Thanks for the great video! and I have some things to ask. The frequency of the reflected signal is the same as the frequency of the feeding signal or a little difference? And how about the relative phase between them?
@@Tom-dn5de Yes, frequency is the same, unless the signal reflects from some non-linear circuit or what not, then it can be different, but for antennas are linear. The relative phase can be calculated exactly depending on the transmission line length, but for practical life VSWR is more useful since it gives information on the steady state. If you're interested in transient phenomena, then the relative phase can be interesting for you.
@@Rfelements Thanks for your explanation, I'm making an output impedance measurement of an RF signal generator using an RF spectrum analyzer, an auxiliary RF signal generator, and a directional coupler. I don't know why an auxiliary generator is used which transmits a wave with a slightly offset carrier frequency into the DUT. This method makes me a lot of confusion. I'm not good at RF but always want to learn about it. Could you explain it to me? This method is from euramet on page 16. www.euramet.org/index.php?eID=tx_securedownloads&p=529&u=0&g=0&t=1653590394&hash=968b4e5de0234a96189d8595ea66236a3bc8b6f5&file=Media/docs/projects/EURAMET-P1461_EM_Final_Report.pdf
@@Tom-dn5de The generator impedance measurement is done indirectly through the setup that's described in the pdf. The measurement signal is AM signal. The generator impedance is derived from the reflection coefficient of the modulated signal.
That largely depends on the application, but generally speaking the smaller the VSWR, the higher the radiation efficiency of an antenna (unless there is a lot of loss within the antenna body, in substrate, or metal).
@@harlemespanol VSWR < 2 is considered a level of good matching for most applications. The choice also depends on the distance you're trying to cover, radio output power, sensitivity of the receiver, and SNR requirements. Based on these you can do a link budget calculation that will give you a good idea if either one of the antennas is enough.
I've seen technician measuring VSWR in the link when they installed wireless communication system.Given that every parts is fixed, why would they bother measuring the VSWR? And if if it's poor, what to do to improve it?
It makes sense if the gear is expensive - you rather check everything well before you turn the system on. In the worst case, the reflection from the load (cable plus antenna and anything in between) can burn the signal source (unless it has some protection in place). If the VSWR is poor, then you have to check each component separately, figure out which one is not matched well and either tweak it or replace for a better matched one.
@@Rfelements Thanks for your time to explain it, I teach engineering subjects so its helpful to learn from experts in the field. BTW, just a follow up questio. For practical purpose, does cutting the T-line a couple of cm or inch from Tx to Antenna have any adverse effect on VSWR? Or does if affect phase at the output?
@@cenonlumabadiii8976 Cutting a cable shorter affects the phase, provided that the transmission line has the same connector and is not otherwise deformed - its characteristic impedance does not change with it being longer or shorter.. Oh, and also the insertion loss, naturally - the shorter the cable the smaller insertion loss.
Hi, good effort. But I think you didn't follow the right way to calculate the reflected power. Cause your input power is in dBm and RL (30 dB) is a unitless parameter. So, how is it possible to subtract a unitless parameter from a parameter that has a unit? Can you explain this, please?
I can because who told that return loss had no units it's there in db's so input power can get added if it is gain and subtracted if it is loss that's it
Hello, I'm looking at an antenna that is good on most of the frequencies it can use except for the one I'm interested in which 617-700, at 617 it spikes around 5 VSWR and eventually comes down to 2.5 VSWR at 680 mhz, should I purchase this antenna or pass on it? Thanks for the video, this helped more than the other sources I've been using.
Hi Harliza, in the video it says -20 dB is considered a very good level of matching, not 'the best'. The requirements on the level of matching vary from one application to another so it's all relative, but we can say that in the world of antenna engineering -20 dB is a pretty good one, considering it means 1 % of energy reflecting back to the signal source..
Considering linear scale, reflected energy = |S11|^2 = ( (VSWR - 1) / (VSWR + 1) )^2 = 0.111 for VSWR = 2. The power of 2 gets us from voltage wave amplitude to energy.. check this page for the formulas www.everythingrf.com/tech-resources/vswr
Use a different antenna. If its specified VSWR is above 2.5, it's a poorly designed product. Generally, it depends on the application if 2.5 is too much. What application / frequency band are you considering?
