but it is worth the time to work through these questions by hand so that you understand the methodology used to produce the computer prediction.
The first thing you ought to do is to print a copy of an azimuthal map projection centered at 65d West and 10d North. For those who don’t know what that is, here is a link to the Wiki page on the subject-
https://en.wikipedia.org/wiki/Azimuthal_equidistant_projection.
More information here- http://www.wm7d.net/az_proj/az_html/azproj.shtml
There are a number of online sources for azimuthal maps, but one I often use is here- http://ns6t.net/azimuth/azimuth.html Go and check it out, and set the origin to 65W, 10N with a radius of 7000 km. Print the map (probably a good idea to print several copies.) Draw a line from the center of the map to the middle of the zone you were given to get your direction, and also figure out what the required beamwidth ought to be for your antenna while you are at it. You could print one copy at 6000 km radius and that would give you a good idea of how many hops you’d need and where they’d take place.
C’mon, guys, work with me here; I promise I would not hand you an impossible problem, nor will I let you dangle too long. Also, I’m not going to beat up on anybody who’s trying; this isn’t about me showing how much I know, it’s about me trying to help you guys learn what I know.
So, you’ve got your map. How many hops and where do they happen?
i guess the struggle is real
“That which is obtained too lightly is esteemed too cheaply”
Keypounder
Looks like around 5369 km, one hop around the gulf of Mexico. That’s using my Google foo.
I think 17m would be the highest frequency too if I was using the muf app correctly.
Really enjoy reading this blog and am trying to learn as I go, thank you.
Gary, welcome, and Thanks for participating. I’m still picking up nuggets here myself; NC Scout has posted a lot of good stuff here.
Actually, it is probably two hops, as the net path is 6000 km, give or take, and the average HF hop is about 3000 km. See anon above, who got it exactly right.
The FoF2 for OK is likely to be the limiter as it is farther from the solar path and the closest ionosonde is probably TX. I’d use Puerto Rico as a proxy for the Carribean. The rule of thumb for 3000 km hops is that the MUF is usually about 3 x the FoF2, so depending on the time of day, you might get 17 (18 mhz) and you might not. If it is open, the advantage is that it usually is less crowded, except on contest weekends when folks take refuge there. The disadvantage is that it is less crowded there and a lot easier to pick out one signal. This can be good or bad. You pays your money and takes your chances.
20 meters is almost certainly going to be open during the day, and it is slightly lower in frequency, so shorter hops ( which have a steeper angle of incidence, and a lower MUF) would be supported as well.
Good job so far, guys, keep working the problem…….
Keypounder
what sort of antenna is-
quick and easy to put up and take down;
allows easy use of multiple bands;
highly directional with high front to back (and sides) ratios;
low visibility;
uses easy to find components that are NOT readily identifiable as being radio related;
I’ll give you guys two, no three, hints-
First, it is NOT a Yagi-Uda or a dipole;
Second, NC Scout has already shown you guys a variant of this antenna;
Third, logically, what takes the least time to erect and get back down, an antenna with multiple lifting points or an antenna with only one?
Get your antenna books out and do some browsing, fellows…..Resistance HQ is desperately waiting for you to solve the problem…..
Keypounder
A vertical half rhombus antenna
Gary-
You’re getting warm- my pick is in the same family.
Again, NC Scout has SHOWED this to you guys before. I remembered seeing it. He wrote about this specific antenna used during his time in the sandbox.
Simple, quick to erect, broadbanded; what is it? If nobody has correctly picked it, I’ll post my pick on Saturday.
End fed
Rhombic is what I meant not rhombus.
Or a sloping vee with terminating resistors. Never made one before but I’ve read each leg should be at least 1 wave length or more. Depending on chosen frequency that’s still going to be a large antenna. Good thing there is help, with practice a team could set up and break down quickly.
If the right terrain is available couldn’t you use a slope facing NWish to mask any signal from going behind you also? Assuming the antenna is at the correct angle to maximize the take off angle.
DING DING DING! We have a winner!
Yes, I would choose a sloping Vee beam antenna with terminating resistors. If you make it more narrow than the classic formula you will get a very nice compact highly directional main lobe with well over 20 db f/b. If you use terminating resistors, the main lobe gain goes down, but you lose the bidirectional pattern, a distinct advantage in avoiding DF efforts.
The longer the wires the more directional it will be, and you can change the half power beamwidth by squeezing or spreading the ends.
Another possible answer would be a long wire with a resistor, but that has more lobes.
I will send NC Scout a copy of the EZNEC azimuthal diagram for a sloping Vee beam and a long wire this weekend.
So what material would you use?
There is a hint in the question statement, and also in my comment on the Beverage article at Western Rifle shooters……
Wire that matches the electric fence, insulators like used on the fence as well. A tree along the boarder to support the antenna with tposts on the wire ends. I guess homemade resistors from dead AA batteries. ( never tried it, need to)
Gary- GMTA. good job!
I have never made a resistor from a dead battery yet, but I’m going to try it shortly, maybe today if I get my other chores done in time. I’d simply bring along a couple of 50 watt 600 ohm resistors, and chuck them in a ditch or a creek when I exfiltrated. If you are going to smuggle in a radio, adding a couple of resistors isn’t a problem.
And yes, I’d use aluminum electric fence wire for the two legs of the antenna. Cut length ahead of time and wound on a couple of reels, I could easily drive the T post grounds, unroll the wire, attach it to the top end insulator with the feedline, which can be more AL wire with separators to hold it about 6″ to 12″ apart (cut up fiberglass stakes for electric fence and electrical tape) or 14 gage landscaping wire.
So we know the frequencies and times we’re going to use (BTW, the VOACAP link NC Scout posted is a great tool to use for that, and it pretty well confirms what I had thought, that under normal conditions as they exist now, 14 and 18 mHz are your go-to bands for daytime long haul propagation. More on that shortly….) and we know what sort of antenna we’re going to use. Next step? Mode.
These problems (and the conversations about the solutions) are like drinking from a fire hose.
I love it, but it’s hard to take it all in.
Nice that I can keep comeing back to them as I start to understand one aspect.
Thanks NC Scout
And to you Keypounder.
It’s like eating an elephant, take one bite at a time, and chew thoroughly!
In any comm problem, ask yourself what it is you have to do. First question, always.
What information needs to go from where to where, how quickly? what are the paramount requirements- Speed? Security? Reliability? Conservation of comm assets? Other factors?
Then ask yourself what suits that task best given the assets you have available. Sometimes the answer isn’t even radio, witness the posts NC Scout has made on field phones. Sometimes the answer isn’t even electronic; Whistles, bugles, semaphore, heliograph, hand signals or hand carried messages all have their place. Do you know what portable communication system broke the Apaches in the late 19th and early 20th century?
Keypounder
The way to solve this problem is first to determine the approximate path and direction, via azimuthal map.
Then determine the MUF along that path, using whatever resources you have available, and select your operating frequency or frequencies and time of operation.
Then select an antenna and feedline that supports both the operational requirements and the frequencies chosen.
We’ve done all those steps.
Now the question is what mode to use?
Is sideband an option?
CW?
I think SSB voice has been ruled out. I think CW would work well if done quickly and accurately. As far as digital I don’t know enough to make a good suggestion.
CW would have to be sent at 75 wpm to meet the required transmit time. Digital is in order.
Here are some sites that may advance your education on what various digital modes look and sound like:
http://www.kb9ukd.com/digital/
http://www.sigidwiki.com/wiki/Category:HF
http://www.w1hkj.com/FldigiHelp-3.21/Modes/
W1HJK’s site shows you what the different modes look like and sound like. More importantly, there is a good discussion on the merits of the various error correcting modes in long distance multipath transmissions. Which modes would you consider?
A PSK mode? It looks narrow enough.
What about CW sent digitally sped up to 70 wpm?
so, what are the requirements?
-Needs to be finished in 20 seconds, which means 75 wpm or more;
-Needs to be highly reliable, which means an error correcting digital mode;
-Needs to be a low signal to noise mode, which means too fast is not desireable either;
-Best to be commonly available, although custom modes are feasible;
What modes that meet the above criteria are currently available on FLDigi?
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