[Boatanchors] Did you ever wonder why?

[Ed Berbari]

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" <beaconeer@elite.net>
To: <boatanchors@mailman.qth.net>
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
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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