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How to identify antennas?

Posted: Aug 14 2019 12:01pm
by izeman
Over the years I bought several antennas from ebay, aliexpress ... Some for 433Mhz projects, some for Wifi (2.4/5.8Ghz). Some for 868Mhz.
Of course after some time, if they where not installed, they ended all in one big box. They are all black or white, but other from that they look more or less the same.
How can i find out what antenna is for what frequency????

Re: How to identify antennas?

Posted: Aug 14 2019 8:05pm
by markz
No markings at all on the antenna's?

I found this site, basically s.o.l. unless you want to open up the plastic housings.
https://physics.stackexchange.com/quest ... signed-for

Re: How to identify antennas?

Posted: Aug 14 2019 9:59pm
by fechter
If there are no markings, it will be hard to know without using some expensive test equipment or ripping them apart.

Re: How to identify antennas?

Posted: Aug 14 2019 10:44pm
by mark5
Run them through a security X-ray machine like they have at criminal courts to try to see the antenna element lengths?

Re: How to identify antennas?

Posted: Aug 14 2019 11:06pm
by billvon
izeman wrote:
Aug 14 2019 12:01pm
How can i find out what antenna is for what frequency????
Borrow a network analyzer and check their S11 across frequencies. You'll see one frequency it is tuned for; it will work best around that frequency.

Alternatively you could do a very manual test with an RF frequency generator, a 50 ohm resistor and a power meter. A spectrum analyzer makes things a little easier.

Re: How to identify antennas?

Posted: Aug 15 2019 3:50am
by amberwolf
One basic test is to measure the actual antenna length, and calculate the wavelength that would be for. frequency is just wavelength inverted (1/x, IIRC) so you can see the frequency based on that.

They'll probably be made for some fraction of their actual wavelength. So you can multiply the frequency by some common ones to guess, so let's say they're 1/4 wavelength and measure out to be for 8.4ghz. That means tehy're rpobably for 2.1ghz, at 1/4 wavelength.

YOu won't know if they're 1/4, 1/2, or whatever, but there are common frequency bands so if the measurement comes out for something that's way higher than you'd expect, divide it by various fractions to see if it comes out to a band you recognize.

If you don't understand what I mean, go to some of the ham radio sites and they'll explain antennas/etc well enough to figure it out. (my brain is too tired to be sure I'm making sense).

Re: How to identify antennas?

Posted: Aug 15 2019 5:43am
by izeman
Damn. I expected those answers, because it googled at bit before.
I don't want (and in some cases even can't) disassemble the antennas. Of course it could just add them to the receiver and see which one works best, but that is a bit to much guess work for me.
I hope the "one or two ring theory" is reliable. As all of the stuff comes from China, I'm not sure they care about this. Take any cover they have and call it a day.
Thanks everybody!!

Re: How to identify antennas?

Posted: Aug 15 2019 8:34am
by kcuf
best antenna is elevation

Re: How to identify antennas?

Posted: Aug 15 2019 9:12am
by izeman
kcuf wrote:
Aug 15 2019 8:34am
best antenna is elevation
Sure. Just tell this to my devices in the basement and I'm fine?? 8) :lol:

Re: How to identify antennas?

Posted: Aug 15 2019 11:28am
by kcuf
don't talk to devices

put em where they do the most good

Re: How to identify antennas?

Posted: Aug 15 2019 1:49pm
by HK12K
If you can find a forum for Ham radio enthusiasts they may be able to identify them just by looking at them.

Re: How to identify antennas?

Posted: Aug 15 2019 2:41pm
by dustNbone
Yup. What you need is radio nerds. Or enthusiasts.

Re: How to identify antennas?

Posted: Aug 15 2019 3:13pm
by izeman
kcuf wrote:
Aug 15 2019 11:28am
don't talk to devices
put em where they do the most good
Your help is really appreciated.

Re: How to identify antennas?

Posted: Aug 15 2019 3:33pm
by fechter
Post some pictures.
Also look closely around the end for any markings.

Re: How to identify antennas?

Posted: Aug 16 2019 5:17pm
by markz
Perhaps find a Ham Radio person in your area, maybe they might be of help.

Look for some Ham Radio forums.

Re: How to identify antennas?

Posted: Aug 17 2019 8:59am
by izeman
I have some 433 and 2.4 antennas and they look EXACTLY the same. So posting pics will make no sense. 🤮

Re: How to identify antennas?

Posted: Aug 17 2019 10:41am
by AngryBob
I realize the trial and error method is not as attractive, but what that will do, is tell you which antenna ACTUALLY WORKS BETTER, rather than a somewhat theoretical view of which one SHOULD.

Re: How to identify antennas?

Posted: Aug 17 2019 7:17pm
by marty
AngryBob wrote:
Aug 17 2019 10:41am
I realize the trial and error method is not as attractive, but what that will do, is tell you which antenna ACTUALLY WORKS BETTER, rather than a somewhat theoretical view of which one SHOULD.
My $20 TV antenna from Radio Shack don't work much better then the piece of wire with alligator clips that I used to have. On my list of things to do is a big TV antenna on the roof. And another internet receiving antenna like this.
Image

That's the internet receiving antenna at mt shop. [Works Great] Want to do the same thing at my house.

Re: How to identify antennas?

Posted: Aug 19 2019 11:08am
by fechter
Just curious what you used the 433mhz antenna for?

I agree a ham radio operator or anyone in the mobile radio business might have test equipment that would identify the frequency.

Re: How to identify antennas?

Posted: Aug 19 2019 11:40am
by izeman
433 was used for home automation sensors. I switched to zigbee mostly now - so i need 2.4ghz antennas to replace the pcb printed ones (for those cases where more range is needed).

Re: How to identify antennas?

Posted: Aug 19 2019 11:35pm
by fechter
If you have a set that works you could range test the various antennas. The one with the longest range will be the one for that frequency.

I was pretty impressed with the range on my 433mHz remote switches.

Re: How to identify antennas?

Posted: Aug 20 2019 4:56am
by izeman
fechter wrote:
Aug 19 2019 11:35pm
If you have a set that works you could range test the various antennas. The one with the longest range will be the one for that frequency.

I was pretty impressed with the range on my 433mHz remote switches.
That was the only approach that makes sense in my case. And is the way i do it now. I was hoping for a more 'engineer like' way. :lol: 8)

Re: How to identify antennas?

Posted: Aug 20 2019 11:34am
by fechter
izeman wrote:
Aug 20 2019 4:56am

That was the only approach that makes sense in my case. And is the way i do it now. I was hoping for a more 'engineer like' way. :lol: 8)
Something like a SWR analyzer or spectrum analyzer would be the preferred engineer approach. Those machines are very expensive so hard to find somebody that has one. Beside ham operators, people who service radio equipment would have them.

Re: How to identify antennas?

Posted: Aug 20 2019 7:00pm
by markz
Yes, those very well may be able to help, not many around anymore.

Not even electronics repair places neither but they probably don't have the equipment.

Think of what people use antenna's a lot, and would have the equipment.
Ham Radio
Radio Repair
Cell Phone Technician
Router Technician
Universities/Colleges/Technical Schools (Maybe put up a poster on campus)
CB Radio Technician Repair




people who service radio equipment would have them.

Re: How to identify antennas?

Posted: Aug 22 2019 3:32pm
by parajared
Presumably you just try them out and see what works best and this is because of impedance. Even among the same manufacturer antenna length varies and that's because loads are different from device to device. The materials used cause resistance and the antenna/device has to match.

For instance when you are building a 433mhz antenna you could broadly say that your antenna would be 70cm for a full wave, 35cm for half wave ect... but in reality you just build your antenna too long and hook it up to a SWR meter. You chop of a little bit of material at a time, annotate your SWR readings, then rebuild.
example
SWR at 40cm = 12
SWR at 38cm = 9
SWR at 35cm = 2
SWR at 33cm = 1.5
SWR at 30cm = 2
SWR at 28cm = 4

You would (in this particular example) re-build your home brew 433 antenna to ~33cm for that exact device because that's the best reading you could get. Your device and antenna have to match. Hope this helps.