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  #1  
Old 05-24-2011, 10:41 AM
trib trib is offline
 
Join Date: Aug 2005
Location: Virginia Beach, VA
Posts: 446
Default Marker beacon antenna question

Reading the many posts on marker beacon antennas from 42" of stripped coax makes this look like a great method. I guess it's obvious, since I can't find any discussion, but it appears that the center wire is the antenna and the shielding is just left alone, attached to nothing. Just wanted to confirm that the shielding isn't grounded or anything that I might have missed. As I see it, the method is to strip the outer cover and shielding for 42" and then mount this stripped cable in the wingtip. A connector can be used for tip removal, but the outer shield is attached to nothing and has no function, other than to provide the shielding for the portion of the cable which runs from the instrument to the 42" unshielded portion.

thanks for the help
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  #2  
Old 05-24-2011, 10:55 AM
DGlaeser DGlaeser is offline
 
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Location: Rochester Hills, MI
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Default You got it

That's exactly what I have. IIRC the shield is grounded at the radio end.
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RV-7A - Eggenfellner H6, GRT Sport & EIS4000, 300XL, SL30, ADI Pilot II, PMA6000, AK950L, GT320
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  #3  
Old 05-24-2011, 11:22 AM
scootwoot scootwoot is offline
 
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Location: Granite Bay, CA
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Where are you putting it? in the wing tip? that was my direction.

Thanks
Bill
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  #4  
Old 05-24-2011, 01:51 PM
vic syracuse vic syracuse is offline
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Default

Yes, you understand it correctly. I have this in the wing tip of the 10 and it works wonderfully.

Vic
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Built RV-4, RV-6, 2-RV-10's, RV-7A, RV-8, Prescott Pusher, Kitfox Model II, Kitfox Speedster, Kitfox 7 Super Sport, DAR, A&P, EAA Tech Counselor/Flight Advisor, CFII-ASMEL/ASES
Mallards Landing, GA (GA04)
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  #5  
Old 05-24-2011, 02:47 PM
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drill_and_buck drill_and_buck is offline
 
Join Date: Feb 2005
Location: Bridgewater, MA
Posts: 303
Default Cowl is also an option

No need to run the expensive coax out to the wing tip.

I put a BNC bulkhead connector on my firewall and then attached the coax to the lower half of my cowl, just below the hinge line.


Mike
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  #6  
Old 05-25-2011, 10:29 AM
trib trib is offline
 
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Location: Virginia Beach, VA
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Is the cowl actually long enough for the 42" antenna? Have you flown with this setup and gotten good reception? It does sound simpler, although I dread drilling another hole through the firewall.
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  #7  
Old 05-26-2011, 07:00 AM
elippse elippse is offline
 
Join Date: Dec 2006
Location: Arroyo Grande, CA
Posts: 938
Default

Quote:
Originally Posted by trib View Post
Reading the many posts on marker beacon antennas from 42" of stripped coax makes this look like a great method. I guess it's obvious, since I can't find any discussion, but it appears that the center wire is the antenna and the shielding is just left alone, attached to nothing. Just wanted to confirm that the shielding isn't grounded or anything that I might have missed. As I see it, the method is to strip the outer cover and shielding for 42" and then mount this stripped cable in the wingtip. A connector can be used for tip removal, but the outer shield is attached to nothing and has no function, other than to provide the shielding for the portion of the cable which runs from the instrument to the 42" unshielded portion.

thanks for the help
This will work even better if you put one or several ferrite beads over the end of the shield where it is stripped to prevent currents from the antenna picking-up currents from the long shield run from getting back into the transmission line and generating nulls and voids in the pattern. This is a poor-man's balun!
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  #8  
Old 05-26-2011, 08:35 AM
PaigeHoffart PaigeHoffart is offline
 
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Quote:
Originally Posted by elippse View Post
This will work even better if you put one or several ferrite beads over the end of the shield where it is stripped to prevent currents from the antenna picking-up currents from the long shield run from getting back into the transmission line and generating nulls and voids in the pattern. This is a poor-man's balun!
So if I understand correctly, you want to feed a 1/4 wave antenna without a ground plane or counterpoise. And, you think a ferrite choke balun will help?

The reason a ferrite choke balun works is because it creates a high impedance which reduces the amount of current traveling through the shield. That's fine if you are feeding a dipole, or antenna with a counterpoise because there is somewhere for that current to go (i.e. the leg of the antenna connected to the shield). If you want to create an antenna out of a piece of coax using a choke balun, put the ferrite bead 1/4 wave away from the feed point (where the center conductor is first exposed). The current would then be radiated by the first 1/4 wave of shield, forming a standard dipole.

For our applications, the easiest thing you could do is ground the shield at the wingtip rib (no it won't setup a ground loop, think about how comm antennas are wired). Any way you skin the cat, you will probably have some sort of pattern distortion. But realize that a marker beacon is a transmitter with a directional antenna firing straight up, and our receivers are intentionally designed to be deaf so that we only hear the beacon when we fly over it. A coat hanger would probably yield good results. Since the antenna at the marker transmitter is aligned with the runway it serves, I'd make sure that a good portion of the antenna runs fore and aft, but IMHO anything beyond that (grounding length, spacing) is gravy.

My .02,
Paige
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  #9  
Old 05-26-2011, 08:28 PM
elippse elippse is offline
 
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Location: Arroyo Grande, CA
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Default

Quote:
Originally Posted by PaigeHoffart View Post
So if I understand correctly, you want to feed a 1/4 wave antenna without a ground plane or counterpoise. And, you think a ferrite choke balun will help?

The reason a ferrite choke balun works is because it creates a high impedance which reduces the amount of current traveling through the shield. That's fine if you are feeding a dipole, or antenna with a counterpoise because there is somewhere for that current to go (i.e. the leg of the antenna connected to the shield). If you want to create an antenna out of a piece of coax using a choke balun, put the ferrite bead 1/4 wave away from the feed point (where the center conductor is first exposed). The current would then be radiated by the first 1/4 wave of shield, forming a standard dipole.

For our applications, the easiest thing you could do is ground the shield at the wingtip rib (no it won't setup a ground loop, think about how comm antennas are wired). Any way you skin the cat, you will probably have some sort of pattern distortion. But realize that a marker beacon is a transmitter with a directional antenna firing straight up, and our receivers are intentionally designed to be deaf so that we only hear the beacon when we fly over it. A coat hanger would probably yield good results. Since the antenna at the marker transmitter is aligned with the runway it serves, I'd make sure that a good portion of the antenna runs fore and aft, but IMHO anything beyond that (grounding length, spacing) is gravy.

My .02,
Paige
Dang! You are so right! I was thinking of a 1/4wave dipole being fed at the center and using the beads to act as a balun and remove the antenna currents. The end of the shield, as you mentioned, should be connected to the airframe for its counterpoise. Another approach would be to use a bazooka balun in series but again that makes the antenna a 1/4wave dipole so that the last 1/4wave of the shield would have to be exposed.

Last edited by elippse : 05-26-2011 at 08:32 PM. Reason: Added bazooka
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  #10  
Old 05-27-2011, 10:29 AM
trib trib is offline
 
Join Date: Aug 2005
Location: Virginia Beach, VA
Posts: 446
Default bulkhead fitting

OK, so now I'm thinking the approach to putting the antenna in the cowl sounds cleaner than the long run out to the wing tip. Looking at the suggestion to ground the coax at the wingtip, my solution is to use a coax bullkhead fitting at the firewall. The bulkhead fitting is grounded by attaching it to the firewall. Drill the hole for the bulkhead fitting for the transition, install the fitting, then the coax shield is grounded simply by connecting the antenna to the bulkhead fitting. The shield from the radio to the bulkhead fitting coax is grounded in the same manner. Does this sound like a good solution?
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