High SWR by itself does not damage transmitters, although sometimes it can contribute to
damage by other causes.
VK1OD has an explanation here:http://vk1od.net/transmissionline/VSWR/damage.htm
The details depend on the design of the equipment and how you use it. I've used my
IsoPole on 2m for 25 years since I adjusted it even the SWR is about 2 : 1. (The adjustment
is corroded so it won't move even if I want to change it: make sure you use some
anti-corrosion paste on all the joints.) I expect any properly-designed FM radio to
withstand SWR of at least 2 : 1 at full power without damage. Most will operate above
that, but might not survive full power operation for long time periods. I regularly use
my radios to measure SWR of 5 : 1 or 10 : 1 while testing antennas, and none of them
have been damaged by it yet.
In my experience the most common cause of transmitter damage is overheating. If you
do not provide enough cooling air to the transmitter heat sink, you can damage a radio
even at a 1 : 1 SWR. The heat generated by the transmitter depends on many factors,
- the output power power level
- the load impedance
- how long you transmit, and how often
The rate that heat is removed from the transmitter depends on:
- air temperature
- rate of air flow around heat sink
The secret, of course, is to remove heat from the transmitter so it doesn't get too hot.
This applies regardless of the SWR: the only time my transmitter failed for heat damage
was in the car on a cold day when the heater was blowing hot air on the radio and I was
using it at high power.
The reason I can measure an antenna with a 10 : 1 SWR without damaging my radio
- I use low power
- I only key the transmitter for a short time while making measurements
Under these conditions, there is no problem with heat build-up. But if you plan to
transmit at full power for half an hour or more, it is a good idea to add an extra fan
blowing on the transmitter heat sink regardless of the SWR.
I don't see any problem operating with an SWR over 1.5 : 1 for your IsoPole. But if
you are concerned, add an external fan and/or monitor the temperature of the heat
sink (with a thermometer or your finger).
Actually, my IsoPole works so well that I rarely turn the transmitter up to full power.
(Probably less than once a year.) But that is also because I live on a hill.