How to protect output RF transistor of FM Tx from accidental reflection?

[Willen]

Heared that, Output transistor is more critical from over reflection and high possibilities of being damage. When I transmitting, cause of some accident like storm, birds etc, antenna can broke down and it is not easy to see that the connection has been brokedown or not. In this situation, sure I will burn my output transistor by over reflection.
So it is too much important to protect output transistor from this kind of situation. Are there any way or idea or shematics to protect output transistor?

 

[unclejed613]

i'm guessing you saw the data sheet for the MRF-232 from the last thread where you were asking what to use for an RF amp? the data sheet says the transistor will work with up to a 30:1 SWR. that's a pretty large mismatch. you could always include a current limiter in the power supply if you're that worried about it. or you could use a power MOSFET (i'll post the circuit later), which is also somewhat "bulletproof", since a MOSFET doesn't have a secondary breakdown region like a bipolar transistor does.

 

[ronsimpson]

You can add a very simple temperature shutdown. (thermostat) Measure the heatsink or transistor temperature.

I have measured the current to the power amplifier. If the current is too high then shutdown for a time.

 

[WTP Pepper]

Google "RF Isolator". It allows your PA to see a stable output impedance regardless of the mismatch on the aerial port. The penalty is usually a reduction of output power.

 

[ronsimpson]

When you have a bad mismatch it is time to shut down and find out why. Fix the antenna.

 

[Willen]

hard to find schematics, recommended by you all.....

 

[unclejed613]

funny you should mention that. here's a MOSFET amp. the MOSFET is 10-15 dollars US (compared to about 50-100 dollars for the MRF-232), and the part is made by Mitsubishi. the part number of the MOSFET is RD15HVF1, and according to the data sheet is impervious to high SWR. the bias control (U1 in the schematic) sets the operating point of the MOSFET. depending where you set it on it's conduction curve, you'll get between 2 and 15 watts out. the transistor must have a heat sink, and this FET has a different pinout than most, GSD instead of GDS. this is so you can mount it on a heat sink without the capacitance effects that the heat sink tab would normally present. it also provides isolation of the parasitic capacitances between G and D. if you try this with a regular TO-220 type MOSFET, the drain tab will act like a big capacitor and kill most of the output signal.

 

[Willen]

funny you should mention that. here's a MOSFET amp. the MOSFET is 10-15 dollars US (compared to about 50-100 dollars for the MRF-232), and the part is made by Mitsubishi. the part number of the MOSFET is RD15HVF1, and according to the data sheet is impervious to high SWR. the bias control (U1 in the schematic) sets the operating point of the MOSFET. depending where you set it on it's conduction curve, you'll get between 2 and 15 watts out. the transistor must have a heat sink, and this FET has a different pinout than most, GSD instead of GDS. this is so you can mount it on a heat sink without the capacitance effects that the heat sink tab would normally present. it also provides isolation of the parasitic capacitances between G and D. if you try this with a regular TO-220 type MOSFET, the drain tab will act like a big capacitor and kill most of the output signal.

-Why the MOSFET is biased as class A amp? I think it would be better in class C, isn't it?
- Should I have to remove or disconnect the attenuator when I am using '200-500mW_Input'?
Nice circuit with additional attenuator and may be nice gain controller by Potentiometer. But it is so sad that I never found such MOSFET. I found 2SC1971 (ONLY one) by searching almost 4 yrs in my country. Online shoping is not possible for me . So I am always searching alternatives...but never found ;(

 

[unclejed613]

it's a class A amp to keep unwanted harmonics to a minimum. a class C amp will generate harmonics, which are difficult to filter out. if you interfere with other important services (especially "safety-of-life" services)and their communications, your government's radio agency (such as the FCC in the US) will descend on you like an angry 800lb gorilla.

leave the attenuator in circuit, as it's also there to provide an impedance match for either input.

 

[davenn]

You can add a very simple temperature shutdown. (thermostat) Measure the heatsink or transistor temperature.
I have measured the current to the power amplifier. If the current is too high then shutdown for a time.

except that the reflected power is likely to kill the transistor or produce unwanted transmitted products long before the temperature rises to any significant level

The standard way to monitor reflected power is to have a RF pickup on the output line of the transmitter, As the reflected power increases, it produces an increasing detected voltage. That voltage is used usually in 1 of 2 ways ...
1) reduce the drive level from the driver stage to the final device or
2) reduce the V+ supply to the final device

both of which have the same effect of reducing the output of the transmitter.
In commercial systems I have worked on that voltage generated by the reflected power is also used to trip an alarm to
signal that there is a feedline/antenna problem

Dave

 

[davenn]

OK here's an example cct with a few of my annotations ......




Dave

 

[Willen]

except that the reflected power is likely to kill the transistor or produce unwanted transmitted products long before the temperature rises to any significant level

The standard way to monitor reflected power is to have a RF pickup on the output line of the transmitter, As the reflected power increases, it produces an increasing detected voltage. That voltage is used usually in 1 of 2 ways ...
1) reduce the drive level from the driver stage to the final device or
2) reduce the V+ supply to the final device

both of which have the same effect of reducing the output of the transmitter.
In commercial systems I have worked on that voltage generated by the reflected power is also used to trip an alarm to
signal that there is a feedline/antenna problem

Dave

Sorry! I pressed ''Unlike'' button by mistake and cannot remove it . Actually your idea is mind blowing!!! You are clear speaker. The cct shown by you is more complicated for me due to my lack of knowledge and also there are some unusual components for me to make in practical.

Are there any cct which can be keep in outside of feed line to monitor over level of reflected power and can warn me? Little simple cct and some usual parts?

I just want to see feflected power level every time, not automatic monitor and not auto-controlled. So i think it will be a simple circuit like simple SWR meter.

 

[davenn]

OK I will edit the image to show the essential bits

If you cant and normally wouldnt be able to continuously monitor the transmitter the system shown is really the best way

The pic is part of a VHF (144-148MHz) commercial radio I once owned and still have the pdf manual for

if you just want to manually monitor the reflected power, without any control over the transmitter just buy a SWR meter
they are not overly expensive and are readily available.

BUT remember the title of your thread was "How to protect output RF transistor of FM Tx from accidental reflection"

a SWR meter is not going to give you any protection

Dave

 

[davenn]

OK ... here's a modified cct showing just for transmit.
have taken out the TX/RX diode switching etc ... power output meter driver etc



Note also the section circled in blue and the text for it.

NEVER put a transmitter on the air without a low pass filter!!!

Dave

 

[Mikebits]

Google "RF Isolator". It allows your PA to see a stable output impedance regardless of the mismatch on the aerial port. The penalty is usually a reduction of output power.

Isolators only become practical at microwave frequencies due to size vs. freq.

 

[Willen]

OK ... here's a modified cct showing just for transmit.
have taken out the TX/RX diode switching etc ... power output meter driver etc



Note also the section circled in blue and the text for it.

NEVER put a transmitter on the air without a low pass filter!!!

Dave

Wow!
But few questions:-
A) What is this line between L209 to Emitter of Q1? Is that a +ve power suppy for final amplifier? I think it is not.
B) There are two markings in red text:-
+ 13.8V
+ 8V Tx
Are they Input voltage for 'Voltage Control Driver Cct? Why you write 'Tx' after +8V? Do you mean 'take +8V supply used in Tx'?
C) Collector Output of Q1 is a +Ve Controlled Supply for Final amplifier? Its voltage will be low when reflected power is high, am I right?
D) Almost...all of these transistor (without final amp) are general purposes transistors? Can I use?

 

[davenn]

Wow!
But few questions:-
A) What is this line between L209 to Emitter of Q1? Is that a +ve power suppy for final amplifier? I think it is not.

Yes it is the + 13.8 V to the final transistor, Q208. the actual voltage is conntrolled by Q1

B) There are two markings in red text:-
+ 13.8V
+ 8V Tx
Are they Input voltage for 'Voltage Control Driver Cct? Why you write 'Tx' after +8V? Do you mean 'take +8V supply used in Tx'?

Those 2 rails +13.8V and 8V TX in the centre of the diagram .... The 13.8 is just a supply rail for Q213
The 8V TX is 8V when TX (Transmit is initiatiated) otherwise that line is 0V

C) Collector Output of Q1 is a +Ve Controlled Supply for Final amplifier? Its voltage will be low when reflected power is high, am I right?

No, the Emitter is the 13.8 output of Q1 .... the 13.8 is being applied to the collector and is being controlled by the voltage/current on the base of Q1 coming from Q213
No, as the reflected power level increases, the drive to the base of Q1 drops, this drops the current flowing through it to the final transistor

D) Almost...all of these transistor (without final amp) are general purposes transistors? Can I use?

The MRF212 (Q208) final transistor is a VHF power transistor rated at ~ 10 - 15W output. This radio easily produces 10W output.
Q1 the 2SD235 needs to be able to handle the several Amps that the final transistor requires.
Q's 211,212 and 213 are just general purpose transistors



Dave

 

[Willen]

Yes it is the + 13.8 V to the final transistor, Q208. the actual voltage is conntrolled by Q1.................
Dave

OK, then collector of Q1 is +13.8V battery terminal, and Emitter of Q1 and Emitter of Q213 are same +13.8V terminal, isn't it?

-Um...again confuse on +8V Tx. Should I have to get this +8V supply from previous stages of Transmitter (oscillator or pre amp) which is operating at +8V? If I am correct, there will be 0 V at final Amplifier when Transmitter is off.
- Can I make this detector and controller cct on same transmitter circuit and always keep connected? Won't it harm for RF out?
- There is a potentiometer, so I think it will little difficult to tune for best and to set suitable level of voltage for controller, won't it? Any idea to measure and tune?
- I will use BD135 or 36 as a Q1, and 1n4148 as a RF rectifier diode. I think OK (?).

Sorry but I will ask again, it's so interesting!! Thank you.