Seriously, I thought about making a line of meters similar to the old Drake and
actually made a couple of prototypes. To be very honest, those can be just as
accurate as any Bird I've seen. Bird is about like buying the name Cadillac
when other cars get you there just as good. That's my opinion of course.
Actually, a transformer coupled sensor is more wide banded than the strip line
type like a Bird. Using a toroidal tranny with about 40-50 turns C.T. on it and
the RF line running through it will go a good bit wide banded. The frequency is
selected with the toroids material like 43, 61 or other. The Bird uses a super
short strip line and the reason behind that is the amount of power range it
has. The lines length has to be shorter as the power goes up or the works can
become overloaded. Or was the way I read about strip line meters. But, for a
narrower bandspread, a stripline sensor is a good meter. On a Bird, the
stripline is formed by the back end of the element being paral
lel with the troughline section enclosed in the "T". By rotating it 180 deg.
gives the fwd. and rev. functions. In other meters, two seperate strips are
used with the rectifiers connected at opposite ends. These strips are placed on
either side of the trough line strip. The fwd. and rev. is then simply selected
by a switch. One good thing about this setup is an SWR meter can be made which
works at the flick of a switch. There's another variance of this where the
strip lines are only used for the SWR meter and the power is measured by
directly measuring the RF voltage off the trough line strip. I actually like
these types better and was what I made in one prototype. I noticed, I think
QRO, used a transformer coupled type for the internal power meter on their
amps. It was mounted at the back end near the antenna relay. One thing here,
the transformer type don't take up as much room as a strip line type. For up to
about 2 KW, the line length is close to 4 inches long plus the p
c board length (say 2-1/2 x 5) where the transformer might take up a space of
1-1/2 - 2 inches max for the sensor. One in an amp has to be wide banded enough
to be accurate over the full bandwidth too.
To calibrate, you can accurately measure the RF voltage off the line with a
scope, calculate the results, then adjust the meters readout accordingly.
That's about the most accurate way I know without having some expensive
equipment. That should get you around 3% or more accuracy I would think, and is
the way mentioned in several books on the subject. To be honest, I'm leary of
adjusting watt meters by having two meters in line and adjusting one from the
other. I've seen meters set this way, and after taking one out of line, get a
different reading with the same amount of power applied. I would think it was
due to the line lengths where a meter and a 3' jumper was taken in and out?
On meters of the same type, let's say we had two meters with +/- 5% accuracy.
What if one meter was at - 5% and the other was at +5%? The difference between
each would be an error of 10% which is a good amount. That's why I think
there's some errors showing between two of the same meter. On the manufacturing
line, they set each meter to the specs and the allowable tolerance they give. A
difference of 10% between two would be easily done this way I would think if
the tech doing it didn't try to set them on the money. I wouldn't doubt if this
isn't the cause between S-meter readings too. 10% dont sound like a lot but on
1 KW it's 100 watts on the difference between two. Also, what if you used a
meter 5% off, and calibrated another. Then, used the newly calibrated one to
calibrate another and the tech sets it say -5% again. That's a -10% error and
can grow each time it's done. I have actually seen this done more than once. A
guy sent off a meter to get calibrated by another shop
. He gets it back and calibrates someone elses meter with it. When I got the
meter off that guy, it had I think an error of about 9% if I recall. I then
sent it back to the manufacturer to have it calibrated and that's what they
told me it was off by. Then, we cant forget those golden screwdrivers out there
on used equipment.
We all could rattle on and on about this, but I just wanted to mention about
accuarcy error and the two types of commonly used watt meters and their
differences.
Best,
Will
*********** REPLY SEPARATOR ***********
On 4/17/05 at 3:10 PM Paul Higginson wrote:
>In message <200504171000150190.07ABAF13@mail.ezwv.com>, Will Matney
><craxd1@ezwv.com> writes
>>Paul,
>>
>>Ohhh, Is that the way you all fudge those readings?
>>
>>LOLOL...Just Kiddin
>>
>>Couldn't resist
>>
>>Best,
>>
>>Craxd
>>
>
>I could make a batch of special amateur spec units , bit like European
>car speedometers :-)))
>
>Regards Paul
>--
>73 de Paul GW8IZR
>(IO73TI Anglesey)
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