3
\$\begingroup\$

I'm really puzzled about this. I have a simple transmission between two ATTiny85's going on with a 433MHz receiver and transmitter. The transmitter is in the basement so the signal is rather weak but here is weird thing. If I hold my USB to TTL cable, that connects the ATTiny to my PC, at a higher position like shown in image #1 I get a steady stream from my transmitter down stairs.

If I put the cable down (touching the cable isolation makes no difference) like shown in image #2 I get no signal AT ALL. Just nothing. I could repeat this behavior several times.

I really don't know much about antennas and radio transmission so this doesn't make sense to me in any way. I guess he USB cable must act as some sort of antenna/repeater but how? It only provides 5v power to the receiver and ATTiny and reads the serial output from the tiny.

High-res: http://i.imgur.com/mmH910U.jpg Image #1 High-res: http://i.imgur.com/y6fIKOo.jpg Image #2

\$\endgroup\$
  • 1
    \$\begingroup\$ Can you try to disconnect laptop from mains? USB ground is connected to notebook, notebook power adapter and whole ground circuit in your house. Thats really huge antenna. \$\endgroup\$ – Kamil Apr 18 '14 at 15:59
  • \$\begingroup\$ It's a PC not a laptop, no battery power there. \$\endgroup\$ – PTS Apr 18 '14 at 16:03
  • \$\begingroup\$ Is this USBasp based programmer for Atmel chips? They do enormous radio mess. \$\endgroup\$ – Kamil Apr 18 '14 at 16:10
  • \$\begingroup\$ Nah its just a USB cable with a PL2303 in the cable that reads the tx line from the tiny or also optionally write to it but the tx line from the cable is not connected. \$\endgroup\$ – PTS Apr 18 '14 at 16:14
  • 1
    \$\begingroup\$ I did, they are all holding in place. Laying it on the desktop and moving the breadboard around leads to a more graduate effect where the signal sometimes gets just very weak and only drops after more subtle movement of the breadboard. \$\endgroup\$ – PTS Apr 18 '14 at 20:33
7
\$\begingroup\$

Yup, all those dangling wires are part of the overall antenna system.

At 434 MHz, half a wavelength is less than 14 inches, or just a little more than a foot. Anything approaching that size makes it no longer a lumped system, so the usual simplifications of this wire connects that signal to here no longer apply blindly.

Your RF receiver ultimately sees whatever signal is different between its antenna input and its ground connection. From the RF point of view, it doesn't really matter whether you think of most of the signal being on the antenna or the ground. Those long wires to your USB converter are acting as antennas, particularly the "ground" wire. Note that ground is actually a rather complicated subject when you don't have a lumped system anymore.

\$\endgroup\$
  • \$\begingroup\$ The short version is that RF and breadboards don't play well together. \$\endgroup\$ – Ben Voigt Apr 18 '14 at 16:02
  • 2
    \$\begingroup\$ @Prof: The reason minor changes in the cable matter is because that changes how good a antenna or not those wires become. Length and orientation and shape all matter. I'd say you have overall grounding problems that this is pointing out. If RF isn't your thing, get someone to help you with that part. This can get complicated. It's too much to get into details on here. \$\endgroup\$ – Olin Lathrop Apr 18 '14 at 16:14
  • 1
    \$\begingroup\$ I see, any recommendations on bloody beginner material to read into? I don't know anyone that has any knowledge about this stuff that I could get help from. \$\endgroup\$ – PTS Apr 18 '14 at 16:17
  • 1
    \$\begingroup\$ @Prof: No I don't, but I'm sure there are books out there on this. This is not a simple subject. I don't know how to explain it other than it takes a while and experience until you eventually get some intuition on how RF works. Even then, experts will still be surprised often enough. \$\endgroup\$ – Olin Lathrop Apr 18 '14 at 16:21
  • 2
    \$\begingroup\$ @ProfessorFartSparkle There's an amateur radio band which the 433 MHz unlicensed band fully overlaps, so you may want to look for amateur radio books that deal with the 70 cm band. It shouldn't be too hard to find such books at your level of understanding. You may also want to browse the Amateur Radio SE and our 70cm-band tag in particular. \$\endgroup\$ – a CVn Apr 18 '14 at 21:26
1
\$\begingroup\$

Im not good at radio devices, so if im wrong - someone please add proper comment and I will remove that answer.

I think the problem is too large impedance of too long ground line.

That long ground path may catch a lot of radio noise/interference sources nearby (few hundred watt pc power supply etc.).

I think if you power it up from separate power supply (like phone USB charger) it should work much better.

Getting power from Raspberry Pi powered by small power supply shall be not a problem too.

You are using very simple radio module, that is not designed to handle noise on ground line.

\$\endgroup\$
  • \$\begingroup\$ Thanks for the tip. I will see how it looks when its connected to the rpi. \$\endgroup\$ – PTS Apr 18 '14 at 20:34
  • \$\begingroup\$ Weirdly the signal is only there when I have a long ground wire and I move that around until I find a sweet spot. When I have a short ground cable nothing gets recieved no matter how I hold the cable. \$\endgroup\$ – PTS May 2 '14 at 16:35

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.