Discussion about setup for antenna RL and Gain measurement

[ifelix]

good antenna rl

Hi guys,
I'm searching for someone that can once and for all give me the right way to measure antenna RL and gain. Now...I'm not interested in the method...I know it but the correct setup. I mean..when I want to measure RL of a small antenna, take as an example a little folded monopole on a little PCB (30 mm x 30 mm area) operating in the 868 MHz band or also a GSM antenna on a PCB; when I heard about RL measurement I've found: take the VNA, connect it to antenna and measure...
BUT...if I take my small PCB antenna and I connect the VNA trought a coax cable placing an SMA connector on the feed point of the antenna I won't found absolutely correct results...the problem is always and always the effect of the cable on the antenna; the dual effect: the cable that acts as a reflector of the incident field radiated by the antenna and the active radiation due to the current flowing on the external face of the coax due to the unbalanced connection of cable to the antenna.
I've found a lot of papers discussing the problem: they say, use a lambda/4 balun which inhibits the presence of the cable wrt the antenna....is this true? Do I need always to use a balun when I measure the performance of my little PCB antennas? It's obvious that if I place the SMA connector on the feed point of the antenna, its presence will affect antenna impedance and so Return Loss and also Gain!!
I've tried more times to take different measures of antenna parameters using a piece of minicoax soldered to antenna feed point and to the other end connected to VNa , or also a semirigid coax with external face soldered to ground plane, but I've found always that the presence of the cable and its orientation affects antenna performance.
Look at the picture attached: this a typical antenna I measure; the red arrow indicates the feed point where i connect the SMA connector by means I connect antenna to VNA. The diameter of the PCB is 35 mm and the antenna operates at 868 MHz. what should be the right way to measure RL and Gain? Now I'm using a coax cable on which I've placed some ferrites along it to mitigate the cable effect but this not enough:the end of the cable towards antenna and the SMA connector still deviates the real antenna performance. Is there another method to make the measurement? How can I do?
Thanks in adv to anyone which will help me!
Bye!

 

[jallem]

antenna r.l

It does not look like the antenna is a balance
sctructure, does it?.........If that is the case
you do not need a BALUN. But what it looks
weird is the coil you have put in the middle, why?
Matching?....You can do with small lumped 402
devices. For other hand the coil and the long
thin traces will be part of the antenna. I would
recommend not to use the coil and use a lumped
inductance at the input of the antenna and
a capacitor for tuning. That would add some
real estate to the antenna so it would be bigger
and easy to tune.

 

[ifelix]

how to measure antenna rl?

Thanks Jallem for your reply...but here the problem is not the design of the antenna...it's just an example...it could be a generic small antenna...the problem is the setup for a correct measurement....do you have any suggestions about it?
Thanks...

 

[jallem]

feed cable in small antenna measurements

OK, if that is the case lets forget about the picture
since is not relevant.
The line of thought would be if you have a balance
structure then you need a differential feeding system.
In other words, if your feeding is a already differential
you do not need BALUN, and if your feeding is single
ended then you need a BALUN or any single ended
to differential transformer.

You were talking about a "monopole" that you connect
and does not have good results. A monopole is not a
balanced structure so it does NOT need a balun.
For dipole, which is a real balanced structure, you will need
a balun. If you connect a coax cable directly to a DIPOLE
the cable becomes an antenna itself. That is what most
of the reference talked about.

For other side you talked about a SMA connector, if the
connector is too big will have an effect over the antenna,
but is not clear in the paragraph below what is the antenna,
a monopole or a dipole........

BUT...if I take my small PCB antenna and I connect
the VNA trought a coax cable placing an SMA connector on
the feed point of the antenna I won't found absolutely correct
results...the problem is always and always the effect of the
cable on the antenna; the dual effect: the cable that acts as
a reflector of the incident field radiated by the antenna and
the active radiation due to the current flowing on the external
face of the coax due to the unbalanced connection of cable to
the antenna.