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  #21  
Old 01-04-2019, 02:18 PM
dlloyd3's Avatar
dlloyd3 dlloyd3 is offline
 
Join Date: Jun 2005
Location: Locust, NC
Posts: 416
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Still have the Van's cheapo foil in the right wing. SL30 still picks up VOR signals good to 80 or so miles (always use GPS so don't pay attention). Still picks up LOC and GS too.
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Dave
RV-7 Tipup, Flying, I0-360,
Hartzell, Dual Skyview
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  #22  
Old 01-04-2019, 06:22 PM
birddog486 birddog486 is offline
 
Join Date: Mar 2016
Location: WI
Posts: 68
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Quote:
Originally Posted by Carl Froehlich View Post
Shoot fire - a lot of churn and some bad information. Some thoughts:
- A Marker Beacon antenna can be a simple piece of wire strung under the engine and ending in BNC bulkhead connector on the firewall. The wire would be 36” or so long (1/4 wavelength). Marker Beacons transmit energy is almost straight up so even a very poor antenna works just fine. I pick up Marker Beacons with this set up miles from airports when at altitude.
- You will find any dipole style VOR antenna mounted inside the cowl to not work. There is just way too much ignition noise when the antenna is only inches away from sparkplug wires.
- You will find any dipole style VOR antenna mounted to roll bars and such to not work. Too much coupling of the high impedance ends of the antenna to ground.
- I did try a temporary dipole VOR antenna run along the top of the fiberglass cabin top in the RV-10. It worked, but poorly. Even there the noise from all the panel switching power supplies was not acceptable.

The cheapest and most effective VOR antenna is a homebrew wingtip antenna. I’ve built many for my planes and others - perhaps $3 of parts. You can get the basic design out of the AeroElectric book. If you do not know how to tune an antenna or have the needed antenna analyzer then don’t build, just buy. You can improve performace by:
- Not running the nav/strobe wire on the antenna leg like Archer tells you to do. Just slide the antenna aft a few inches.
- Strive to change the dimensions to extend the antenna as far into the wingtip as it will go.
- Mount the antenna on a light piece of angle that is connected to the end rib, not glassed into the wingtip. The wingtip just slips over the antenna.

My wingtip VOR antenna works out to 100nmi and picks up the LOC/ILS at well beyond practicable range - both far more than I will ever need.

Carl
Carl, do you have any pics of the vor antenna attached to the wing rib? I'm trying to visualize it but the antenna seems to flimsy to be mounted like that.
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  #23  
Old 01-05-2019, 09:21 AM
BMC_Dave BMC_Dave is offline
 
Join Date: Mar 2017
Location: Seattle, WA
Posts: 277
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Quote:
Originally Posted by Carl Froehlich View Post
What airplane uses carbon fiber for the wingtip? This is gross overkill.
Carl
Gross overkill? Lol, oh my.
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  #24  
Old 01-31-2019, 01:17 AM
XPPilot XPPilot is offline
 
Join Date: May 2012
Location: Dallas, TX
Posts: 147
Default Cheap NAV Antennas?

I am still looking for a picture of antennas you build with dimensions.
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  #25  
Old 01-31-2019, 07:24 AM
rv7charlie rv7charlie is online now
 
Join Date: May 2006
Location: Pocahontas MS
Posts: 3,862
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Try this recent thread; also sign up for the Aeroelectric Connection email list & visit their site. There are plans for an Archer clone on the site.

http://www.vansairforce.com/communit...d.php?t=168066
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  #26  
Old 02-10-2019, 12:30 PM
keitht keitht is offline
 
Join Date: Jun 2016
Location: coupeville wa
Posts: 60
Default Antenna for legal IFR

Trying to make decisions about what equipment to install for a legally capable IFR capable RV-7A without taking out a mortgage to buy an expensive GPS navigator. I went through the FARs and relevent ACs and the conclusion I came to was that the Minimum Equipment List and cost would be a TSO’d VOR/ILS nav/com and associated installation. This requires a TSO’d antenna according to my read of the regulations. While a stripped coax may be made to work clearly it wouldn’t stand up to examination as meeting any objective requirments for a “certified” antenna. Unbalanced, no isolation for vertical polarization, undefined VSWR, undefined beam pattern - to name just basic parameters. So if the basic idea is to have a VOR/ ILS capability for Flight under VMC conditions it is in persuit of our certification in the “experimental” category but not suitable or legal for flight in IMC conditions. I am persuing purchasing a second hand certified antenna on Ebay for a fraction of the new price and doing my own recert process. This may turn out to be a wasted effort - will update as progress is made.
KT
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  #27  
Old 02-10-2019, 12:47 PM
rv7charlie rv7charlie is online now
 
Join Date: May 2006
Location: Pocahontas MS
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I'm far from being an authority, but my understanding is that while GPS must be TSO for IFR even in homebuilts, VOR/ILS can be whatever 'meets the requirements'. Perhaps someone with more intimate knowledge will jump in.
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  #28  
Old 02-10-2019, 03:48 PM
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Carl Froehlich Carl Froehlich is online now
 
Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 2,028
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Quote:
Originally Posted by keitht View Post
Trying to make decisions about what equipment to install for a legally capable IFR capable RV-7A without taking out a mortgage to buy an expensive GPS navigator. I went through the FARs and relevent ACs and the conclusion I came to was that the Minimum Equipment List and cost would be a TSO’d VOR/ILS nav/com and associated installation. This requires a TSO’d antenna according to my read of the regulations. While a stripped coax may be made to work clearly it wouldn’t stand up to examination as meeting any objective requirments for a “certified” antenna. Unbalanced, no isolation for vertical polarization, undefined VSWR, undefined beam pattern - to name just basic parameters. So if the basic idea is to have a VOR/ ILS capability for Flight under VMC conditions it is in persuit of our certification in the “experimental” category but not suitable or legal for flight in IMC conditions. I am persuing purchasing a second hand certified antenna on Ebay for a fraction of the new price and doing my own recert process. This may turn out to be a wasted effort - will update as progress is made.
KT
Sorry - your assumptions are wrong. Only the IFR GPS navigator needs this certification.

Antennas are not rocket science. For me, my homebrew VOR/LOC/ILS wingtip antenna is first choice over any “TSO” product, especially one purchased on eBay. I do however measure and tune each antenna for optimal performace. If you buy commercial, this step is typically not done.

Carl
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  #29  
Old 02-10-2019, 04:37 PM
keitht keitht is offline
 
Join Date: Jun 2016
Location: coupeville wa
Posts: 60
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Carl,
Thanks for the reply. I think the ebay antenna is a long shot for a number of reasons ( Dorne & Mangolin DMN-42 towel rail type) but we shall see.
I looked at the Bob Archer design for the wingtip and had the following concerns. The design looks optimized for the VHF band for VOR and Localizer with a gamma match. The gamma match will be way off at the 300 Mhz glideslope band so performance is problematic for glideslope. I also looked at the “whisker”
type antenna that have a balun and matching network and work quite well across the two bands. I haven’t seen any beam pattern data for the Archer antenna but would suspect that it would have a R cos theta shape off of the wingtip with a null off the other wingtip. I would also be concerned that the rejection of the vertically polarized wave component is not adequate for either VOR or localizer.
I thought about designing a center fed microstrip phantom slot but the dimensions are quite large and placement would be a big issue. Another big issue is not having access to all the test equipment any more so I feel the need to go with a tested design. The Archer design is appealing - cheap to build, hidden in the wingtip and seems to have a lot of success but the glideslope performance is a question in my mind. If you have data on VSWR and polar signal strength plots for the Archer I would be very interested. Smith chart plots are OK.
Maybe I am just over thinking this issue but better now that after committing to a bad installation.

KT
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  #30  
Old 02-10-2019, 05:19 PM
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Carl Froehlich Carl Froehlich is online now
 
Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 2,028
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KT,

Sorry again, but your concerns are an order of magitude below practical antenna performace. For example:
- Polarization. Take your VHF comm handheld radio and listen to any tower or aircraft communications. Rotate the handheld 90 degrees. Still hear the station, right? I routinely communicate with a vertical antenna to a horizontal antenna receiving station. VHF is not just line of sight. Consider the same handheld radio works inside your closed hangar. Why? Does it work better outside, sure. Performance is a mission driven objective, not an antenna test range measurement.
- Beam pattern. Sure, some effect but remember that the objective is adequate signal to noise ratio. My wingtip picks up VORs around the compass rose and out to 100nmi at altitude, and LOC/ILS beyond practical range.
- Gamma match. Resonance is resonance. A third harmonic and the same a gamma match L/C ratio that brings the antenna input impedance to match the coax will be the same.
- Gamma match eliminates the use of a balun. I’ve been using gamma match antennas with coax feed for close to 50 years.

Now a word of caution, any antenna is only as good as the installer tuning it for the application and mounting conditions. I put an antenna analyzer on every antenna I install (verifies the antenna and feed line). For the wingtip antenna I modify the dimension to use the full wing rib to wingtip outer edge length to enhance performance. From there I tune the antenna to resonance and then adjust the gamma match to achive the best match to the 50 ohm RG-400 coax - perhaps a 15 minute evolution. If you don’t know how to do this, buy the Archer product and follow the installation instructions.

Carl

Last edited by Carl Froehlich : 02-10-2019 at 07:19 PM.
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