Has anyone tried this return loss bridge? - Page 1
Forum section: electronics / testgear

author: KD0RC
author_rank: Regular Contributor
timestamp: January 03, 2015, 09:52:04 pm
content: I came across this return loss bridge on ebay for $42 USD. Specs look good (maybe too good?) for the price. Has anyone seen this one or dealt with this Hong Kong seller (aix81)?http://www.ebay.com/itm/2MHz-1500MHz-RF-Reflection-Bridge-Directional-VSWR-SWR-Bridge-Antenna-Analyzer-/181598814923?pt=LH_DefaultDomain_0&hash=item2a482202cbThanks,Len

author: 4cx10000
author_rank: Regular Contributor
timestamp: January 03, 2015, 10:14:12 pm
content: No, but for $42 it could be worth a try

author: KJDS
author_rank: Super Contributor
timestamp: January 03, 2015, 10:19:55 pm
content: It is a truth universally acknowledged that DUT port return loss is usually similar to directivity.I believe the specified RL of 19dBI don't believe the spec'd directivity of 40dB

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 04, 2015, 01:00:28 am
content: 6dB insertion loss?I have a few resistors that can do that... would you like to buy a bridge as well?For slightly higher price, you can have authentic Minicircuits product with considerably improved performance, e.g.,http://www.minicircuits.com/MCLStore/ModelInfoDisplay?14203330497850.07344025725867709Tim

author: G0HZU
author_rank: Super Contributor
timestamp: January 04, 2015, 01:50:10 am
content: My guess is that it will have a similar (classic) circuit to the Wiltron 60N50 RLBridge. This works over 5-2000MHz and boasts 40dB directivity and about 6dB insertion loss. There is an optional version of the Wiltron bridge that delivers 46dB directivity.The loss is about 6dB for the 60N50 Wiltron bridge because the circuit uses a broadband resistive bridge inside.I seriously doubt that the seller will have managed to replicate the performance of the Wiltron 60N50 but it isn't 'that' difficult to get very good performance from a homebrew return loss bridge over a very wide bandwidth.What would concern me would be the dodgy looking box it is built in and also the lack of proper performance plots. So I would either ask for reliable performance data or be prepared to be a bit disappointed if you buy it and test it formally.

author: c4757p
author_rank: Super Contributor
timestamp: January 04, 2015, 01:53:35 am
content: Quote from: T3sl4co1l on January 04, 2015, 01:00:28 am

author: G0HZU
author_rank: Super Contributor
timestamp: January 04, 2015, 02:03:24 am
content: Here's a link to what a typical RLB looks like inside.There's a few (ugly) homebrew ones and also some commercial ones including the Wiltron/Anritsu 60N50.The Advantest TR4172 spectrum analysers that feature in the link had an impedance measuring option available from the factory back in the 1980s that used the 60N50 along with the TR4172's inbuilt tracking generator.So if you bought the option you got a 60N50. I've got a couple of Advantest TR4172 analysers here that have the option but the 60N50 wasn't included in the sale. Long since lost...But I have made my own RLBs for it and I have measured impedance with it using the Tgen, the RLBridge and the impedance option. So this particular analyser can mimic a VNA in this mode and measure impedance with the 60N50.http://www.wb.commufa.jp/ja2djh/html/e_rlb.html

author: KD0RC
author_rank: Regular Contributor
timestamp: January 04, 2015, 02:17:45 am
content: Thanks for the insights. I have a home built RLB, but having never seen a commercial unit, I am always wondering how well mine works. The RLBs that I checked out on the Mini-Circuits site were around 90 USD, but when I looked again this evening, I see that there are some nice ones for closer to 50 USD. I don't really need one, I am just using it to learn a bit more about RF and was thinking of getting one to compare with my home built efforts.My challenge now is to flatten the return loss response. If I hold my thumb over the open end of the unit and adjust my tongue angle just right, I can get it to around 30 dB from 5 MHz to 450 MHz. From there it rolls off to around 18 dB at 600 MHz and 10 dB at 750 MHz. I assume that the capacitance of my thumb is the culprit, and that I should try a couple pf of capacitance across the transformer.Without the thumb trick, it is at around 35 dB at 80 MHz, 20 dB at 365 MHz and 10 dB at 750 MHz.Ironically, the single most expensive component in my bridge is the square aluminum tubing that I used to enclose it!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 04, 2015, 02:22:06 am
content: FWIW, thumbs vary from fractional pF (dominant, long distance) to maybe 10-100pF and 100-10k in series (touching), so don't forget to include the resistor if you're going to add much to the circuit.How about THAT for a "rule" of thumb!Tim

author: cncjerry
author_rank: Unknown
timestamp: January 04, 2015, 02:28:21 am
content: I've made my own and bought from minicircuits. It is very hard to stay flat over that range using a full machine shop and a small fortune in test equipment. You are not going to get 40db isolation with a plastic box, forget it. You need separate compartments within the box as well. Now I must admit that the POS RLB that came with my cheapo Asian VNA works incredibly well to 500M in a plastic box but that is using some funky coax transformers so it is well shielded.This design is easy to replicate and the boards are beautiful:http://www.makarov.ca/vna_bridge_0703.htmtMakarov and Paul who designed the VNA are both great guys. I am just about done building the N2PK VNA. I built a few of his RLBs and they work well to about 250M. After that I got some ripple in the response but it is such a low frequency that it wouldn't impact anything other than a broadband test.If you decide to go that route and don't have a drill press, PM me and I will machine the boxes for the cost of shipping.Jerry

author: KD0RC
author_rank: Regular Contributor
timestamp: January 04, 2015, 03:05:00 am
content: Quote from: T3sl4co1l on January 04, 2015, 02:22:06 amThanks Tim!!! I have been having no success with a straight gimmick capacitor. Your comment about a resistor got me to try wrapping the gimmick wire around a 328 K ohm resistor, and that nearly did it! I will goof around with the amount of capacitance to see if I can get it to match the thumb trick. If I can get it a bit closer, I will post before and after pics of the plot on my SA.Quote from: cncjerry on January 04, 2015, 02:28:21 amThanks for the offer Jerry, I really appreciate it! For now I am going to pursue the RLB I already built. I have looked at the N2PK pages before, and he sure does good work...

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: January 04, 2015, 10:47:39 am
content: Last year I built an RLB based on the deigns by VK3DIP, they use 4 "pig nose" ferrite cores with coax through them, built in a Hammond die cast box. Mine works from 350 KHz to 1.5 GHz. They are easy to make, see attachments.

author: KD0RC
author_rank: Regular Contributor
timestamp: January 04, 2015, 02:30:37 pm
content: Quote from: Solder_Junkie on January 04, 2015, 10:47:39 amNice work! That is beautifully done. Can you share a return loss (or VSWR) plot of it with a 50 ohm resistive load? I would be interested to see how it compares to mine.Len

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: January 04, 2015, 03:06:55 pm
content: Len, I'll run a plot tomorrow when I'm back home. If you Google VK3DIP you will find various versions, using a variety of ferrites. I can't remember if I used type 43 or 61. Using PTFE coax helps avoid a molten centre, although I guess RG174 would suffice.Dave

author: KD0RC
author_rank: Regular Contributor
timestamp: January 04, 2015, 03:38:08 pm
content: Quote from: Solder_Junkie on January 04, 2015, 03:06:55 pmThanks Dave, I really appreciate that. I have looked at Paul's page before, and he has done some pretty cool things to get the bridge to work at higher frequencies. I particularly like how he trimmed the pin of an N connector so that he could solder the SMD resistors directly on. Talk about zero lead length!Len

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: January 05, 2015, 09:20:57 am
content: Here are plots using the RLB, file "RLB.png" is with a 50 Ohm termination, the second "X50N.png" is the plot of a Diamond dual band ham radio antenna for 2m and 70cm, it is on the end of a 10 metre length of RG213.The RLB is a design by VK3DIP and I checked the ferrite binocular cores I used, they are type BN43-202. The coax is RG110 and the analyser is a Rigol DSA815/TGDetails of various RLBs by VK3DIP are here:http://www.yagicad.com/Projects/ARRLB.htmDave

author: KD0RC
author_rank: Regular Contributor
timestamp: July 11, 2015, 04:18:26 am
content: I finally got back to trying to improve the UHF directivity of my homebrew RLB. I made a high tech gimmick cap that is so low in value that I can't measure it on my Brymen 869 or my cheap AADE (Almost All Digital Electronics) L/C meter. I got almost exactly the same results that I got by placing my thumb over the open end of the bridge. The resistor I tried did not make any difference. The thing that finally worked was two pieces of telco wire about 3/4 inch long twisted together. In earlier attempts, I used too little wire. So, while this is not commercial grade, it is a definite improvement and makes the unit usable to just over 550 MHz. I sacrifice a little directivity at around 80 MHz, but gained a bunch at the higher frequencies.

author: KD0RC
author_rank: Regular Contributor
timestamp: July 11, 2015, 10:24:02 pm
content: Here is the final gimmick soldered in place. I got a bit better performance by stripping one of the wires so that I have a shorter capacitor. Now this is usable a bit past 600 MHz, but I won't go past 500 MHz with what I am doing.

author: IconicPCB
author_rank: Super Contributor
timestamp: July 12, 2015, 06:32:51 am
content: to dampen some resonances ( in the gimmick cap area ) get some soft iron filings and plain old window putty.Knead the filings into the putty and apply.Make the construction symmetrical ( Ref port to Test port ) for that improved directivity and bandwidth.

author: KD0RC
author_rank: Regular Contributor
timestamp: July 12, 2015, 02:35:14 pm
content: Quote from: IconicPCB on July 12, 2015, 06:32:51 amThanks for the suggestion, I appreciate it!

author: IconicPCB
author_rank: Super Contributor
timestamp: July 12, 2015, 10:28:39 pm
content: No worries.In the early eighties I used to work for a cable tv equipment manufacturer. Have not thought about this gear for a while. The posting brought back memories.