[ARDF] Attenuators for ARDF...

[Jay Hennigan]

On Sun, 17 Oct 2004, Matthew Robbins wrote:

> I've been planning to make several tape-measure beams both as gifts
> and to have some available for people who show up at an event with no
> gear of their own.
>
> I have several of the offset attenuators off Joe's website, but I've
> been trying to decide if there is a better option.  I've been
> considering building a step attenuator.  I am attaching a .jpg (that
> might not make it) of an 1/2 inch aluminum conduit box that I've been
> trying to use in the aft boom of the TMB (between the driver and
> reflector), sort of as a handle and a convenient place to hold the
> attenuator and controls.  The radio will go aft of the reflector to
> balance the CG.

Binary attachments to mailing lists are not a good idea.  Put the picture
on the web and include a link.

> Does anyone know how much attenuation is needed for ARDF?  Is the
> two-button arrangement on some imported gear two steps of attenuation
> or something else? What are the two levels of attenuation?  I know my
> Bryan Ackerly radio has 15db steps, and I usually use 5 or 6.  Closer
> than 24m it switches to more, but I don't usually need it at that
> point.  So 5 is 75db, and 6 is 90db.

90 dB should be plenty, but without an offset or modifying the receiver
you aren't going to get that much with a step attenuator in the real
world.  The radio will pick up signals through the case, the outside
of the co-ax, the headphone leads, etc. sufficient that the yagi and its
step attenuator are no longer the primary signal source, and thus you
have no directivity.  A typical commercial ham HT or receiver is nowhere
near well enough shielded to be used in this manner.  Try taking your
rig, completely disconnect the antenna, attach headphones, and get within
about 50 meters of a one-watt transmitter.  You'll hear it loud and clear.

If a cheap enough receiver can be obtained that is easily modified, try
reducing the IF gain and/or injection in steps within the receiver itself.

--
Jay Hennigan - CCIE #7880 - Network Administration - jay at west.net
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