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Marker beacon antenna question

trib

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

Vic
 
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
 
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.
 
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!
 
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
 
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.
 
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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?
 
Bare?

So do you strip everything off the 42" of coax? I understand you remove the outer coating and shielding, but also the little plastic covering the inner wire?
 
Although insulation will technically change the resonant length of an antenna, the effect is so slight, that it won't make any difference in this application...it's probably a good idea to leave the insulation on the center conductor just to make sure it doesn't inadvertently short out.

As for the bulkhead connector idea, I used one on my wingtip. I also saved some weight by using RG-174 coax.

Paige
 
So do you strip everything off the 42" of coax? I understand you remove the outer coating and shielding, but also the little plastic covering the inner wire?

So a couple of questions...

First, if the MB frequency is 75 MHz, isn't 1/4-wavelength closer to 1m (39.37") than 42"? Where'd the 42 come from? (other than being the answer to life, the universe and everything).

Second...any tricks for stripping off the outer jacket and shield from 3' of $G-400? Just "split" the jacket lengthwise with a razor blade or something?
 
I used a coax stripper which I bought for other radio and antenna wiring. $15 at your local radio shack.

here's the link http://www.radioshack.com/product/index.jsp?productId=2062637

this will cleanly strip as many layer of the cable as you need. It's great when installing the bnc connectors.

Have one of these for stripping for BNC/TNC connectors...but how did you use this to strip 40" of jacket and shield off the coax?
 
Receive antenna lengths are not critical

Instead of thinking of it as a piece of coax just think of it as a coax going to a antenna. Your razor blade approach will work fine for removing the outer insulation, then go through the process of removing the outer conductor with picks (carefully) and cutters to expose the antenna (center conductor) of the desired length.

I personnally have the little "boat" under the fuselage but if I were going this way I would install it in the wing tip because it is coming off far less often than the cowl and the operational environment is more antenna friendly.

Bob Axsom
 
I'm probably just dumb, but why would you want a good antenna for a marker beacon? You want to know when you are over it -- this isn't DX; that is, something that you don't want to hear at a distance.

Check out the commercial antennas -- they look to me like "almost deaf" is a fair descriptor.
 
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