On 1/9/22 2:40 PM, Jeff Green wrote:
Thank you, I was afraid of that.
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From:
On page 61: "Splitting up the winding into two rings suitably spaced provided a reasonable compromise between simplicity of construction and constancy of antenna input impedance. However, instead of 125 ohm ¡À15% the impedance actually varies from 50 - 300 ohms."
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So, Any idea how the input impedance is measured and how did Collins verify the above data?
Thank you.
You'd measure it (today) with a VNA, making sure that you pick a drive level that's reasonable for the receiver (i.e. if your receiver input can only tolerate -50dBm, then better not put -10 dBm in).
The trick is that active devices often have impedances that vary with operating point.
Let's assume you know the gain of your receiver very accurately. That is, you've turned off any AGC, and you can measure the output for a given input level.? Then, you can put known impedances in series with the input, forming a voltage divider (Zrcv/(Zrcv + Zseries)), and from a set of measurements, you can determine what Zrcv is. You will need both resistive and reactive series Z, because typically, you can only measure the magnitude of the signal, not the phase.
