# Antenna theory

Hi sorry to hijack this thread as i’m unable to create my own post and this topic hopefully fits with my question.

Can someone explain how Rx works ? I keep reading a higher Dbi antenna will have a higher Rx and be able to hear more than a lower Dbi.

But then I read that an antenna will be inactive when not beaconing and the sent beacon must reach the receiving antenna in full as in make contact with their antenna.

So in my head if there were two targets side by side 4dbi and 10dbi then if the archers was able to shoot long enough he would hit both… and he didnt have the reach he would miss both as they are just inactive targets.

How does Rx work or does it have some kind of drawing power ?

I have moved your post to a new topic. If you can’t create one yourself that means you didn’t spend the minimum amount of time required on the forum.

That doesn’t excuse hijacking another topic for your questions…

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Appreciated.

Imagine an antenna in the middle (hole) of a donut. For the 4dbi antenna the donut will be high and have a rellatively small circumference. For a 10dbi antenna the donut will be flat but have a much larger circumference. Both have the same volume (= amount of energy).
If you are further away from the antenna (but at the same level) you will be inside the 10dbi donut but not inside the 4dbi one.

The disadvantage of the 10dbi antenna is that regulations allow a limited amount of transmission power that is measured at a certain distance from the antenna. At the measurement point the power of the 4dbi antenna might be at the limit and at that time the 10dbi antenna will exceed the limit so the transmission power must be dialed back to stay legal resulting in a smaller circumference for transmissions.

Sorry just in case im failing to understand this. Is this description explaining the transmit pattern or is it describing Rx?

The debate we are having is how a 10dbi antenna can witness/hear more than a 4dbi antenna and the usually answer comes down to its Rx.

But most of us believed when the antenna wasn’t sending/beaconing it was inactive and in order for my antenna to witness another beacon their beacon had to reach my antenna in full. Which then makes me wonder where the Rx comes into play as surely it then just comes down to whether or not the sending beacon can reach me or not regardless of what my Rx is.

With the above image i threw together.

The 10 dbi was able to hit both the 3dbi and 15dbi and they received the beacon

the 6dbi fell short of hitting either the 3dbi and 15dbi and they failed to receive the beacon

so how does the higher Rx of the 15dbi help over the 3dbi if the sending signal needs to reach the antenna ?

Does the 15dbi have some kind of drawing power to hear weaker/shorter signals ? as we had been led to believe the antenna is inactive if its not beaconing.

thanks been trying to figure out why a higher dbi has a greater ability to hear beacons as it doesnt add up if it’s inactive.

Antenna are mostly passive, but the gain is both for the TX and RX direction…
but maybe you would benefit first to read something like

Yes this is where I’m getting lost with how the higher Rx gain can benefit for witnessing/Hearing if the antenna is passive.

I will have a read through those thanks as so far i’ve not been able to find much about Rx and how it works only tons of articles explaining how the gain helps with transmitting.

If you have a transmitter (TX) and two receivers (RX), one RX with an antenna of 10dbi and one with 3dbi, and they both have LOS and same distance from the TX, ones RX level will be 7db greater than the other.

But read the articles first.

Think telescope. Also passive and yet you can view things further away.

Hi thanks for the further replies i’ve now had a look through the above articles but didnt explain how the witnessing works unless i’ve missed it or not clever enough to understand it.

Even with your latest reply it still doesnt explain how it benefits to be able to see a beacon thats further away if the beacons needs to actually reach/hit the antenna to be counted as a witness ?

If the beacon fails to reach both the 3dbi and the 15 dbi physically and falls short what benefit would it be for the 15 dbi to be able to see it over the 3dbi not being able to see it ?

Unless what you are saying is the the signal doesnt need to actually reach the antenna to be counted as a witness and just being able to see it is all that matters which would make a lot more sense and explain how Rx is beneficial to these RF antennas.

Appreciate the followups

Beacons and witness sound like you are thinking Helium, not LoRaWAN with TTN?

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So now the ones level is 7bd greater than the other, now if we increase the distance between the RX with 10db antenna and TX and still have a greater or equal RX level than the one whit a 3db antenna., up to a point.

A = Attenuation
f = frequency
d = distance
This is for free space

Yes Helium

This is TheThingsNetwork, not Helium. Please use their forum for Helium related questions regarding beacons etc.

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I am puzzled by this.

Why does an antenna need a witness for it to work ?

Or is this along the lines of;

“If a tree falls in the forest and no one is there, does it still make a sound?”

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An antenna doesn’t, however a certain crypto based LoRaWAN network (Helium) needs witnesses as proof of presence.

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Yes, but you must know, the price for the gain in reception is a narrowed reception field, or transmission direction!

I think the see the problem here : 15 dBi Antenna 868Mhz Helium HNT Miners Bobcat Miner Antenna / LMR400 / Arrester | eBay

You should ask your antenna vendor to provide data in the form of radiation pattern charts, and impedance matching charts.

He won’t provide them because this isn’t a 15 dBi antenna, and he’s just made up the numbers.

You should revert to using the antenna that came with (or specified with) the gateway you bought.

If you just add more dBi without knowing what you’re doing you’ll degrade performance (as evidenced by the premise of this thread) and very likely be breaking the law with too much Tx power