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WORKS OFF OF THE MAGNETIC PORTION OF THE RF SIGNAL.  WILL THE
MAGNETIC PORTION PROPAGATE LIKE THE ELECTRIC PORTION?  A ON PAGE 58
OF THE JANUARY 2022 ISSUE OF QST, KAI SIWIAK, KE4PT, EXPLAINS THAT
WHAT WE COMMONLY CALL A MAGNETIC LOOP IS MORE PROPERLY CALLED A
SMALL HF LOOP ANTENNA.  ITS CLOSE NEAR FIELD IS MAINLY A MAGNETIC
RESPONSE, BUT IT ALSO HAS A SMALLER YET SIGNIFICANT ELECTRIC FIELD
RESPONSE.  THE SIZE OF THE NEAR ELECTRIC FIELD DEPENDS ON THE LOOP
CIRCUMFERENCE.  HOWE ASK DAVE HAMS PERSEVERING THROUGH ALL SORTS OF
ISSUES TRACKING DOWN INTERMOD Q OUR CLUB HAS BEEN OPERATING A
REPEATER FOR OVER 30 YEARS AT THE SAME LOCATION.  OF LATE, WEVE BEEN
GETTING INTERFERENCE THAT HAS GROWN WORSE.  WEVE TRACED THE AUDIO
WERE HEARING TO A COUNTRY MUSIC FM STATION WHOSE ANTENNA IS NEAR OUR
REPEATER.  THEY SAY THERES NOTHING WRONG WITH THEIR EQUIPMENT.
WHERE DO WE START?  A ITS VERY LIKELY THAT THE FM STATION IS
CORRECT.  FURTHER, THE INPUT FILTERS FOR YOUR REPEATER THE CANS ARE
SO NARROWBAND THAT THEY WILL NOT LET ANY OF THE FM SIGNAL INTO YOUR
REPEATER.  THE PROBLEM IS INTERMOD.  THE FIRST STEP IS TO ADD A
CTCSS TONE REQUIREMENT TO YOUR REPEATER INPUT.  THAT WAY, EVERY TIME
ONE OF YOUR CLUB MEMBERS TRANSMITS ON THE REPEATER INPUT FREQUENCY,
THEY ARE INCLUDING A SUB AUDIBLE TONE THAT THE REPEATER MUST HEAR
BEFORE IT WILL WORK.  USUALLY, INTERMOD WILL NOT INCLUDE THIS
FREQUENCY.  IF IT DOES, CHANGE THE CTCSS FREQUENCY.  THIS COULD
SOLVE THE PROBLEM, AND IS WHAT MANY REPEATER OPERATORS HAVE DONE.
IF THE INTERMOD BLOCKS LEGITIMATE INCOMING SIGNALS, YOU HAVE MORE
DETECTIVE WORK TO DO.  THE FM STATIONS SIGNAL IS BEING ADDED TO
ANOTHER STATIONS FREQUENCY IN A NON LINEAR DEVICE, AND THE OUTPUT
FREQUENCY IS EQUAL TO YOUR INPUT FREQUENCY.  SUBTRACT THE FM
STATIONS OUTPUT FREQUENCY FROM YOUR REPEATERS INPUT FREQUENCY.  LOOK
AROUND FOR A SOURCE OF THIS FREQUENCY, WHICH COULD BE A COMMERCIAL
REPEATER OR SOME OTHER SOURCE.  OTHER THINGS CAN ALSO ADD UP OR
SUBTRACT DOWN TO YOUR REPEATERS INPUT FREQUENCY.  THIS IS WHERE IT
GETS DIFFICULT.  FOR EXAMPLE, NO MATTER HOW GOOD THE FM STATION IS,
AS CLOSE AS IT IS TO YOUR INPUT ANTENNA, THERE WILL BE HARMONICS.
AND WHERE IS THIS NON LINEAR DEVICE THAT IS MIXING ADDING AND
SUBTRACTING FREQUENCIES WHEN MORE THAN ONE SIGNAL IS PRESENT?  IT
COULD BE ANYTHING, SUCH AS A LOOSE BOLT SOMEWHERE ON THE TOWER, AN
ISSUE WITH SOMEONES COAX, OR JUST ABOUT ANYTHING.  YOU CAN TRY
MOVING THE REPEATERS ANTENNA A FEW FEET.  THE SEARCH FOR THE
INTERMOD SOURCE CAN BE EXTREMELY FRUSTRATING.  ARRL HAS A TECHNICAL
INFORMATION SERVICE TIS ARRL.  ORG THAT CAN HELP WITH DEEPER
PROBLEMS.  INSTALLING A VERTICAL IN A SMALL SPACE Q IM CONTEMPLATING
A DX COMMANDER ANTENNA.  I HAVE A SMALL YARD WITH ONLY ABOUT 18 20
FOR RADIALS.  THE GRASS IS NOT HEALTHY AND WILL NEVER GROW OVER THE
RADIALS.  CAN I COVER THEM WITH GRAVEL?  WHAT ABOUT PUTTING
ARTIFICIAL TURF OVER THEM?  WILL EITHER APPROACH INTERFERE WITH
THEIR FUNCTION?  A COVERING THEM WITH AN INCH OR TWO OF GRAVEL IS
FINE.  THE ARTIFICIAL TURF WILL NOT

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