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Re: [TenTec] Orion-II 15m TX Spur?

To: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Subject: Re: [TenTec] Orion-II 15m TX Spur?
From: Ken Brown <ken.d.brown@hawaiiantel.net>
Reply-to: ken.d.brown@hawaiiantel.net,Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Mon, 10 Jul 2006 10:21:53 -1000
List-post: <mailto:tentec@contesting.com>
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.


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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