On Sun, 2009-12-13 at 08:42 -0500, Phil Sussman wrote:
> >From my experience, an SWR bridge does indeed provide approximation; however,
> the science of a simple device mirrors the more complex. In other words,
> rocket science is not needed. For years I used a cheap CB matchbox on 10
> meters in series with a Bird 43 wattmeter, just for kicks.
>
> I discovered, to my surprise, the readings of SWR were identical. Minimum was
> minimum no matter the device. Sure you can create higher degrees of precision
> and limit the amount of interaction, but at the end of the day all you need
> is
> a good dip when tuning -- another 1 or 2 db isn't significant.
The problems are that without good directivity, that null may not
actually be a match. Poor directivity couples forward power to the
reverse power indicator and while there is always a null indication
possible, its not necessarily at true nulled reverse power. Say if the
directivity is only 10 dB that means the forward power is coupled at 10
dB down and then you can have a situation in tuning where the reverse
power is 10 dB down from the load and out of phase from the leaked
forward power to get the null, but the reverse power is truly 10% of the
forward power or a significant SWR of 1.92:1. If the directivity is 20
dB the situation is better with the SWR of 1.22:1. Collins designed for
30 dB directivity or better. Which means the null shows a SWR of 1.065
or better.
Most rigs work fine into a 1.22 SWR, but not necessarily at 1.92 SWR.
You might find different results if you reverse the order of CB meter
and the Bird.
>
> 73 de Phil - N8PS
>
73, Jerry, K0CQ
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