Determine antenna frequency? - Page 1
Forum section: electronics / rf-microwave

author: onesixright
author_rank: Frequent Contributor
timestamp: July 20, 2016, 07:43:13 pm
content: Hi,I have a few SMA antenna's (pretty cheap). Yet im curious if somehow you can measure the frequency they wheredesignedfor? They are no outer markers, they look all the same. But apparently one is a 433Mhz, while the other is a 2.4Ghz.Any thought?Much obliged!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 20, 2016, 07:49:07 pm
content: If they are the same, or similar, design, the 433MHz one will be about five times larger.Can you observe anything about their design or structure? Covered in a plastic shell? Metal foil on sticky tape? Spring wire in free space?Tim

author: Kilrah
author_rank: Unknown
timestamp: July 20, 2016, 07:54:53 pm
content: He said they all look the same, I guess the usual short whip. They can indeed be any frequency element in there, I have some on 433MHz, 2.4, 5.8 that all look exactly identical, some just having 3/4 of wasted empty space.But it's usually easy to pull the cover off and determine the freq by looking at the actual element.

author: onesixright
author_rank: Frequent Contributor
timestamp: July 20, 2016, 07:56:53 pm
content: I added a picture.Now i'm more curious if and how you canmeasureit, opposed to "eye-ball" it.

author: Kilrah
author_rank: Unknown
timestamp: July 20, 2016, 08:01:07 pm
content: Network analyzer...

author: Tony_G
author_rank: Super Contributor
timestamp: July 20, 2016, 08:16:13 pm
content: I got these 2 from China:You can see the one with brown tape on it is the 433MHz one - It's slightly longer and if you zoom in at the tip:The 433 MHz one has 2 little rings - I have no idea if this is a standard or not because I usually just stick it on my RF Network Analyzer and look at the reflection plot.433Mhz:915MHz:This helps if you have a network analyzer but not so much if you don't. You can do the same sort of thing with a spectrum analyzer and an SWR bridge or directional coupler.TonyG

author: onesixright
author_rank: Frequent Contributor
timestamp: July 20, 2016, 08:31:38 pm
content: Quote from: Tony_G on July 20, 2016, 08:16:13 pmHi Tony, thanks.Could you elaborate how this would work with a SA (i do have one)?I assume that you need to send a sweep signal to the antenna and then you measure it? Which means you need to send a pretty broad spectrum?

author: Tony_G
author_rank: Super Contributor
timestamp: July 20, 2016, 08:57:30 pm
content: The key is the SWR bridge or directional coupler.If your SA has a tracking gen then this is much easier - Mine doesn't so what I did before I go the network analyzer was:1. Attach signal generator to the RF Input port2. Attach antenna to the DUT port3. Attach SA to the SWR Output Port4. Set SA to trace hold max trace A5. Sweep generator across 0 - 1GHz rangeThe resultant plot should show where the antenna reflects the least energy and these will be the points where it has the best load matching - It should look like this:Pleased note that this was setup quick and dirty (you'd expand range, align amplitude etc) and I took a photo because I haven't yet reinstalled the 7440 utilities due to some forced NVIDIA fun time...TonyG

author: Kilrah
author_rank: Unknown
timestamp: July 20, 2016, 09:00:32 pm
content: The key requirement that wasn't mentioned before is that a signal gen is needed too

author: Tony_G
author_rank: Super Contributor
timestamp: July 20, 2016, 09:03:59 pm
content: Nice! Thanks Kilrah, forgot to point that out because I was sort of assuming that the SA would have a tracking gen - Then when I wrote that I realized, wait I don't have a tracking gen on my SA, doh!in fact my sig gen (8657B doesn't have a sweep mode) so I use VEE to automate it normally - This time I just set the frequency increment to be 1MHz and then held down the up button.As I said, quick and dirty.TonyG

author: onesixright
author_rank: Frequent Contributor
timestamp: July 20, 2016, 09:12:48 pm
content: Yeah, i was just thinking about the sig. gen. My AWG is max 60Mhz. So thats not gonna work. And my SA (MDO3000) doesn't have the TG.Anyway, thanks for the "instructions" I put it on my to-do-wanna-do listAs with many of these things, the frequencies go up, so does the price!

author: Kilrah
author_rank: Unknown
timestamp: July 20, 2016, 09:17:27 pm
content: Here's a cheap solution:http://seeedstudio.com/RF-Explorer-Signal-Generator-(RFE6GEN)-p-2074.html

author: Tony_G
author_rank: Super Contributor
timestamp: July 20, 2016, 09:33:59 pm
content: I don't know if you've used it but the page says:QuoteMight be a good cheap stand in till you can get some better gear - Though there may actually be older HP stuff available on Ebay that would cost around the same but have beteer specs, use 10 times the power and weight 100 times moreTonyG

author: SNGLinks
author_rank: Regular Contributor
timestamp: July 20, 2016, 09:47:38 pm
content: You can use a noise source instead of a tracking generator. This give a general idea using an RTL SDR as a spectrum analizer.http://www.rtl-sdr.com/rtl-sdr-tutorial-measuring-filter-characteristics-and-antenna-vswr-with-an-rtl-sdr-and-noise-source/

author: Kilrah
author_rank: Unknown
timestamp: July 20, 2016, 09:50:00 pm
content: I actually have both the RFE generator and the 6G spectrum analyzer, frankly a good bit of kit for the price, tracking works great if you've got a bit of time. Has its limitations of course, but is incredibly handy. I've also got an HP8593A, but given that I'm travelling a lot and spend more than half of the year away from "home" where the 15kg beast is is the cheap kit gets a lot more use...