[Elecraft] Re: MFJ259

[Don Wilhelm]

Jack,

How about putting a sharp tuned circuit between the detector output from the
bridge and the AD8307 - a single parallel tuned circuit with a variable cap
and a winding on a toroid can serve with high Q over a wide frequency range
(depending on the variable capacitor size) - not much different than tuning
the receiver.  An input link on the toroid can match the 50 output of the
bridge and a third link can match the input impedance of the AD8307.

Of course the detector need not be so exotic if a narrow band signal source
is used (like a QRPp transmitter), and that can simplify not only the
bridge, but the detector too since more power is available.

Ideas and alternatives abound - all with tradeoffs for accuracy, ease of
construction, ease of use.

Yes, the number of knobs grows, and likely limits use at the top of a tower,
but half wavelengths (or multiples) of coax are handy in that situtation and
allow the measurements to be made on the ground - ideally by a ground crew
member and the antenna tuning done by the person at the tower top.

73,
Don W3FPR

> -----Original Message-----

>
> Don:
>
> I've been  giving the subject a bit of thought as well and a noise
> bridge came to mind rather quickly. I have an old MFJ 202 noise bridge I
> bought 25 years ago.
>
> However, I believe a broad band detector such as the AD8307 will fail.
> The null condition is rather narrow frequency wise, and the noise
> generator detector output is broad, so the different in total power at n
> AD8307 detector between no null and perfect null will be tiny,
> particularly if the bridge is constructed to have good balance, which
> is, of course, the objective.
>
> This is a case where one needs a narrow band tuned detector, I believe.
> Or, a narrow band source.
>
> Jack K8ZOA
> www.cliftonlaboratories.com
>
>
> Don Wilhelm wrote:
> > Those who have both an Elecraft N-Gen and a KX1 have everything needed
> > except the bridge elements to construct and use a relatively
> compact version
> > of that Improved Noise Bridge described by John Grebenkemper in
> that August
> > 1989 QST article.
> >
> > The N-gen is the noise source and the KX1 can serve as the
> detector - listen
> > for the null.  Of course, it is easy to construct the noise
> generator, so
> > those with only a portable receiver like the KX1 can have an accurate
> > impedance meter once it is calibrated.
> >
> > Detectors other than a receiver can also be used - an AD8307 based power
> > meter should work just fine and fit into a small case.
> >
> > The 'disadvantage' of the noise bridge is there are two knobs
> to deal with,
> > but when a deep null is achieved one can read both the resistive and the
> > reactive component of the unknown impedance.
> >
> > 73,
> > Don W3FPR
> >
> >
> >> -----Original Message-----
> >>
> >
> >
> >> Dave G3YMC wrote:
> >>
> >>
> >>> I would also make a plug for a noise bridge. I built the one by KI6WX
> >>> which appeared in QST in August 1989 (available to download from the
> >>> ARRL site). If you have a suitable portable rig to couple to it does
> >>> most of what the fancy analysers can do at a fraction of the cost.
> >>>
> >> It should not be too difficult to design a fairly simple receiver
> >> drawing a
> >> small amount of current to be enclosed with a noise bridge to
> >> make an 'easy
> >> to carry' instrument. Top of head thinking.
> >>
> >> 73,
> >> Geoff
> >> GM4ESD
> >>
> >>
> >>
> > --
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