WiFi antenna

WillemW

Hello all,
Can someone tell me what the gain (dbi) is of the original WiFi antenna of the Asiair Plus?
Can I use a 10 dbi wifi antenna for my Asiair Plus to increase wifi range? Thanks.

w7ay

WillemW Can I use a 10 dbi wifi antenna for my Asiair Plus to increase wifi range?

Keep in mind that to establish a WiFi connection, the ASIAIR needs to not just transmit well, but also have a decent signal-to-noise ratio (SNR) in the receive direction. Increasing the gain of the antenna will not help SNR if the interference is coming from same direction as the increased directivity of the antenna.

From all the complaints that I have read, I highly suspect that peoples' problems are caused by the receive direction (not enough SNR), and not by the transmit direction (not enough power). I.e., living in suburbs that are saturated with WiFi devices. The only help in this case is to increase the transmit power of the WiFi router. It could simply be that your neighbor already has his/her router set to much higher power than your router (yes, you can change the power levels from many WiFi routers). If your drop outs are intermittent, they could come from other sources in your own house (since it is in the same general direction as your router) that use the same "license free" 2.4 GHz ISM band -- cordless telephones, microwave ovens, weather stations, soil moisture sensors, and even nearby Bluetooth devices. Of course, it could also come from any of your neighbor's devices as well.

Drive the ASIAIR away from your neighborhood to a clear field in the countryside, to see if it works better. If it does, then the receive direction is your problem, and a higher gain antenna may not be the cure, if the interference is arriving at the ASIAIR from the same direction as the increased gain of your antenna.

If receive SNR is the problem, a higher gain (i.e., higher directivity) antenna only helps if the interference is coming from a different direction as the direction of the antenna's main lobe.

Secondly, antenna gain is achieved only by increasing its directivity (i.e., conservation of energy). If the signal is stronger in one direction, it has to be weaker in a different direction. By definition, an isotropic dipole radiates equally in all direction, so, the solid angle of the directivity pattern is reduced by 10 decibels (the "i" in dBi refers to an isotropic reference antenna), or a factor of 10 smaller. The gain of a "10 dBi" antenna is thus about 0.4 * pi steradians.

With a higher gain antenna, it becomes even more important that the antenna does not move through the night. If your current dropouts are caused by your mounting the antenna on the OTA, the problem could become worse, not better. Mount your antenna to a stationary point, like the tripod leg.

That being said, since it is not isotropic, a typical short dipole (probably what came with the ASIAIR, although ZWO neglected to publish the radiation pattern of the ASIAIR) has a gain of about 2.2 dBi. So the gain that you achieve with a 10 dBi antenna is probably only 7.8 dB better than what exists today.

Thirdly, keep in mind that antennas (again, by conservation of energy) works as an inverse square law. If all else (interference, average path loss, etc) is equal, an extra 6 dB of antenna gain will only improve the distance by a factor of two (not a factor of four), and as mentioned in the first paragraph, if your problem is caused by interference in the direction of you router or tablet, then distance will not improve one iota anyway.

Chen

Swizerlan

What could help though is a 5dbi on a extension cable. Mount it to a tripod leg so you can aim at your house. This way you wont have spotty connection if the scope slews away from you and blocks signal. I do this with an external router which had a way stronger signal than the aa+

w7ay

Swizerlan Mount it to a tripod leg so you can aim at your house.

That assumes that the interference is not coming to the ASIAR's WiFi receiver from the direction of the house.

The best is still to connect directly to an Ethernet dongle that is connected to an iPad, so there is no wireless connection at all (a Raspberry Pi 4 Ethernet port is good to over 900 Mbits/sec, how fast the ASIAIR firmware transfer data from the camera becomes the limiting factor). This is a also a good solution out in the field, since you can avoid bringing a travel router with you.

Next best is to connect the ASIAIR directly through Ethernet to your router.

I had been connecting the ASIAIR to an eero Mesh router that is right next to the ASIAIR. Even that is sub-optimal since that mesh router is still using wireless to talk to the rest of the mesh network. While smaller (sub-APS frame) downloads in a blink of an eye, a full frame ASI6200 image still takes a palpable time to download.

So I bit the bullet recently, and pulled an Ethernet cable between the telescope and the eero mesh router that is in the house and that gives me the best compromise between complexity and convenience for me. My tri-pier is semi-permanently outdoors (haven't move it, or the mount, from its location for more than a year and a half now), so the Ethernet cable is now a permanent fixture as well.

Each person's situation is different, but it is worth knowing the possible failure mechanisms so one can make an intellegent decision on what is the "best" personal solution.

Chen

w7ay

Swizerlan What could help though is a 5dbi on a extension cable.

Be careful about the antenna too -- sweep it using a VNA to see what the VSWR is like around the 2.4 GHz band and the 5 GHz bands.

The cheap (Amazon) antennas are usually not designed to really be efficient dual band antennas. To make the VSWR match on both bands (so that the WiFi transmitter does not burn up due to reflected signal, instead of a real matching network, a fixed resistor is placed in parallel with the antenna. If you cut open one of the antennas, you can often spot the resistor - or measure the impedance with a DC ohmmeter -- if the DC resistance is near 50 ohms, the manufacturer had placed a resistor as a matching network).

A resistor to match really zaps the power.

I nearly fainted when someone on the ASIAIR Facebook page recently suggested cutting the resistor to get a much better range. I highly discourage that because the resistor is there to keep the relections down. Without the resistor, the failure rate of the WiFi radio in the Raspberry Pi will climb quite astronomically [sic].

Unfortunately, there are too many cowboys out there playing pretend-RF-engineer.

If you are curious, you can model what a single piece of wire does at 2.4 GHz and 5 GHz with an antenna design program that I wrote for the Macintosh, and see what it looks like with an extra resistor (and also read out the efficiency of the antenna with the resistor):

http://www.w7ay.net/site/Manuals/cocoaNEC/index.html

Since I recenty bough a third generation ASIAIR (the one that comes with an antenna), I think I will measure its antenna on a VNA (Vector Network Analyzer), and post it here. For what its worth, since I don't intend to use the ASIAIR's WiFi antenna port, I currently have a 50 ohm terminator on that ASIAIR.

Chen

w7ay

I just measured the antenna that came with my third generation ASIAIR (note: no guarantee that yours is the same, since ZWO just buys the antenna in the open market).

The antenna is measured on a LibreVNA vector network analyzer (up to 6 GHz).

Electrical Engineers among us (lots of engineers in this hobby, for some reason :-) will notice that the Smith Chart shows that the antenna impedance is confined well within a 2.0:1 circle.

Here is the Return Loss in detail from 2 to 6 GHz, with the cursor resting at about 2.4 GHz (showing about -37 dB reflected signal):

And here is the cursor at 5 GHz showing return loss of about 30 dB.

Notice that the antenna is actually designed for the 2.4 GHz band (there is a deliberate broad return loss around 2.4 GHz to 2.6 GHz region), while being somewhat "OK" for 5 GHz band (no broad dip, but the return loss is small enough already).

A Return Loss of 37.4 dB corresponds to a VSWR of 1.03:1 and a return loss of 29.6 dB corresponds to a VSWR of 1.07:1. Anything below 2.0:1 is completely safe for a normal transmitter.

A 30 dB return loss says that if you transmit 100 mW, only 0.1 milliwatt is reflected back to the transmitter chip.

Chen

w7ay

Whoops, I made a mistake.

The annotated values in the S11 graph are for actual measured power (in dBm), not relative to the input power (which was set to -10 dBm). So the actual return loss at 2.4 GHz is really more like 27.4 dB and the return loss at 5 GHz is more like 19.6 dB.

They correspond to VSWRs of 1.09:1 and 1.23:1 respectively. Still well within the 2.0:1 VSWR.

Chen

Swizerlan

w7ay so based on your findings could you suggest a replacement with better performance?

w7ay

Swizerlan so based on your findings could you suggest a replacement with better performance?

Unless the interference source that is arriving at the ASIAIR's antenna is coming from a different direction as the station mode router, and the antenna is fixed mounted (i.e., not mounted where the direction can move during the night), I would never recommend a higher gain antenna.

Since an antenna is a passive device, higher "gain" of an antenna is achieved only by reducing the solid angle the antenna radiates (i.e., the constant amount of energy is focused on a smaller part of the sphere). By the Reciprocity Theorem, that transmit direction's solid angle is the same solid angle for the receive direction. Since I suspect that most problems are caused by the ASIAIR's WiFi receiver (and not the router's WiFi receiver), changing the antenna will not get rid of a problem if the interference source remains in the same general direction as your station mode router. Any extra directivity will cause even worse problems if the antenna is allowed to move around while the mount is tracking. I see a lot of non-engineers on the Facebook page mount their ASIAIR on the OTA; and you wonder why they have problems; and on top of they, they install a higher gain (a.k.a. higher directivity) antenna.

In the case of the antenna that came with my ASIAIR, the VSWR is well within acceptability, so there is no reason to swap it for a different antenna for that reason either, unless your interference is coming from a neighbor who is in a different direction from your own router, and the antenna is fixed mounted.

Personally, WiFi problems do not bother me. I use a direct Ethernet cable that is connected directly to a mesh router. For my third generation ASIAIR, I even have the antenna removed, and replaced by a 50 ohm SMA terminator. That way, nobody sees its SSID from more than 10 meters away. (Remember never to remove the antenna without replacing it with a 50 ohm terminator, or the WiFi chip will see infinite VSWR.)

Chen

Swizerlan

w7ay I understand not recommending a different gain, but is there an antennae of better quality in the same gain that would do better with 5ghz

w7ay

Swizerlan there an antennae of better quality in the same gain that would do better with 5ghz

A return loss of 20 dB is not good enough for you??? With that kind of return loss, the reflected power is only 1% of the forward power!

Chen