FlightAware Discussions

New User - Question on Indoor Antenna

I bought one of those $4 PCB ADS-B antennas from eBay and drilled a hole in the top of it and used thread to attach it to my (nylon) window screen. I put it in the middle in case the frame of the screen (I think it’s painted aluminum) causes interference and it works well. I can see planes about 200 miles out from the top floor of my apartment building. As others have said height is important but you also have to remember that with your antenna being inside the building itself is going to block a lot of signals unless you can get the antenna above the roof. Before I got the PCB antenna I used one of those homemade spider antennas which worked well but not as good as the PCB one does. There are several videos and websites that explain how to make a spider.

Same here.

Have you tried this 8-legged Spider? For me this performed better than PCB antenna.

QUICK SPIDER - No Soldering, No Connector

I see lots of pictures of people placing their antenna right next to something else. Try to keep the antenna some distance away from other objects. That distance depends on the dimensions of the antenna and frequency. (As a rough estimate I try to keep at least 1 wavelength away i.e. 27cm for 1090MHz). Closer than that, the object is in the near field of the antenna, and will significantly change the radiation pattern of the antenna and detune it, making it less effective. Wikipedia and lots of RF-sites have better explanations.

How about Cell Phone antennas? See photo of Samsung Galaxy S8 phone below. It operates on many cell bands including LTE-U 5Ghz, GSM 800 & GSM 950. The last two are very close to 1090 Mhz.

The size of this phone is 65mm x 145mm

How many mm spacing of these 7 antennas with each other and with other metallic objects in this phone a visual estimate gives you?


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Those kind of antennas are electrically smaller than 1/4wave, so different factors in the near field dominates, which means that the radiation pattern does not get influenced, but great care still has to be taken when you design that device. The antenna is not an add-on, but is inegrally designed into the device, so the proximity of other components near the antenna is taken into consideration when the antenna is designed. If you remove one of those antennas and try use it somewhere else, you will find its resonance frequency is totally different from what it was within the device, normally a lot higher, as nearby components tend to make the antenna electrically longer. That is why the calculated dimensions for a collinear antenna in free air, is different from the dimensions you get by doing something simple to it, like putting a PVC sleeve over it. If you put a sleeve over it, your resonant frequency is totally different, and you have to change the dimensions to bring it back to resonance.
If you look at the datasheets of those antennas, there are a lot of rules that has to be followed, to make them work.
Here is a typical app note on antennas in these devices.

The bottom line is, if you don’t have an VNA or antenna-analyzer to test (and retune) your antenna in its final position, keep it as far as possible from any other objects. 1 wavelength and you are in the transition space, 2 wavelengths or more are better.



Yes, I fully agree.

The cell phone antennas are designed by specialists.

Prototypes are precision built, tested in-situ by these specialists using precision instruments, and dimensions adjusted to get required resonance, gain and other parameters.

Once the prototype is finalized, the final dimensions are used for mass production.


Sometimes they don’t get it quite right!

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На одном из форумов обсуждались и сравнивались стоковые антенны.
Вот результат сравнения:

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Trial Run Results for Three Types of Whip Antennas

Trial Run Results for Three Types of Whip Antennas

Trial Run Results for Three Types of Whip Antennas


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да именно это самое исследование.

I used a paper clip and it worked just fine. Unscrewed the whip, straightened out the clip, wound about 4 turns around the screw base as tight as I could with my fingers, and then bent the tail straight up & trimmed it off to the correct length. I was routinely getting 175-ish miles and 700-800 aircraft per day with it sitting on top of an empty 1qt paint can in my 2nd floor bedroom window.