Hi Trevor,
How is the Orion output coupled to the spectrum analyzer? If your
spectrum analyzer sample is not coming from some kind of coupler with
known characteristics, then the level of the spur relative to the
desired signal is not accurately known. Since you speak of moving the
spectrum analyzer to another part of the house, that suggests you just
have a sense antenna on the spectrum analyzer input. If that is the
case, you cannot state relative signal strengths with any accuracy.
There are many things that can mislead you in that kind of setup. The
spectrum analyzer input may be overdriven and producing the apparent
spur internally. The spur may be generated in some other piece of
equipment that is picking up the Orion's TX signal and radiating the spur.
To make this measurement properly you need to have no antenna connected
to either the Orion or the spectrum analyzer. The Orion should feed a 50
ohm non-inductive load and the sample going to the spectrum analyzer
should be at a level that is not enough to overdrive the spectrum
analyzer. All of the connections and cables should be coax, so that you
have a closed system, not radiating or picking up signals from the outside.
The two most straightforward systems would be as follows:
1) A series of attenuators directly from the Orion output to the
spectrum analyzer input. The total attenuation must add up to an amount
such that the spectrum analyzer is not over driven. The attenuator
connected directly to the Orion has to be rated at 100 watts or more,
and the subsequent ones have to handle what comes out of the first one.
If all the attenuators have flat frequency response over the range you
are making measurements, no corrections need to be applied to state the
relative level of any spurs.
2) A coupler in line between the Orion and a 50 ohm termination. This
could be something like a tap off element in a Bird 43 wattmeter. To
state relative levels, you need to know the characteristics of the
coupler and apply corrections to your spectrum analyzer readings.
You could make your own sampling coupler by running a short length of
wire on the inside of the shield of a piece of coax, that runs from the
Orion to the test load. The exact attenuation at any given frequency
will be unknown unless you have a way to measure it or calculate it. So,
the relative level of any spur detected would still not be accurately
known, and if you're not careful you could damage your spectrum
analyzer's front end. At least you could make it a closed system using a
home brew coupler. You may not be able to accurately state relative
levels, but at least you know where the signals are coming from.
DE N6KB
Trevor wrote:
During the checkout of my new O-II I observed a significant spur on
11.8mhz while transmitting on 21.105mhz
under the following conditions:
Barefoot - no linear amp.
"Out of the box" default settings - both VFO's on 21.105mhz - mode LCW
on both - (A or B has the same spur)
Tune - advance power level from 1w to 55w spur is ~30db down - spur
disappears at 50w through 100w.
With amp (SB-220 or HF-2500) at 1kw there is no 11.8mhz spur however
should I wish to run QRP or reduced power up to 55w barefoot I have the
spur.
My spectrum analyser is 3 feet away from the rig so I took it to another
part of the house and saw no difference in the spur amplitude.
Anyone else see this? No spurs found on any other bands but have'nt
completed my checkout yet.
-Trevor AB5EU
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