I think that you should try and reduce the interference from the motor first. You are peobably not only disturbing your own communication link, but a lot of other HF equipment as well. You have to comply with FCC in the US and the EMC Directive in Europe and I doubt that your equipment will pass an EMC test based on these requirements.

When this is done, you will surely have a better range. And is possible that this increased range will reduce the blind spots.

Are the motors DC low voltage motors? Is there any PWM inverter involved?

The motor is a universal, 115 vac.  I'll try shielding it and its leads and see what happens.

Bob,

I think that you should try ferrite cores in the motor leads. And also a capacitor across the brushes. You may need to talk to someone that knows about EMC and filtering to arrive at a good solution.

This instalation would never meet EC or CISPR EMI requirements if it is affecting a UHF receiver but the addition suppressors to the power lead, right at the motor would probably solve the problem.

You will find though, that the interference at 900MHz is less than 418MHz, but that's not the issue.  There should not be that much at 418MHz.

It may be that your RF module has a susceptibility issue. The interference from the motor may be getting into your IF at a much lower frequency. Most filtering on small modules is on the front end (i.e. just after the antenna). Additionally, if your module is located immediately adjacent to the motor, the magnetic field of the motor may be directly coupling into the module effectively bypassing the shielding the module has. US regulations do not require a product be tested for susceptibility to external intererence - European regulations do.

A module in 915 MHz ISM band will have some reductions in dead spots due to the shorter wavelength and the possibility that your module will not be completely within a null spot. Additionally, many modules at this frequency are Frequency Hopping reducing the size of potential null spots. You can also select a receiver with antenna diversity (two antennas spaced slightly apart) - very effective at eliminating null spots.

More good input, thanks.  I'm going to try the 915 Mhz band and see if that is any better.  I'll also work on the motor shielding and suppresion.  The board is fairly close to the motor.  Moving it and its antenna away from the motor helps.