TECHNICAL NOTES

(THIS DOES NOT APPLY TO CONVENTIONAL TRACTORS)

1987 FREIGHTLINER CABOVER
   
WHEN INSTALLING A PAIR OF WILSON FGT SERIES ANTENNAS ON TRACTORS OR    
VEHICLES WITH INADEQUATE GROUND OR COUNTERPOISE, BUILD A HARNESS USING   
TWO (2) LENGTHS OF RG8X CUT TO EXACTLY 18 FEET.  IN MOST CASES, THE    
IMPEDANCE MATCHING LINKS DO NOT COME INTO PLAY IN THIS PARTICULAR   
SITUATION.  THIS MATCHING METHOD HAS BEEN USED CONSISTENTLY WITH MUCH   
SUCCESS.  THERE IS A DOWNSIDE TO THIS SCHEME, HOWEVER.  THE BANDWIDTH   
OF THE ANTENNA SYSTEM IS SEVERELY COMPROMISED.  FOR EXAMPLE, WITH A    
PAIR OF 4 FOOT SILVER LOADS THE SYSTEM WILL TUNE THE FREQUENCIES FROM   
26.965 - 27.405 AT 1.7:1 SWR OR LESS ACROSS THESE 40 CHANNELS.  ANYONE   
HAVING WORKED WITH THESE ANTENNAS AT ALL KNOWS THAT IN A CONVENTIONAL   
CONFIGURATION WITH THE 75 OHM HARNESS THE BANDWIDTH OF THESE ANTENNAS   
IS EXTREMELY WIDE, ON THE ORDER OF 1.5 - 2.0 MHZ., STILL MAINTAINING   
AN SWR BELOW 2:1.  CUSTOMERS USING THE ABOVE RECONFIGURED MATCHING   
SCHEME REPORT THAT RANGE IS EXCELLENT, LEADING US TO CONCLUDE THAT IF   
ONE IS WILLING TO SACRIFICE BANDWIDTH FOR EFFICIENCY, ABOVE AVERAGE   
ANTENNA PERFORMANCE CAN BE HAD.  PRESENTLY, ONE DRIVER USING THIS SET-   
UP WITH A COBRA 29 & A PALOMAR 225 IS TALKING CONSISTENTLY FOR DIS-   
TANCES UP TO 40 MILES ON GROUND WAVE. (NO SKIP)  HE PULLS MOBILE HOMES   
USING AN 87 FREIGHTLINER CABOVER.  THIS TRACTOR HAS VERY LITTLE GROUND   
SURFACE IN REGARD TO REALIZING ANY PERFORMANCE FROM A DUAL ANTENNA    
SYSTEM UNLESS THIS PARTICULAR HARNESS IS USED.  WORKS EXTREMELY WELL.

AS CAN BE EXPECTED THE REDUCED BANDWIDTH RESULTS IN A HIGHER "Q"
FACTOR WHICH WOULD EXPLAIN THE EXCELLENT EFFICIENCY OF THE SYSTEM,
NARROW THOUGH IT MIGHT BE. EVEN AT 1.7:1 NEARLY 92% OF AVAILABLE
TRANSMITTER POWER IS BEING ABSORBED BY THE LOAD. EXCELLENT PERFORMANCE
CAN STILL BE HAD SOME 20 - 40 CHANNELS FARTHER UP AND DOWN BUT SWR
WILL MOST LIKELY BE IN THE NEIGHBORHOOD OF 3:1 AT THE BAND EDGES
120 CHANNELS WIDE. EVEN AT THE EDGES TRANSFER EFFICIENCY IS STILL
75%, ASSUMING NEGLIGIBLE LOSSES IN THE LINES. THE Q COULD BE REDUCED
BY USING PHYSICALLY LARGER TOP-LOADED RADIATORS, BOTH IN DIAMETER AND
IN OVERALL LENGTH. THIS WOULD TEND TO KEEP THE SWR OF THE SYSTEM
FROM EVER SEEING 3:1 AT 120 CHANNELS WIDE AND DEPENDING ON THE ANTENNA
DESIGN COULD VERY WELL HOLD THIS TO 2:1 OR LOWER. MOST RADIO OPERATORS
WORRY TOO MUCH ABOUT SWR AS IT IS.

A SINGLE ANTENNA WOULD BE NO HELP HERE AT ALL....3:1 WOULD BE THE BEST
ONE COULD HOPE TO ACHIEVE. MOST INDIVIDUALS WOULD END UP JUST CUTTING UP
THE ANTENNA, RUNNING WIRES ALL OVER THE PLACE, ETC.. THE EXPERIENCED
INSTALLER KNOWS THAT IN SITUATIONS SUCH AS THIS THAT DUAL ANTENNAS ARE
THE WAY TO GO HERE. ONE WAY TO SOLVE THE PROBLEM WITH A SINGLE ANTENNA
WOULD BE TO USE A 1/4 WAVE TRANSFORMER/MATCHING SECTION. HAD THE CUSTOMER
GONE WITH A SINGLE ANTENNA I WOULD HAVE SIMPLY INSERTED A PIECE OF 50
OHM CABLE CUT FOR ONE OR ANY ODD MULTIPLES OF A 1/4 WAVELENGTH FOR THE
FREQ IN USE. 1.3-1.4:1 WOULD BE ACHIEVABLE WITHIN A LIMITED RANGE OF
SPECTRUM. SINCE THIS ARRANGEMENT IS MORE BANDWIDTH RESTRICTIVE THAN A
PAIR OF THE SAME ANTENNAS IT'S PROBABLY BETTER THAT WE WENT WITH THE 
DUAL ARRANGEMENT TO BEGIN WITH. 

JUST TO DEMONSTRATE THE CONTRAST, THE ORIGINAL HARNESS INSTALLED INTO
THE TRACTOR INITIALLY WAS THE STANDARD, DUAL 18' PIECES OF RG-59. THE
ANTENNAS MENTIONED ABOVE WOULD NOT TUNE BELOW 3:1 WHETHER THE LINKS
WERE ATTACHED OR NOT. AS A MATTER OF FACT, THE SWR INCREASED ABOVE
3:1 WHEN THE LINKS WERE GROUNDED. RE-TUNING WAS NO HELP. THIS SHOULD 
TELL YOU QUITE A BIT ABOUT THE ANTENNA SYSTEMS FEEDPOINT INPUT 
IMPEDANCE. MFJ ANALYZER CONFIRMS THAT THE FIGURE WAS AROUND 16 - 17 OHMS. 
ONE FURTHER NOTE: IF AN AMPLIFIER OF ANY APPRECIABLE POWER HAD BEEN USED
WITH THIS SYSTEM AND THE 75 OHM HARNESS DAMAGE TO THE AMPLIFIER WOULD 
ULTIMATELY OCCUR. THE OTHER CONCLUSION ABLE TO BE DRAWN FROM THIS 
INSTALLATION WAS THE FACT THAT ON CABOVER TRACTORS (OR ANY OTHER VEHICLE
WITH INSUFFICIENT COUNTERPOISE) THE TYPICAL FEEDPOINT INPUT IMPEDANCE
IS ABOUT HALF OF WHAT IT IS WHEN THE SAME ANTENNA IS MOUNTED ON SAY 
A CONVENTIONAL TRACTOR RIG. NUMBERS ON THESE USUALLY RUN IN THE NEIGHBOR-
HOOD AROUND 30 - 36 OHMS. IF YOU UNDERSTAND WHAT'S GOING ON HERE WHAT'S
HAPPENING IS READILY APPARENT. 

AGAIN, THE TRACTOR IN THIS EXAMPLE WAS A 1987 FREIGHTLINER CABOVER
(DO NOT INSTALL THIS CONFIGURATION BY DEFAULT INTO ALL TRACTORS)	   
292 RADIO SHOP..................I-40 EXIT 292 HOPI TRAVEL PLAZA


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