dumb questions about 434 MHz transmission lines and matching - Page 1
Forum section: electronics / rf-microwave

author: gardner
author_rank: Regular Contributor
timestamp: August 09, 2019, 04:44:53 pm
content: I am working on a super low budget project using cheap OOK/ASK 433.92 MHz modules from China. The project has some sensors that can be periodically read by an ATTiny85 and the data sent to a base station using a VirtualWire OOK codec.The quality of the cheap Chinese 434 MHz transmitters is all over the map, and the documentation about their design is pretty scarce to non-existent. I think most of them are designed to match a 1/4wave whip at ~35R or are matched to ~50R or perhaps not designed with any particular matching impedance. The transmitters all seem to be designed to be single ended and most of them have to nearby RF ground.For my application I am driving a 1/2 wave dipole made out of two sections of 3mm copper wire. After messing with little baluns wound onto cores scrounged from CFL lights I have settled on using a twisted pair taken from a CAT5e cable as both a transmission line and current balun. I can use a 150mm length of UTP soldered on one end directly to the dipole elements, and on the other, connected to the radio via a 2.5mm header connector. It seems to work alright.The link budget I'm working with is pretty narrow. I believe I am getting ~ -96 dBm at the location where the receiver needs to be, as measured with my RFExplorer in perfect weather conditions. The receiver sensitivity is ~ -106 dBm so bad weather, vegetation or cobwebs on the antennas could put me close to the limit.Is there anything truly terrible about my UTP transmission line design that could be corrected cheaply? Would I be better off to use 50R coax for the transmission line? Is there a compact antenna design much better than a 1/2 wave dipole I should consider?Thanks.

author: tautech
author_rank: Super Contributor
timestamp: August 10, 2019, 09:12:48 am
content: What range are you striving for ?These keyfobs that are rated to 200m I managed to get 500m from it with just a decent receiving antenna.The receiver is just a cheap multi channel unit too and its antenna was just coiled wire until I broke it out into coax and a BNC socket on its enclosure then a 3m run of RG58 to a tuned J-Pole.First I'd check yours are 433.92 MHz and then if you have a reflection bridge sweep your antenna and adjust to get max emissivity at 433.92 MHz.Sure without a VNA it's luck to get antennas spot on however from what I learnt tuning mine getting the emissivity peaked at the frequency of interest was the biggest bit of the battle.I wrote up my battles and learnings here:https://www.eevblog.com/forum/rf-microwave/antenna-project-log/

author: Lord of nothing
author_rank: Super Contributor
timestamp: August 11, 2019, 04:12:20 pm
content: The point it its on the Amateur Radio Band where also Repeater are in use the could put out a huge output Power so the Device should us very good Filter to not get maxed out by the Repeater.

author: gardner
author_rank: Regular Contributor
timestamp: August 12, 2019, 12:32:43 am
content: Quote from: tautech on August 10, 2019, 09:12:48 amI am looking for about 500m from the farthest one, and 150m from the closest one, all through ~150m of bush, plus some distance across open water. Using the antenna design and feed line I mentioned, I have usable reception currently, at these distances. The fact that it is somewhat close to the limit of usability is what makes me seek a potential design improvement.Interesting write up about your 500m requirement. In my case, I don't really have the option of a really good receive antenna. I need something that can be installed unobtrusively in a screened in porch. At present I have a dipole hot-glued to a plastic card thumb tacked to a wall with a feedline of UTP to the radio a few hundred millimeters away.The regulatory framework I am operating in is the same as for wireless thermometers and I have a few dozen mW cap -- although I don't know if the cheap Chinese modules exceed that or not -- and transmit much less that 1/10% of the time, at intervals above 60s. It's in quiet area for 433MHz spectrum, but is bound to tolerate other users of the spectrum without problems, and the project components are fine, even in an urban area vis-a-vis tolerating interference.The only RF instrument I have that operates in the 433mhz band is an RFExplorer. I thought about one of those cheap Chinese $80 Nano-VNAs, but even that would add costs to the project that I am unsure are worth it. I will likely fabricobble an RF power meter if I can rummage out a suitable diode from my junk bin. I don't think I have a germanium diode -- maybe a small enough schottky diode so that the junction capacitance is super low. I wouldn't spin up a Mouser order just for a few BAT81s. That would at least allow me to compare the raw power output of different transmitter module options.

author: tautech
author_rank: Super Contributor
timestamp: August 12, 2019, 12:40:18 am
content: Is a Yagi an option ?

author: gardner
author_rank: Regular Contributor
timestamp: August 12, 2019, 03:44:36 am
content: Quote from: tautech on August 12, 2019, 12:40:18 amProbably not. At the base station, the antenna has to pick up three stations that are ~120 degrees apart. Also, a 3 or 4 element yagi-uda would be a bit bulky. It would have to be mounted outside and would require a longer, better feedline -- like 75-ohm RG59.I was vaguely considering building a 4 element colinear that could hang on the wall. That would have all its gain on the horizon, which would be good for the base station.At the remote sensors, only the closest one is located where a yagi-uda could be placed. The farthest station is on a floating swim raft which can drift around a bit, particularly directionally, so a more omni-directional design is needed. For this one, it would likely be hard to outdo a dipole.

author: tautech
author_rank: Super Contributor
timestamp: August 12, 2019, 04:09:42 am
content: Quote from: gardner on August 12, 2019, 03:44:36 amUnderstood as that was the consideration for the antenna I finally chose with the help of the RF guys on the forum.Run the numbers for a J-Pole, the link for its calculator is in the thread above.Like any antenna there are near field effects you need to consider and adjust for to get optimum results.I put mine in a plastic waste pipe tube with a cap on the top and adapters to threaded galv pipe on the bottom so on which to mount it but as yours is 433MHz it will be a bit more compact than the 315MHz unit I built.