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 > >> > >> > >> > > -- > > No virus found in this outgoing message. > > Checked by AVG Free Edition. > > Version: 7.5.446 / Virus Database: 268.18.11/721 - Release > Date: 3/13/2007 > > 4:51 PM > > > > _______________________________________________ > > Elecraft mailing list > > Post to: [email protected] > > You must be a subscriber to post to the list. > > Subscriber Info (Addr. Change, sub, unsub etc.): > > http://mailman.qth.net/mailman/listinfo/elecraft > > > > Help: http://mailman.qth.net/subscribers.htm > > Elecraft web page: http://www.elecraft.com > > > > > _______________________________________________ > Elecraft mailing list > Post to: [email protected] > You must be a subscriber to post to the list. > Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/subscribers.htm > Elecraft web page: http://www.elecraft.com > -- > No virus found in this incoming message. > Checked by AVG Free Edition. > Version: 7.5.446 / Virus Database: 268.18.11/721 - Release Date: > 3/13/2007 4:51 PM > -- No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.446 / Virus Database: 268.18.11/721 - Release Date: 3/13/2007 4:51 PM _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
