Transcript
RF in the SHACK and OTHER GROUNDING MYTHS NCARC NOVEMBER 2009 MEETING PRESENTED BY: DAVE – W0LEV
A POSSIBLE GOOD GROUND? A ground rod is an 8 foot long copper coated road pounded 7 ½ feet into the ground. It should be in a place that is moist from time to time. You cannot put it in dry sand and expect it to work. The ground rod wire, for a power system ground, should be #8 wire or larger. Construction Plans will provide a specification or the National Electrical Code can be consulted. You could connect a toaster between the hot side of the power line and a ground rod. Since the neutral of the power line is grounded you should have a complete circuit. Connect the toaster normally and make a piece of toast and time the toaster until the toast pops up. Then, connect the toaster to the HOT side of the power line and to the ground rod and make a piece of toast. If the time is within 25%, comparing the first piece and the second piece then you have a good ground. You should fully understand electricity and the safety needed when doing this test BEFORE you attempt it. If the toaster time is too far off then drink a quart of liquid, wait an hour or so, and then wee on the ground rod and make sure you wee about ¾ of the quart out (do not have the toaster connected or operational). Wait 1 hour and re‐test with the toaster. This will test to see if the soil needs to be wet. Getting a second person to do this and weeing one hour after you wee, and then waiting 1 hour after the second wee would be a better test.
THREE TYPES OF GROUND • SAFETY: ASSURES NEUTRAL AND “GREEN” ARE AT THE SAME POTENTIAL SO YOU DON’T GET SHOCKED – NOTHING TO DO WITH RF IN THE SHACK
• LIGHTNING: PROVIDES A RETURN FOR LIGHTNING INDUCED RETURN CURRENTS SO YOU DON’T TAKE LIGHTNING DAMAGE TO YOUR HOUSE OR EQUIPMENT – NOTHING TO DO WITH RF IN THE SHACK
• RF: PROVIDES A RETURN FOR INDUCED RF CURRENTS (HEAVEN FORBID EMP) ‐ NOTHING TO DO WITH RF IN THE SHACK
THEN WHAT IS A “GROUND”
RF IN THE SHACK? • IF YOUR ANTENNA IS WORKING AS INTENDED, YOU DARN WELL BETTER HAVE “RF IN THE SHACK”!!
WHAT IS RF IN THE SHACK • WHAT IT IS: GETTING “STUNG” OR “BURNED” FROM ABOUT ANY CONDUCTOR WHILE TRANSMITTING THE TRANSMITTER MAY EXPERIENCE RF FEEDBACK IN THE AUDIO CIRCUITRY THE PRESENCE OF HIGH RF FIELDS IN THE SHACK •
WHAT IT IS NOT: DEPENDANT ON “GROUNDING”
RF IN THE SHACK? • DO WE STILL HAVE “RF IN THE SHACK”?
MY TYPICAL CAMPING STATION GUARANTEED RF IN THE “SHACK”
OH‐OOOHH, MAYBE THIS ISN’T SUCH A GOOD IDEA • GET “BIT” OR RF‐BURNED BY TOUCHING EVEN THE SMALL MICROPHONE CLIP (at 80‐watts) • CAN NOT OBTAIN A MATCH TO THE WIRE • RADIO RF OUTPUT IS IRRATIC AND UNSTABLE • RF INTO THE AUDIO CAUSES “RF FEEDBACK” • RADIO AUDIBALLY SQUEELS WHEN THE MICROPHONE IS KEYED.
SOLUTION BIG FAT CONDUCTOR FROM RADIO CHASSIS TO JEEP SHEET METAL NOTE: RUBBER TIRES DON’T CONDUCT RF ENERGY
NOTHING HAS BEEN “GROUNDED”!
MODEL OF THE CAMPING SETUP
TRANSMITTER TRANSMITTER
NO SINK FOR RETUTN CURRENTS (RADIATIVE OR NON‐RADIATIVE)
VEHICLE SKELETON IMAGE PLANE (Jeep)
RETURN CURRENTS TRAVEL THROUGH FAT CONDUCTOR
NOW, WHAT IS RF IN THE SHACK? • SIMPLY: AN ATTEMPT OF THE RF FIELDS GENERATED IN THE INTENDED ANTENNA TO SINK IMAGE OR RETURN CURRENTS ‐ THE ANTENNA IS INCOMPLETE – THIS IS WHAT CAUSES THE BURNS AND BITES. • THE PRESENCE OF HIGH RF FIELDS IN THE SHACK – THIS DOES NOT BURN AND BITE.
JUMP ROPE ILLUSTRATION THINK FIELDS AND WAVES
PROVISION MADE FOR IMAGE OR RETURN CURRENTS
NO PROVISION MADE FOR IMAGE OR RETURN CURRENTS
IS “GROUNDING” REQUIRED TO ELIMINATE “RF IN THE SHACK”?
SUPPLY THE REST OF THE ANTENNA, DUMMY • IN A RADIATIVE MANNER – THE OTHER HALF OF THE DIPOLE
• IN A NON‐RADIATIVE MANNER – THE VERTICAL MONOPOLE IMAGE PLANE
CAN A GOOD DC GROUND BE A GOOD RF “GROUND”? • ZERO LENGTH – MAYBE – BUT WHY • ¼‐WAVELENGTH LONG CONDUCTOR – NO BUT, THE ¼‐WAVELENGTH “GROUND” CONDUCTOR BECOMES A PART OF THE ANTENNA! IT SUPPLIES A CONDUCTIVE RETURN PATH FOR THE (NOW) RADIATIVE RETUREN CURRENTS!
WHAT DOES THE RADIATING ? Mobile Resonator Coaxial Feedpoint (center conductor to resonator, braid to tower)
Does This Matter ???
Tall Tower
Hint: What is the Radiation Resistance (if you answer 50‐ ohms, you flunk)
Why ?? Hint: Without the tower, would there be “RF In the Shack”?
THE FACTS •
FACT: A severely electrically shortened “radiator” (the mobile resonator) presents a very LOW radiation resistance (notice this is resistance, the real part, not the reactive part)
• •
RESISTANCE: A) Frequency Independent (R) REACTANCE: A) Frequency Dependent (L or C)
B) Can transform types of energy B) Can not transform types of energy
•
The radiation resistance is generated as a result RF fields on a conducting structure and is dictated by the physics of those fields coupling to free space ‐ radiation (we’ll end EM theory there).
•
In a radiating structure, what is responsible for producing radiation? Voltage? Current? Magic? Why? Why should I care? What does it matter? All? None? Black Magic? Witchcraft? The Large Hadron Collider?
OHM’S LAW • What does OHM’S LAW have to say about this? Yes, OHM’S LAW, remember? •
It doesn’t apply to just resistors! Does Ohm’s Law require a “ground”?
• V = IR
Simple linear algebra. REMEMBER IT !!!!!!!!
I had this on my Novice Exam in 1959!! Its worth remembering!!!!! •
Low Resistance dictates
high current and low voltage
THEREFORE •
ANSWER: THE TOWER
Low‐Z Feedpoint
RF Current Generates fields which couple energy to free space ‐ radiation
The Mobile Resonator Contributes an Insignificant Amount of Energy to the Radiated Fields of the Structure.
QUESTIONS
