On Tue, 2008-06-24 at 20:08 -1000, Ken Brown wrote:
> The Bird Thruline RF Directional Wattmeter model 43 accuracy is
> specified as +/- 5% of full scale reading.
Good points.
>
> That means that if you have a 500 watt element (model 500H) measuring
> the output Ten-Tec Hercules II amplifier, and the reading is at exactly
> full scale, the actual power could be anywhere from 475 watts to 525
> watts. It also means that if you rotate the element to read reflected
> power, and you see a reading of two and a half divisions (which is 25
> watts on the 500 watt scale) the actual reflected power is somewhere
> between 0 and 50 watts.
> That corresponds to a reflected power percentage between 0 and 50/475 =
> 10.5%, or expressed in terms of VSWR 1.0:1 to 1.95:1.
>
> Even if the reflected power indication is zero, 5% of full scale is 25
> watts,
> so the percent reflected power could be as high as 25/475 = 5.3% or an
> SWR of almost 1.6:1.
>
> You say you have an SWR of exactly 1.0:1? Yea, sure you do. You're going
> to trim your antenna because 1.5:1 SWR is too high? Go ahead, I'll just
> operate with it as it is.
>
> Using the smallest standard HF element, the 50H, a zero reading of
> reverse power could be as high as 0.05 * 50 = 2.5 watts. If the forward
> power using the 500H element was indicating exactly full scale, a zero
> reading with the 50H element for reverse power could be as much as
> 2.5/475 = 0.53%, expressed in VSWR almost 1.16:1.
Two problems. I have more confidence that the meter will indicate
something with some reflected power, but the meter indication won't go
to zero, providing Bird still gets good diodes with virtually no
detection threshold. Ham built meters with silicon, germanium, or
schottky diodes will have a threshold on the indicated power.
The other serious problem with the reverse power slug is its
directivity. That's when the load is truly matched, yet the reverse slug
is indicating some power. I think a Bird's slug is rated at 30 dB
directivity and that means the minimum reverse power is 0.1% of forward
power. 500 watts forward, then 1/2 watt reflected though the load is a
known perfect match. So you slip in a more sensitive slug and fry it.
>
> This will make it very difficult to determine the VSWR of one Bird 43
> Thruline meter, using another one cascaded to measure the reflected
> power from the second meter, since the insertion VSWR of the Bird 43 is
> rated at less than 1.05.
The insertion VSWR may be rated that low, but the lower the frequency
and the power, the tighter the coupling has to be and so the insertion
VSWR can't help but be higher. I was told it was more significant by a
ham (with BSEE) doing amplifier stage production tests at a military
contractor's facility. He was upset by the SWR seen by the first Bird
caused by the second Bird.
>
> DE N6KB
>
>
73, Jerry, K0CQ
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|