[Boatanchors] Did you ever wonder why?

[Duane Fischer, W8DBF]

Upon looking at Chuck Dachis book, "Radios By Hallicrafters", this arguement
does not hold up. The following receivers all had an antenna trimmer: SX-73,
SX-88, SX-99, SX-100, SX-101, SX-110, SX-111, SX-129, SX-133, SX-140, H-13,
R-19, RC-10 and more. 	
	
Note that the antenna trimmer appeared only in the models with a crystal filter,
or the SX models. None of the entry level, lower priced models, had the antenna
trimmer. 	
	
The eleven SX models listed above: seven were double conversion and four were
single. 	
	
The feature was used on general coverage as well as Ham band only models. From
the lower priced SX-110 to the higher priced SX73 and SX-88. 			
	
Maybe the question should be: Why did only the models with a crystal filter have
an antenna trimmer? 	
	
Duane W8DBF	


	
 

----------
From: Philip Atchley <beaconeer@elite.net>
To: boatanchors@mailman.qth.net
Subject: [Boatanchors] Did you ever wonder why?
Date: Friday, April 09, 2004 12:54 AM

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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