On Mon, Sep 30, 2024 at 08:27 AM, Raphael Wasserman wrote:
There is nothing wrong with the suggested Laudet's model. Besides, other sources support the same model for unshielded SMA.
Loudet's model requires a 0R source or ideal voltage source. If the source impedance is higher than 0R, then it does NOT replicate Martin's loop impedance measurement. The Smith chart linked below shows the modelled output impedance of Loudet model / low pass L-network when non-zero source impedance, e.g. 5R and 50R is connected to the input.
/g/loopantennas/photo/297909/3836118?p=Name%2C%2C%2C20%2C1%2C0%2C0
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The Loudet model / low pass L-network's inability to use a 50R source reduces it suitability for adoption as a practical dummy aerial. On the other hand, the proposed high pass topology agrees well with Martin's loop impedance measurement over 1-20 MHz - see .
I looked at the graphs of your article. Your presented a graph of S21 frequency ?response of 2nd order LPF which is unrealistic ¨C to much attenuation starting ?already ?at low frequencies, however the selected cutoff frequency of your dummy antenna elements is correct about 28 MHz.
Are you referring to Fig. 10? I have no experience with active loops' frequency responses and so, cannot tell if the depicted gain-slope are realistic or not. However, I will add a paragraph admitting that the measurement may not accurately replicate a particular active loop's frequency response, especially when the simple circuit has been designed for impedance and not for frequency response. However, the dummy is still useful for comparing different pre-amps' relative gain at a particular frequency because it can mimic the actual loop impedance, e.g. if pre-amp X's gain is higher than pre-amp Y at 5 MHz when measured with the dummy, then there is no reason why the real-world gain performance will be dissimilar.
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What should be the correct gain-slope rate (say, in dB/octave) at low frequencies, say, over 1-6 MHz? If you, Martin or someone else can show me a graph of the expected frequency response, there is a possibility of adding a amplitude correction network before the dummy for correction (similar to amplitude equalizers inside CATV amps). Alternatively, it can be implemented using a correction table for the signal source, inside the test software. :-) But, that will be version 2!
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I do not whether ?you selected correctly source and load impedances for this dummy antenna. How well they match with ?realistic SMA ?
Are you referring to Fig. 10? If yes, the source impedance is the VNA's; i.e. 50R. The load impedances are those presented by the inputs of LZ1AQ, M0AYF, MLA-30+ and PA0FRI pre-amps.?
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Your point is ?how come your measurements do not coincide with Ehrenfried's loop measurement. Well, probably you have been ?comparing apples with oranges¡
Actually, fig. 9 coincides with Martin's measurement beyond my expectations over 1-20 MHz. It is the 30 MHz resonance point that diverges. Martin has a hypothesis for the divergence: "
The (loop's) resistive component increases near resonance? The (loop's) radiation efficiency increases with frequency, as the antenna becomes a greater proportion of a wavelength in size? "
Surely a simple high-pass L-network cannot be expected to be able to approximate the two effects hypothesized by Martin.
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73, Chin-leong Lim, 9W2LC
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