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Coaxial Dipole

 
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slk23
Antenna Theory Regular


Joined: 11 Sep 2013
Posts: 13

PostPosted: Wed Sep 11, 2013 12:23 am    Post subject: Coaxial Dipole Reply with quote

Do coaxial dipoles really work well without any impedance matching? For example: http://www.hamuniverse.com/bazooka.html
The arms are quarter-wavelength except the inner wire is shortened to account for the velocity factor of the coax.

And here is a variation of the coaxial dipole which connects the inner and outer conductors of the coax dipole arms at the ends:
http://www.g4cqm.co.uk/YAGI%20DESIGNS/2M%20YAGI/coaxialdipole.html

I built a coax antenna based on the G4CQM design but not in a Yagi. It is simply a T made of RG400 with the feed coax conductors connected to the outer shield of the arms at the center of the T. At the arm ends the center wire and shield are connected. I did a non-quantitative test with a 1090 MHz aircraft transponder and it seemed to work as well or better than the quarter-wavelength monopole that is typically used for aircraft use. I'd really like to lab test this antenna but I don't have access to an analyzer or even a VSWR meter that works at this frequency.

Here's a picture of my antenna temporarily taped to the seat belt in the empty backseat for flight testing. I can't get embedded pictures to work so here's a link:
http://www.flickr.com/photos/26676700@N06/9722530483/


Comments would be appreciated.

[]


Last edited by slk23 on Fri Jun 06, 2014 10:49 pm; edited 1 time in total
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bigSteve
Antenna Wizard


Joined: 14 Mar 2009
Posts: 265

PostPosted: Fri Sep 13, 2013 5:18 pm    Post subject: Reply with quote

Yes coaxial dipoles work. Essentially you are just using the outer shield of the coaxial cable as the arm of the dipole. The center conductor doesn't really do anything and if it was removed, it wouldn't matter.

On your setup: so you have two arms in the seat belt? ANd the center conductor of your feed coaxial cable goes to one arm, and the outer/shield of your feed coaxial cable goes ot the other arm? If so - that is perfectly valid and will work. What frequency are you going for? Are you ensuring the length is roughly a half wavelength?
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slk23
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Joined: 11 Sep 2013
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PostPosted: Fri Sep 13, 2013 6:03 pm    Post subject: Reply with quote

Thanks for the reply. I should have been more specific -- my question was really about impedance matching and whether a coaxial dipole needs a balun. I'm also curious about the G4CQM design with the center wire connected to the dipole arms at the arm ends. What do you think about their claim that the center wire is beneficial?

Quote:
The G4CQM 50Ω coaxial dipole can be described as a coaxial structure and non-folded dipole made from two open conductive cylinders, each approximately λ/4 long and potentially benefits from the stub compensation of a folded dipole by passing an insulated wire through the axis and connecting it to the ends of both limbs.


In my photo the arms are roughly vertical on the seat belt for vertical polarization. It's a half wave dipole cut to the correct length for 1090 MHz with the connections as you described except the inner wire is connected to the outer shield at the arm ends.


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bigSteve
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PostPosted: Sat Sep 14, 2013 5:18 pm    Post subject: Reply with quote

A balun is used to make the current on both arms of your dipole equal. This is important for polarization purity. With no balun, current will also flow on the outside of the coaxial cable feed wire. Now, this isn't necessarily a hugely bad thing, as you'll still get radiation.

In your case, you wouldn't do any impedance matching unless you could view the mismatch loss/smith chart with a Vector Network Analyzer. I think you'll be ok without it. I would keep the arms shorter than a half-wavelength - maybe closer to 0.46 lambda without a balun, just from experience (resonance typically occurs as less than a half-wavelength).

In addition, if the seatbelt is worn by a person, their body material will also absorb a bunch of the energy the antenna would want to transmit or receive (humans bodies are good at absorbing electromagnetic energy).

On the claim of benefits from using the inside coaxial line tied to the ends - it sounds interesting and could potentially be of use. However, the claim goes on:

"potentially benefits from the stub compensation of a folded dipole by passing an insulated wire through the axis and connecting it to the ends of both limbs."

The thing won't radiate at all like a folded dipole, as the inner arm is completely shielded from the outside world by the outer coax. So that claim, and the use of the word "potentially" makes me a bit skeptical.
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slk23
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Joined: 11 Sep 2013
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PostPosted: Mon Sep 16, 2013 4:56 pm    Post subject: Reply with quote

Thanks for the comments Steve. I shortened it to 0.46 lambda and appears to work very well: my transponder was replying to received interrogation signals at the same rate or better than it did with the 1/4 wavelength monopole.

And to clarify, the antenna was on the seatbelt for testing only. The permanent installation will place it in the nose of the airplane. It's a composite structure (resin, fiberglass, and foam) so there shouldn't be much energy absorption.
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erum
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Joined: 10 Dec 2013
Posts: 1
Location: Azerbaijan

PostPosted: Tue Dec 10, 2013 6:41 am    Post subject: Reply with quote

-6dB is equivalent to a VSWR of 3, which is generally about as high as you want to go for mismatch loss. Some people will expect better, but it really depends on the application.
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Rack201
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Joined: 17 Dec 2013
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PostPosted: Tue Dec 17, 2013 11:49 am    Post subject: Reply with quote

I struggle to understand why many coaxial dipole designs call for joining the shield to the inner core at the arm ends. I am a complete beginner, but doesnt this in effect short the elements together and double the effective length of the antenna?
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