Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Hot to match HF loop antenna?

Status
Not open for further replies.

AndreyG

Full Member level 4
Joined
May 10, 2010
Messages
196
Helped
27
Reputation
54
Reaction score
27
Trophy points
1,308
Location
Vancouver, Canada
Activity points
2,946
HF (8.2MHz) loop antenna is resonant at 6.3 MHz (lower than required) without any capacitors across terminals. How to re-match it to make it resonant at 8.2MHz?
Antenna is driven by a MOSFET driven by on-off digital pulses - like a switch. Antenna is connected to Drain of MOSFET and there is current-limiting resistor in MOSFET Sink - that's all the circuit.
 

Are you talking about resonance of the whole circuit (e.g. with MOSFET Cds) or unloaded antenna?

Did you consider a parallel connected inductor (with sufficient high SRF) to shift the resonance upwards?
 
Or - depending on exact output configuration, add capacitance in series with the loop. Please show a schematic, without one we can't predict the DC and AC signal paths and how the resonance is achieved.

Brian.
 

Of course FvM, Inductor in parallel! How can I forget? Thank you FvM!

- - - Updated - - -

Please find attached schematic.
I see now how to tune it higher - add parallel inductor. What impedance to match it to? It is not 50 Ohm, FET will present dfferent impedance in 'on' and 'off' states.?
 

Attachments

  • Exiter.jpg
    Exiter.jpg
    57.9 KB · Views: 162

Those values make theoretical resonance at 7.15MHz, ignoring the capacitance of the FET.
You can make it resonate at higher frequency by reducing the value of C1. Somewhere around 110pF would be more appropriate.

Brian.
 

Brian,
I guess I did not make myself clear. The L and C values for the antenna are values derived from measurements of existing antenna. I can not reduce value of C1.
Andrey
 

What's the role of L2?

It is part of the circuit. I believe it is there to compensate capacitance of mosfet but can not say for sure. I tried shorting it, changed it's value. It did not result in improvement.

- - - Updated - - -

Also note, mosfet on the schematic is not correct part number. I do not have correct part in this library.
 

If the objective is to maximise the current in the inductor (the radiating part) at 8.2MHz, from the values given either reduce the value of R1 to about 240R or, counter intuitively increase the value of L2, somewhere around 6.8uH should do it.
The circuit is a little like a resonant power converter circuit. What happens when the 'receiver' is close to the antenna, it may be that in the intended applcation that the circuit is off resonance until power is coupled out of the circuit.
 

Without knowing the actual MOSFET type and e.g. its output capacitance, it's impossible to estimate the circuit behavior. L2 can be part of a matching network, but it needs a shunt capacitor at the MOSFET output to achieve an up transformation. There's also a risk of MOSFET breakdown when feeding an inductive load.
 

Actually the resonance of the antenna (3.3uH in parallel with 150pF) is 7.1MHz, and not 6.3MHz.
Adding a parallel 10uH inductor to the terminals of the antenna it will bring the resonance to 8.2MHz.
In this situation, don't expect the antenna will radiate the same as if is resonating itself on 8.2MHz, without adding any external components.
 
vfone, this is correct - antenna by itself resonates at 7.1MHz, when transmitter connected - at 6.3MHz.
Why after matching with parallel inductor antenna will not radiate the same - because of losses in matching inductor?
 

Best way to use such a circuit would be to tune it such that you get zero voltage switching at the FET. If the self resonant frequency of the coil is already too low, then adding a parallel inductance would be the proper way to tune it..
 

This is valid for any antenna type. A resonant antenna (on a desired frequency) works better (or much better) than a non-resonant antenna with a matching network used to bring the input impedance to other value.
Loop antennas anyway have poor efficiency, and on top of this, if they are not resonating on the desired frequency make things even worse.
 

Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top