{"id":9940,"date":"2019-03-28T16:24:18","date_gmt":"2019-03-28T20:24:18","guid":{"rendered":"http:\/\/www.engineeringradio.us\/blog\/?p=9940"},"modified":"2019-03-29T17:20:21","modified_gmt":"2019-03-29T21:20:21","slug":"emergency-transmitter-replacement","status":"publish","type":"post","link":"https:\/\/www.engineeringradio.us\/blog\/2019\/03\/emergency-transmitter-replacement\/","title":{"rendered":"Emergency transmitter replacement"},"content":{"rendered":"

Bad weather or other disasters can strike any time of year.\u00a0 Around these parts, the most dangerous weather events occur from early spring through late summer.\u00a0 In the past twenty years or so, we have had tornadoes, hurricanes, micro bursts, flooding events and so on.\u00a0 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.<\/p>\n

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.\u00a0 That is what happened in great City of North Adams, Massachusetts<\/a> when the tower that held the cell carriers, the 911 dispatch, and the local FM radio station came down in an ice storm.\u00a0 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.<\/p>\n

That will cover the city of license, provided there is electricity…<\/p>\n

What if there where an event that was so devastating that the electrical power would not be restored for months?\u00a0 Think about hurricane Maria in Puerto Rico.\u00a0\u00a0 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.\u00a0 It can happen.<\/p>\n

With that in mind, I began poking around and thinking about how I would get something back on the air.\u00a0 In the face of massive disasters, AM and FM radio is still the most effective way to communicate with the general public.\u00a0 Radios are still ubiquitous in homes, cars and businesses.<\/p>\n

\"Bext<\/a>
Bext 30 Watt FM exciter<\/figcaption><\/figure>\n

In a short period of time I came up with a couple of solutions.\u00a0 First, the frequency agile Bext exciter uses a single solid state rectifier feeding 24 volts to the power supply board.\u00a0 The audio input includes a mono balanced line level input which can be fed by a computer sound card or some other simple source.<\/p>\n

\"Bext<\/a>
Bext 30 Watt FM exciter power supply<\/figcaption><\/figure>\n

From there +12, +15 and +20 VDC are created to run various circuits.\u00a0 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.<\/p>\n

\"Bext<\/a>
Bext 30 Watt exciter power supply voltage<\/figcaption><\/figure>\n

The power output is about 22 watts, which is not bad.\u00a0 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.<\/p>\n

\"6<\/a>
6 volt, 435 Ah batteries<\/figcaption><\/figure>\n

Then I though about the deep cycle batteries in my barn.\u00a0 These 6 volt, 435 Ah units have been around for a couple of years, but last I checked, they still held a charge.\u00a0 Other deep cycle batteries from things like golf carts, fork lifts, campers, boats etc could also be pressed into service.\u00a0 The point is, 24 VDC should not be impossible to create.<\/p>\n

To keep a charge on the batteries, this solar panel will work:<\/p>\n

\"225<\/a>
225 Watt, 36 volt solar panel<\/figcaption><\/figure>\n

This setup would require some sort of 24 volt DC charge controller, which I found on Amazon for less than $15.00 US.\u00a0 This charge controller has selectable 24\/12 VDC output and also has two USB ports which would be handy for charging hand held devices.<\/p>\n

I measured the power draw while the exciter was running 20 watts into a dummy load, it draws 120 Watts.<\/p>\n

The final part would be some sort of antenna with transmission line.\u00a0 For this situation, a simple wire center fed dipole hung vertically would work well.\u00a0 This can be fabricated with two pieces of copper wire and a few insulators.<\/p>\n

\"Simple<\/a>
Simple dipole antenna<\/figcaption><\/figure>\n

The lengths of each wire can be calculated as follows:<\/p>\n

Approximate length in feet: 234\/f (MHz)<\/p>\n

Approximate length in inches: 2808\/ f (MHz)<\/p>\n

Approximate length in cm: 7132\/f (MHz)<\/p>\n

For the FM band, maximum length of wires needed will be 32 inches (81 cm).\u00a0 Insulators can be made of anything that does not conduct RF; PVC, ABS, dry wood, dry poly rope, etc.<\/p>\n

\"Emergency<\/a>
Emergency FM band dipole, cut to 88 MHz, lowest FM frequency<\/figcaption><\/figure>\n

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.\u00a0 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.\u00a0 This will create a balun of sorts.<\/p>\n

My emergency FM kit consists of:<\/p>\n