I was looking at the picture and drawing of the test setup on the web
site. They do not show any type attenuator in the drawing.
Even making a differential measurement, trying to read to 1db on a log
scale with 10db per division is not realistic. You just can't "eyeball"
it that close. Especially if you can't switch back and forth between the
two signals. Let alone giving tenth of a db gain specs out of those
measurements.
Using the vernier db attenuator on the 141, if it has not been
calibrated the linearity can be off without too much trouble for making
those kind of accuracy measurements. That adds to the error.
It is also easy for the log scale on those to have errors if they are
not properly calibrated. Not saying that theirs was not but who knows.
The biggest error on the analyzer would come from trying to eyeball the
same screen position on the 10 db scale.
I assumed that they used the log scale and not the linear scale, which
would be easier to eyeball for the same spot. But since no attenuator
was shown the log scale would have to have been used.
By the way if you look at the picture of the test site closely on the
right side it looks like a set of large guy wires for a large tower or
power line? Not good for a test range.
My whole point is that giving gain specs to a tenth of a db with a super
test setup would even be iffy let alone all the obvious errors at first
glance here.
73
Gary K4FMX
Rick Karlquist wrote:
> I believe someone else said they used a step attenuator.
> Secondly, they were doing differential measurements between
> a dipole and a SteppIR, not absolute measurements of power.
>
> In any event, the 141 series can be quite accurate if you know
> what you are doing and let the instrument fully warm up. There are
> several adjustments (eg gain and offset) that need to be made. If these
> are properly made they are really quite accurate over a narrow amplitude
> range (much better than a couple of dB). I have adjusted many of them and
> checked them with calibrated signal generators which in turn had been
> verified with power meters. If you read the specs on the 141 series, it
> may seem like they are not so accurate. Having worked for HP/Agilent for
> 25 years, I can assure you that may considerations go into setting
> those specs, and in many cases they are very conservative.
>
> Rick N6RK
>
>
>
>
> Gary Schafer said:
>
>>Not even a step attenuater in the circuit. On top of that using the log
>>scale on a 141T spectrum analyzer! Lucky to get within a couple of db
>>with that setup alone.
>>
>>I am not saying their gain figures are right or wrong. Just a poor way
>>of testing.
>>
>>73
>>Gary K4FMX
>>
>>
>>W7TMT wrote:
>>
>>>Here is how the testing is described on the SteppIR site...
>>>(http://www.steppir.com/, click the Field Test button)
>>>----
>>>
>>>"We started by placing a reference dipole at one end of an antenna
>>>range and a receiving dipole at the other, with both antennas being of
>>>an equal height of 48'. From the reference dipole station a 100 watt
>>>CW signal was transmitted via an Icom 706 transceiver to the receiving
>>>dipole, with the signal strength characteristics being recorded by an
>>>HP 141-T spectrum analyzer. This was done for each frequency to be
>>>tested. SWR measurement was accomplished with a Bird model 4391 RF
>>>Power Analyst. The reference dipole was then replaced with the SteppIR
>>>Yagi test antenna. The two antennas were on level terrain, separated
>>>by a distance of 705 feet. A CW signal was transmitted from our
>>>SteppIR Yagi, beamed directly at the receiving dipole, with the HP
>>>141-T acting as the "receiver". The differential between the recorded
>>>gain of the reference dipole and that of the SteppIR Yagi (in the
>>>exact same conditions) determined our actual forward gain at a given
>>>frequency. We also performed front to rear tests. Front to rear is the
>>>response from the back of the antenna at it's least favorable point.
>>>Front to back, on the other hand, is simply the response of the
>>>antenna at exactly 180 degrees. In many cases this is not the worst
>>>case response from the back of the antenna. We feel that specifying
>>>front to rear measurement is much more useful to our customers. Also,
>>>keep in mind that front to back varies with the angle of arrival of
>>>the particular offending signal."
>>>
>>>-----
>>>
>>>Similar to the technique used by N0AX and K7LXC for the triband beam
>>>comparisons detailed in their book "HF Tribander Performance, Test
>>>Methods & Results, 2nd Edition".
>>>
>>>73
>>>Patrick
>>>W7TMT
>>>
>>>-----Original Message-----
>>>From: towertalk-bounces@contesting.com
>>>[mailto:towertalk-bounces@contesting.com] On Behalf Of Tom Rauch
>>>Sent: Wednesday, March 16, 2005 11:08 AM
>>>To: jimjarvis@ieee.org; towertalk@contesting.com
>>>Subject: Re: [TowerTalk] steppIR Gain--test method
>>>
>>>
>>>
>>>>If I recall correctly, Mike and Jim have their own range, with the
>>>>unit under test on one tower, and an HP spectrum analyser connected
>>>
>>>to
>>>
>>>
>>>>a dipole on another tower, many wavelengths distant. Tower heights
>>>
>>>
>>>>were on the order of 60'. A calibrated attenuator was used with the
>>>
>>>
>>>>SA.
>>>
>>>
>>>Well, the many wavelengths distant is already a problem.
>>>Measurements would be much more accurate just out of the near field. I
>>>don't know why anyone would measure an antenna in the null of the
>>>pattern, and when a horizontal HF antenna is at 60ft and the path is
>>>long there is a null along the horizon! Bad measurement protocol.
>>>
>>>Measurements would also be more accurate with a directional antenna at
>>>the receiver also, and a good meter instead of a spectrum analyzer.
>>>
>>>Measurements like this are always a problem. Look at the silly claims
>>>of the RAIbeam and that little two element hex quad thingy. It's
>>>commonplace that measurements made in test ranges that aren't really
>>>test ranges are a few dB off.
>>>
>>>
>>>
>>>>Theoretically, the steppIR could be a bit better than a canonical
>>>>yagi...in practice, it's hard to tell, because the differences are
>>>
>>>too
>>>
>>>
>>>>small to matter.
>>>
>>>
>>>Bingo....and probably too small to measure accurately also.
>>>
>>>73 Tom
>>>
>>>_______________________________________________
>>>
>>>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>>>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
>>>any questions and ask for Sherman, W2FLA.
>>>
>>>_______________________________________________
>>>TowerTalk mailing list
>>>TowerTalk@contesting.com
>>>http://lists.contesting.com/mailman/listinfo/towertalk
>>>
>>>
>>
>>
>>
>>_______________________________________________
>>
>>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
>>any questions and ask for Sherman, W2FLA.
>>
>>_______________________________________________
>>TowerTalk mailing list
>>TowerTalk@contesting.com
>>http://lists.contesting.com/mailman/listinfo/towertalk
>>
>>
>
>
>
>
>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|