[RFI] Re: rfi-digest V1 #70

Cortland Richmond Cortland Richmond" <cortland_richmond@qmgate.optilink.dsccc.com
22 May 1998 09:56:53 -0800

                      RE>rfi-digest V1 #70                         5/22/98

Bob Wanderer wrote in issue V1 #70:

>>From: Bob Wanderer <aa0cy@nwrain.com>
>>Subject: RE: [RFI] Spurious signal levels
>> When I had a spurious problem on 14 MHz 
>> back in the late 70's, I called the FCC 
>> Laurel, Md. Lab) to find out whether the 
>> measurement was into a dummy load or into 
>> the antenna itself (as the levels were 
>> different and into the dummy load less 
>> than the limits discussed below).   They 
>> couldn't answer. I eventually was able to 
>> solve the problem (brought SWR from 3:1 to 
>> 1.7:1 by modifying the angle of the radials 
>> on the trapped 1/4-wave vertical). I lost a 
>> bit of respoect for the Commission's 
>> technical representatives. Anyway, what's 
>> the answer?
>> 73, Bob AA0CY

The FCC measures transmitter spurious products at the output with the transmitter operating into its designed load.

However... our responibility, IMO, is to _radiate_ signals whose spurious products meet the requirements. 

For type-acceptance/certifcation, the FCC would usually use 50 ohm ports, but for determining our compliance with the Rules, they'd use a spectrum analyzer or receiver, and an antenna.  This makes the transmitter output impedance irrelevant.

The requirements may READ the same, but they're different measuremnts.  

(This raises another question; many spectrum anaylzers have a dynamic range insufficient to detect low-level spurious signals in teh presence of other, strong signals -- but that's the FCC"s problem, and maybe someone's defence!)

Cortland Richmond, KA5S

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