Antenna length! - Page 1
Forum section: electronics / projects

author: Things
author_rank: Regular Contributor
timestamp: June 29, 2012, 08:57:42 am
content: Hey guys, new here, but been into electronics for a fairly long time. One downside to this is I was always self taught - so I know the practical side of it very well, but suck at the theory!I have a cheapo weather station, and keep having issues with it dropping out! It's outdoor sensor is wireless, and is almost line of sight out the window, but still!I have opened up the receiver once, and tried replacing the antenna wire with a longer one of a precise length, however it doesn't seem to help. It says it's nominal operating frequency is 433Mhz, pretty common, but I can't tell for sure as I have no way of measuring!Could someone possibly help me out with calculating the antenna wire lengths for this frequency? I would prefer to not have to make it directional, but I suppose it's not out of the question.EDIT: The wire I have on it now seems to be about 30cm, which looks like the half wavelength. Originally it must have had 1/4 wavelength, but going to 1/2 didn't seem to help ... maybe even made it worse!I must also note that there are water tanks between the RX and TX, however it is not very large at all, I can almost just see the sensor setup from out the window the RX is sitting on. Is it possible the water is interfering?Cheers,Dan

author: Psi
author_rank: Super Contributor
timestamp: June 29, 2012, 09:13:00 am
content: I gave up trying to get RF stuff to workThe issue with antenna length is that often a circuit is "tuned" for the antenna it was designed with.That's what some of those variable caps/inductors are doing.Without a RF meter/spectrum analyzer it's hard to build/fix/modify RF gear and it just gets harder over 100Mhz.Don't take that as discouragement though, RF engineers get paid heapsya just have to learn the theory.RF design books are probably a good place to start. I'm sure someone on here will recommend one, or help you with your antenna issue.

author: Things
author_rank: Regular Contributor
timestamp: June 29, 2012, 09:55:14 am
content: Lol tell me about it! If it wasn't for the fact I had to run thru a window, I woulda thrown a wire on it long agoI have tried moving it literally right outside the window, see if it's any better.Dan

author: notsob
author_rank: Frequent Contributor
timestamp: June 29, 2012, 10:13:35 am
content: I'm not sure how far you wish to go with this, however if you want to 'see' what is happening (and have $100 to spare) try this RF spectrum unithttp://www.seeedstudio.com/depot/rf-explorer-433m-p-784.html?cPath=174

author: vk6zgo
author_rank: Super Contributor
timestamp: June 29, 2012, 11:06:23 am
content: Quote from: Things on June 29, 2012, 08:57:42 amIf it has a 1/4wave antenna mounted on the unit,extending it to 1/2 wavelength is not going to help,& indeed may make things worse.End fed antennas of different wavelengths present widely differing impedances to the device they are connected to,with mismatched impedances causing loss of signal.One thing you can try is hanging a 1/4 wave wire off an "earthy" part of the receiver.This is often used with Handheld VHF & UHF Ham radios to improve the performance of the antenna,If your antenna is a screw on type using an SMA connector,you can probably connect this wire to the outside part of the connector,otherwise you may have to take "potluck" at finding a earthy point on the receiver body.If the water tank is metal,it could be shielding your antenna from the signal,whether it has water in it or not.If it is plastic,it should not shield much,unless it is full of water.Another possibility is that you are getting two out of phase signals at your antenna which are cancelling.One could be direct,& the other reflected or diffracted by the tank or something else.Is the receiver portable?If you can take it up close to the sender,& see what results you get.then walk around to see where it starts to get dodgy,you may be able to find a better position for it or the sender.If the receiver is not portable.can you move the sender around in the same manner?

author: Things
author_rank: Regular Contributor
timestamp: June 29, 2012, 12:30:35 pm
content: Thanks for the infoI actually moved the sensor just outside my window, but noticed it wasn't transmitting at all!After bringing it inside, I did a bit of digging and found that the TX unit would work fine when I disconnected the solar/UV/light sensor panel!On this panel however, is a reset switch. I did a bit of continuity probing with my DMM and found that the reset pin was actually around 1.2V unpressed, and 0.4V pressed. So every time I plugged it into the TX unit, it'd just hold the uC in a reset!I'm not exactly sure what's going on with it though, it doesn't appear to be a simple button with a pullup resistor, so I have just disconnected it entirely for now, as it's not difficult to just remove the batteries if it needs a reset (Very rarely anyway).I'll do a bit of range/dead spot testing like you said and see how it goes.The tanks are plastic, but are completely full of water.Dan

author: G7PSK
author_rank: Super Contributor
timestamp: July 02, 2012, 05:28:45 pm
content: Are your window frames metal and is the glass coated for heat retention as this can also cause attenuation and I expect that the transmitter is not very power full, another thought as the unit is 433mhz are there any other transmitters in the area a lot of baby monitors etc and low power pmr's are also 433 as well garage door openers etc.

author: ee851
author_rank: Regular Contributor
timestamp: July 03, 2012, 01:56:57 pm
content: After you have checked what vk6zgo said about antenna lengths, there are a couple other things to check:- Both antennas, TX and RX, should be more or less parallel. If one is horizontal and the other vertical, the radio signal won't pass readily from one to the other. If one is horizontal, make the other one horizontal too. (or vertical, whichever it happens to be)- In some towns, buildings have walls that are built of plaster that is laid atop metal netting material--aluminum netting, for example. If the sizes of the holes in the netting material are much smaller than the wavelength of the radio signal, the netting material might block the signal. You can test for this by putting the TX and RX both outside with no metallic objects between them.