Chavdar,

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In order to "complicate" this matter more.

The current theory about usage test simulators for receiving antennae postulates:

"these equivalent circuits ( Thevenin and Norton ) may be applicable to transmit system but not suitable for receive antenna system since the receiving antenna is actually a constant power source.

An accurate model for a receiving antenna should be a power source rather than a voltage or a current source".

Thus, the current NEC or EZNEC tools are for birds unless we are modeling transmit anteenae?

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Raphael

To Dan,

When the loop length is ?close to the wavelength there are standing waves ?- in your last data? from NEC (thanks for that) the ratio Imax/Imin is? 1.4 – the max. ?current ?being at the loop side and min. current at the load. I plotted ?the current vs. segment# and I can ?see a typical ?standing wave pattern. The current variation is not very large so with fixed current source measurement setup we can expect even at these frequencies to get a glimpse of the frequency response of the antenna factor . ??

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About loop as transmission line

Yes, I think that the loop can be considered as a ?transmission line. ?But since the loop is so short compared to the wavelength ?we cannot observe standing wave patterns ?at low frequencies when loop is shorted or opened. ?For me closed and open loop are both standing wave cases. ?By the way if we terminate one side of the small loop with? characteristic impedance of this transmission line ??we will get something? like traveling wave antenna?? - ??Flag antenna ?is a typical example. Flag becomes unidirectional when load R = Zc of this ?tr. line. ?But I will stop here – my knowledge in antenna theory is very very limited, I am just a standard NEC user.

? Chavdar lz1aq

?Hi Raphael,

I am sorry , ignore previous message #20287 it is wrong.

?The ?Current source resistances must be much higher than the anticipated LOAD resistance , ?so 2 KOm which are used are 20 times higher than the maximal expected input impedance of the amplifier (e.g. 100 ohms with input filter at highest frequency) .? NEC data show that the loop deliver ?constant current in very wide frequency range. ??So we will get frequency response of the loop amplifier if we assure constant current at its input . This is very simple? - even we can get almost correct frequency response without inductance at all – just we must feed the input with constant current . ?2 KOm will give current 5% less at 100 Ohm load ?compared to 6 Ohms load.? To increase the accuracy we can use e.g. 20 K? and the error will be 0.5% .? The problem is that 20 K will reduce the signal input substantially? with existing signal generators? and also parasitic capacitances of the resistors cannot be ignored e.g. ?1 pF at 30 MHz? = 5.5 kOm. ?The parallel ?inductance will go into action at low frequencies where its impedance is comparable to R load.

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Again: the internal resistance of the current source must be much higher than the anticipated ?R load (not the? impedance of inductance) . ??

Chavdar

At 7 MHz a 113 inch long .5 inch solid or tubular conductor (either one due to skin effect) the AC resistance (owing to skin effect) is 50 millOhms (0.05 Ohms).

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Verification would be welcome.

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Value from "Round" tool used here:

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de AA5CT Jim

I would agree with Fred.

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Despite what many articles and videos on the internet may suggest, including many professional ones, it is not easy to make practical and repeatable IMD measurements, especially at the IMD levels we are interested in.

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I've struggled with this, and have my own built band pass and notch filters and crystal oscillator based test sources. But even things like dirty connectors, especially things like BNC test leads, can cause results to vary considerably.

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The first step is to check that your IMD test rig is capable of producing results that are at least 10dB better than the device you are testing. Even this isn't really good enough, but it's something to aim for. Then try measuring known devices, such as the Mini-Circuits PGA-103+ or GALI-74+.

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I've always found these perform pretty closely to their published specification, and I tend to use one as a sanity check, before I try measuring something else.

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Regards,

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Martin

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On Mon, Oct 7, 2024 at 04:19 PM, Fred M wrote:

Hi Chin-Leong Lim ( the first time that i got right)

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Besides the TinySA, whose own IP3 is 17 dBm at best (thats your absolute limitation), the Signal generators Output Stages and even the Splitter with its Ferrite nonlinearity are also a source of Intermodulation which limit your measurement dynamics. Again, IP measurement of high IPs is demanding, dont't fall in your own traps.

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regards, Fred

Dave,

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As far my acknowledgement of usung VNA for measurement small impedances or resistors of 0.1 ohms value allows to reach about 0.04 % accuracy.?

However, for low resistance values that you mentioned shall be used a different method by connecting between CH0 and CH1 ports of VNA ( so called I- method by measiring S21 or current through your resistor). This technique has 0.01 %.

I guess you can measure ohmic resistors not radiation resistance

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Raphael

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Chavdar,

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I am sorry I forgot to mention another point. I can understand why you have been using Norton equivalent circuit for a prove how great works active small loop antenna in short current mode. The low resistive input of amplifier cancels the reactive influence of loop antenna and regardless what series arms in simulator are used - we will get flat ( frequency independent ) frequency response.

On other hand using Thevenin circuit it does not? prove the above said because you do not have a control mechanism allowing the source voltage to follow the frequency in acordance to Faraday' law of induced current. So, the frequency response is not "frequency flat" ( indepenent from frequency ) more but skewed down with the frequency increase.

My understanding has been usage of dummy aerial circuit with a receiver in our technical business always? ignores that applied test signal shall follow Faraday's law.?

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Raphael?

Hi Chavdar,

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With all my respect to you spending a lot of time exploring loop antennae with amplifiers I am in disagreement with your "simulator" design.

For all practical means we have been using as a source voltage generators, thus for testing active antennae their equivalent circuit? should be Thevenin's circuit.

Yes, we can transform from Thevenin to Norton circuit but it is pure theoretical move.

We agreed that value of current source in Norton's is related to voltage source in Thevenin's as I sc = V oc / Z ant because of equal power condition requirement for this transform.

Using as series arms in your simulator relatively high in value resistors 1 Kohm, it cannot be? anticipated that really was created Norton's circuit because the current source does not reach the required current level? when 1 Kohm >> Z ant in frequency range of interest from f lc ( low-cut off ) to up to f0 ( resonance point ).

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Regards,

Raphael

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Hi Martin & Fred,

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Thanks for the article links. I will dig into them later.

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At this moment, I don't have notch filters for the 5-30 MHz range that I want to measure. To quickly verify whether the IP is internal to the TinySA, I will try to increase its attenuation to see if the displayed IMD reduces.?

What I find hard to understand is why the measured OIP3 is 30 dBm at 20 MHz (see graph), but progressively drops as the frequency pair is lowered, and finally reaching 16 dBm at 5 MHz. My expectation is internally generated IMD should be quite constant over frequency. On the other hand, if the external & internal IMD are combining constructively and destructively at different frequencies, as Martin has speculated, I would expect the OIP3 trace to be wavy. Or did I miss something?

Anyway, more investigation is required to understand the setup's eccentricities.

At the project onset, I thought it would take 1 month max to build a working active loop, but now it has already taken a year of my life ?. What a deep rabbit hole!

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73, Chin-Leong Lim, 9W2LC

To Raphel

Sorry , 10- 20 times larger from? the loop impedance.

The Northon is ok - the current source is the short circuit current.? Zloop >>Rin so we ignore current through the loop.

Chavdar lz1aq

On Sun, Oct 6, 2024 at 07:32 PM, <biastee@...> wrote:

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What's your test setup, and how do validate it ?

FY6900 function gen, Minicircuits ZSC-2-4 hybrid combiner, & TinySA. I admit I haven't validated the setup.

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Hi Chin-Leon Lim
Measurements of high IP3 values are challenging. It is very likely that the measured IM products will be created in Tiny SA itself, not in the DUT. Your limitation is the internal IP of the Tiny SA

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regards, Fred

Hi Leong,

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OK thanks for that.

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From experience, I don't think you will be able to achieve the required performance of your test setup without building fundamental frequency notch filters to place on you analyser input.

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Ideally you need to be able to measure signal levels below -100dBm, in the presence of test signals at 0dBm or more. Even with a high end SA, this is asking a lot.

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Steve and Everett wrote this useful practical guide for Siglent.

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https://www.siglenteu.com/application-note/inter-modulation-distortion-imd-testing/

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I think the variation you are seeing is due to the external and internal IP generation aiding and abetting each other as the phase and amplitude relationships vary with frequency.

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In the old days of analogue TV transmitters, we used to deliberately generate IP products at low level, and inject them into the RF transmission chain, to null out IP's that were being produced in the high power amplifier stages. We called this feed-forward correction.

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Regards,

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Martin

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In receive mode the resistance increase due do the skin effect could be neglected for simplifiaction. It is disproportionanally lower than the frequency dependent increase of the inuctive reactance.

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regards

Fred

Hi Martin,

73, Chin-Leong Lim, 9W2LC

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On Sun, Oct 6, 2024 at 10:52 AM, Martin - Southwest UK wrote:

Good work, but the LZ1AQ IOP3 looks far too low.

Hi Martin,

Sharp eyes! ?

You r correct, the LZ1AQ OIP3 looks too small. I think the problem lies in the large variation with freq - the OIP3 varies from 16 dBm at 5 MHz to 30 dBm at 20 MHz. In contrast, the competition - MLA-30+, M0AYF & PA0FRI have flatter OIP3 variation.

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My normal sanity check is to check that the IOP3 is typically 10 to 15dB higher than the P1dB value.

What's your test setup, and how do validate it ?

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73, Chin-Leong Lim, 9W2LC.

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