SWR Bridge by BG7TBL - A look inside the box + Attenuator pad set. - Page 1
Forum section: electronics / testgear

author: Fraser
author_rank: Super Contributor
timestamp: July 28, 2015, 01:41:30 pm
content: I have seen the BG7TBL SWR Bridge advertised on eb*y for some time and decided that it would be interesting to have one to play with. The unit costs little so it was worth getting one just to see what lives inside.I should state that I am not providing any sort of recommendation to buy this item as I am currently unable to test it due to enforced rest due to illness. This Post is just showing you what lives inside the case as I could not find any pictures of such on the internet. Sorry I cannot provide test results at this time. That is sadly beyond my control.The SWR bridge comes with a set of attenuators mounted on a neat little PCB. This is because it is really designed to go with a 500MHz NWT RF sweeper. As such it is rated at 500MHz maximum frequency (untested by me)I bought my set here for around £17 delivered to the UK.http://www.ebay.co.uk/itm/131505535293?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AITYou can also buy the unit from other eb*y sellers and some even provide test data that I will include in this post.It is available on its own for £19 here.http://www.ebay.co.uk/itm/0-5-500mhz-50-ohm-SWR-Bridge-Standing-Wave-Ratio-Amateur-Radio-/131494123837?pt=LH_DefaultDomain_3&hash=item1e9da9193dThere is also a different version available that has a greater frequency capability. It is different in that it has an external 50 Ohm load. I may well buy that unit for a teardown as well. As it is from the same BG7TBL source I suspect the build quality will be very similar. This unit is priced reasonably as well (~£14 delivered)http://www.ebay.co.uk/itm/131505535293?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AITThe attenuator pads that are provided on the neat little PCB are 0dB, 6dB and 40dB. As can be seen in the images, the design is all SMD and the construction is decent.Upon opening the SWR bridge it is immediately obvious that this unit uses a traditional, well known design as found in many SWR/Return Loss bridges, such as those from Wiltron. I will attach a picture of a Wiltron bridge for comparison. Do not expect Wiltron performance from the BG7TBL unit though(Apples and Oranges)The images of the revered Wiltron 60N50 return loss bridge were harvested from this interesting web page:http://www.wb.commufa.jp/ja2djh/html/e_rlb.htmlIn the BG7TBL you can see the three 49.9 Ohm SMD resistors and two ferrite cores wound with very thin coaxial cable. The SMA connectors are the usual cheap Chinese products and have been soldered to the PCB and also at the locking nut (which was good to see). The soldering between the connectors and PCB is not wonderful but appears functional. PCB mounted connectors would have been better but this would have complicated construction in a case.The best way I can describe this SWR bridge is 'Functional' rather than beautiful. DIY bridges are very similar but perform to the required standard for amateurs, and this unit should be considered no different. It is not in any way intended to be a 'professional' product.Take a look at the pictures and graphs that I have harvested and see what you think.Aurora

author: Fraser
author_rank: Super Contributor
timestamp: July 28, 2015, 01:42:10 pm
content: Internal Pictures

author: Fraser
author_rank: Super Contributor
timestamp: July 28, 2015, 01:42:39 pm
content: Wiltron 60N50 bridge internal pictures

author: Fraser
author_rank: Super Contributor
timestamp: July 28, 2015, 02:02:50 pm
content: As a side note.I use professional quality directional couplers when testing for forward and reflected RF levels. They come in many forms and some are affordable on the secondary market. Minicircuits offer reasonably priced directional coupler modules as well.Aurora

author: Fraser
author_rank: Super Contributor
timestamp: July 28, 2015, 02:52:58 pm
content: PA3BWE has a web page showing this design of bridge. I attach the schematic from that page.http://pa3bwe.milatz.nl/?p=320He built a bridge specifically for his Rigol DSA815 Spectrum Alalyser.Aurora