Just checking in with a question - Page 1
Forum section: electronics / testgear

author: Buzz239
author_rank: Contributor
timestamp: December 07, 2013, 03:51:42 pm
content: Hello to everyone in the group,I recently purchased a RIGOL DSA815 TG. I was eager to try the VSWR function of the spectrum analyzer using a return loss bridge. I built a return loss bridge from a common schematic found on the web. My first attempt at this yielded a 1.06 : 1 VSWR from 100 kHz to 55 MHz After 55 MHz the VSWR went up to 1.1 : 1 getting progressively worse as the frequency increased. Having no reference to start with, so I purchased a ZFDC – 20 – 5+ coupler from MINI CIRCUITS. This device fared a little better with a 1.1:1 transition at 88 MHz. After building several different circuits I was able to achieve a 1.1:1 VSWR transition at 235 MHz.My question is; if the ZFDC – 20 – 5+ coupler from MINI CIRCUITS, says it is good from .1 MHz to 2 GHz, why is that transition point only go to 88 MHz and not to 2 GHz? I can post schematics and screen data if you like.TNX Gary, KF9CM

author: Mr Simpleton
author_rank: Unknown
timestamp: December 07, 2013, 04:38:30 pm
content: Gary, I do not have the 815 but after reading the user guide... I assume you do a proper open calibration, and then try to verify with a 50 ohm load? What type of load do you use ?If you write a more descriptic title of your posting, I am sure you do will get more answers too

author: Buzz239
author_rank: Contributor
timestamp: December 07, 2013, 06:21:30 pm
content: Thanks for the reply,Yes I do a proper open cal ans a use both a BNC and N connector dummy load good to 6 GHz.The first attachment is the ZFDC – 20 – 5+ coupler from MINI CIRCUITS.The second attachment is the resultant full scan of that coupler.The third is my Return loss bridge.The forth is the full scan of my Return loss bridge.I don't know how to word it other than if I use the coupler from MINI CIRCUITS on a 2 meter antenna, I'll never see 1.0:1 VSWR no matter how efficient the antenna is.G

author: Mr Simpleton
author_rank: Unknown
timestamp: December 07, 2013, 09:40:25 pm
content: Interesting pics... Now a couple of reflectionsYou will never ever see a VSWR 1:1 that means Zero reflected power...But looking at the traces, I see you do have 1 MHz RBW, and from what I have seen previously with the DSA815 the internal noise is pretty high so please try to reduce the RBW to say 1 or 10 kHz as this will lower the noise floor, i.e. better sensitivity. With higher RL the lower the return signal and the higher sensitivity needed to correctly measure this.Now RL 30 dB equals to VSWR 1.065:1 which I consider pretty good! Not knowing how good your "dummy load" are, most are not even close to this! You may have to look at precision standars to see better than this!As for antennas, a RL of 20dB i.e. 100 watts gives 1 watt reflected is to me good enoguh... not sure when you need better that this??Keep us posted on your progress!I am sure there are others here on the forum that has a better and deeper understanding than I have in this issue.CheersLasse SM5GLC

author: VK5RC
author_rank: Unknown
timestamp: December 08, 2013, 03:25:31 am
content: ?Another factor may be the cables/connectors, once you start getting fussy RL,30bB, SWR <1.1 it gets complex fast, I tend to think of BNCs as a bit rough much beyond HF.It sounds like you have done well though.One pair of good used SMA cables set me back AUD200, I don't use them often (all have limits on life) but I get quite different results than my home made SMA cables.regards Rob

author: larry42
author_rank: Regular Contributor
timestamp: December 08, 2013, 03:32:32 pm
content: Quote from: Buzz239 on December 07, 2013, 06:21:30 pmThe short answer is that you're never going to see 1.00:1 VSWR, you're doubly never going to see it without a vector network analyzer and a full cal kit, but thankfully you don't need 1.0:1 VSWR"Yes I do a proper open cal ans a use both a BNC and N connector dummy load good to 6 GHz." - they might be "good" - but that can mean anything up to 1.2-1.5:1 VSWR (note that BNC is not even a calibration standard connector - only N is available with real NIST tracable calibration)You don't need 1.0:1 VSWR for running efficiently at 2m. even 1.5:1 VSWR will result in 90% of your power getting to the antenna, and in any case, VSWR doesn't say anything about the efficiency of the antenna...

author: Buzz239
author_rank: Contributor
timestamp: December 10, 2013, 12:08:23 pm
content: I would like to thank everyone for their comments on this project . I've run some more tests and I have posted the results below. I ordered some N connector 10 db attenuators so I can balance the bridge out. I made some side-by-side comparisons using the ZFDC – 20 – 5+ coupler and my bridge. They were set up first using a 50 ohm load with 30 kHz bandwidth and then with a 75 ohm load. Any comments would be appreciated.Gary

author: KD0RC
author_rank: Regular Contributor
timestamp: March 01, 2014, 05:31:48 am
content: Hi Buzz, I also just got an 815 and first thing I did was build a RLB from the ARRL site. Here is a pic of 1 - 500 MHz with a cheap 50 ohm load. It seems real usable from 1 - 250 MHz or so, and I can squeak by at 450 MHz. I do not have a precision 50 ohm load, so it is just an old 50 ohm network terminator from when networks ran on coax (10 base - 2). I was surprised that my bridge works so well considering it cost about $5.00 to build!I do get a slightly different trace sometimes, so I am not sure if the little transformer is moving around on its leads or something, but I don't think it affects the accuracy much.The second pic is a homebrew 2 meter J-Pole. It is tuned a bit low, but with the RLB, I was able to tune it closer to 146 MHz.73,Len, KD0RC