question about matching a component antenna

Hi all,

is there a case whereby you can't tune/match it with components for the component antenna?
it is a ready made 2.4GHz helical antenna, and the only tuning I can do is by adding
the 0402 lump capacitors and inductors components. I prepared T and TT on the line, but I still can't get the intended resonant at 2.4GHz. Anyone experience this? what can we do ? really need help . desperate to match it.

thanks,
spid3rx

Resonance of the antenna doesn't have anything with the matching network, which is necessarily only to get maximum power transfer and good VSWR.
Usually to match a helical antenna, 3 lumped components in the matching network are more than enough to get good VSWR.
Try to measure first the antenna impedance using the VNA, and using a Smith chart software find the suitable matching network.

vfone,

is that means that the ready made antenna had already been design to be resonates at 2.4GHz even when the VSWR/ S11
-6dB bandwidth that I measure with our PCB board is not on 2.4GHz?
it is just the power loss due to mismatch but it should resonates at 2.4GHz ?

---------- Post added at 02:14 ---------- Previous post was at 01:53 ----------

I tried the method to measure the antenna impedance together with the components, housing, flex as the whole kit.
by using Smith chart software, I am able to determine the suitable lump components. however, when I place the
lump components, the values from the Smith are totally different from I would expected and not even close based
on the software results. I suspect the parts surrounding the antenna is affecting the results. is this common? what
can I do ?

right now, I am just evaluating the smith characteristic and try the lump values.

million thanks

If the antenna was sold as 2.4GHz antenna, definitely should resonate on 2.4GHz, without any other external components.
Usually they are 50 ohms antennas, but sometimes require some external matching network to get good VSWR.
At 2.4GHz (relative high frequency) any small parasitics of components (which were not taken into account during simulation) will change the performance of the matching network. Have to tune practically these MN components in small steps, until reach the best VSWR.

vfone,

thanks for you input again. yes the antenna was sold as a 50ohm 2.4GHz antenna, but when I placed it on the whole system
with external components, the measured resonance frequency was on 3GHz. The matching doesn't really tune it back near to
the 50ohm characteristic in smith. furthermore there are parallel resonance in the S11 plot which I suspect is coming from
external components. I guess I have to tune is practically one by one until I can get the best S11.

so my question is if my S11 can't tune to 2.4GHz, the antenna will still radiates at 2.4GHz but with much loss. correct?

thanks

Yes, the antenna still radiates, but less energy.
As a tip to avoid parallel resonances try to use in the matching network capacitors with SRF greater than 3GHz.

vfone,

can you explain abit on your tip? really interested to know deep on this as I found out the parallel resonances are
due to the external flex which is very near to the antenna. should I look into shunt or series kind of capacitor matching?
I tried before a 22pF series and 10pF series, but the effect in the smith chart is minimal. I end up just added a series
0ohm to connect the TL because the capacitor effect is minimal.

I had been advised for the matching circuit, to use a shunt capacitors, I have to avoid using high value capacitors
namely because it will act like a short circuit to ground because of 2.4GHz. They told me that the shunt capacitors
are good to be less than 1pF, else the 2.4GHz signal may be shunt to ground. is this true?

thanks vfone, you had been really helpful to most of my posts.

thanks

Yes is true, you already explained at the end of your post what mean avoiding low SRF capacitors (high capacitance value).
good luck

The good news is that it is easier to lower the frequency of a too-high antenna (capacitive loading), than it is to raise the frequency of a too-low antenna. The bad news is that helical type antennas are pretty stubborn about retuning their frequency!