Impedance matching to Q branch

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petergriffin
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Joined: Sat Mar 21, 2020 10:27 am

Impedance matching to Q branch

Post by petergriffin » Sat Mar 21, 2020 10:37 am

I want to use a tuned ferrite antenna with my RTL-SDR. From what I understand at the point of resonance, only the real (resistive) impedance remains. This should be a few ohms for the resistance of the wire, capacitor and ferrite losses.

The input pins of the RTL-SDR are about 3300 ohms, how do I go about matching these two ?

I was thinking of a negative feedback amplifier, I've seen some examples of this online but it is generally used for broadband matching when there is no capacitor. Can this also be used for a tuned antenna ?

Here is an example of an amplifier used in this way, the rest of the circuit is irrelevant. I won't post a 1000 pictures but a google of "broadband magnetic antenna schematic" shows this is a recurring method.

Image

Brightnoise
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Location: Netherlands, Nuenen
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Re: Impedance matching to Q branch

Post by Brightnoise » Wed Apr 08, 2020 9:17 pm

ActAnt-RTL.jpg
ActAnt-RTL.jpg (23.14 KiB) Viewed 2388 times
Nobody reacts; I will do.

Your frequency of interest is not mentioned. I assume you want to receive medium wave.
Other frequencies are possible with the proposal described below.

The dongle direct sampling input impedance is 50 Ohm in parallel with the bias-T inductor and can be used down to 200 kHz. The sensitivity of the RTL-dongle is constant from 1 MHz to 14 MHz, but below 1 MHz it is reduced according a first-order slope due to the input transformer IN the dongle, after the pre-amp and the low-pass filter, directly at the input of the second chip (R820T) in the dongle.

A tuned loop (medium wave, common L-C value in this range: L = 200 uH and tuning capacitor variable from 20 pF to 400 pF) is at the resonance frequency a high impedance.

Solution #1 (preference is solution #2!)
In transistor radios with a ferroceptor there is a coupling coil on that ferroxcube rod with only a few windings that gives noise matching to a transistor with a few milli amps DC-collector current. Here the interface is just that transistor (BF199 or even a simple BC107 is ok) with collector current 5 mA and output from collector via a capacitor to the dongle input.

Preferred solution #2
Easier than a bipolar transistor is the use of a junction FET like J-309 or BF245b: without coupling winding (!) the LC-antenna circuit can be connected to the gate of the FET (and the source as ground). The output (drain) can be connected to the dongle. Activate the bias-T and there is your active antenna with a noise limited sensitivity as good as the passive L-C antenna circuit allows. Of course you need to separate the antenna a few meters from your computer.

When you need more detailed information, just ask.

Brightnoise
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Stromeko
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Joined: Wed May 01, 2019 6:32 pm

Re: Impedance matching to Q branch

Post by Stromeko » Fri Apr 10, 2020 7:00 am

petergriffin wrote:
Sat Mar 21, 2020 10:37 am
I want to use a tuned ferrite antenna with my RTL-SDR. From what I understand at the point of resonance, only the real (resistive) impedance remains. This should be a few ohms for the resistance of the wire, capacitor and ferrite losses.
In a pinch you could try a 9:1 balun in reverse. An active antenna is likely the better choice, lots of places to buy one and lots of schematics on the net if you want to build your own (search for active ferrite rod).
petergriffin wrote:
Sat Mar 21, 2020 10:37 am
The input pins of the RTL-SDR are about 3300 ohms, how do I go about matching these two ?
That depends on what version of the dongle you use and whether you drive it single-ended or differential. In the case of an RTL-SDRv3 you really should match to 50Ohm, since the diplexer around the tuner is already built in and matches to the respective end points.

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