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Slot antenna

 
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slk23
Antenna Theory Regular


Joined: 11 Sep 2013
Posts: 13

PostPosted: Sat Jun 07, 2014 6:59 pm    Post subject: Slot antenna Reply with quote

Hello,

I'm going to try making a slot antenna, which I understand is complementary to a dipole, for 1090 MHz. In a thread about dipoles bigSteve advised:

Quote:
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)


Should I do the same for a slot antenna and make the slot 0.46 lambda long? Or make it a half-wavelength?

The diagrams I've found on the web show the feed point approx. 1/20 lambda from one end of the slot to get a 50 Ohm impedance. Does that sound right?

And finally, a question about shielding people from the antenna's radiation. I plan to mount the slot antenna on the belly of a small airplane behind the passenger seat. I only need the antenna to radiate below the airplane (ideally in a hemisphere) and although the transmitter is low duty cycle I'd like to protect the passenger from the radiation. Would a thin metal mesh or screen on the seat back work? Would that screen have to be connected to aircraft ground?

Thanks in advance.

-Steve
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slk23
Antenna Theory Regular


Joined: 11 Sep 2013
Posts: 13

PostPosted: Sun Jun 08, 2014 4:42 pm    Post subject: Reply with quote

Yesterday I built a 1090 MHz transponder slot antenna as best I could with information found on the web. However, the only ground plane I have available on this fiberglass composite airplane is a small aluminum access panel cover on the belly. It's only ~0.6 lambda compared to the recommended minimum of 0.75 lambda.

Since I don't have access to any test equipment I decided to give the antenna a field test. At first it seemed promising: the transponder reply light was flickering, which indicates it's responding to signals received from the FAA radar facility approx. 30 miles away. But as I was climbing away from the airport the tower controller asked me to 'recycle' my transponder, meaning they weren't seeing the signal from it. Since I could see the reply light blinking I have to assume the antenna's performance was poor, its location was blocking the signal, or both.

I landed at a nearby airport and switched the transponder connection to a dipole I made previously. That antenna works great and I need to find a location to install it in permanently. On a small airplane it's hard to find space, especially when there are health concerns about the antenna's proximity to people. The transponder has a fairly low duty cycle but it can transmit at up to 200W.

If anyone has the equipment and time to bench test the slot antenna please let me know -- I'd be happy to mail it to you Smile

Thanks for reading.

For those interested, here's the airplane:
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bigSteve
Antenna Wizard


Joined: 14 Mar 2009
Posts: 265

PostPosted: Mon Jun 09, 2014 2:39 am    Post subject: Slot Antennas Reply with quote

For the slot antenna, what really matters is the perimeter, which should be about 1 wavelength. So if you make a thin slot, it should be about 0.49 lambda (factoring in the width of the slot).

As for feed point, lambda/4 = 0.25 lambda would be the midpoint, which would be a bad location as the impedance would be infinite. Lambda/20 = 0.05 lambda seems pretty short to me, I might recommend 0.1-0.15 lambda.

If you want to back the slot antenna with metal, you are essentially making a cavity backed slot antenna. In that case, you want the distance between the metal and your slot antenna to be about lambda/4 (in that case, reflected energy comes back more or less in phase).

If the metal backing is very close, you will essentially short out the slot.

What type of connection does your slot antenna use? I could probably test it out for you, particularly if you can get it to California.
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slk23
Antenna Theory Regular


Joined: 11 Sep 2013
Posts: 13

PostPosted: Mon Jun 09, 2014 7:36 pm    Post subject: Reply with quote

Thanks for the reply bigSteve.

The slot antenna I made has a female BNC but I could easily change it to SMA.

I will take some measurements to see if there's space in the installation for a lambda/4 cavity backing. I assume the cavity is composed of metal sides and back and not just a metal back.

Side question: how important is polarization? The 1 GHz signal from the ground stations is vertically polarized. How bad would it be to place a dipole antenna horizontally in the airplane? I understand that in theory it wouldn't work due to the 90 degree polarization difference. But in application would it greatly affect the antenna performance?

.
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bigSteve
Antenna Wizard


Joined: 14 Mar 2009
Posts: 265

PostPosted: Wed Jun 11, 2014 2:09 am    Post subject: Reply with quote

You could have 20 dB of polarization mismatch loss. It's tough to estimate the impact of that, it depends on the degree of "purity" of the polarization of your antennas, distance, transmit power, etc. Generally you should try to match the polarization, but sometimes you can get away with not.
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james5566
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Joined: 25 Oct 2014
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Location: aaaaaa

PostPosted: Sat Oct 25, 2014 8:47 am    Post subject: Reply with quote

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). ?????????????
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