Franklin-Spider Antenna (for advanced users)

ab cd

Senior Member
#1
SIMULATION RESULTS
Gain = 3.7 dBi
SWR = 3.1
Impedance = 155 - j 8.18 ohms
HIGH IMPEDANCE/SWR - WILL NOT GIVE GOOD RESULTS WITHOUT IMPEDANCE MATCHING

Franklin-Spider Simulation.jpg



IMPEDANCE MATCHING BY "TRANSMISSION LINE+CAPACITOR" TECHNIQUE
Using Smith Chart:
Capacitor to be inserted = 3.8 pF (4 pF will do)
Coax length (from antenna) where Capacitor should be inserted = n x λ/2 + 80.8mm Coax of VF=0.85,
where n is any integer.

Franklin-Spider Smith Chart.jpg




ACTUAL IMPLEMENTATION

This is a two step SWR improvement process.

STEP (1): Trim RG6 feed coax at the pigtail till value of R becomes close to 50 ohms.

STEP (2): Add a capacitor in series with core wire of coax to nullify +X component of impedance. Location of capacitor: between pigtail and filter.

By doing the STEP (1) ONLY, impedance changes from 155-j8.2 (TP1 on Smith Chart) to 50+j38.3 (TP2 on Smith Chart). This results in changing SWR from 3.1 to 2.1, which gives a big improvement in performance.

Doing STEP (2) after doing STEP (1) results in impedance becoming 50+j0, and SWR=1, a perfect match.


Note: Unfortunately I have miss-calibrated my antenna analyzer using a 75 ohm resistor. Now it is not resetting to factory calibration of 50 ohms. I have ordered a 50 ohm dummy load resistor. Waiting for delivery to re-calibrate the Analyzer with 50 ohms load. I therefore could not actually do the impedance matching. Following only show the steps I will do when my Analyzer is re-calibrated.


Franklin-Spider -1.jpg



Franklin-Spider - 2.png


Franklin-Spider - 3.png
 
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ab cd

Senior Member
#2
In my first post, the antenna Analyzer was miscalibrated, and showed 45 ohms reading where it should have shown much higher (around 100 ohms) reading. I was therefore not able to do the impedance matching. I ordered a 50 ohm load resistance to be able to calibrate the Antenna Analyzer.

Now I have received 50 ohm load resistor, and calibrated the Antenna Analyzer.
Conducted impedance matching (feed coax trimming+insertion of capacitor) after calibrating the Antenna Analyzer


INSERTION OF CAPACITOR, AFTER TRIMMING THE COAX TO A POINT WHERE R = 50 OHMS (OR CLOSE TO 50 OHMS) AND X IS +VE
Insert Capacitor - 1.png




RESULTS AFTER INSERTION OF CAPACITOR:
VSWR = 1.16
R=50.2 OHMS X= -7.5 OHMS

Insert Capacitor - 2.png



CAPACITOR INSERTED = 2.7 pF
AN EMPTIED TV SPLITTER HOUSING IS USED TO INSERT THE CAPACITOR

Insert Capacitor - 3.png
 
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