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Kitfox vertical fin antenna opportunities

 
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nuckolls.bob(at)aeroelect
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PostPosted: Wed Feb 10, 2021 6:12 pm    Post subject: Kitfox vertical fin antenna opportunities Reply with quote

I'm hearing stories that builders are experiencing
'satisfactory' performance with vhf comm antennas
embedded in the structure of a Kitfox vertical
fin.

I'd like to better understand the physics of
this location a bit better. I've attached a
photo of a Kitfox vertical fin sans fabric.

Would you folks with access to a 'naked'
structure tell me what dimensions apply to
the items labeled in the attached photograph?

Thanks!

P.S. Names and email addresses of individuals
flying Kitfox, embedded off-the-shelf antennas for VHF
comm would be useful too.



Bob . . .

Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"


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Kitfox_fin_structure.jpg
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Kitfox_fin_structure.jpg


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Eric Page



Joined: 15 Feb 2017
Posts: 241

PostPosted: Thu Feb 11, 2021 10:02 am    Post subject: Re: Kitfox vertical fin antenna opportunities Reply with quote

nuckolls.bob(at)aeroelect wrote:
Would you folks with access to a 'naked'
structure tell me what dimensions apply to
the items labeled in the attached photograph?


A couple of notes... This installation would only be possible in a Kitfox with the optional airfoiled tail, which adds wood ribs to the steel structure. Otherwise the vertical stab and rudder are flat sided and there is no room to pass an antenna vertically within the stab. When installed, the ribs are only there to shape the fabric and are not structural.

I bought my project from its first builder, and the tail ribs were already installed. It was very messily done, but the ribs are firmly attached so I did not remove/replace them. Their location is specified in the build manual, but accuracy obviously depends on the builder. These measurements are from my example:

A: 6-1/2"
B: 6-3/8"
C: 6-1/8"
D: 6-3/8"
E: 19" (sits on 3/8" dia steel tube)
F: 22-1/2"
G: 27" (widest rib; sits on 7/8" dia steel tube)
H: 6-1/4"

The ribs taper from 3-3/4" wide at the bottom of "A" to 1-7/8" wide at the top of "H".

I'm attaching some photos of how I did my installation. I fabricated a wood block with an angled end that would direct the antenna with equal spacing between the forward and aft tubes in the stab. The wood block has a slot in the bottom end to receive the antenna, which is secured with a fillet of caulk.

Each rib has a 1" slot, through which the antenna elements pass, again secured with a small dab of caulk to prevent chafing. The center feed point falls in the space labeled "B" in your photo, so the coax will be supported by Adel clamps on diagonal steel member "F". The feed point is secured against vibration by a vertical strip of wood bonded to the ribs above and below, and attached to the feed point with a dab of caulk.

The bottom element of the antenna will be secured by pinching between two small wood blocks, with their inner surfaces lined with a thin rubber sheet for grip. This will be done after the horizontal stab is installed, as the antenna passes through the open center section of the stab.

The 1" dia tube near the bottom of the antenna is aluminum; it's the elevator push-pull tube.

Quote:
Names and email addresses of individuals
flying Kitfox, embedded off-the-shelf antennas for VHF
comm would be useful too.

I can't help with that, unfortunately. I roughly copied the installation done by another builder who used this antenna. He reported on the TeamKitfox forum that he hears aircraft in the pattern at an airport 70 miles away from his, has good comms with ATC and has had no complaints about his transmissions.

As I posted in the other thread, I'll try to get a VNA sweep once I have the coax installed and the weather improves so I can roll the fuselage outside. It's snowing sideways for the next several days.


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IMG_1267.jpg
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Finished Installation
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IMG_1267.jpg



IMG_1099.jpg
 Description:
Bottom Mounting Block
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IMG_1099.jpg



IMG_0965.jpg
 Description:
Vertical Stab Rib Slots
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IMG_0965.jpg



IMG_0963.jpg
 Description:
Top Mounting Block
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 Viewed:  2027 Time(s)

IMG_0963.jpg


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user9253



Joined: 28 Mar 2008
Posts: 1907
Location: Riley TWP Michigan

PostPosted: Thu Feb 11, 2021 2:49 pm    Post subject: Re: Kitfox vertical fin antenna opportunities Reply with quote

It took me awhile to realize that the vertical black object going through the
middle of the ribs is actually the com antenna and not structural steel. Is that
a black plastic box at the bottom? What is inside of it? And what is the
purpose of the triangular part that you are holding? Thanks
Theoretically the steel tubes will block radio waves fore and aft. But one can
not argue with the success of others. The installation looks good and I think
that you will be happy with the antenna.


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Joe Gores
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Eric Page



Joined: 15 Feb 2017
Posts: 241

PostPosted: Fri Feb 12, 2021 10:43 am    Post subject: Re: Kitfox vertical fin antenna opportunities Reply with quote

user9253 wrote:
It took me awhile to realize that the vertical black object going through the
middle of the ribs is actually the com antenna and not structural steel. Is that
a black plastic box at the bottom? What is inside of it? And what is the
purpose of the triangular part that you are holding? Thanks
Theoretically the steel tubes will block radio waves fore and aft. But one can
not argue with the success of others. The installation looks good and I think
that you will be happy with the antenna.

Yes, the antenna is sort of a fabric tape with, I presume, copper foil elements bonded to it, then encased with a layer of fiberglass and resin. It's about 1" wide and perhaps 0.05" thick.

The "box" and triangular object are the same item: a small block of wood (painted black). It's half of the clamp for the bottom end of the antenna.

The antenna's feed point is a ~1" cube in the center of the tape, located just behind the diagonal steel tube. The BNC is just visible in the "Finished Installation" photo.


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