Using a Siglent to measure Reflection Loss - Page 1
Forum section: electronics / testgear

author: alannz
author_rank: Contributor
timestamp: December 12, 2020, 08:18:26 pm
content: Hey all,I have recently purchased a Siglent (thank you Rob) and are using it to measure return loss of a "rubber ducky" type antenna.The issue I'm having is that it seems to under read in frequency by about 20Mhz compared to my other VNA's (2).But if I hold on to the ground of the antenna, the reading is about the same - but then it is very difficult to get a stable reading.What do you find is the best method for getting an accurate reading from this kind of antenna.Someone made a suggestion I should make a metal box about the same side as the radio and attach the box to ground?Your thoughts?

author: rvalente
author_rank: Frequent Contributor
timestamp: December 12, 2020, 08:25:59 pm
content: First things people will ask:What antennaEquipment modelGeneral setup

author: alannz
author_rank: Contributor
timestamp: December 12, 2020, 08:41:29 pm
content: Antenna = Random Antenna off handled radios, but usually either VHF (150Mhz) or UHF (450Mhz).The equipment I have is a Siglent SSA 3015 with a SWR Bridge attached.I not sure if that helps?

author: alannz
author_rank: Contributor
timestamp: December 12, 2020, 08:48:38 pm
content: The antenna (DUT) are directly attached to the SWR bridge

author: tautech
author_rank: Super Contributor
timestamp: December 12, 2020, 09:25:42 pm
content: Quote from: alannz on December 12, 2020, 08:48:38 pmYou sent me this pic:There is a good chance you have some proximity effects influencing measurements.

author: alannz
author_rank: Contributor
timestamp: December 13, 2020, 03:16:17 am
content: Thanks for posting the photo, what I'm after is the best process people have found for making these measurements.I have found, if I leave it in "mid air" I get a stable reading, but it appears to be up to 20Mhz out for the DIP as measured in other VNA'sIf I hold it, like in the photo the dip jumps all around the place.There must be a way to get a good stable reading and thats the information that I'm after.Thanks again.

author: tautech
author_rank: Super Contributor
timestamp: December 13, 2020, 07:23:01 am
content: Step by step screenshots with SVA1032X in SA mode without the Reflection Loss feature active however the following are critical steps required for a correct initial setup.Siglent N type bridge.Preset to ensure any previous settings are removed.Full sweep and default settings:Connect bridge.TG ON with default output. For some DUT's the TG stimulus level need be adjusted.Store Ref trace.Normalize.Adjust Normal Ref Pos if requiredConnect DUT.DUT = 2.4 GHz WiFi whip.Peak Marker>Search>Peak Type = MinMarker> set to desired frequency.Adjust Span as requiredTune ^^^ to suit your tastes.

author: tautech
author_rank: Super Contributor
timestamp: December 13, 2020, 10:08:02 am
content: Quote from: tautech on December 13, 2020, 07:23:01 amWhich of course leads to the UNCAL warning above....one of the little traps when making adjustments after Normalize.Easily remedied by removing the DUT and returning to the TG menu, saving the Ref trace and performing Normalize again.SVA models with VNA mode are simpler to use in this respect after adapters and cables to the DUT have be included into a calibration where thereafter adjustment to VNA settings have no effect while you zoom in on the measurements required.Same DUT as above with similar settings but in VNA Log Mag mode.

author: TurboTom
author_rank: Super Contributor
timestamp: December 13, 2020, 11:58:51 am
content: Why don't you increase TG output to 0dBm to get a better noise margin? There's hardly anything (especially when testing an antenna) that you can damage with the higher level.In the photo published by @tautech: Do you need the SMA elbow as a gender changer? This arrangement with objects / wires in close proximity will completely de-tune your antenna with unpredictable results. Use an SMA line instead. Do the calibration of the measurement setup with a terminator at the end of the line. Swap in the Antenna and, if possible, suspend it in mid-air using a glass fiber rod or the like to support it. Keep it several wavelengths away from any objects in the plane of emission. This configuration should give you better results.Is it correct that @tautech is testing the identical antenna at 2.4GHz and you used a 1~500 MHz RLB to test it? I'm not surprised that you don't get the expected result...Further up the page, you specify the antena is designed for the 150/450MHz range which would match your RLB. But then I've got to ask, why do you normalize your setup over the full frequency range of the SA? There are a few details in your strategy that I don't quite get...

author: virtualparticles
author_rank: Regular Contributor
timestamp: January 18, 2021, 02:30:19 pm
content: The "Rubber Duck" antenna has no built-in ground plane. It depends on the equipment to which it is attached and to some extent, the user's head. For cell phone design, we used a gel type "human head" model to check the ground counter-poise of the antenna. The RF currents running in the skin of the cranium of the user are an important part of the antenna design. The Rubber Duck is not quite as bad but you'll have to pick a configuration close to real world application and stick with it. Don't expect to get great return loss.

author: CDN_Torsten
author_rank: Regular Contributor
timestamp: January 18, 2021, 11:18:36 pm
content: Asvirtualparticleshas indicated, this is about the environment surrounding the antenna.Bringing objects close to your antenna will shift the resonant frequency.In your setup I would recommend at the very least not using a right-angle SMA just before the antenna - try to connect it vertically.The test should be done with nothing in the vicinity of the antenna, or, it should be tested in the environment that it will be used in.