An isolator or circulator is for isolating a transmitter from the load where RF will travel with virtually no attenuation in one direction (towards the antenna) and RF going the opposite direction as in reflected power or energy from nearby transmitters will be bypassed to a load. An isolator does not specifically reduce harmonics and in fact its a non linear device that will create harmonics.
What you need is a low pass filter or band pass filter. These are commonly placed after an isolator to reduce or eliminate harmonics created within the isolator. BTW an isolator and low pass filter come stock inside your Quantar transmitter.
Most isolator mfrs offer a low pass filter and here are a few examples, although these prices are sky high. I'm used to paying in the $30-$35 range for a Telewave low pass filter to put on the output of an isolator.
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You can also use a secondary low pass filter from a repeater and here is a common one out of a GE Master III series repeater. I use these for various projects and they should knock down a third harmonic by 60dB or more. I think the price is a little high on this one but $60 would be a fair price if the seller takes offers.
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Sorry, but with my limited knowledge of this topic its the most information I can offer at this time.
