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.
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.
By 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 deliery 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.
Since it’s the half wave segment that’s causing the mismatch wouldn’t be a better approach to tweak the stub to achieve matching?
Also a coil matching would allow for tube mounting, this stub is not very practical for outdoor use
Comparison of Performance
ONLY STEP-1 (Trimming of RG6 coax) is done, which resulted in bringing down SWR from 3.1 to 2.1
STEP-2 (Capacitor insertion) NOT yet done.
Further improvement expected after capacitor insertion.
I’m still using the one I made ages ago as it’s the best performing antenna I tried. It far outperforms the normal spider, and gives marginally better results than the commercially made sandpiper antenna I have. I have it mounted in the loft using an LNA4ALL connected directly underneath it.
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 FEED COAX TO A POINT WHERE R = 50 OHMS (OR CLOSE TO 50 OHMS) AND X IS +VE
abcd567, consider using a gimmick capacitor next time, e.g. http://213.114.137.49/begin/gimmik-0.htm. They were common in the vacuum tube era (I’m showing my age) but are still a great choice whenever a small pF capacitor is needed.
