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  #11  
Old 01-26-2022, 02:00 PM
TASEsq TASEsq is offline
 
Join Date: Aug 2017
Location: Melbourne, Australia
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Hi All,

Thanks for all the help.

I will see if i can borrow an analyser. It looks fairly straight forward:
https://youtu.be/aBtYlL-OLoE

Do I have to analyse it with the full run of coax to the panel? Or would it be ok to get it resonating properly at say 113mhz using just the coax from the antenna to the wing root? (As the fuselage doesn’t exist yet )

To summarise everyone’s suggestions:
1. Ground a piece of angle to the wing rib with screws / nutplates (I have some j channel like Carl used - great idea)
2. Keep the gamma match triangle dimensions the same (5”) as well as the thickness of the insulator material.
3. Extend the spanwise arm to help it reach the outer parts of the wingtip and reduce the length of the chord wise arm.
4. Use steel hardware to connect the shield to the ground side and nylon (with any type of nuts I assume) on the capacitor.
5. Use an analyser to tune.

A question - say I was going for 113 but the analyser showed it resonating at 120mhz. Which way would you go with the chord wise arm? Shorter or longer? Similarly, if the impedance was say 55ohms would you make the insulator shorter or longer?

@carl - is yours in a rv14? If so, would you know the dimensions of the 2 arms you used and also the length of the insulator by any chance?
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http://www.tasrv14.blogspot.com.au/

Last edited by TASEsq : 01-26-2022 at 02:15 PM.
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  #12  
Old 01-26-2022, 02:18 PM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is offline
 
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Location: Dogwood Airpark (VA42)
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Trent.

I made wingtip antennas for RV-8A, 8, 10 and a 14. The dimensions are whatever I could get out into the wingtip.

If you end up with the resonant frequency too high, you trailing element is too short. I suggest starting with it a little long and trim to the desired frequency (in the middle of the VOR band, around 113mhz).

This is an iterative process. Trim the trailing element to get close, then tune the gamma match to get the lowest SWR on the analyzer. Repeat as needed. This typically takes just a few minutes.

You can have a pigtail on the antenna to connect to the analyzer to do this or do it at the end of the coax run. The reading will be a little different but way in the grass for what we need.

One note - I add a piece of piece of 3/8" split nylon conduit on the outboard edge of the trailing element as something to keep the antenna from rubbing against the inside of the wingtip. I attach with small zip ties into small holes in the aluminum. A few dabs of RTV would work just as well.

Carl
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  #13  
Old 01-26-2022, 02:29 PM
TASEsq TASEsq is offline
 
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Location: Melbourne, Australia
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Also,

I found some “Bakelite” here. Is this what is used?
https://www.swiftsupplies.com.au/fab...s-250mm-square

Or would a fibreglass board like this work?
https://www.swiftsupplies.com.au/iso...s-250mm-square

Or can I just lay up some normal fibreglass with west systems etc to 0.032”?
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Trent Stewart
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http://www.tasrv14.blogspot.com.au/

Last edited by TASEsq : 01-26-2022 at 02:32 PM.
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  #14  
Old 01-26-2022, 02:30 PM
chaskuss chaskuss is offline
 
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Quote:
Originally Posted by Carl Froehlich View Post
108-137mhz
Carl, does that cover the VOR/LOC range? Or all the aviation bands [Com, Nav, GPS, ELT, etc] ? I ask, because Bob suggested getting your EAA Chapter to buy an analyzer. I need to know what specs the analyzer needs to check all aviation antennas. Sorry I didn't specify that the first time.

Charlie
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  #15  
Old 01-26-2022, 02:39 PM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is offline
 
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Quote:
Originally Posted by chaskuss View Post
Carl, does that cover the VOR/LOC range? Or all the aviation bands [Com, Nav, GPS, ELT, etc] ? I ask, because Bob suggested getting your EAA Chapter to buy an analyzer. I need to know what specs the analyzer needs to check all aviation antennas. Sorry I didn't specify that the first time.

Charlie
All the VHF bands, and HF if you do that in your plane.

The one I listed will do everything you need. Note, this is an antenna analyzer, not an SWR bridge.
https://www.dxengineering.com/parts/mfj-259d

For this NAV antenna the VOR/LOC band is 108-118mhz.

Carl
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  #16  
Old 01-26-2022, 02:43 PM
TASEsq TASEsq is offline
 
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Quote:
Originally Posted by Carl Froehlich View Post
This is an iterative process. Trim the trailing element to get close, then tune the gamma match to get the lowest SWR on the analyzer. Repeat as needed. This typically takes just a few minutes.
Thanks Carl,

Iím definitely going to give this a go if I can get my hands on an analyser.

Iíll extend the spanwise arm into the wingtip as far as I can, and Iíll make the chordwise arm long. Iíll trim it short to get it close to 113 using the analyser.

I see on spruce 2.0:1 is listed as the SWR for a purchased antenna. If mine was higher than this, would I shorten the Bakelite? Is it worth making this also a little longer at the start?
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  #17  
Old 01-26-2022, 02:52 PM
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Carl Froehlich Carl Froehlich is offline
 
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Quote:
Originally Posted by TASEsq View Post
SNIP

I see on spruce 2.0:1 is listed as the SWR for a purchased antenna. If mine was higher than this, would I shorten the Bakelite? Is it worth making this also a little longer at the start?
After the antenna length is about right, adjust the Gamma match for lowest SWR on the analyzer. You can experiment by just loosening the screw holding the Bakelite between the two pieces of aluminum as this simulates a thicker insulator (less capacitance). You can also experiment with changing the aluminum pieces of the capacitor sandwich (shorter is less capacitance, longer is more)

All this is ~$1 of material. Play around with it.

Carl
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  #18  
Old 01-26-2022, 04:14 PM
TASEsq TASEsq is offline
 
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Interesting.

So the Bakelite length is not really that important? Itís the length of the aluminium piece on top?

Thank you for your advice.
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  #19  
Old 01-26-2022, 04:26 PM
TASEsq TASEsq is offline
 
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Carl: I found my answer - thanks for all your guidance.

https://vansairforce.net/community/s...56&postcount=7
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  #20  
Old 01-26-2022, 10:31 PM
TASEsq TASEsq is offline
 
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Does the long arm need to be parallel to the ground plane arm? (The most outboard arm running fore/aft parallel to the rib?)
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