I have an ESP-07S which does not have an on-board ceramic antenna but instead a micro-coaxial connector.
Now I wonder if it is possible to use a matching cable which I cut off the shield from in the far end and use as a WiFi antenna?
If this is possible it would make it rather much simpler than using one of these WiFi antennas which attach to the mini connector.
If so, what would be the optimum length of the exposed inner conductor?
The free air wavelength of the WiFi at 2.4 GHz is about 125 mm, so for instance a quarter length "antenna" would be some 31 mm long. Or is it a half wavelength it should be?
BosseB:
I have an ESP-07S which does not have an on-board ceramic antenna but instead a micro-coaxial connector.
Now I wonder if it is possible to use a matching cable which I cut off the shield from in the far end and use as a WiFi antenna?
If this is possible it would make it rather much simpler than using one of these WiFi antennas which attach to the mini connector.
If so, what would be the optimum length of the exposed inner conductor?
The free air wavelength of the WiFi at 2.4 GHz is about 125 mm, so for instance a quarter length "antenna" would be some 31 mm long. Or is it a half wavelength it should be?
Certainly possible. It's called a "sleeve" antenna. Think of a shirt sleeve. The 1/4 wave length is correct. The shield on the coax becomes the other 1/4 wave length of a dipole. You need to have a way to make the RF think the shield is only a 1/4 wave length long.
More research on your part is necessary to see how you want to accomplish this.
Paul
I presume the antenna requirement for a 2.4GHz ESPxxx is the same as for a 2.4GHz nRF24L01+ module.
To reduce the size I sawed the PCB antenna off some nRF24s and soldered a short length (1/4 wave) of ordinary insulated thin wire to it. Seems to work fine, but I have only needed to use it over a range of a few metres.
...R
Sleeve antenna search got me to several good places like this tutorial.
But there is one thing I don't understand fully and this concerns the length of a quarter length dipole section.
From the WiFi frequency and the speed of light one would think it is about 31 mm. But in several places I have read they set it to 25 mm, which is about 80% of the length I calculate.
Is this due to the slower speed of the EM wave in a coaxial cable?
But in any case I don't need an optimized antenna, just one that can work OK on my workbench towards a router in the same room.
Try it and see if it works?
It will not cost much in time or materials to change it if it doesn't.
Then you can tell us the results and post pictures of what you did.
...R
I will do that as soon as I get back home on Tuesday...
Currently I do not have the tools needed to handle the tiny coax connector and cable.
But I have been annoyed about the clumsiness of having a big WiFi antenna hooked to the ESP with a 15 cm long cable while doing firmware development and testing...
BosseB:
Sleeve antenna search got me to several good places like this tutorial.
But there is one thing I don't understand fully and this concerns the length of a quarter length dipole section.
From the WiFi frequency and the speed of light one would think it is about 31 mm. But in several places I have read they set it to 25 mm, which is about 80% of the length I calculate.
Is this due to the slower speed of the EM wave in a coaxial cable?
But in any case I don't need an optimized antenna, just one that can work OK on my workbench towards a router in the same room.
I don't know the context of the two lengths. The shorter may be for an ideal wire, infinitely thin and radiating in free space.
The shorter one is based on actual wire (copper?) radiating in air. The dielectric constant of air slows the wave. The diameter of the wire slows the wave. End result is a longer wire. If you had insulation on the wire, it would be even longer.
Paul
Well it would be shorter if the speed is slowed, right?
It fits the bill concerning the numbers quoted, in empty space the wavelength of 2450 MHz (middle of the 2.4 GHz WiFi band) is 122.4 mm, thus a quarter wavelength antenna would be 30.6 mm. 31 is then a good value.
But some pages state the length needs to be 25 mm, which would be true if the speed is reduced to 82% of light speed.
This happens in coaxial cables for instance, I seem to remember from my physics classes in university that one could get about 75% speed inside a waveguide or coaxial cable.
I am back home and have tested the sleeve antenna solution from one of the pages I found.
I used a 200 mm WiFi coaxial cable purchased readymade with a connector in each end.
Did as follows:
- Cut the cable in half
- Stripped off 35 mm of the outer plastic cover
- Then pushed back the shield over the remaining plastic cover, thus exposing the inner conductor
Note that the shield only reached 26 mm back so one should really strip 40 mm) to get 31 mm on this part too. - Cut the exposed conductor down to 31 mm (quarter wavelength of the WiFi frequency)
- Replaced the bulky WiFi antenna with this cable on my ESP-07 prototype board
After this was done I fired up the device in AP mode and used the WiFi Analyzer app on my phone to check the field strength of the AP. Turns out that it was almost equal to the WiFi router I also have in my study.
So it seems like this is the simplest way to get a compact antenna solution!
I attach an image of the solution where you can see the unused half of the cable below the assembly.
Question:
Is there some way to read the signal strength of the current connection using the ESP8266 WiFi library?
Answer: Yes, see below my next post.
Nicely done! Thanks for the picture and dimensions others can follow.
Paul
Thanks!
And I also found the way to read the signal strength:
long rssi = WiFi.RSSI();
SerialDebug.print("RSSI:");
SerialDebug.println(rssi);
The result on my debug channel:
dhcp client start...
ip:192.168.119.208,mask:255.255.255.0,gw:192.168.119.1
RSSI:-46 <== Here is signal output
STA ADDR = 192.168.119.208
So the new antenna reads similar to the app WiFiAnalyzer on the phone regarding the strength of the WiFi signal from my router, which the app sets at -40 to -43 dB when the phone is located close to the ESP-07S.
Concerning the WiFi signal strength reading:
Once I have executed:
long rssi = WiFi.RSSI();
how can I convert rssi to a C string (array of char)?
BUMP! - Edit
I have now found the simple way to do it as shown below....
I want to create a function used to check the strength on external command and then send it as a C string over a TCP channel.
So I need a function that will stuff the value into a char array supplied to it as a pointer like so:
int GetWifiSignalLevel(char *buf)
{
int res = 0;
int wista = WiFi.status();
// write the received signal strength from the access point into buf
if ((ESPConf.mode == WIFI_STA) && (wista == WL_CONNECTED))
{
int rssi = WiFi.RSSI();
itoa(rssi, buf, 10);
}
else res = wista;
return (res);
}
Works as expected.
Using the above function and adding it to my command interface I have now tested the efficiency of the antenna.
It turns out that when it is positioned as shown in the photo I get about -49 dB. But raising it perpendicular to the board gives me -37 dB an improvement of 12 dB!
So it looks like the antenna works fine and is directional as a dipole would be (in a disk shaped pattern).