Resolving the Clandestine Radio Question

Continuing on from this original question from Keypounder, several close answers were posted, and generally the logic was in the right direction. That being said, here’s the correct answer:

“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 Slobovia 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 Slobovia, 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?

>Keypounder sez>  OK.  this is about a 6,000 kilometer short path, which
means about 2 hops.  One will be hitting the ionosphere about 1500 km
away, over the Carribean.  The next will hit the ionosphere somewhere
over the central US.  You could do this easily on either 40 or 80 meters
at night, but 40 and 80 meter antennas are big, and it is hard to get
them high enough off the ground to get good low angle propagation.  For
longer haul comms, we need to be looking at 10 mHz and up.  The higher
the frequency, the easier the contact as long as the band is open.  At
this time of the year and at this stage in the solar cycle, what are the
FoF2 readings over the south central and central USA?

Checking the Austin TX, Boulder CO and Idaho Falls ionosonde data, we
find that the FoF2 around local noon is between 5 and 6 mHz.  Puerto
Rico or Florida will give me a pretty good idea of what can be expected
for the first bounce;  these readings are around 6mHz, too.  The rule of
thumb is that the MUF will be around 3x the FoF2, so the maximum useable
frequency is going to be somewhere around 15 to 18 mhz, barring solar
activity.  For this purpose, we want to use as little power as possible,
which means as high a frequency as possible, but no higher than
propagation will allow.

I would expect that 20 meters (14 mHz)would be open for this path, and
maybe 17 meters, at about 1 pm in Colorado, or about 2000 Zulu;  we
still have good ionization over the Caribean at that time, so my
frequency band choice is 20 meters primary, with a backup of 17 and 15
meters if there is solar activity, in the digital portion of the 20
meter band.  (14.060 to 14.080)

Q>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.

>Keypounder sez>  So, we need a directional antenna that is relatively
narrow in transmit mode, low profile, easy to take down quickly,
unidirectional with reasonable gain and a good front to back/side ratio.
Ideally this would be something that does not look like an antenna at
all.  My choice would be a terminated Vee-beam fed with window line; a
secondary choice would be a long wire. Reasons include:
– easy to fabricate in the field;
– forgiving of layout and construction errors;
– can be made using common electrical fence materials;
– When properly configured, capable of high front to back and side
ratios with reasonable gain.
-Easy to install in the field, and very quick to take down.

The feedpoint would be strung from the tallest tree I could get a rope
into on the south side of one of the plots with the least visibility
from the road or other houses.  An 8 hectare plot is about 20 acres, or
around 880,000 sq ft; this is about 900+ feet on a side, so I could use
up to 900′ legs.

If you look at an azimuthal map centered on the specified location in
Venezuela (see http://ns6t.net/azimuth/code/azimuth.fcgi) you will see
that the ‘intermountain west’ runs from about 305 degrees to about 328
degrees true bearing from 10 d N/65 d W.    This means that your antenna
should not have a 1/2 power beamwidth pattern any tighter than 23
degrees. Realistically, 30 or 35 degrees 1/2 power beamwidth is probably
a good idea to allow for inaccuracies in pointing, and the center line
direction should be about 315.5 degrees true bearing.

Classic amateur radio designs are intended to cover the maximum azimuth
possible with the maximum gain. From the Wire Antenna book, vol 1, page
5-2 figure 3, we see that a 23 degree 1/2 power primary lobe requires a
leg length of 3 wavelengths with an angle between the two legs of the
antenna of 60 degrees.

However, although the gain is decent, it is very broad in azimuth, with
lots of relatively high powered lobes off the sides and rear.  Once
again, the difference between amateur radio requirements, and resistance
operator requirements becomes apparent.  For OUR use, a better solution
would be to spend some time with EZNEC and look for a vee-beam solution
that provides reasonable gain with fewer sidelobes and better front to
back and sides to reduce the probability of being DFed.

EZNEC shows that a pair of 370′ long wires, feedpoint up 50′ high, with
500 ohm resistors to ground at the lower ends, and those ends separated
by 130 feet, an included angle of ~21  degrees, gives only about 4 to 5
db of gain, but much more importantly yields a very well defined beam
with side and back lobes down well over 20 db and a half-power beam
width of about 38 degrees.

Now, we need to figure out how to use a magnetic compass to set the
antenna legs. Magnetic bearing = True bearing – magnetic declination;
we consult the declination maps shown at
https://maps.ngdc.noaa.gov/viewers/historical_declination/  and find
that the declination is about -12.3 degrees.  To be really sure, since
the local declination can vary considerably, one could check the compass
bearings against various stars, but this will do especially since your
antenna has ample beamwidth. So, true bearing for the center of the vee
beam is 315.5 degrees -(-12.3) =~ 329 degrees to the centerline. Add 11
degrees for one leg and subtract 11 degrees for the other;  the ground
rods should be driven 360 feet from the feed point and at a bearing of
340 and 318 degrees respectively.

I’d use high strength aluminum electric fence wire for this antenna.
(When my transmission was complete, I’d re-roll the antenna wire onto
the rolls it came off and throw it in the back of the truck with the
rest of my electric fencing materials, insulators and such.) Ideally,
I’d use 450  ohm ladder line and a tuner, but I could use 14 gage
landscape wire for a feedline.  Lay this out with two of your helpers,
and drive ground rods at the terminus of each leg.  Attach the 20 watt
500 ohm carbon resistors to the each of the lower ends of the wire and
the ground rod.

Q-What mode will you use for transmitting the message?  If digital,
which specific mode and why?

I’d use PSK 250, because of the high transfer rate and low power
requirements; 20 watts will do nicely.

Q-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?

As stated above, I’d study the ionosonde data from Florida and Puerto
Rico, as being indicative of the first ionospheric reflection
conditions, and the ionosonde data from Texas and Colorado for the second.

Q- What will your cover story be if you are stopped by Venezuelan security
forces?

We’re just on our way to install some more electric fence!

Q-What are three non-radio related personal essentials that you should
bring with you? (arms of any sort are not on this list.)

Insect repellent;
water disinfection tablets;
a good hat!

And that’s my answer, NC Scout!

A long wire would be another antenna possibility, as it also uses only
one pick point.  Everything else starts to look too much like an
antenna.  With this setup, you can leave the wire down on the ground
until the minute before you want to transmit, then pull it up into the
air, transmit, then drop it again and roll up the wire.
Yagi or quad antennas look like exactly what they are.

And there you have it. Where the Technical meets the Tactical, right were we want to be.