general question about antennas

Thread Starter

Kardo22

Joined Mar 12, 2014
46
Hi,
I'm looking at some designs (sorry but can't share these) and I have some questions about antenna design (which I know very little about).

There is a a board that controls RF communication and is connected to an antenna board which has a connector where an antenna is connected.
Some antenna boards (depending on frequency) also have an antenna trace (picture attached). But why only some? If the antenna that goes to different frequency versions is different then why is the trace needed for some?
Frequencies are like 900MHZ, 133MHz etc if I remember correctly.

Sorry that I can't give more information.
 

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Papabravo

Joined Feb 24, 2006
14,403
Without more details it is quite impossible to give you a precise and meaningful answer. Generally speaking, do you understand the effect of impedance discontinuities that a signal may encounter on it's way from the source to the transmission line to the antenna and to free space? The answer to your original question is probably connected to dealing with those issues.

You do know the security guys probably monitor public forums like this
 

Thread Starter

Kardo22

Joined Mar 12, 2014
46
Without more details it is quite impossible to give you a precise and meaningful answer. Generally speaking, do you understand the effect of impedance discontinuities that a signal may encounter on it's way from the source to the transmission line to the antenna and to free space? The answer to your original question is probably connected to dealing with those issues.

You do know the security guys probably monitor public forums like this
Thanks for trying.
Impedance is something I don't get that well.

But explaining simply, why is there that trace to GND only for some frequencies?
Different frequency products mean different antennas and different RF boards (IC and filters are on them). Antenna connector and the trace in question is on a separate PCB from IC and filter.
I think it was for 900MHz but not for others. Is that so different from 133MHz?
 

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Delta prime

Joined Nov 15, 2019
490
Hi there. A general question will get a general answer. :) The dielectric constant of a material IS slightly frequency dependent. Therefore, the trace height and width you choose for 1 GHz will be a slightly different impedance at 10 GHz .
For standard PCB FR4 material, the dielectric loss will become a concern around 0.5 to 1 GHz. The RESISTANCE, however, does become important when you have higher current lines. For example: If you have 1 Amp going on a 6 mil wide trace of 1 oz copper for 1 inch of length, that's .1 Ohms of resistance. You'll have a drop of about 0.1V and around a 60C temperature. If you can't handle that 0.1V drop, you need to obviously widen the trace or thicken the copper.
a trace has certain inductance her unit of length, and, correspondingly, certain capacitance, proportional to length. These L and C are inductance and capacitance per unit length. So, the actual inductance of a wire segment would be L = L * length; same for C.
Now consider a sine wave coming into the trace. Waves propagate at the speed of light (in particular dielectric/air media it is about 150ps/inch). At every and each moment the particular charge deviation (waveform) interacts with a section of wire equal to the corresponding length of this wave. Slower frequencies have longer wave lengths, while faster frequency components have proportionally shorter lengths. So, what do we have? Longer waves "see" a longer trace and therefore a bigger L, and bigger capacitance C. Shorter (higher frequency) waves "see" the shorter effective line length, and therefore smaller L and C. So, both effective L and C are proportional to wavelength. Since the impedance of the line is Z0=SQRT(L/C), the dependence of L and C on length cancels, and that's why waves with different frequencies "see" the same effective impedance Z0
 

Papabravo

Joined Feb 24, 2006
14,403
...
I think it was for 900MHz but not for others. Is that so different from 133MHz?
The difference is like night and day. If you don't understand impedance and transmission lines there is very little point in trying to educate you in a forum post.
In the simplest terms:
  1. In a section of constant impedance, a signal will propagate at some fraction of the the speed of light. e.g. 0.69c would be 69% of the speed of light.
  2. At each discontinuity, some faction of the signal will propagate, and some fraction will be reflected
The goal of the designer is to maximize the fraction of the signal which is propagated, and minimize the fraction which is reflected. That's it in a nutshell. The section of trace on the PC board has inductance and capacitance. It will present a low impedance to some set of frequencies and they will be shorted to ground. Other frequencies will be passed to the antenna. Your mission should you choose to accept it is to determine which frequencies are passed to ground and which frequencies are passed to the antenna.

Good Luck
 

Thread Starter

Kardo22

Joined Mar 12, 2014
46
The difference is like night and day. If you don't understand impedance and transmission lines there is very little point in trying to educate you in a forum post.
In the simplest terms:
  1. In a section of constant impedance, a signal will propagate at some fraction of the the speed of light. e.g. 0.69c would be 69% of the speed of light.
  2. At each discontinuity, some faction of the signal will propagate, and some fraction will be reflected
The goal of the designer is to maximize the fraction of the signal which is propagated, and minimize the fraction which is reflected. That's it in a nutshell. The section of trace on the PC board has inductance and capacitance. It will present a low impedance to some set of frequencies and they will be shorted to ground. Other frequencies will be passed to the antenna. Your mission should you choose to accept it is to determine which frequencies are passed to ground and which frequencies are passed to the antenna.

Good Luck
thanks
that was helpful
 

sagor

Joined Mar 10, 2019
248
That trace in your drawing could be a simple inductor which provides DC ground to the antenna, but blocks the RF. That provides protection from static coming back down to the device.
It could also be part of a tuned circuit, using the capacitance of the PCB, especially if there is a ground plane, to provide some tuning compensation for a given antenna.
 

Janis59

Joined Aug 21, 2017
1,221
RE:""have an antenna trace. But why only some?""
Because any real antenna is far more effective than pcb antenna. But pcb version is cheaper and smaller.
 
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