Richard Clark wrote:
On Sat, 2 Feb 2008 08:45:27 +0000, Ian White GM3SEK
wrote:

As with the AN-URM120, the Bird design adjusts the coupling so that all
detector diodes in all the inserts are operating at the same RF levels,
so they can all share the same meter scales. The difference is that
instead of physically moving the same insert inward or outward to adjust
the coupling, Bird do it by selling us more slugs :-)

Hi All (Congratulations Ralph),

There are more differences than that. The Bird suffers more from
parts erosion than the URM120 as the URM has bigger elements. With
the bigger elements, the geometries are held to a tighter precision.

It is the coupling link that moves in the URM, not the element. The
link rides on a cam that is stepped for the 4 different power ranges
used. If you remove the knob from the slug, and then the cover plate,
you have access to an adjustment screw that provides the fine control
over the depth of penetration. The smaller PDF at the boat anchors
web page is the more useful of the two offered.

I have calibrated a pile of both the Birds and the URMs, and the URMs
always required less maintenance, and rarely needed adjustment. The
Birds, on the other hand, always arrived out of calibration.

One source of error in the VHF/UHF region was that the rubber gasket
inside the N connector (and for that matter, for any N connector) will
accumulate thread debris. It should be used as a visual correlative
to the amount of similar debris bridging the Teflon insulator. This
debris can whack out these meters and degrade connections.


Thanks for the clarification, Richard.


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek