[Boatanchors] Did you ever wonder why?

[jackiv@juno.com]

Phillip, that is why I installed an antenna cap in the SX-25 that I have
(bought new in oakland ca, 1945) I opened up all of the ant trimmers and
let the front panel mounted cap do the job, really works fb!  i labelled
it with transfer letters, looks original too.
old jack

On Fri, 9 Apr 2004 04:54:17 -0000 "Philip Atchley" <beaconeer@elite.net>
writes:
> Hi All,
> I was just sitting here listening to the Hallicrafters SX-71 and (as 
> I often
> 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
> high wire balancing act of juggling present technology, ultimate
> performance, desired features, cost of production, parts 
> availability, "real
> 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
> either a High Pass Filter with a cutoff above the broadcast band, OR 
> what I
> 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 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 offset
> 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 of
> 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 left
> 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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