[Amps] Intermittent Bird element

[Will Matney]

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 parallel 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 pc 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 at mail.ezwv.com>, Will Matney 
><craxd1 at 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)