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AirNav RadarBox and RadarBox24.com => AirNav RadarBox and RadarBox24.com Discussion => Topic started by: abcd567 on March 11, 2017, 04:59:22 PM

Title: Franklin-Spider Antenna (for advanced users)
Post by: abcd567 on March 11, 2017, 04:59:22 PM

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

(https://c1.staticflickr.com/1/706/33325660395_b8f282ea37_c.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.

(https://c1.staticflickr.com/3/2863/32512178073_8b58fd9f58_c.jpg)



ACTUAL IMPLEMENTATION
This is a two stepped 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 4pF 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.
(https://c1.staticflickr.com/4/3902/33197922161_b59383de56_c.jpg)


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.


(https://c1.staticflickr.com/1/711/32483024854_7513cc404d_o.png)

Title: Re: Franklin-Spider Antenna (for advanced users)
Post by: abcd567 on March 22, 2017, 06:09:26 PM

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
(https://c1.staticflickr.com/3/2833/33505778796_c042bfaea0_o.png)



RESULTS AFTER INSERTION OF CAPACITOR:
VSWR = 1.16
R=50.2 OHMS   X= -7.5 OHMS
(https://c1.staticflickr.com/3/2909/33505778506_70b4ccedeb_o.png)


CAPACITOR INSERTED = 2.7 pF
AN EMPTIED TV SPLITTER HOUSING IS USED TO INSERT THE CAPACITOR
(https://c1.staticflickr.com/3/2932/33505778576_0848e94736_o.png)