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is antenna matching with resistance recommended?

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spid3rx

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I tried to tune a bluetooth antenna and found that with series inductor, it manage to shift to the horizontal line of the smith chart. It just need another 130ohm shunt resistance to horizontally move to the center of 50 ohm impedance.

Usually the matching involves inductors and capacitors, but I had not tried resistor before. My question is whether adding a shunt resistance is recommended and will it degrade radiated power? A good or a bad move ?

thanks!
 

Hi spid3rx,

Matching like that, you would achieve that a transmitter would not see a reflected wave. But a part of power will be lost (both receiving and transmitting characteristics will be worse).
Instead, you can use standard matching techniques (e.g. quarter-wavelength line).
Regards

Z
 
Thanks Zorro,

Can you elaborate more on how the standard matching techniques? quarter-wavelength line ? do you mean by tuning the antenna length to
achieve quarter wavelength?
The BT antenna I have is ready made, the only thing left to do is just by using lump matching components.

thanks
 

In general, not a good thing. It introduces loss to the system and degrades NF and efficiency. Don't do it just for better matching.
 
Thanks cnm. New findings from my test, I tried to measure conducted power after the matching, it has been reduced with the resistance in place. Really not a wise move.

I also realized one thing, this time without resistor, the higher i place for series inductor matching, the higher the loss regardless of the matching that I observed through
the network analyzer, I found out that 2.7nH has less than 1dBm loss compared to 4.3nH in series respectively. Is this observation hold true when doing matching ???
any thoughts ?

thanks
 

Hi spid3rx,
Matching with lumped elements will improve the matching for sure but it aint guarentee efficient power transfer from source to antenna as mentioned above that u ll loose power across lumped elements.

For impedance matching in antennas,mostly transformers are used as they provide matching with no loss. Use quarter-wavelength transformer or their are other methods aswell mentioned in all microwave books. for example you can check Microwave engineering book by David.M pozar to have idea about these matching techniques.
/SC
 
Theoretically reactive matching circuits using LC components have very low losses.

What cannot be ignored is the Missmatch Loss:
Missmatch Loss[dB] = -10*LOG [1 – Γ²]

So, whatever no-loss matching components you use, always will be a loss in the circuit if you get reflections.
 
spid3rx said:
Can you elaborate more on how the standard matching techniques? quarter-wavelength line ? do you mean by tuning the antenna length to
achieve quarter wavelength?
Click to expand...

If you connect a quarter-wave line to a load with impedance Z2, the impedance seen is Z1=Z0^2/Z2.
In your case, Z2 is near 81.25 ohms, so you can get the Z0 necessary (63.7 ohms) for getting z1=50 ohms.
Regards

Z
 
Hi,

You improve the VSWR but you decrease the efficiency. There is no free launch. But when VSWR and/or bandwidth is very important you can use it.

Cheers
Hadi
 
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