VSWR and Quality Factor
 

Hi to everybody,
I got the VSWR of an antenna from a Vector Analyzer and I'd like to know
(and how) if is possible to calculate the Quality Factor from it.
Thanks,
Massimo

Not sure what you mean by Quality Factor -- but from URL:
http://www.antennex.com/preview/vswr.htm

"As a rule of thumb, any accurate VSWR reading under 2:1 is probably not
worth the effort to achieve if the other elements of your antenna system are
the best you can make them."
--
73- Cambio - Keyboard To You (:-)

"Massi" wrote in message
...
Hi to everybody,
I got the VSWR of an antenna from a Vector Analyzer and I'd like to know
(and how) if is possible to calculate the Quality Factor from it.
Thanks,
Massimo

On Mon, 9 Feb 2004 13:37:29 +0100, "Massi"
wrote:

|Hi to everybody,
|I got the VSWR of an antenna from a Vector Analyzer and I'd like to know
|(and how) if is possible to calculate the Quality Factor from it.
|Thanks,
|Massimo

Do you have the R +jX data, or only VSWR?
|

Do you have the R +jX data, or only VSWR?
|


I have also the Smith Chart and the R+jX data for every frequency (I'm
working about between 10 MHz and 15 MHz)

"Massi" wrote in message ...
Hi to everybody,
I got the VSWR of an antenna from a Vector Analyzer and I'd like to know
(and how) if is possible to calculate the Quality Factor from it.
Thanks,
Massimo


You mean the "Q" of the antenna, or how narrow or broad-band it
is.


I would do it like this:

1. Place a short or an open at the end of the test cable,
where the antenna is placed. Measure the return loss in dB, at the
center frequency of interest. This will be the return loss that will
include the losses of the cable too (ideally, it will be pretty close
to 0dB). Let's call this the RLshort.

2. Replace the antenna, and then find the upper and lower
frequencies (immediately off of the center frequency, F_upper and
F_lower) where the
return loss is = RLshort - 3dB.

3. The "Q" of the antenna would be
Fcenter/(F_upper-F_lower).


This all assumes a decently lossless antenna, mind you.

As you can see, it's easier to do this if you change the
measurement from VSWR to Return Loss on your network analyzer.


Dr. Slick

Massimo
VSWR and Q have nothing to do with one another.
73
H.
NQ5H

"Massi" wrote in message
...
Hi to everybody,
I got the VSWR of an antenna from a Vector Analyzer and I'd like to know
(and how) if is possible to calculate the Quality Factor from it.
Thanks,
Massimo

On Mon, 9 Feb 2004 17:22:28 +0100, "Massi"
wrote:

|
| Do you have the R +jX data, or only VSWR?
| |
|
|
|I have also the Smith Chart and the R+jX data for every frequency (I'm
|working about between 10 MHz and 15 MHz)

For simple resonant antennas, (dipole for example) you can evaluate
the slope of reactance v. frequency and calculate Q.

Consider that the antenna has the properties of a series RLC circuit
with L and C as unknowns and R known. The antenna has a constant L and
C and the reactance you measure at any frequency is determined by
their sum.

By picking two frequencies, preferably spaced equally about the
resonant frequency, and setting up two equations with two unknowns you
can solve for L and C and then using the reactance of either,
calculate Q as X/R. You can use Excel's solver, Mathcad or brute force
to solve the equations.

Another related method is to use the following:

Let

t1 = deltaX / deltaF

t2 = t1 / (4 * pi)

Q = t2 * omega / R

Whe

deltaX = the reactance difference at two frequencies

and

deltaF = the difference between the two frequencies

omega = 2 * pi * Fres

In another posting on a different subject, Steve Best offered this:

Begin Quote

Near the antenna's resonance, a different method must be used to
determine antenna Q. One method is to determine Q from the antenna's
1/2 power impedance bandwidth as follows:

Q = 2 / BW

where BW is the bandwidth determined from the 1/2 power SWR points
where the ZO used to determine SWR is the antenna's feedpoint
resistance. The 1/2 power SWR is equal to 5.828.

Another method is to determine Q directly as follows:

Q = A (B + C)

where A = w / (2 R), w = 2 pi F

B = dX/dw

C = |X|/w

End Quote.

So contrary to other posts, VSWR and Q *are* related and you have all
of the data you need to make the calculation.

Wes Stewart N7WS


|

Thank you very much indeed to everybody,
I used the "1/2 power SWR is equal to 5.828" and the "return loss way" to
find the Q factor and and value is the same and it is reasonable.
Massi

I think Wes has given you a nice explanation about _how_, but I'm
curious _why_ you want to know the Q of an antenna. (Beware...some
antennas behave as multiple coupled resonators, and it can be
misleading to try to assign a Q to a whole system of coupled
resonators. It makes some theoretical sense, at least, if it behaves
as a single resonator over the frequency of interest, though.)

Cheers,
Tom

"Massi" wrote in message ...
Hi to everybody,
I got the VSWR of an antenna from a Vector Analyzer and I'd like to know
(and how) if is possible to calculate the Quality Factor from it.
Thanks,
Massimo

On 10 Feb 2004 10:07:45 -0800, (Tom Bruhns) wrote:

|I think Wes has given you a nice explanation about _how_, but I'm
|curious _why_ you want to know the Q of an antenna. (Beware...some
|antennas behave as multiple coupled resonators, and it can be
|misleading to try to assign a Q to a whole system of coupled
|resonators. It makes some theoretical sense, at least, if it behaves
|as a single resonator over the frequency of interest, though.)

Good point. I was going to mention Yagis, etc. and point out the
pitfalls but figured I'd gone on long enough. [g]