Focal Length Analogy

Focal Length Analogy

Focal Length Analogy

Focal Length Analogy

700 Hz filter photo.
Credit N1JR

Un­less you have a pan­a­dapt­er, you can’t work sta­tions that you don’t hear.  This is why sav­vy op­er­a­tors sav­vy op­er­a­tors pre­fer a 650 to 750 Hz pass­band when con­di­tions per­mit.  And not just for ca­su­al CW band scan­ning.  It’s al­so per­fect for hunt­ing dx, and even scan & pounce con­test­ing.

For these op­er­a­tors, the K3’s de­fault (norm) 400 Hz pass­band set­ting is much too nar­row for nor­mal use; kind of like view­ing the world thru a drink­ing straw … while rid­ing a bi­cy­cle!

“To use an analo­gy from my past in­ter­ests in pho­tog­ra­phy, this fil­ter is like look­ing through a wide angle 35 mm lens in­stead of the 200 mm tel­e­pho­to.”  -- N1LQ

The slide­show below ex­pands on this “drink­ing straw” analo­gy.  It lik­ens fil­ter band­width to the focal length of a cam­er­a lens.  (Or more pre­cise­ly, to the re­sult­ing an­gu­lar field of view for a giv­en fo­cal length.)  The re­sult­ing im­ag­es pro­vide a vi­su­al com­par­i­son which may be in­tu­itive­ly eas­i­er to grasp than com­par­ing a ta­ble of num­bers (i.e. fil­ter band­widths), or com­par­ing in­di­vid­u­al pass­band roll-off plots.

Even though the im­ag­es are quite con­vinc­ing, it’s ac­tu­al­ly the usage sug­ges­tions – in the cap­tions – that pro­vide the most com­pel­ling case for 700 Hz.  (In or­der to see the cap­tions, you may need to:  scroll down fur­ther; or zoom out; or re­size your brow­ser win­dow; or re­ori­ent your mo­bile de­vice from land­scape to por­trait).


 Slide­show …

A Visual Comparison of 8-Pole CW Filters
Usage Suggestions Pertain to “Open” HF-Band Conditions
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Note that …

  1. The three nar­row­est fil­ters are clus­tered fair­ly close to­geth­er.  Spe­cif­i­cal­ly, the ra­tios of the widths of ad­ja­cent fil­ters at -6 dB, are:
    1. 118% between the 250 and 400
    2. 126% between the 400 and 500
    3. 143% between the 500 and 700
    4. 135% between the 700 and 1,000
  2. The 700 Hz fills a “siz­a­ble” (193%) gap be­tween the 500 and 1,000 Hz.
  3. The pro­por­tions of the image used for this pres­en­ta­tion were not al­tered to bol­ster our case for 700 Hz.
Image courtesy of www.spacetelescope.org
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Updated 1047 UTC Saturday 3 MAY 2014
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