Impedance measurements with antenna analyzer - Page 1
Forum section: electronics / rf-microwave

author: laus
author_rank: Contributor
timestamp: March 23, 2019, 08:26:58 pm
content: I'm trying to measure the impedance with an antenna analyzer but I don't understand some of the numbers.I know that impedance (Z) is made up of resistance (R) and reactance (X). I always thought the (R) relates to dc resistance and any of the ac "resistance" is (X). So it's puzzling to me why R changes so drastically when I add a ferrite.Am I doing something wrong or what am I missing?

author: Bud
author_rank: Super Contributor
timestamp: March 23, 2019, 08:49:53 pm
content: Your understanding is fundamentally incorrect to start with. Think of a matched dipole antenna. It would have Z=50+j0, with r being 50 and x being 0 Ohm. However its DC resistance will be infinite.

author: laus
author_rank: Contributor
timestamp: March 23, 2019, 10:44:02 pm
content: Quote from: Bud on March 23, 2019, 08:49:53 pmWhat is non-reactive resistance (R) then?My understanding is that any resistivity force from capacitance and inductance is the reactive resistance (X). On your resonant dipole antenna example the 50 ohms (R) is caused by the radiation resistance, right?In my "circuit", is the ferrite bead causing the wire to radiate and therefore increases the resistance, or what is going on here?

author: ahbushnell
author_rank: Frequent Contributor
timestamp: March 23, 2019, 11:54:01 pm
content: Quote from: laus on March 23, 2019, 10:44:02 pmThe R represents the energy lost in the circuit. It could be due to energy radiating away or it could energy lost in the resistance of the system or in the ferrite core. If the wires are near a metal surface there are eddy currents that loose energy which would also be lumped into your R.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 24, 2019, 01:31:16 am
content: Quote from: ahbushnell on March 23, 2019, 11:54:01 pmWe call it "radiation resistance" in antenna terminology, where the power applied to it is converted intoradiated Electromagnetic power.To the external circuitry, it "looks" like a resistance (V & I in phase), so it is convenient to regard it as a "resistance".The ratio of "real"(heat loss) resistance to radiation resistance is a good guide to antenna efficiency.Similar concepts are used in electric motors where the current increase when the motor is loaded looks likea parallel resistance, but obviously is not, as the electrical power is not lost, but converted to mechanical power.

author: laus
author_rank: Contributor
timestamp: March 24, 2019, 02:40:50 am
content: Thanks everybody to help me work this through and make me understand it correctly.So is the non-reactive resistance (R) where energy gets converted and leaves the "circuit" (i.e. heat, electromagnetic radiation, etc) and reactive energy that gets stored in in the circuit (i.e. capacitors & inductors) that later may be released into the circuit again?What I messed me totally up was that I always thought frequency change would only affect the reactance and that the resistance is independent from frequency. I guess I was totally wrong there.So about actually measuring the inductance of a ferrite bead, is is just the Z value of the analyzer the way I wired it up?

author: ahbushnell
author_rank: Frequent Contributor
timestamp: March 24, 2019, 02:24:40 pm
content: Quote from: laus on March 24, 2019, 02:40:50 amA ferrite bead of that type is used for removing noise at high frequency (> 1 MHz). They use a lossy ferrite. That would be a combination of the bead inductance, losses and wire inductance and losses and some radiation.Yes R is the way the circuit loses energy of all forms. (I guess I should say power instead of energy). The R and Z represent the Thevenin's equivalent circuit.Imagine a simple circuit of a capacitor in series with a resistor. At very high frequency the capacitor looks like a short and the power would flow out the resistor. But at DC you would measure an open circuit.Andy

author: radiolistener
author_rank: Super Contributor
timestamp: March 27, 2019, 02:05:20 am
content: Actually your analyzer shows impedance of real and imaginary components connected in series (equivalent circuit is resistor and capacitor/inductor connected in series):Z = R + jXSeries notation is used for antenna analyzers by default because it is more easier to express.There is also exists parallel notation, but it is more complex to express, so it is used rarely.If X < 0 then imaginary part represents resistance of capacitance at specified frequencyIf X > 0 then imaginary part represents resistance of inductance at specified frequencyIf X = 0 then there is no reactive component at specified frequency (capacitance is compensated with inductance)When X=0 it doesn't means that there is no capacitance or inductance, it just means that both components are compensated, so the RF energy didn't reflected back to the source.Real part represents active resistance R at specified frequency (losses): R = Rr + Rg + RcIt consists of:Rr - radiation resistance (radiation loss)Rg - ground loss (loss from heating the earth below the antenna and in conductive objects nearby)Rc - conductor resistance (heat loss due to resistance)Rc depends on the frequency because high frequency current flows mainly through skin layer, and that layer depth depends on the frequency, seeskin effect.In your case, you're increasing Rg component with ferrite core. It means that there is RF energy loss to heat ferrite core. Also you can see inductance component of the wire.

author: E Kafeman
author_rank: Regular Contributor
timestamp: March 29, 2019, 10:02:56 am
content: It is perfectly explained by radiolistener so I have nothing to add as text.Instead, as I easy have the possibilities to show different types of Ohmic curves relative frequency can maybe some graphical illustrations be of interest.Below curves are measured for a recently designed dipole-antenna, tuned for 2400-2500 MHz.As can be seen does both resistance and imaginary part, reactance, vary with frequency.From these can Return Loss and VSWR be calculated or can both R and jX be mapped in a Smith-chart where a typical resonance loop can be seen.

author: tautech
author_rank: Super Contributor
timestamp: March 31, 2019, 09:46:08 am
content: Quote from: laus on March 23, 2019, 08:26:58 pmAs Bud says you have to first get your head around what you're trying to achieve.Impedance is just one parameter to consider and there are several more important as other replies have outlined.When I recently ventured down this path too I also had a lot to learn about the basics and in particular as to which really matter in terms of dramatically affecting antenna performance for the hobbyist.That RigExpert you have looks like it can give you enough info to get an antenna to work quite well however I learnt through lots of fumbling around the key is to first get your antenna close to the desired resonant frequency.Much then falls close into place with just a few tweaks to get other measurements to desirable levels.I've outlined a project that might have some content to help you on your RF path:https://www.eevblog.com/forum/rf-microwave/antenna-project-log/Study the screenshots carefully.

author: Milmat1
author_rank: Contributor
timestamp: April 11, 2019, 02:20:47 pm
content: Remember the ferrite bead is very frequency dependent, and the amount of inductance in the circuit is increased in a non linear fashion over the freq range.The circulating current in the ferrite can vary and the looses will vary accordingly,I think your main quiry is why does the DC resistance value change with frequency. And that seems a little off at first, however the discrepancy lies in how the antenna analyzer operates.