antenna mismatch detection circuit

[maniana]

what is antenna mismatch

Could someone explain how this circuit works?
Below is a short description from an ICOM service manual regarding this circuit but I don't understand a few things:
1. why the voltage before the inductor corresponds to the forward power and the voltage after the inductor corresponds to the reflected power?
2 why the combined voltage is minimum when antenna is matched?
if I short the antenna the voltage should be lower, shouldn't it?
whot is there is no antenna?

"The APC detector circuit (D14, D17, L39) detects forward
signals and reflection signals at D14 and D17 respectively.
The combined voltage is at minimum level when the antenna
impedance is matched at 50 Ω and is increased when it
is mismatched."

 

[kspalla]

rf antenna mismatch

Can I ask you to share the schematic of this power detector and freq of operation so that it can be analised in simulator.

 

[maniana]

if detector circuit theory

Hi,
Below is the corresponding part of the schematic.
F=150MHz

I saw also a different version of the same circuit where the diodes in the detector are not shorted.

I simulated it in Spice and it works better at F around 110MHz.

I have a theory about it but wouldn't want to suggest anything on the forum until some clever guy explains it to me first.

 

[vfone]

directional coupler & power & circuit & lumped

This is the schematic of ICOM IC-2200H amateur-radio FM transceiver.

The circuit is forming a lumped element directive coupler, and is used not only to prevent for high VSWR, but also to prevent transmitting higher power than a specific limit (65W in the case of IC-220H).

The circuit is calibrated for a combined DC level between forward and reflected waves, and it will vary the RF power for any change of this level.

 

[maniana]

antenna missmatch

vfone, thanks for your reply.
I know what the circuit does, but I don't understand the principle of its operation.
I know lumped versions of directional coupler, but they look like transformers, they have several windings, at least primary and secondary , here we a have a simple inductor.....
I dont understand how the power on one side of the inductor can be the forward power and on the other side the reflected power?
the diodes form a simple peak voltage detector, how can the detector be sensitive to the reflected power and not the forward power?

My theory was that the inductor and capacitors form a lumped equivalent of lambda/4 line, but in this case wouldn't there be an impedance transformation? would the impedance before and after the inductor be different?

 

[FvM]

mismatch vswr antenna spice model amplifier

The shown schematic, if exactly representing the real circuit, involves only a summation of scalar voltages at the left and right side of a Pi-filter. In this case, it can't work as a lumped element directional coupler. If someone knows a way how it could though, please tell.

But depending on the filter Q, which I didn't calculate, it's possible that the voltage is minimal in matched case. This would be less information, than a directional coupler can provide, but meaningful anyway.

 

[vfone]

directive coupler

The schematic IS representing the real circuit, because the second posted picture is part of the service manual of the transceiver.
This kind of power leveler and protection circuit is very common in two-way radios, probably 40 years back, when appeared the first solid-state RF power amplifiers.
The circuit even is not looking as in theory, is a coupler.

 

[maniana]

rf antenna detector circuit

vfone,
would you shine some light on how it works and how to design such a coupler for any frequency, bandwidth, coupling factor ?

It is IC2200H , but if you have a look at IC2720 schematic (attached) you will see that they use this circuit (though a bit different) AND a coupler- so why double detection of the same thing?