portable antenna VSWR

[sophiante]

What sort of VSWR do one have to accept (in general) on a UHF whip antenna mounted on a handheld two-way radio with almost no ground plane?
My concern is the load presented to the PA rather than radiation efficiency.

 

[VEBill]

The designer of the radio would have worried about that; you shouldn't have to. The designer probably allowed for as much as 3:1 while maintaining output. Typically, the PA will be fully protected with power turndown.

Why are you worried ?

 

[VEBill]

Further to previous: Are you designing a portable transmitter? (Sorry if I didn't understand the nature of your question last time.)

 

[photistor]

The reflected power that the PA can handle dedpends on the PA design. Most PA's should be desinged to handle a short (or open) at the antenna port. If you were to use an isolator, than the reflected power is "absorbed" into the load in the isloator and the PA is protected. Typically an isolator will present a 2.5:1 or better load to the PA (depends on the specs of the isolator). Then the reflected power only comes into play as far as transmitted power (this translates into range). An isolator will add insertion loss to the transmit path. Typically in a two-way radio the Tx & Rx paths are "switched" so you would want to make sure the isolator was in the Tx path only.

 

[sophiante]

To VE1BLL, yes I am designing a portable transmitter, and to photistor, I don't have room for an isolator.
The problem is the amount of "damping" needed in the PA transistor to allow for a high load VSWR without instability. And also the PA power/current under these conditions.

 

[VEBill]

For HF designs, I've seen 'ballasted' transistors where there was a built-in resistor (in the transistor) that provides a degree of protection to mismatches. I'm not sure if it applies to UHF.

In general, you should study existing (successful) designs and see how others have done it. There should be reference designs on the websites of companies selling the applicable transistors (Motorola, etc.).