RF signal through a metal container

Thread Starter

Sonite

Joined Dec 6, 2023
5
I have a steel container utilized for beer fermentation, housing an iSpindel hydrometer sensor immersed in the liquid. This sensor transmits data through 2.4GHz WiFi. To enhance signal strength outside the vessel, would it be beneficial to affix two passive omni 2.4GHz antennas to the lid, connecting them through a small hole—one on the interior and one on the exterior? Alternatively, should I contemplate drilling a hole of a specific size and covering it with plastic?
 

MaxHeadRoom

Joined Jul 18, 2013
28,513
I would think that any hole drilled may jeopardize the integrity of any air locks that may be in place, unless completely sealed.
 

Thread Starter

Sonite

Joined Dec 6, 2023
5
Certainly, you are correct. A hole that isn't sealed and can't withstand internal pressure might compromise the functionality of my container. Do you have any thoughts or answers regarding the strength of the RF signal in response to my question? I know nothing about signal theory.
 

nsaspook

Joined Aug 27, 2009
12,782
Certainly, you are correct. A hole that isn't sealed and can't withstand internal pressure might compromise the functionality of my container. Do you have any thoughts or answers regarding the strength of the RF signal in response to my question? I know nothing about signal theory.
If this is it that's pretty nice: https://www.homebrewfinds.com/hands-on-review-ispindel-digital-wifi-hydrometer/

If you are getting a good signal (from normal slot antenna seal leakage) at close range, my plan would be to install a dedicated wfi AP for the sensor(s) as close as possible to the tank or other sensor.
 

Thread Starter

Sonite

Joined Dec 6, 2023
5
If this is it that's pretty nice: https://www.homebrewfinds.com/hands-on-review-ispindel-digital-wifi-hydrometer/

If you are getting a good signal (from normal slot antenna seal leakage) at close range, my plan would be to install a dedicated wfi AP for the sensor(s) as close as possible to the tank or other sensor.
Yes, it's a similar one. :) https://www.opensourcedistilling.com/ispindel/ and as you suggested I will have a dedicated WiFi AP in the near vicinity outside the vessel. But I'm still very curious as to if a passive antenna could fetch the signal bouncing around on the inside and passively send it to the outside.
 

nsaspook

Joined Aug 27, 2009
12,782
Yes, it's a similar one. :) https://www.opensourcedistilling.com/ispindel/ and as you suggested I will have a dedicated WiFi AP in the near vicinity outside the vessel. But I'm still very curious as to if a passive antenna could fetch the signal bouncing around on the inside and passively send it to the outside.
Your metal container was not designed to be a Faraday cage.
1701894027477.png
The metal to metal pressure seals are likely insulators (of rubber or plastic) that make long RF transparent gaps between electrical connections like clamps. Those gaps provide a type of slot antenna for signal propagation.
https://www.allaboutcircuits.com/technical-articles/a-brief-introduction-to-slot-antennas/

The metal parts near the gap already act as a passive antenna to transfer RF in and out. Could a passive antenna tuned to the WFI range be better, very likely but with digital encoding, if it's 100% good now, reducing the link loss won't make it better.
 

Thread Starter

Sonite

Joined Dec 6, 2023
5
Your metal container was not designed to be a Faraday cage.
View attachment 309298
The metal to metal pressure seals are likely insulators (of rubber or plastic) that make long RF transparent gaps between electrical connections like clamps. Those gaps provide a type of slot antenna for signal propagation.
https://www.allaboutcircuits.com/technical-articles/a-brief-introduction-to-slot-antennas/

The metal parts near the gap already act as a passive antenna to transfer RF in and out. Could a passive antenna tuned to the WFI range be better, very likely but with digital encoding, if it's 100% good now, reducing the link loss won't make it better.
I have this Product: https://www.hembryggeriet.se/brewly-ccf-30l-conical-cooling-fermenter-jastank-med-chiller-1025 and am about to access the link you provided. As you mentioned, it seems reasonable to assume that the silicone sealing creates a prolonged opening through which RF may escape. However, for enhanced performance, wouldn't it be necessary to insulate the clamps as well? Currently, the clamps are in direct contact with both the lid and the container. Another consideration is that, given the presence of a cooling spiral in the liquid, could this contribute to a "weak connection" between the lid and the container?
 

nsaspook

Joined Aug 27, 2009
12,782
I have this Product: https://www.hembryggeriet.se/brewly-ccf-30l-conical-cooling-fermenter-jastank-med-chiller-1025 and am about to access the link you provided. As you mentioned, it seems reasonable to assume that the silicone sealing creates a prolonged opening through which RF may escape. However, for enhanced performance, wouldn't it be necessary to insulate the clamps as well? Currently, the clamps are in direct contact with both the lid and the container. Another consideration is that, given the presence of a cooling spiral in the liquid, could this contribute to a "weak connection" between the lid and the container?
What does enhanced performance actual mean in the current sensor configuration? Is there some actual deficiency that needs to be addressed?
The clamps grounding the lip won't affect the slot antenna RF aperture by any measurable amount at 2.4GHz. At 2.4GHz, that unsealed large container gap and coil (that large cooling coil has minimal effect at 2.4HGz) are unlikely to have sufficient signal loss to affect connections to a near AP.
 
Last edited:

Ya’akov

Joined Jan 27, 2019
8,973
Welcome to AAC.

Beware of the naïve error of premature optimization until you have a problem don’t set about fixing one you might have. This is the rôle of prototypes—the provide empirical data you can act on.

In your case, measure the effect of the container on the signal. It is very likely to attenuate it, possibly considerably—the question to answer is: does that have any practical effect on the operation of the system.

If “no”, then spend your time and money elsewhere. If “yes”, then characterize the issue using the empirical data and set about optimizing the design with a roadmap instead of guessing at things.

On your original question, yes—you can certainly devise an arrangement where an antenna inside feeds one outside to increase the signal strength outside. In a case like yours I would expect the best route to be using a sealed feedthrough* and a pair of ¼λ antennas with the inner one being something very flexible—even just a silicone insulated wire with a small weight on it to keep it stretched out.
*you might be best served by a bulkhead mount, teflon insulated SMA connector with the soldered side facing in and the threaded facing out. Teflon heat shrink tubing combined with acetic acid free RTV would protect the beer from the solder joint, and vice versa.

The outer being a piece of piano wire of suitable gauge. The inner one might also be stainless steel wire in a coil, which will make it much lower profile. In any case, of it comes to that, you can address the problem.
 

Thread Starter

Sonite

Joined Dec 6, 2023
5
Welcome to AAC.

Beware of the naïve error of premature optimization until you have a problem don’t set about fixing one you might have. This is the rôle of prototypes—the provide empirical data you can act on.

In your case, measure the effect of the container on the signal. It is very likely to attenuate it, possibly considerably—the question to answer is: does that have any practical effect on the operation of the system.

If “no”, then spend your time and money elsewhere. If “yes”, then characterize the issue using the empirical data and set about optimizing the design with a roadmap instead of guessing at things.

On your original question, yes—you can certainly devise an arrangement where an antenna inside feeds one outside to increase the signal strength outside. In a case like yours I would expect the best route to be using a sealed feedthrough* and a pair of ¼λ antennas with the inner one being something very flexible—even just a silicone insulated wire with a small weight on it to keep it stretched out.
*you might be best served by a bulkhead mount, teflon insulated SMA connector with the soldered side facing in and the threaded facing out. Teflon heat shrink tubing combined with acetic acid free RTV would protect the beer from the solder joint, and vice versa.

The outer being a piece of piano wire of suitable gauge. The inner one might also be stainless steel wire in a coil, which will make it much lower profile. In any case, of it comes to that, you can address the problem.
Thanks! I might be stupid. But I have a hard time understanding your answer. The issue is real. A close repeater to the vessel will solve the signal to the router,. But I'm still curious if I could build something to mitigate it by using other solutions. And I dont have a repeater. But I do have 2 zigbee (2.4Ghz) antennas :)
 

nsaspook

Joined Aug 27, 2009
12,782
Thanks! I might be stupid. But I have a hard time understanding your answer. The issue is real. A close repeater to the vessel will solve the signal to the router,. But I'm still curious if I could build something to mitigate it by using other solutions. And I dont have a repeater. But I do have 2 zigbee (2.4Ghz) antennas :)
I can tell you from long experience in the field (military) using things like very effective but still lossy back to back antenna passive repeaters to bridge blocked RF links, your max possible net gain won't be worth the effort of trying to replace a local AP that's $20 on the used market. You already have a passive repeater in the form of the slot antenna.
 
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