[Boatanchors] Did you ever wonder why?

Wed, 14 Apr 2004 22:11:37 -0600

WHAT???  No HQ 180 in the collection?    HI HI

Ed Berbari wrote:

>Phil,
>
>I understand your lament about not having the antenna trimmer on many
>receivers, but I would point out that in my entire collection Hammarlunds
>(HQ 100,110, 120, 129, 140, 145, 150, 160, 170) that all of them have an
>antenna trimmer.  For the most part these were not low end receivers in
>their respective eras.
>
>Ed, W9EJB
>
>
>----- Original Message -----
>From: "Philip Atchley" <[email protected]>
>To: <[email protected]>
>Sent: Thursday, April 08, 2004 11:54 PM
>Subject: [Boatanchors] Did you ever wonder why?
>
>
>  
>
>>Hi All,
>>I was just sitting here listening to the Hallicrafters SX-71 and (as I
>>    
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>often
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>>do) musing over why engineers do things the way they do (as an electronics
>>technician I've worked with engineers in the past 8^)
>>
>>Yes, I know that bringing a new model (of anything) to market is actually
>>    
>>
>a
>  
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>>high wire balancing act of juggling present technology, ultimate
>>performance, desired features, cost of production, parts availability,
>>    
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>"real
>  
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>>estate" (room for components, heat dissipation etc), market demand and
>>ultimately resale price and profit ratios.
>>
>>However, there is one control that isn't a really high priced feature, nor
>>difficult to implement, that I feel is very important in any receiver.  It
>>has MORE OFTEN THAN NOT been left out by Hallicrafters, Hammarlund and
>>others.  Oddly enough, this item is often included on lower end products
>>like Heathkits, Knight Kits and others.  It will also be found on the
>>military R-390x receivers but NOT on the top ranked Hammarlund SP-600.
>>
>>What is this lowly control?
>>
>>It's the variable capacitor in the first selective stage of the receiver,
>>THE ANTENNA TRIMMER.  As many readers of this list realize, switching in
>>different antennas, each having different impedance's, reactance's etc can
>>drastically change the tuning of the input stage of the receiver.  It is
>>almost guaranteed that except at one specific frequency, any given antenna
>>will NOT be a perfect match to the receiver, and often at no point on the
>>dial if you're using random antennas.
>>
>>1.  For stronger signals this won't make any difference in what you hear,
>>just what the "S" meter reads.  But on a very weak signal it can make a
>>significant difference in whether you can copy the station or not.
>>
>>2.  For single conversion sets it can make quite a difference in overall
>>image rejection too!  It probably makes some difference in image response
>>    
>>
>on
>  
>
>>a double conversion set, but images aren't usually a big problem on those
>>sets.
>>
>>3.  I've found that on some sets, down in the tropical bands it can also
>>help reduce QRM from the broadcast band.  (Though, for this task I find
>>    
>>
>that
>  
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>>either a High Pass Filter with a cutoff above the broadcast band, OR what
>>    
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>I
>  
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>>use, a decent antenna tuner like one of the MFJ 300 Watt units having a
>>larger Higher Q coil than receive only tuners.  That's because the usual
>>"Tee" tuner makes a very effective High Pass filter.  I've found one of
>>    
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>the
>  
>
>>above antenna tuners can completely eliminate BCB QRM in the tropical
>>bands).
>>
>>4.  While costs are always a factor in equipment design, the cost of the
>>manufacture adding the antenna trimmer is actually very small, easily
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>offset
>  
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>>by the additional performance that could have been realized.
>>
>>5.  IN FACT, this SX-71 has (to my mind) a totally useless tone control!
>>The receiver already has so much selectivity (even in the normal position
>>    
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>of
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>>the bandwidth switch) that audio is just a little muffled and has no real
>>highs to filter out with the "high cut" tone control.  That control is
>>    
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>left
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>>at the full treble position all the time.  An antenna trimmer would have
>>been a better use of panel space and the small variable capacitor probably
>>only pennies more than the tone potentiometer.
>>
>>So, did you ever wonder?
>>
>>73 from the "Beaconeers Lair".
>>Phil, KO6BB
>>
>>DX begins at the noise floor!
>>Merced, Central California
>>37.18N  120.29W  CM97sh
>>
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>>
>>    
>>
>
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>$$ For vintage radio info, see the HCI web site $$      
>http://www.w9wze.org    
>
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