Today we present another article for this round in our non-fiction writing contest – by PrepperDoc
In a grid-down scenario, many will turn to radio for strategic information and advance warning of coming adversaries. Hand-held VHF walkie-talkies will be quite range-limited if their usual repeaters go down also. CB radio may allow 5-10 mile range, and marine radio slightly more, both depending on antenna height.
Short wave (high frequency, “HF”) ham radio will uniquely allow direct two-way communications both locally and over statewide and nationwide distances. Many are planning on this as their primary long distance communications. Unfortunately, if they are relying on DC inverters for AC power, they may be stunned to find overwhelming hash interference coming from the high frequency switching Field Effect Transistors (FET) in their own inverter! In that situation, they may have to temporarily turn off their inverter power source, and turn to battery-powered low-power (“QRP”) ham radio transceivers, or simply monitor battery-powered AM/FM/shortwave broadcast receivers.
I learned this the hard way. Unaware of this problem, I had a very elaborate solar power system installed, using widely available robust and powerful equipment (Out Back Power, www.outbackpower.com). The first time I tested my setup on “emergency battery power,” I was surprised to have significant, wideband hash noise that was coming from my own power system. Adding hefty inline EMI (electromagnetic interference) power line filters right inside the inverter AC panel boxes calmed down the interference to just above background level, very tolerable. Not all inverters produce the same level of interference. A friend who has a MagnaSine 4kW inverter (Magnum Energy, magnumenergy.com) doesn’t seem to have the problem.
Now aware of the problem, and concerned for allies in my area (whom I want to be able to hear my signal!), I did experiments on a couple of different small inverters and some possible filtering solutions, measuring the received signal or interference at 3.7 MHz, in the middle of the 80-meter ham band that can be used for statewide or national communications after sundown.
I used a venerable Heathkit SB-102 vacuum tube transceiver as the test receiver, and to somewhat quantify the amount of interference versus typical signal strength, I measured the interference using the S-unit meter display of the receiver. S-units are a relative and uncalibrated indication of how strong a received signal is, created by automated receiver circuitry that tries to automatically adjust the speaker volume despite widely varying signal strength.
Different receivers may grade the same signal quite differently. My receiver seems to grade signals rather conservatively (giving fairly low S-values). However, each increase of 1 S-unit reflects a significantly easier to hear signal. Background atmospheric noise randomly moved between S1 and S2, while black clouds and lighting ten miles away produced S6 lightning crashes. Such nearby lightning will generally wipe out almost all signals. Strong and easily readable voice single sideband signals were reading S3 or S4 on my receiver.
The Table below presents my results of interference signal-strength measurements:
RECEIVED SIGNAL LEVEL IN S UNITS
|Power Source:||Household AC||Modified Sine Wave Inverter ||Pure Sinewave inverter |
|No powerline filter||Background S1-S2
Nearby lightning S6
|S3 hash||S6 hash|
|Schaffner EMI filter , ungrounded||S1-S2 (barely noticeable)||S3 hash|
|Schaffner EMI filter , grounded to household power ground||S1-1/2, not discernible||S2 hash, somewhat above background noise|
|Homemade jerry-rigged EMI Filter ||No improvement at all (S3)||No improvement at all (S6)|
Surprisingly, the cheaper modified-sine wave inverter creates significantly less wideband (hash) interference than the low-power but higher quality sine wave inverter (which may have faster-switching FETs). Both of these inverters are relatively small, portable, relatively low-priced inverters that can easily be stored in a small Faraday cage at home and might well be chosen as a grid-down backup device. Both produce adequate power for a small vacuum tube ham radio (relatively EMP-resistant) with roughly 75-100 watts output. The 2500-watt modified sinewave inverter can start and run a refrigerator compressor. The interference from either would drown out most signals without power filtering. Both inverters were on a different floor from the receiver (which had a coax transmission line to an outside wire antenna) and a long extension cord went from inverters to the room with the ham radio. Interestingly, the interference seemed to be conducted via plugging the receiver into the output AC power from the inverters — simply having the inverter-powered extension cord in the same room as the receiver did by itself not cause problems.
Interference from solar power inverters has been widely recognized and comes in two flavors, usually simultaneously: differential mode (signal on hot and cold AC wires is 180 degrees out of phase) and common mode (signal on hot and cold AC wires is the same, and significant versus ground). 
A jerry-rigged homemade attempt at a power line filter  was completely ineffective. Apparently one needs higher inductance and capacitance values, and likely a split capacitor with the center tap grounded to reduce common-mode interference.
Placing an inexpensive 20-amp rated commercial EMI filter inline virtually eliminated the interference from the modified sine wave inverter, particularly when the ground tap on the filter was connected to my house wiring ground. The same filter reduced the more potent interference from the sinewave inverter to an acceptable — but still noticeable — level when the ground connection was used. Without the ground connection, such filters reduce only differential-mode interference; the ground connection increases the possibility of reducing common-mode interference.
This type of harmonic interference from the fast switching pulses of a DC-to-AC inverter will vary from brand to brand, and should become weaker at higher frequencies. I did not test to see if it is troublesome at CB frequencies (27 MHz) but I would expect much less of a problem, and I would doubt that it would be a great problem at 2-meter frequencies. However, if you are depending on an inverter for your sound-signature-safe alternate power supply, and wish to have long-range ham radio communications, it would be wise to test your setup and/or purchase some form of EMI filtering and add it right at the output of your inverter before long power runs to portions of your house or your ham radio equipment. It’s an easy, cheap and effective fix.
 Whistler PRO2500W 2500 watt power inverter, 12 volt car battery source. Inverter ungrounded.
 Morningstar SureSine300. 300watt (peak 600 watts 10 minutes). 12 volt car battery source. Inverter ungrounded.
 Schaffner FN2030A-20-06 Power Line Filter, rated 20A AC, purchased from Mouser Electronics, www.mouser.com, details at: http://www.mouser.com/ProductDetail/Schaffner/FN2030A-20-06/?qs=%2fha2pyFadujyPQkwXmbiEp4v1SwIQD%2faY%2fbAgajTvcbbNdiK%2fOAztg%3d%3d
 Homemade filter consisted of series open wire coils approximately 20 microhenries (40 turns, 1.5″ dia) and a homemade shunt capacitor on the load side made with 80 sq. inch aluminum foil plates separated by 20 pound copy paper, measured as approximately 2000 pF.
Prizes for this round (ends July 10 2015 ) in our non fiction writing contest include…
- First place winner will receive – A case of Yoder’s Canned Bacon (12 cans, $169.95), a case of Future Essentials Canned Green Coffee Beans (12 cans, $143.30 value), and a case of our Future Essentials Canned Breakfast/Cold Cereal Variety with Milk (12 cans; a can each of Raisin Bran, Rice Krispies, Corn Flakes, Apple O’s, Whole Grain Frosted Wheat’s, Cocoa Rice Krispies, Honey & Nut O’s, Fruity O’s and Frosted Flakes, as well as three (3) Cans of Powdered Milk Substitute (18 oz. each) – (a value of $62.90) all courtesy MRE Depot and a WonderMix Bread Mixer courtesy of FoodPrepper.com a $300 value. Total first place prize value over$674.
- Second Place Winner will receive – A Royal Berkey water filter, courtesy of Directive 21 (a $283 value) and an autographed copy of 31 Days to Survival…
- Third place winner will receive – A gift certificate for $150 off of Hornady Ammo courtesy of LuckyGunner Ammo.