People, who travels in remote places due to either job or hobby, like geologists, hunters, archaeologists, biologists, and just those living in places with extremely low population density, understand the importance of telecommunications.
In a century of cell networks, Internet, and satellite phones it looks like communications are easy and abundant. Its true only from the first sight. If you dig deeper, it becomes clear, that territories with cell phone coverage do not comprise the whole planet, not even the land, excluding oceans. Cell phone companies build the network only in places where they can get some profit. Of course, remote Alaska forests, bogs, mountains and deserts are not so populated to cover them continuously by cellular networks. One could say that direct replacement of cell coverage is satellite communications. Although, they have some peculiarities. First of all, the price for a satellite terminal is not affordable for everybody. Secondly, satellite phones have troubles working from deep canyons or dense wet forest. Thirdly, it is just some kind of a phone, which means you need to press some right combinations of buttons, wait for response, and get used to inevitable communication delays.
Taking into account all these problems, we would like to come back to HF communications. The market is full with different radios. We shall omit CB (Citizens’ Band) and VHF/UHF (PMR or LPD) due to their communication range limitations. Let us talk about the most complex range of 10-200 km (6 – 125 miles), the most common cases of distance of a traveler or a small group from the base camp or a local town. It looks like everything is obvious: take any portable HF radio, connect to antenna and a battery, take a mic, push PTT button and call your respondent on a working frequency (of course, only if you have a license). But it is not that simple. Putting all the mentioned parts together in the best case scenario you will get a pile of equipment of 5 kg (10 lbs) weight. It is due to the fact that communication during daytime for the mentioned distances will require at least 10 Watt of power (better 50 Watt) using full size antennas on a low frequency bands from 2 to 7 MHz. When a traveler will have a look at all this pile of equipment, a desire to take 4-5 cans of SPAM instead overcome all communication needs. May be it is wrong, but this is in the human nature and for most of the travelers it is too late to change. This is why we should think how to make a necessary set of equipment, which can be invaluable in emergency situation or everyday life, lighter and easier to use. How can this be achieved?
It may look strange, but the reason for making all this so complex is in the laziness of human nature. People do not want to learn the Morse code, while even this primitive technique would allow to decrease the size of an antenna, required power and as a result battery capacity and weight. Why is it so? Because for CW (communications using Morse code, telegraph) one needs a bandwidth 10 times lower than for voice communications, as a result one would need to transmit 10 times less energy for the same level of signal. Another thing which contributes here is the human brain, which is effectively a supercomputer capable to discern the signal of the same level as noise and transform it into the useful information. Due to all these factors mass of the equipment (which way simpler for CW) and batteries is lowered significantly. After the WWII there were a lot of former military CW operators working for civil services during the peaceful time. Unfortunately, in spite of all the progress of electronics current automatic Morse code decoders are far from perfect and not even close to a professional communications operator. In real life RFI conditions electronics cannot recognize the signal from the noise and reconstruct the message from incomplete information, as human CW operator can do.
The history goes in spirals and Physicists and Electronics Engineers figured out how to make computers to form a signal which is not convenient for a human ear to comprehend, but is suitable to be decoded by not very powerful electronic device at signal noise ratios way lower to allow a human brain even to detect the presence of the extremely low bandwidth signal. Many of you heard about such communication protocols as JT65, JT9, and FT8. “Computer will replace a man!” – this was a slogan from late 80’s and this is happening right now.
The proposed device I called HF-pager, as it resembles this now mostly historical communication device as close as possible, at least in operation principles. It composed of small box which is connected to a smartphone with Android operation system. The small box is in reality a QRP (low power) transceiver “Uleima” working at 3.5 MHz (80 m) band. Due to the fact that smartphone already has all the necessary AF amplifiers, the radio becomes simpler. All we need to do is to transform audio signals with phase modulation into low bandwidth radio signal and transmit it with power 1-2 Watts. On the receiving end radio signal will be transformed into audio and passed for processing to the smartphone, which will show decoded message on the display with the signal waterfall.
Initially software sends a header, consisting of a callsign and a control sum. Later the message text is being transmitted. If in the receiving smartphone header was decoded without errors, then the received message text is save into Inbox folder as on a normal cell phone or e-mail client. If there was and error during header reception, then the transmitter will continue to send a header until it will receive a confirmation.
Perhaps, you have already got used to the network indicator on your cell phone. When you see those bars you have no doubt in your connection. How one can see that HFPager has a reliable connection and no information was or will be lost? HFPager has a special function for that, but due to the fact that there is no “HFPager network” and no base stations, you can check the existence of the channel only activating a beacon, which will periodically (with a period from 15 minutes to 3 hours, according to settings) send a test packet. So coming back to the camp you can have a look at the smartphone display and if you see green squares of control periods you can be sure that both devices are OK, just no message was received.
How HFPager can be useful in a travel or an expedition? Here are few practical messages from the tom of our head:
- We reached destination, everything is OK.
- Fishing is bad, soon be back.
- Bring more food.
- John is sick, need doctor. -26 letters, spaces and signs.
- We stay for another week.
- Bring equipment with the second group.
- Battery is dead, expect no communication.
- Wait for us at bridge in any case. -34 letters, spaces and signs.
Even in the worst case such message will be delivered in 12-15 minutes. Probably, you agree that some of this messages can seriously influence your endeavor.
Software interface looks like this:
Signal Propagation (distance, season, time of day, geographic latitude)
The main application of HFPager is communication in sparsely populated regions on the distances from 50 to 200 km (30-125 miles). The most suitable for this task is 80 m band. Waves of this frequencies can propagate both by direct wave (along the earth surface) and through reflection from lower layers of ionosphere, for example E or F2, sometimes D. One should use NVIS (nearly vertical incident skywave), when reflections happens on altitudes of 90-120 km and almost perpendicularly to the ionosphere layer.
The hight of reflecting ionosphere layer depends on a level of ionization, which, in turn, depends on a season, time of day, and sun activity.
On a picture one can see propagation paths depending on ionization of different ionosphere layers. Of course this is very simplified picture. In reality these processes are way more complex and one can read more about it in many books. The main thing to remember is the following: near communication is better when the ionosphere is strongly radiated by Sun, and wise versa when it is dark and cold you can have good communication on far path and bad one with nearly located respondent. Here is an example how signal level (really, signal to noise ratio) changes during the Spring on a 100 km path between Uglich and Pereslavl’.
Blue and red colors indicate signals at full-sized and shortened dipoles.
It should be emphasized, that the best propagation happens on dusk and dawn. It can be from few minutes to an hour, depending on latitude, when ionosphere gives us few extra dBs of signal. Close to Uglich (57.5239° N) this time is about an hour before the dawn and about an hour after dusk, see picture below.
One of the most important advantages of HF waves is their ability to diffraction – possibility to go around of obstacles comparable in size with the wavelength. Due to this property the direct (land) wave communication is possible even in mountainous or hilly regions.
RFI (radio frequency interference)
RFI is the most “painful” topic for modern HF communications. It was always important, but in recent decades the amount of RFI increased significantly due to wide spread of switching power supplies. Power supplies of Wi-Fi routes, laptops (notebooks), TVs are producing a lot of noise. Even modern gas fueled electrical generators will produce enormous RFI together with the desired AC power. Also, you should not situate your antennas close to power lines. For example, 10 KV line produces significant RFI on a distance of 50 m, but 110 KV line gives troubles even at 300 m.
To transmit a message using IFSK modulation transmitting power requirements are rather low with respect to standard 100 Watt of a usual amateur radio transmitter. Because we want to use light radio with a light battery all we can rely on is few Watts, but with good antenna, a sensitive (about 1 muV) receiver, and low RFI this power is enough for our distances. For decoding of the message by HFPager signal noise ratio can be as low as -25 dB. Let us compare with a voice signal: in order to receive it one needs signal noise ratio about +6 dB, which means HFPager has 1000 times advantage. Therefore, for HFPager one can use way less powerful transmitter or/and shorter antenna.
DC input: 12.6 V.
During transmission consumed current is around 700-850 mA.
Receiving mode current is way lower, around 10 mA.
The major problem is smartphone power consumption. It is usually way higher and depends on the device, as well as their battery capacity. It is a good idea to have some powerbank and delete/suspend all unnecessary applications and services, turn on power saving for the screen and make sure that application (HFPager in our case) can still run in such a mode in order to receive messages. Not all smartphones provide this capability out of the box, some work only when display is on.
The device is pretty forgiving to the external temperatures (both high and low). There are two things you need to remember.
- Batteries are chemistry driven devices and chemical processes are going on differently at different temperatures. Some batteries when exposed to low temperatures can freeze and stop providing necessary current (good idea at freezing temperatures to keep at least one spare battery under your jacket or in some other warm place to prevent freezing). At extremely high heat their self discharge current grow and some batteries can discharge by themselves.
- Crystal stabilized oscillator used in HFPager is pretty stable, but there is an adjustable capacitor in its circuit, which has negative TCC (Temperature Coefficient of Capacitance), which can result in slight frequency drift with change of device temperature. This can decrease signal to noise ratio as the tuning will not be perfect.
The full scale field tests at wide range of temperatures has not been performed yet. Current experience during the Summer and Winter at the Central part of Russia is quite positive.
It also should be taken into account that final amplifier stage is situated inside of the plastic transmitter case, which can theoretically result in overheating at long time continuous transmission. Though, in practical applications such issue never revealed itself.
Packing for transportation.
Well known “Uleima” transceiver (upper middle pocket with a battery and headset in the upper right pocket) together with an Android based smartphone (upper left pocket) comprise an HFPager.
Lower pockets contain a twisted pair feeder (lower middle pocket) and legs of shortened dipole antenna with extension coils (lower left and right pockets).
Perhaps, the most important part of any communication device (especially on HF bands) is an antenna. You can consider and antenna your path from one point to another.
If it is wide well paved highway you will get to your destination in few hours. If it is some not maintained gravel or even dirt road you can easily spend a day or more. The same thinking should be applied to antennas. The main thing which any HF communication device user have to understand is that antenna is not just piece of wire. This is tuned resonant system, which has specific parameters, defining what fraction of applied power will be radiated (and where), what amount of incoming power will be transferred to receiver and how much RFI noise will be added. This is why one should carefully follow antennas user manual recommendations. Antennas should be located in a free space, do not touch conducting materials and better be as far from them as possible. The length of an antenna is critical parameter, it should not be changed just for the sake of convenience. Height of the installation should be as close to recommended one as possible.
The most efficient antenna for our purposes is a full-size half-wave dipole, which would have length about 36 meters (188 feet). Such an antenna can be provided by request.
Shortened Dipole 22 m.
After some testing it was decided that small decrease in transmitted power is well compensated by lower mass of an antenna and convenience of installation in the field. “Uleima” by default is supplied with shortened (22 m = 72 feet) antenna with extension coils to make the use more practical. Efficiency of such an antenna, although lower, is enough for most of the cases.
One can find more details about the application in the User manual.
Here we shall shortly glance through its pretty intuitive and simple interface
main part of which is the Waterfall
The application allows to choose the rate of transmission from 1.46 Bd to 23.44 Bd.
Modulation: 18-tone IFSK, with forward error correcting Reed-Solomon code RS(7,15), superblock by 4 RS blocks with interleaving.
One can choose manipulation frequency, usually it is 1100 Hz. There is a possibility of automatic control of the channel parameters, as well as possibility to transmit GPS coordinates (received by smartphone). Respondent after reception of the coordinates can easily see the point on MAPS.me electronic map, if it was previously downloaded. Application can automatically request a confirmation receipt so you will be sure that your message was received successfully.
Size and Composition of the Device (The First Version)
The first two samples were produced in two different variants.
The one with internal battery (standard LiFeP 26650 element: 4 cells, 3.2 V each).Weight of such device is 540 gram (1.2 lbs). Size: 150x75x50 mm.
The second modification was designed for operation with external battery.Its weight is 270 gram (0.6 lb) and size 136x67x19 mm.
Reference oscillator frequency: 3,580,436 Hz (can be changed by change of crystal).
Transmission frequency = Reference oscillator frequency – manipulation frequency. For example, of manipulation frequency is 1100 Hz, transmission frequency is 3579336 Hz.
Modulation is SSB (single side band) with lower side band (LSB) (0K10J2B)
Filter bandwidth: 3578800-3579500 Hz=700 Hz
Output impedance: 50 Ohm
Output power: 1.6-1.8 Watt
Sensitivity: better than 0.5 muV
DC input: 12.6 V±2 V.
Transmission: 700-850 mA.
Reception: 10 mA.
Input AF signal: 1.5 V
Output AF signal: 0.2 V
Mass: 0.54 kg (1.2 lbs)
Size: 150x75x50 mm
Laws and regulations.
The transmission frequency of the HFPager device belongs to the 80m amateur radio band. It has to be emphasized that transmission in HF bands is allowed only to licensed commercial or amateur operators. For example, in the USA it is regulated by FCC and all documentation can be found in the Part 97 of Title 47 of the Code of Federal Regulations (see links here: http://www.arrl.org/part-97-amateur-radio). Violation of this regulations can result in high fines and loss of operator’s license.
Transmission at 80m amateur band requires at least General class amateur license in USA. Please, follow this link to learn more: http://www.arrl.org/getting-licensed
- Bad visibility of display in bright sunlight (depends on smartphone)
- No reception – reload the app.
- Simplex – you don’t know whether your respondent will be receiving you next second or will start transmission.
- Transceiver does not start transmission (no audible relay click). Check the volume level on the smartphone, perhaps it was lowered and VOX is not activated. Another possibility is depleted transceiver battery.
- Some smartphones stop receiving when the screen is off. Usually you can set the screen to be always on, but the battery of the smartphone will discharge faster.
- You should agree about settings in advance and set Rate, Receive and Transmit Frequencies, FEC.
- Always enter you call sign!
- Electronic maps have to be downloaded in advance, not in tundra!
- It is helpful to find out dusk and dawn times in your travel location, for example in Google, then you will avoid loosing your battery energy in vain and will use optimal propagation times.
Ionograms can be found here: http://space-weather.ru/index.php?page=ionogrammy
Participation in testing and experiments with HFPager.
This is not recreational but practical project. At the moment a chief of the project is Eugenii Slodkevich, UA3AHM, who is looking for partners to continue experiments and testing. The following three settings are going to be tested: stationary, portable, and mobile. There are two variants of software: for MS Windows and Android OS. Programs are provided to partners under following conditions:
For stationary tests using Winpager software:
- location not further than 500 km from Uglich, Yaroslavl region, Russia;
- at least Russian 3rd category amateur radio license or equivalent (giving permission to transmit on 80 m amateur radio band);
- low RFI QTH (station location);
- availability of transceiver with output power 5 Watt on the amateur radio bands: 160 m, 80 m, 40 m, with corresponding antennas;
- some experience with digital modes (JT65, JT9, FT8 or others);
- account in Telegram messenger for fast communication