Emergency transmitter replacement

Bad weather or other disasters can strike any time of year.  Around these parts, the most dangerous weather events occur from early spring through late summer.  In the past twenty years or so, we have had tornadoes, hurricanes, micro bursts, flooding events and so on.  All of that got me thinking about what would happen if a tower came down, or a transmitter building was destroyed by fire, wind, water, etc.

If past events can predict future performance, there would ensue a mad scramble to replace damaged equipment and or get some type of temporary antenna into service.  That is what happened in great City of North Adams, Massachusetts when the tower that held the cell carriers, the 911 dispatch, and the local FM radio station came down in an ice storm.  Fortunately, we had a single bay Shively antenna at the shop that we trimmed up and installed on a temporary pole with 200 watts TPO.

That will cover the city of license, provided there is electricity…

What if there where an event that was so devastating that the electrical power would not be restored for months?  Think about hurricane Maria in Puerto Rico.   After that event, the infrastructure was so devastated that there was not even the possibility of getting a fuel truck to deliver diesel for the emergency generators at the hospital in San Juan.  It can happen.

With that in mind, I began poking around and thinking about how I would get something back on the air.  In the face of massive disasters, AM and FM radio is still the most effective way to communicate with the general public.  Radios are still ubiquitous in homes, cars and businesses.

Bext 30 watt exciter
Bext 30 Watt FM exciter

In a short period of time I came up with a couple of solutions.  First, the frequency agile Bext exciter uses a single solid state rectifier feeding 24 volts to the power supply board.  The audio input includes a mono balanced line level input which can be fed by a computer sound card or some other simple source.

Bext 30 Watt FM exciter power supply
Bext 30 Watt FM exciter power supply

From there +12, +15 and +20 VDC are created to run various circuits.  The heat sink cooling fan is the only thing that runs on 120 VAC, which is old and I might replace with a 24 VDC unit.

Bext 30 Watt exciter power supply voltage
Bext 30 Watt exciter power supply voltage

The power output is about 22 watts, which is not bad.  That will certainly get out well enough from a high spot and provide good coverage when the power is out because all the other in band RF noise generators will be off.

6 volt, 435 Ah batteries
6 volt, 435 Ah batteries

Then I though about the deep cycle batteries in my barn.  These 6 volt, 435 Ah units have been around for a couple of years, but last I checked, they still held a charge.  Other deep cycle batteries from things like golf carts, fork lifts, campers, boats etc could also be pressed into service.  The point is, 24 VDC should not be impossible to create.

To keep a charge on the batteries, this solar panel will work:

225 Watt, 36 volt solar panel
225 Watt, 36 volt solar panel

This setup would require some sort of 24 volt DC charge controller, which I found on Amazon for less than $15.00 US.  This charge controller has selectable 24/12 VDC output and also has two USB ports which would be handy for charging hand held devices.

I measured the power draw while the exciter was running 20 watts into a dummy load, it draws 120 Watts.

The final part would be some sort of antenna with transmission line.  For this situation, a simple wire center fed dipole hung vertically would work well.  This can be fabricated with two pieces of copper wire and a few insulators.

Simple dipole antenna
Simple dipole antenna

The lengths of each wire can be calculated as follows:

Approximate length in feet: 234/f (MHz)

Approximate length in inches: 2808/ f (MHz)

Approximate length in cm: 7132/f (MHz)

For the FM band, maximum length of wires needed will be 32 inches (81 cm).  Insulators can be made of anything that does not conduct RF; PVC, ABS, dry wood, dry poly rope, etc.

Emergency FM band dipole
Emergency FM band dipole, cut to 88 MHz, lowest FM frequency

I recommend to cut the wires slightly long, then trim little bits off of each end while watching the reflected power meter on the exciter.  To keep RF from coming back down the shield of the transmission line, make 8-10 turns, 6-8 inches in diameter of coax as close to the antenna as possible and secure with a wire tie.  This will create a balun of sorts.

My emergency FM kit consists of:

  • Bext Frequency agile exciter
  • 30 feet, RG-8 coax with N male connector on one end
  • 4 ten foot RG-58 BNC male jumpers
  • 1 four foot LMR-400 N male jumper
  • Dipole antenna, cut long
  • Solar charge controller
  • Small basic tool kit; hand tools, plus DVM and soldering iron
  • Power cords, extension cords
  • 300 watt 12VDC to 120VAC inverter (pure sine wave)
  • 20 feet audio wire
  • Various audio connectors; spade lugs, XLR male and female, RCA, 1/4 TRS, etc
  • Various RF connectors; PL-259, N, BNC, etc
  • Bag of 12 inch wire ties
  • 3 rolls of 3M Scotch 88 electrical tape
  • 100 feet of 3/8 inch poly rope

This is all kept in a sturdy plastic storage bin from the Home Depot.  If needed, the batteries and solar panel are stored in the barn along with an assortment of other goodies.

Will it ever be needed?  Well,  I hope not.  However, it is much better to be prepared to restore services than wait for somebody to show up and help.  Sitting around complaining about the government does not relieve those people in need during and after a disaster.

Emergency Communications

In this modern day and age, we take electronic communication for granted. Imagine being plunged into a world were there are no phones, cellphones, internet, email, television or even radio. Back in the day when I served aboard ships, we called that being underway.

Way, way back in the late 1980’s and early 1990’s, those that served at sea were at the mercy of the Fleet Post Office.  I will say, the FPO did a very good job routing the mail to the appropriate place, however, sometimes weeks or even a month would go by without mail.  When the mail finally did arrive, it all smelled the same.  Everyone’s wife or girl friend put some sort of scent on the outgoing, but since those letters mingled tightly packed in the same bag for weeks, often in hot humid tropical Pacific air, those scents blended together and became the Westpac Mail Smell, which permeated everything, even the letters from my father.

What will happen if people can’t sign on Facebook?

Fortune favors the prepared.

Communications loss in ordinary circumstances

Loss of utility company power, phone service and internet service can happen at any time for a variety of reasons.  The worst case scenario will occur when such loss is coupled with a natural disaster, which are often a major disruption of normal life.  Loss of information, especially at critical moments, can make a bad situation much worse.  In a situation where all normal means of communication are not functioning, something will fill that void, most likely the rumor mill.  That could be bad.

For information gathering, there are many options.  A good AM/FM shortwave radio is a decent start.  I would recommend a quality shortwave radio that has AM/USB/LSB options.  During run of the mill storms and power outages, many radio stations will remain on the air with emergency generators.  The key is to figure out which stations are staffed and offer good timely information.  NOAA all hazards radio can be good source of weather information, however, their transmitters can remain off the air for weeks or months at times.

One might ask “Isn’t this overkill or alarmist?”  I suppose that depends.  In the December 2007 ice storm, we had no power for seven days. In the aftermath of several major Northeast hurricanes and winter storms, some people had no power for more than two weeks.  Not only no power, but no cable, phone or internet either.  In situations like that, having some form of connection to the outside world can make a big difference.

Communications loss in less than ordinary situations

Other situations and scenarios may require more effort.  Prolonged information shortages could be triggered any number of national or global situations.  Shortwave receivers are not only for listening to international broadcast stations but also for tuning into the amateur radio (AKA “Ham”) frequencies as well.  Amateur radio is often used for emergency communications on a local and national and international level by governments and the Red Cross when other systems are out.  National and international communications are often heard on the HF band; 3-30 MHz.  The Amateur radio primary emergency voice nets are:

  • 3791.0 USB VOICE PRIMARY International, DX, and Emergency/Disaster Relief
  • 5371.5 USB EMERGENCY Emergency/Disaster Relief
  • 5403.5 USB EMERGENCY Emergency/Disaster Relief
  • 7185.5 USB VOICE PRIMARY International/Regional and Emergency/Disaster Relief
  • 14346.0 USB VOICE PRIMARY International/Regional and Emergency/Disaster Relief
  • 18117.5 USB VOICE PRIMARY International/Regional and Emergency/Disaster Relief
  • 21432.5 USB VOICE PRIMARY International/Regional and Emergency/Disaster Relief
  • 24932.0 USB VOICE PRIMARY International/Regional and Emergency/Disaster Relief
  • 28312.5 USB VOICE PRIMARY International/Regional and Emergency/Disaster Relief

These are voice channels from the ALE website. If there is not traffic on these frequencies, tune around a little bit.  In addition to voice nets, the amateurs also use something called ALE, which stands for automatic link establishment.  This is a data system that can be decoded on a listen only basis with a computer and some free software, for those so inclined.

For local amateur communications, 2 meter and 70 cm repeaters are often pressed into service.  For those, a VHF/UHF scanner is required.  Get a trunking scanner for 800 MHz police/fire dispatch as well.  Make sure that all radios can operate on 12 volts DC.  For this application, the size of the solar panel and battery is moderate, as receivers do not use much current.

Another option is a wide band USB radio for a lap top computer like a WinRadio WR-G315e.  These devices can be power by the USB outlet on the computer while the computer itself is charged with a solar panel.  For this route, some research on lap top solar chargers is needed.  The DC power requirements vary from lap top to lap top, so I can only offer general advice here.

With any receiver, a good antenna will greatly improve performance.  If there is room for an outdoor antenna, any length of wire strung up in a tree, away from power lines will work well.  For indoor setups, some type of receiving loop will work best.

Prolonged loss of communications in extraordinary circumstances

For longer term situations, gaining access to vital information and communications may become more problematic.  First of all, electronic communications require electricity.  Long term disruptions to the electrical distribution system could occur by either natural or man made events.  When those events happen, those that are prepared will be in a better position to survive if not thrive.  Things like ad hoc computer networks and amateur radio can facilitate two way communications.  In order to use amateur radio, one needs to get a license first.  This is a pretty easy thing to do and most other amateur radio operators won’t talk to you without a valid call sign.  Not only will they not talk to you, they will likely track you down and report you to the FCC.  That is the nature of the hobby, like it or don’t.

Amateur radio set ups can be very simple and not terribly expensive.  An used HF radio can be purchased on eBay for a moderate sum.  A simple multiband vertical antenna will serve general purposes.  For those that are interested in HF Link, a newer radio will work better.

Wireless ad hoc computer networks can be set up to establish a quasi internet over a moderate sized area.  WiFi WAN networks can be locally established using 900 MHz, 2.4 GHz, 5.8 GHz and 24 GHz license free channels.  Depending on the frequency, those links can be used for point to point medium to long haul links, or to establish local links to laptops and wireless devices:

  • 900 MHz: lower speed data rates, long haul, good to moderate building and vegetation penetration
  • 2.4 GHz: Limited channel availability, high atmospheric absorption, moderate speeds, low vegetation and building penetration
  • 5.8 GHz: High number of channels available, potential interference issue with TDWR radar systems, moderate to high speeds, line of sight only
  • 24 GHz: Large bandwidth, high speed, point to point back haul, line of sight only

Once the information is obtained, distribution to the greater public becomes a problem.  A very simple webserver (Apache, Nginx) with a light weight, simple index page containing vital information, news, weather, etc can be set up on a laptop and all HTTP traffic directed to the default index page.  This type of set up could be run off of a battery charged by a solar panel.  The issue here would be obtaining the information to put on the web page.

The Emergency FM Replacement Antenna

Hurricane season is here. This time of year makes me fondly remember hurricanes of the past and the things we had to do to get stations back on the air; walking a mile down a sandy spit of land, wading through swamp water to get to the transmitter shack, being threatened with arrest by the Connecticut National Guard, blow drying RF modules with a hair dryer, sleeping in a camper for a week…  Ahhhh, good times, great times!

The one thing that I did learn, if the disaster is big enough, expect none of the normal services to be functioning.  That includes things like gas stations, fuel delivery, grocery stores, restaurants, hotels, UPS, roads, bridges, telephone service, internet service, etc.

It is not a far fetched scenario for the main FM transmitter site to be out of commission and will not be available or accessible for some prolonged period of time.  There might also be mitigating circumstances such as catastrophic tower failure, destruction of transmitter building, flooding, or other major infrastructure disruptions.  In those situations, calling the broadcast supply vendor of choice for a replacement might not be an option.

It has happened before…

All of these things got me to thinking about how to fabricate a reliable FM broadcast antenna from simple materials available on hand.  The FCC allows for temporary operation with an emergency antenna in part 73.1680, which reads:

(a) An emergency antenna is one that is erected for temporary use after the authorized main and auxiliary antennas are damaged and cannot be used.

(b) Prior authority from the FCC is not required by licensees and permittees to erect and commence operations using an emergency antenna to restore program service to the public. However, an informal letter request to continue operation with the emergency antenna must be made within 24 hours to the FCC in Washington, DC, Attention: Audio Division (radio) or Video Division (television), Media Bureau, within 24 hours after commencement of its use. The request is to include a description of the damage to the authorized antenna, a description of the emergency antenna, and the station operating power with the emergency antenna.

(1) AM stations. AM stations may use a horizontal or vertical wire or a nondirectional vertical element of a directional antenna as an emergency antenna. AM stations using an emergency nondirectional antenna or a horizontal or vertical wire pursuant to this section, in lieu or authorized directional facilities, shall operate with power reduced to 25% or less of the nominal licensed power, or, a higher power, not exceeding licensed power, while insuring that the radiated filed strength does not exceed that authorized in any given azimuth for the corresponding hours of directional operation.

(2) FM, TV and Class A TV stations. FM, TV and Class A TV stations may erect any suitable radiator, or use operable sections of the authorized antenna(s) as an emergency antenna.

(c) The FCC may prescribe the output power, radiation limits, or other operating conditions when using an emergency antenna, and emergency antenna authorizations may be modified or terminated in the event harmful interference is caused to other stations or services by the use of an emergency antenna.

In this situation, making a circularly polarized antenna would be overly complicated, so either a horizontally or vertically polarized antenna would be the most likely scenario.  There are a few antenna types that readily lend themselves to field expedient fabrication.

These are, in no particular order:

Of these, the 1/2 wave wire dipole is the easiest to construct.  Cut two wires, length (in feet) determined by the formula 234/Frequency (Mhz).  Attach one wire to the center conductor and one to the shield, stretch to the wires out and tune for minimum SWR by cutting or adding small lengths to the ends.  The total length for such an antenna would be approximately five feet and it could be mounted horizontally or vertically.  The issue with a wire dipole would be bandwidth and power handling capability.

A 1/2 wave dipole made from tubing would have better bandwidth and power handling, but tubing is a little harder to work with when it comes to tuning the antenna.

Frankly, if one is going to go through the trouble of using tubing to create an emergency antenna, the the J-Pole (end fed antenna with a 1/4 wave matching section) is probably the best.  This antenna is easier to tune, does not need to work against a ground plain, has good bandwidth and a low take off angle, meaning more power is radiated out toward the horizon, giving it a good deal of gain over both a ground plane or dipole antenna.  Additionally, when using standard RG-8, RG-214, LMR-400 or other similar transmission line, a well matched antenna might be able to accept about 1 KW of input power, which would net approximately 4.4 KW ERP.  Not an insignificant sum, especially in an emergency situation.

Vertical radiation pattern for J-pole (1/2 wave end fed) antenna
Vertical radiation pattern for J-pole (1/2 wave end fed) antenna
1/4 wave ground plane vertical radiation pattern
1/4 wave ground plane vertical radiation pattern

There are many J-Pole antenna calculators available on line, but many of them include a 20 inch or so section of tubing below the tuning stub that can be electrically coupled to the supporting structure.  This configuration defeats the main advantage of the antenna, creating a good deal of upward radiation.  It is a better idea to use a non-conductive support piece and keep any conductive materials at least 1/2 wave length or greater from the radiating portion of the antenna.

The basic j-pole antenna looks like this:

J Pole (1/2 wave vertical antenna) diagram
J Pole (1/2 wave vertical antenna) diagram

The radiating part of the antenna starts above the tuning stub.  Basically, the 1/4 wave stub is shorted at the bottom, the feed point is adjusted away from the shorted end until a 50 ohm impedance point is found.  The center conductor of the coax is attached to the 3/4 wavelength section, while the shield is connected to the stub. The critical distances are the tuning stub length and the distance of the feed point from the shorting section.  I created an excel spreadsheet (.xls) that can be used to create all the lengths required to fabricate one of these antennas.  That spreadsheet can be had here: J Pole Calculator

Having a few moments of time to spare, I thought it would be fun to build one of these and put the analyzer to it.  I think testing things in the real world is a good exercise and I always enjoy working with antennas anyway.  Looking in the basement, I found some 3/4 inch copper tubing, a tee, an elbow and a few end caps.  The complete list of parts is thus:

Part Amount Use
¾ copper tubing 78-96 inches (196-244 cm) (frequency dependent) Main section
¾ copper tubing 26-32 inches (66-82 cm) (frequency dependent) Tuning stub
¾ copper tubing 2.5-3 inches (6.35-7.62 cm) (frequency dependent) Tuning stub short
¾ copper tubing 2 inches (5.08 cm) Mounting section, bottom of T to MIP threaded adaptor
¾ copper T section 1 each T section for joining main section to tuning stub
¾ copper 90 elbow 1 each Elbow
¾ copper end cap 2 each End cap on tubing
¾ to 1 inch copper MIP threaded adaptor 1 each Antenna Mounting
1 inch PVC FPT threaded adaptor 1 each Insulating mounting connection
1 inch PVC Approximately 20-25 inches (50-65 cm) Insulating mounting material
1 inch stainless steel hose clamps 2 each Attaching the coax to the antenna feed point
RG-8, RG-214, LMR-400 or other transmission line As needed, including 5-6 turns, six inches in diameter to form RF choke at feedpoint RF choke needed to keep RF off of coax shield

One important detail to remember when using the above spreadsheet, the measurements are to the closest side and not the center.  Thus, if something measures 2.5 inches, it is metal to metal.  Some basic soldering skills are required, but assembly is relatively straight forward.  In a pinch, almost any conductive material could be used including aluminum, brass, steel, EMT, rigid conduit, or even iron pipe.

Parts cut to size for j-pole antenna on 87.9 MHz
Parts cut to size for J-pole antenna on 87.9 MHz
j-pole antenna assembled
J-pole antenna assembled
J-pole antenna on the antenna testing range
J-pole antenna on the antenna testing range

I made this particular J-pole antenna on 87.9 MHz because I didn’t feel like chopping up all my 3/4 inch tubing.  Cutting and soldering the tubing took about a half an hour.  Designing and fabricating the feed point system another half an hour.  I’ll throw another hour in for rounding up the parts, tools, etc.  Thus, entire antenna was constructed in about two hours.  I used my AIM 4170D to find the proper feed point.  If I were going to actually use this antenna, it would then be a matter of finding a mounting location and running the transmission line.

J-pole antenna analysis results
J-pole antenna analysis results

Actually, I was less than happy with this. While the antenna is nice and broad across several channels, there is 16 ohms inductive reactance that is impossible to get rid of. That gives an SWR of 1.4:1, which is not great.  With that kind of load, I would be reluctant to run more than a couple of hundred watts into this antenna. The interesting thing is, that graph is the first one, with everything set as calculated in the spreadsheet.  After that, I could make the impedance and reactance worse, but not better.

Still, in a pinch, I would use this antenna until something better could be found.


As promised, a picture of the feed point:

J-pole feed point connections
J-pole feed point connections

The hose clamps are not optimum, I am sure a better way to attach the feed line to the antenna can be fabricated, but again, I was thinking of an emergency situation and the parts which may be available from local sources.