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Author Topic: Simple emergency transceiver for use with NVIS antenna - ideas?  (Read 3070 times)
HS0ZIB
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Posts: 553




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« on: July 14, 2017, 12:31:58 AM »

I live in the country of Myanmar (Burma), which suffers from occasional tsunamis, cyclones, floods and earthquakes.  It is also the second-poorest country in Asia.

I'm a radio and telecoms engineer (hands-on), and I'm working (free of charge) to provide the Ministry of Communications and local communities in danger zones with a low-cost, simple transceiver system that can be used with a NVIS antenna to facilitate medium-range (eg 100-500 miles) communications, in times of disaster, where mobile phone communications are disrupted (either through infrastructure failure, or due to too many users).  There is minimal fixed line internet or telephone service in Myanmar - everything relies on the mobile networks.

I previously built and tested NVIS antennas with great success at my old QTH in Thailand.  I use the design from DX Engineering:

https://static.dxengineering.com/global/images/instructions/WP-NVIS-Rev3.pdf

To simply the use of the transceiver system, (and to keep the cost down), I'm going to develop a system which uses an everyday mobile phone as the means to speak and listen.  The mobile phone connects with the emergency transceiver using the bluetooth function of the phone.

At the transceiver side, the bluetooth function is achieved by using the 'guts' of a bluetooth boom microphone and headset, which can be bought for a few dollars in the local electronic shops.  These boom headsets have a 2-way bluetooth function, normally receiving the music from a laptop or mobile phone and delivering that to the ear pieces, and transmitting the audio from the microphone back to the laptop or mobile phone.

It's a very easy way to get the 2-way wireless functionality at a low cost.  The ear piece output connects to the transmitter audio input, and the receiver audio output connects to the microphone transmitter of the bluetooth device.

That just leaves the transceiver....

This is where I need some advice.  Simplicity suggests that I should use a simple AM transmitter with perhaps 5-10 watts of RF output power, and receiver, that can use a rechargable battery that is powered by a small solar panel.

The transmit frequency will typically be in the 60 meter band.

I value your suggestions about whether an AM tx is the best idea for this project.  I think using SSB or digital modes would significantly increase the cost and complexity of the project.

Your comments please! 
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G3RZP
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« Reply #1 on: July 14, 2017, 03:48:32 AM »

For the range you want, I think you need at least 100 watts for relatively reliable communication, especially since you are limiting the choice of frequency. That makes SSB much more desirable than AM because of power consumption, size, weight, cost and signal to noise ratio, although you will need a clarifier control unless you use an automatic clarifier.

Back in the mid 1970s, Burma Railways had an extensive HF 4 channel SSB network with selective calling: if you want selective calling, you could perhaps get hold of the handbook for that equipment  for ideas if the railways people still have it. (Decca Communications KW2000CAT was the transceiver)
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HS0ZIB
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« Reply #2 on: July 14, 2017, 05:39:48 AM »

Thanks for the suggestions.  I did a quick coverage prediction using VOACAP, (although they don't offer NVIS as an antenna option).  It suggests that 50 watts should provide a reasonably high % coverage on 60mb over the typical path distances for NVIS systems.

Coincidentally, I see some reasonably low-cost, 50 watt simple AM HF rigs on Ebay.  I've contacted the manufacturer to see if they can 'tweak' their design to provide me with a test unit for my band of interest.  (Also need to check if that rig has an output filter to minimise harmonics etc).
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WB6BYU
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« Reply #3 on: July 14, 2017, 07:45:11 AM »

Quote from: HS0ZIB

I did a quick coverage prediction using VOACAP, (although they don't offer NVIS as an antenna option). 



The "Dipole at 5m" option should be adequate for your purposes.


But you can do much better than trying to copy the AS-2259 antenna, which actually is limited
on the lower bands because it had to work up to 12 MHz.  You can use the same construction
techniques, but make one element resonant on 60m instead, so you don't need to use a tuner.
That also reduces losses in the coax.  (The original AS-2259 mast doubled as a low-loss, rigid
coax feedline to a wide-range tuner at the base.  It isn't nearly as efficient with a longer length
of standard coax feedline.)

In fact, you only would need the single 60m element for your purpose, but you could add a wire
for another band in place of rope guys in the other direction if you want.  Or you can use two
dipoles to radiate a circularly polarized signal, as long as the antennas are always set up to have
the same sense.  If the 60m element is too long, you can add loading coils.  Certainly provide
enough rope on the ends so the wire ends can be kept as high off the ground as possible, which
reduces ground losses.


For a transceiver, I'd consider something like the BITX, which  uses relatively common parts
that were available from local shops in India.  You can make it crystal-controlled, and the
output power can be increased by running the finals from higher voltage (like 24V), so 3
12V batteries (one for RX, two in series for TX) would power the rig and could still be
charged from a 12V solar panel.  I think that SSB will give you better coverage for the
same battery draw.
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HS0ZIB
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« Reply #4 on: July 14, 2017, 03:47:04 PM »

This SSB transceiver kit looks very interesting.

http://ea3gcy.blogspot.com/2013/01/iler-40-4-5w-qrp-ssb-monoband.html

I'd have to re-engineer it to operate on 60 metres, since 40 metres would probably be above the MUF after dark.
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KA9UCN
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« Reply #5 on: July 14, 2017, 09:48:53 PM »

A simple dipole placed . 1 wavelength above a wire on the ground that is cut to the desired frequency + 10% makes a very good NVIS antenna. The wire might not be needed if soil monster and minerals are sufficient. It will however ensure proper ground reflection and can be moved th the opposite side of the desired direction of transition to allow some gain and directivity. If it is placed directly under the antenna. The radiated pattern will be omnidirectional. The kit you have referenced has a very high review on E-ham.

Many years ago I used a full wave loop .1 wavelength above ground at 10 watt. It performed very well and 10 watt. It was enough to work whatever NVIS signals that I was able to hear with very good signal reports.
The full wave loop is not as quick and easy but it worked well. I do not believe you will have any problems with 4-5 watt.

   The antenna that you have suggested basic design was developed by the military and has proven to be easy and dependable over average terrain. If the terrain was to be extremely dry and sandy. A reflector wire 1.1 wavelength is centered and placed directly under the radiating element. This is seldom needed if soil monster is average.

   If more power is required. There are many Chines bare bone amplifier kits for very little money on AlieExpress.

Joe KA9UCN
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WB6BYU
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« Reply #6 on: July 16, 2017, 02:05:28 PM »

This SSB transceiver kit looks very interesting.

http://ea3gcy.blogspot.com/2013/01/iler-40-4-5w-qrp-ssb-monoband.html

I'd have to re-engineer it to operate on 60 metres, since 40 metres would probably be above the MUF after dark.


The conversion effort would be about the same for that or a BITX, which is pretty similar (but
uses discrete transistors rather than some of the ICs.)  You can get pre-built BITX boards from
China which would simplify construction.  In either case, you could probably leave the IF frequency
(including the crystal filter) where it is for one of the other band options and just put the proper
frequency crystal in place of the VFO / VXO, then change some of the filter capacitors and coils. 
Or you might find a standard frequency crystal that you can buy off the shelf that is close enough,
especially if you have some frequency flexibility on 60m.

Actually, choosing the right IF frequency might be worth considering for a bit.  A frequency in the
4.5 - 6.5 MHz range or so would make it more difficult to mix to 60m with low spurious outputs (though
you can buy 5.185 MHz crystals off-the-shelf, so  you might be able to skip the oscillator entirely
if you used that frequency.  You also wouldn't want to use a 10 MHz IF with a frequency in the low
end of 60m, as your VFO / VXO would be very close to the operating frequency.

None of these considerations would be a serious impediment, but if the kit / assembled product
comes with different IF ranges for different bands, it is possible that some other band might work
better for you than the 40m verision.


Looking at the VOACAP plots, having 10 or 20 watts would be a useful step up from 5W, especially
when the band is marginal, so being able to run the BITX final off of 24V for higher output could be
an advantage.  There are a number of similar rigs around - the choice really comes down to what
is available in quantity and suits your needs.
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AH7I
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« Reply #7 on: July 16, 2017, 07:36:20 PM »

I would look first at what is already successfully deployed and what has been tried and failed.
The ministry may know.
How delicate can this thing be?
-bob
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VK4FFAB
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« Reply #8 on: July 17, 2017, 05:08:16 AM »

This SSB transceiver kit looks very interesting.

http://ea3gcy.blogspot.com/2013/01/iler-40-4-5w-qrp-ssb-monoband.html

I'd have to re-engineer it to operate on 60 metres, since 40 metres would probably be above the MUF after dark.


The conversion effort would be about the same for that or a BITX,

Actually, choosing the right IF frequency might be worth considering for a bit.  A frequency in the
4.5 - 6.5 MHz range or so would make it more difficult to mix to 60m with low spurious outputs (though
you can buy 5.185 MHz crystals off-the-shelf, so  you might be able to skip the oscillator entirely
if you used that frequency.  You also wouldn't want to use a 10 MHz IF with a frequency in the low
end of 60m, as your VFO / VXO would be very close to the operating frequency.

With the BITX 40m SSB take the LO highside of the IF, 17mhz LO - 5mhz desired = 12mhz IF, its easy enough to do that in software for the Si5351a, then you just need to reroll the LP and BP filters by changing cap values only, why rewind inductors if you do not have to, remove the BFO and use a 2nd output on the si5351a to set the BFO frequency. (Assuming sideband inversion, its late and im not thinking, if not just roll with the xtal bfo thats there)

As far as regiggering goes, its probably the simplest solution and requires minimal construction, change a few cap values, remove 1 part from the bfo solder in 2 wires and a little Arduino Foo and its done. Ummm, 7mhz is still going to be a pain isnt it. Im going to bed LOL
« Last Edit: July 17, 2017, 05:24:46 AM by VK4FFAB » Logged

VK6HP
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« Reply #9 on: July 17, 2017, 05:42:38 AM »

The OP didn't mention much about the envisaged nature of the emergency link.  Is this largely a "peer to peer" system, or one in which there are a few high power base nodes usually communicating with the field stations?  The answer is important in specifying the field stations.  For example, the Royal Flying Doctor Service in Australia got away with incredibly simple HF transceivers (watts of AM output) in its early days because the heavy lifting on the link was largely done by the base stations, which had large antennas, sensitive receivers and big transmitters.  Mind you, the super-regenerative field receivers were quite sensitive too, at least for the era.  The favoured field antenna for these 2-8 MHz (approx) systems for many years were modest horizontal wires with a counterpoise, the latter tuned by reference to a light bulb if I recall the historical articles correctly.

As others have noted, it's pretty easy and cheap to get 10W of SSB these days but the operational nature of the system is probably the driver of whether you want 10W or 100W.
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HS0ZIB
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Posts: 553




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« Reply #10 on: July 18, 2017, 04:04:59 AM »

Quote
I would look first at what is already successfully deployed and what has been tried and failed.
The ministry may know.

LoL, to put it politely, the ministry knows squat!  That's why I advise them Smiley  This is one of my routes to being issued with a ham license in Myanmar.  I'm an RF engineer by qualification and profession, (although it's not my profession in Myanmar).  I'm currently advising the ministry on low bandwidth digital comms, APRS/vehicle tracking, STEM projects, linking VHF rigs via internet (Echolink style), HF email (Winlink style), duplex and simplex repeaters etc.  Lots of fun projects Smiley

I chatted with Javier, who sells the ILER-40 rig that I linked to.  We discussed about running it on 60 metres and this is easy using the add-on synthesized oscillator.  But that adds to the price.  So he will provide me with a rig that operates on the WRC-15 band, and using an internal VXO. I'll build and do some drive tests using different antennas.

The envisaged usage is as VK6HP described in his post ==> several decent base stations with which the small, simple rigs can communicate with.  So as mentioned, the base stations can do the 'heavy lift' work.

I'm over in Bangkok next week, so can order Javier's rig.  (Can't order from Myanmar, banking system is ... er... not so good!). Oh, I'm also getting a WSPR beacon tx to do some propagation and antenna tests.

I'll update when I start these new projects.
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KM1H
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« Reply #11 on: July 18, 2017, 04:01:55 PM »

Quote
Thanks for the suggestions.  I did a quick coverage prediction using VOACAP, (although they don't offer NVIS as an antenna option).  It suggests that 50 watts should provide a reasonably high % coverage on 60mb over the typical path distances for NVIS systems

Is that 50W the AM carrier or PEP which is ~ 4X the carrier.  The typical 100W ham rig runs a 20-25W carrier and 80-100W PEP on AM.

In Burma you will have to deal with year around T storm static which gets pretty intense at 5 mHz. VHF FM may be a better choice.

Carl
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HS0ZIB
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« Reply #12 on: July 18, 2017, 04:42:34 PM »

Quote
In Burma you will have to deal with year around T storm static which gets pretty intense at 5 mHz. VHF FM may be a better choice.

Hmm, having lived in this region for 15 years, I can say that I definitely don't have that problem Smiley It's never been a major issue with my ham activity.
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KM1H
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« Reply #13 on: July 19, 2017, 02:40:56 PM »

Quote
Hmm, having lived in this region for 15 years, I can say that I definitely don't have that problem Smiley It's never been a major issue with my ham activity.

No problem when you turn the radio off during a storm or use a very inefficient antenna. Also AM bandwidth just multiplies the noise in the receiver over SSB and CW.

When I used to float around in the Bay of Bengal for the USN the static was horrendous at times.
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HS0ZIB
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« Reply #14 on: July 20, 2017, 04:39:13 AM »

Quote
When I used to float around in the Bay of Bengal for the USN the static was horrendous at times.

Hmm, maybe I've lived here too long, gone 'local' and can't hear the static any more Smiley
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