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 Post subject: Franklin-Spider Antenna (for advanced users)
PostPosted: Wed Mar 08, 2017 12:20 pm 
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abcd567 - FlightAware user avatar

Joined: Sat Feb 15, 2014 3:08 am
Posts: 1862
Location: Toronto CYYZ
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

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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.

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ACTUAL IMPLEMENTATION

This is a two-step SWR improvement procedure.

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.


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Last edited by abcd567 on Sat Mar 11, 2017 12:23 pm, edited 2 times in total.

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 Post subject: Re: Franklin-Spider Antenna (for advanced users)
PostPosted: Wed Mar 08, 2017 3:29 pm 
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aldoir - FlightAware user avatar

Joined: Sat Oct 29, 2016 5:54 pm
Posts: 79
Location: Curitiba/Brazil
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


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 Post subject: Re: Franklin-Spider Antenna (for advanced users)
PostPosted: Sat Mar 11, 2017 11:43 am 
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abcd567 - FlightAware user avatar

Joined: Sat Feb 15, 2014 3:08 am
Posts: 1862
Location: Toronto CYYZ
METHOD OF INSERTION OF CAPACITOR

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- - - - OR - - - -

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 Post subject: Re: Franklin-Spider Antenna (for advanced users)
PostPosted: Sun Mar 12, 2017 2:31 am 
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abcd567 - FlightAware user avatar

Joined: Sat Feb 15, 2014 3:08 am
Posts: 1862
Location: Toronto CYYZ
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.



FlightAware Antenna + FA Filter + ProStick (Orange)
http://flightaware.com/adsb/stats/user/ ... stats-6396
Messages (6 day's average) = 624,404
Aircrafts (6 day's average) = 2,705


Franklin-Spider Antenna + FA Filter + ProStick (Orange)
http://flightaware.com/adsb/stats/user/ ... stats-5252
Messages (6 day's average) = 501,112 (80% of FA Antenna)
Aircrafts (6 day's average) = 2,623 (97% of FA Antenna)


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 Post subject: Re: Franklin-Spider Antenna (for advanced users)
PostPosted: Tue Mar 14, 2017 12:00 pm 
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caius - FlightAware user avatar

Joined: Sat Mar 07, 2015 4:51 pm
Posts: 249
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.


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 Post subject: Re: Franklin-Spider Antenna (for advanced users)
PostPosted: Thu Mar 16, 2017 11:49 am 
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abcd567 - FlightAware user avatar

Joined: Sat Feb 15, 2014 3:08 am
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Location: Toronto CYYZ
caius wrote:
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.

Yeah, I remember, and it was without impedance matching:

You, April 18, 2015: https://forum.planefinder.net/threads/ads-b-diy-antenna.23/page-141#post-4243

Me, April 07, 2015: https://forum.planefinder.net/threads/ads-b-diy-antenna.23/page-131#post-3797
.


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 Post subject: Re: Franklin-Spider Antenna (for advanced users)
PostPosted: Mon Mar 20, 2017 4:13 am 
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abcd567 - FlightAware user avatar

Joined: Sat Feb 15, 2014 3:08 am
Posts: 1862
Location: Toronto CYYZ
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
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RESULTS AFTER INSERTION OF CAPACITOR:
VSWR = 1.16
R=50.2 OHMS X= -7.5 OHMS
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CAPACITOR INSERTED = 2.7 pF
AN EMPTIED TV SPLITTER HOUSING IS USED TO INSERT THE CAPACITOR
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 Post subject: Re: Franklin-Spider Antenna (for advanced users)
PostPosted: Wed Mar 22, 2017 9:55 pm 
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nu3e - FlightAware user avatar

Joined: Tue Apr 21, 2015 8:42 am
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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.


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 Post subject: Re: Franklin-Spider Antenna (for advanced users)
PostPosted: Sun Mar 26, 2017 10:54 am 
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abcd567 - FlightAware user avatar

Joined: Sat Feb 15, 2014 3:08 am
Posts: 1862
Location: Toronto CYYZ
nu3e wrote:
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.

Good tip. Thanks nu3e. I will try the gimmick capacitor next time.

Gimmick Capacitor Image Source: from web page linked by nu3e
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