[TenTec] Colliins SWR Bridge

[Phil Sussman]

Yes Jerry, I did reverse the two but the result was the same.
Note that I used RG-213 between the two and RG-58 on either side.
I tried different lengths of RG-213 from 12in to 45in without
much difference. Let best match always was the deepest by far.

Yes, it is possible to 'fool' the input when the output impedance
is way of of tolerance. However, I've used an MFJ antenna match to confirm
my findings at a given frequency. The MFJ I use has both SWR AND impedance
meters. I wasn't concerned with the forward power reading as 'peak' and
'average' and 'weighted' all offer different readings. However, the best
dip (eg. match with a tuner) was always consistent among devices, regardless of 
quality. Funny but true, eh?

73 de Phil - N8PS

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Quoting "Dr. Gerald N. Johnson" <geraldj at weather.net>:

> 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
> 
>