crazy readings - Page 1
Forum section: electronics / rf-microwave

author: p.larner
author_rank: Frequent Contributor
timestamp: February 10, 2024, 02:35:12 pm
content: i built this,see pdf,it works ok for upto about 5 watts but reads garbage above,is it due to the coils not being shielded,it reads ok with pots to set the signal to the meters,the fwd readings seem to track my diawa meter but the ref goes nuts!,only on this tho not the diawa.

author: A.Z.
author_rank: Super Contributor
timestamp: February 10, 2024, 03:49:51 pm
content: and, the question is ?

author: p.larner
author_rank: Frequent Contributor
timestamp: February 10, 2024, 04:37:44 pm
content: why is it doing that,is it lack of shielding between the two coils,its fine at<10w.

author: Andy Chee
author_rank: Super Contributor
timestamp: February 10, 2024, 08:36:29 pm
content: what precisely do you mean by "reads garbage" and "goes nuts"?

author: radiolistener
author_rank: Super Contributor
timestamp: February 10, 2024, 08:42:39 pm
content: Quote from: p.larner on February 10, 2024, 02:35:12 pmIt's not clear from your information what is going on, but since it fails when power is high, most of all this is an indication that toroid cores are saturated. In order to handle higher power you're needs to redesign toroid to keep it bellow saturation level at power which you're want to use.You're needs to look at toroid material specification in order to select proper material and and size of toroid core for selected frequency, inductance and current.Be careful, applying too much power to a transformer or coil can damage its core in case if it reach saturation level. The core can be melted or physically damaged and even can lead to a fire...

author: A.Z.
author_rank: Super Contributor
timestamp: February 10, 2024, 10:04:22 pm
content: and then, one may consider the wiring, looong leads and RF aren't nice

author: p.larner
author_rank: Frequent Contributor
timestamp: February 10, 2024, 10:24:04 pm
content: toroids are ft50-43,If its inline from my atu and diawa meter,Both power and swr track more or less the same on both meters,over about 5 watts if i adjust the atu for a 3-1 missmatch on the diawa,the homebrew one reads about 2-1 max and stays like that,even if i tune for a 10-1 swr,it tracks fine bellow 5 watts ok tho,is it because there is no shieldng separating the toroids?,there is only about 35mm separation and they are in the same plane,cheers.+73.

author: Andy Chee
author_rank: Super Contributor
timestamp: February 10, 2024, 10:31:34 pm
content: Quote from: p.larner on February 10, 2024, 10:24:04 pmWhen in close proximity to each other, toroid coils behave differently to solenoid coils Long story short, toroids are self-shielding.In other words, toroid shielding is NOT your problem. Look elsewhere for your problem (e.g. core saturation, too many turns, etc)

author: radiolistener
author_rank: Super Contributor
timestamp: February 10, 2024, 11:22:53 pm
content: Quote from: p.larner on February 10, 2024, 10:24:04 pmno

author: vk4ffab
author_rank: Frequent Contributor
timestamp: February 10, 2024, 11:49:17 pm
content: Quote from: p.larner on February 10, 2024, 10:24:04 pmHow to align an SWR meter.1. Connect 50ohm load to the OUT and a transmitter to the IN and apply the maximum power the meter is going to see. 5w, 100w, 1 bazillion w. ETC.2. Adjust the FWD power pot for maximum deflection on the FWD meter.3. Reverse the IN and OUT connections4. Apply power again and this time adjust the REF pot for maximum deflection.If you have a 3:1 load, connect it and it should read 1/2 scale on both meters.JOB DONE.Again, this has to be done at the MAXIMUM power and will make the readings at MINIMUM power slightly off. It sounds to me like you have aligned the meter at 5W and then at 100W wonder why its reading wrong.An FT50 sized toriod is fine to about 1KW in an SWR application. Take a look in any commercial SWR meters, they use essential a stack of 2 FT37 size toriods. The issue has nothing to do with saturation. Though, shielding the RF portion from the meter portion should be a priority. Put the bridge in a metal box and the meters outside that box and it should be fine. Just align it right.Also PICTURES of what you have build and done actually help. The link to the schematic is useless in this case if its user error rather in construction. We are not mind readers. If its all just flapping about in the breeze and you have wires everywhere, all bets are off and it could be anything. Put it in a metal box first, then align it.

author: p.larner
author_rank: Frequent Contributor
timestamp: February 15, 2024, 01:28:12 pm
content: Well ive taken the advice given and put it inside a metal box,just a couple of questions should i use coax ie rg 174 from the sensor to the board with the cal resistors then from that to the meters and use some 0.1uf decoupling caps to ground from the meter feeds to ground at the meter end?.thanks in advance.

author: MarkT
author_rank: Frequent Contributor
timestamp: February 15, 2024, 09:31:50 pm
content: Is it that surprizing that a tiny QRP SWR meter doesn't handle QRO?

author: vk4ffab
author_rank: Frequent Contributor
timestamp: February 15, 2024, 09:49:22 pm
content: Quote from: p.larner on February 15, 2024, 01:28:12 pmShould not be necessary, Just twisting the pairs that go to the meters should be good enough.

author: p.larner
author_rank: Frequent Contributor
timestamp: February 17, 2024, 02:35:01 pm
content: well ive replaced the diodes with a pair of oa91 diodes with matching vf,the ref response is way off,its so bad to be useless,with an swr of 6-1 it will read about 1.3-1,is it just a poor design?,if so has anyone plans for a better sensor i can build?,its also mounted in a metal box with the wires to the meters decoupled to ground with 0.1uf caps.

author: A.Z.
author_rank: Super Contributor
timestamp: February 17, 2024, 02:52:45 pm
content: post detailed images of the coupler and the meter

author: vk4ffab
author_rank: Frequent Contributor
timestamp: February 17, 2024, 11:45:21 pm
content: Quote from: p.larner on February 17, 2024, 02:35:01 pmBig chance the germanium diodes are fake and they are more like really bad schotkey's breaking down with a very pronounced knee. Just use plain old rectifier diodes and accept the extra loss and calibrate the meters accordingly.After changing the diodes, did you re-align the meter?I looked over that schematic 3 times in the duration of your build and it looks fine, the issue will be one of build error.POST PICTURESso we can look for obvious errors. SWR meters are all pretty much the same, I use the schematic over on kits and parts, 10:1 turns and component values only hold true for 5W, if building for higher powers, the resistors need to be beefed up and the turns ratio changed.

author: p.larner
author_rank: Frequent Contributor
timestamp: February 18, 2024, 10:38:22 am
content: so what turns ratios are needed for say 10w-100w and 1000w?,the ref voltage seems off,for example if the atu is tuned to give a 6-1swr on my diawa meter on the homebrew one the most it reads is about 1.5-1,should i add a couple of more turns on the lower torroid?,it voltage swing on ref between say a 2-1swr and 10-1 swr is too small,how can i increase it?.,i cant post pics as its already fitted in the steel box but its built exactly as the pdf using ft50-43 toroids.

author: Andy Chee
author_rank: Super Contributor
timestamp: February 18, 2024, 10:44:15 am
content: Quote from: p.larner on February 18, 2024, 10:38:22 amYou can take a photo when you open the steel box to add/remove turns.Once we analyse your photo, we can suggest a course of action.

author: p.larner
author_rank: Frequent Contributor
timestamp: February 18, 2024, 11:24:26 am
content: i have spare toroids and pcb and 0a47 diodes,i may build another sensor then take pics,what number of turns do you recomend on the toroids andy?,i would like it to be calibrated for 10w 100w and 1kw if pos using a multiway switch and pots to cal on each range.

author: A.Z.
author_rank: Super Contributor
timestamp: February 18, 2024, 06:53:35 pm
content: and... still no pics, ignoring the requests, now WHO is the TROLL ?

author: p.larner
author_rank: Frequent Contributor
timestamp: February 18, 2024, 10:17:29 pm
content: can you not read,i have already stated i cant take pics yet as its still in the metal box!.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: February 19, 2024, 06:45:08 am
content: Quote from: p.larner on February 18, 2024, 10:38:22 amEverything depends on the meter movement. You want a turns ratio that gives you full meter deflection into a 50ohm load at full power, using resistors capable of dissipating 5 times the rating of the meter movement, probably not accurate, but good enough for the girls I go out with.I can tell you what it is for me, not much use for you, so with a 100mA meter 10:1 is just fine for 100W as long as the 50ohm resistors can dissipate the power, though I am not to sure how critical things are here, I could simulate it and find out, but never needed to know, I just put in bigger than I think is needed, maybe a couple of watts.Some math to bookmark:)RF Power Converstionshttps://www.apogeeweb.net/tools/rf-unit-converter.htmlOhms Law Calchttps://ohmslawcalculator.com/ohms-law-calculatorVoltage Transformer Calc, this is for power transformers, but i think the values hold true for us at RF.https://goodcalculators.com/transformer-calculator/That is everything you neef to make your own calculations.

author: Andy Chee
author_rank: Super Contributor
timestamp: February 19, 2024, 07:14:10 am
content: Here's an easy procedure to determine what the likely full scale deflection (FSD) of your unknown meter is:

author: p.larner
author_rank: Frequent Contributor
timestamp: February 21, 2024, 02:00:19 am
content: fwiw the meter is a twin needle jobby from aliexpress 13 quid from there as oposed to 27 from fleabay!,i asked the seller the meter specs and its 100ua for fsd,not had time to get onto it againe as been busy with hospital appointments,73.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: February 22, 2024, 01:50:34 am
content: Quote from: p.larner on February 21, 2024, 02:00:19 amDo you have a link to the meter? The only one I could find with my search was 500uA that alone would explain the very low readings you are getting. I have asked you a couple of times if and how you aligned the meter, something you never answered. We are assuming you aligned it right and during alignment it showed correct readings. I put it in bold its not like you could miss it.

author: p.larner
author_rank: Frequent Contributor
timestamp: February 22, 2024, 06:18:28 pm
content: https://www.aliexpress.com/item/1005006309028399.html?spm=a2g0o.order_list.order_list_main.32.217d1802FHn6W8Thats the link to the meter,they told me it was 100ua for fsd.,for the cal i had my diawa inline set the rig for 60w into a dummy load and adjusted the cal pots accordingly,same for swr,it reads power fine its just the reflected reads way low,ie there is little difference in the meter swing from say 1-1 thru 10-1.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: February 23, 2024, 03:24:14 am
content: Quote from: p.larner on February 22, 2024, 06:18:28 pmWhen you get a chance, measure both meters, and see what current is required for each one to go full deflection, they might even be different. I would not trust the seller to know what it does.

author: A.Z.
author_rank: Super Contributor
timestamp: February 23, 2024, 09:30:48 am
content: Quote from: vk4ffab on February 23, 2024, 03:24:14 amWell, he may just swap the meters connection and repeat the measurement

author: p.larner
author_rank: Frequent Contributor
timestamp: February 25, 2024, 03:53:54 am
content: well i redid the bridge using an idea from an old arrl book,i used 1millihenry chokes on the feeds to the meters and 20 turns on each toroid,it seems to be working ok ish now although the ref sensitivity is down compaird to my diawa meter but its usable now.73+thanks.