[HBR] The long, SLOW HBR project

[Walt Hutchens]

Donald Haworth posted:

> Thanks for the update, Walt.....Kinda reminds me of an old Republic Pictures
> serial...."next week....Chapter ten..."The Deadly 10 Meter
> Drift..........",  BTW, I am a HUGH fan of Republic Pictures......

Half hour, black and white ... matinee admission only a quarter.   Ran
before the feature?   I forget -- that was a LONG time ago!

> Anyway, how about some pics of your radio?  Would love to see it!  I feel
> like its an old friend, after following its' progress.

I will try to get pics up this week.

> -I have a question for you-or anyone else, for that matter.  Given a typical
> rf amp-6ba6-what would be the best l/c ratio for the (input ) tuned
> circuit?  For example, an inductance of -say 280uh with a capacitance of
> 365...versus a cap 0f 150pf and a much higher L of 650.....both will
> resonate at the wanted frequency...but which combination will give the best
> transfer of energy to the tube?

Almost no energy is transferred to the tube.   A bit is dissipated in the
grid circuit including the tank circuit itself.

The grid VOLTAGE resulting from a specific signal depends on the Q of the
grid tank circuit and with modern materials and techniques, it's pretty easy
to get as much as is useful.   The higher L tank can deliver higher voltage,
but that's only needed in specialized cases.

The problem in HF communications receivers is rarely one of having enough
gain to hear a weak signal.   It is instead being able to pick that signal
out of the noise.  

1. Atmospheric noise -- worst on the low bands, most of it is gone by the
time you get to 10M.

2. Other signals of all sorts.   This includes all the trash generated in a
modern household by computers and computer chips in household devices and
carried around the neighborhood on power and phone lines.

3. Noise generated in the radio itself.   This includes both the noise
caused by electrons moving and noise resulting from signals (including
unwanted signals) and external noise combining because the radio isn't
perfectly linear.  

Assuming you have a decent outside antenna with a proper feed line so the
in-house noise sources aren't a big deal then up through 40M the problem for
weak signals and uncrowded band conditions will be atmospheric noise.   The
best you can do is have the passband no wider than the bandwidth of the
desired signal -- say 6 kcs for AM, 2.8 kcs or so for SSB, and as little as
a few tens of CPS for CW.

When these bands are crowded with strong signals (say evenings on 80 and 40)
then the unwanted signals replace atmospheric noise as the main issue.
Again, however, an appropriately narrow passband is the best you can do.

Now things get more complicated.   The simple picture is you can cut your
passband down (with a filter -- sharp IFs, a crystal filter, etc.) at any
convenient stage.   HOWEVER the lower bands generally supply so many very
strong signals -- I once lived where I could measure 0.5 VOLTS at the
connector on my antenna! -- that when the gain you need to copy (say) 1 uV
is applied, one or more stages in your receiver can be overloaded to the
point of generating new unwanted signals.

(If you have to amplify the 1 uV signal to 1 V at the speaker, that's a
voltage gain of a million.   Applying the same gain to the broadband signal
from the antenna mentioned above would deliver 500,000 volts at the speaker
terminals ... talk about overload!)

Think of how an overdriven linear amp sounds.   That's the wanted signal --
the guy's voice -- plus a bunch of trash generated in his flat-topping
amplifier.   If you want to hear signals clearly, your receiver must be a
series of properly adjusted linear amplifiers, all the way from the antenna
jack to the speaker or headphones!

In the receiver situation, however, the overdriving signal can be anything
that can get to the overloaded stage.   A poor quality receiver might be
hitting the 1st IF amp with a 100 kcs bite of 40M at night.   If you are
operating such a receiver in the upper part of the band here in Virginia,
World Christian Radio with (is it?) 100 kw in the Nashville area beaming
toward Europe is part of what is reaching that 1st IF and that stage IS
being overloaded.   Similar conditions can occur on 80M due to the great
number of phone stations in the upper part of the band and also on 160 if
you have nearby BC stations in the upper end of that band.

(You might think that AGC would help with this, but if the unwanted signals
are filtered out anywhere before the AGC detector, then World Christian
Radio may be invisible to the AGC.   Only if the IN-passband signals are
strong enough to kick up the AGC does it help at all with the
OUT-of-passband ones.   This is where manual gain (or AGC threshold)
controls come in handy.

Overloaded stages -- often the 1st IF but can also be the first or second
mixer or even the RF stage -- can combine two signals that you think are out
of passband to create new IN passband signals that will ride right along
with the station you're trying to hear.   Once this happens no filter can
separate the trash from the wanted signal.

For example strong stations at 3820 and 3840 that aren't filtered out before
they get so strong that they overload a stage will give you trash signals at
3800 and 3860.   If you are listening at either frequency, you will have ON
FREQUENCY interference.

The ideal way to beat this would be to put the sharp filter at the antenna
but that can't be done since the combination of 'very sharp' and 'tunable'
is not practical.  

The practical solution has four parts:

1. Don't couple more signal than needed from the antenna.   Once you have
coupled enough signal to the first stage (usually the RF amp) to hide its
internal noise, you don't want any more.   Some receivers fix the coupling
at a fairly low level, others provide variable coupling or an attenuator of
some kind.   

Coupling the maximum signal you possibly can from the antenna to the first
receiver stage would only be a good idea with a poor quality receiver -- one
that doesn't have enough gain and thus wouldn't hear much of anything,
otherwise.   (Some of those designs with a 6SA7 or 6BE6 mixer as the first
stage might need all the sig they can get.)

2.  Until you get past the sharp filter don't use more amplification than
needed to hide the noise of the next stage.   Use too much amplification and
that next stage may be overdriven by unwanted signals that you haven't yet
filtered out.   That's how 3820 + 3840 -> 3800 & 3860.

Ever notice that top quality receivers are easier to listen to under
thunderstorm conditions?   If your front end isn't linear, then 3810 signal
+ crash energy at 3820 -> noise at 3800 and 3830.   3820 signal + crash
energy at 3840 -> noise at 3800 and 3860 ... and so on, for the entire band
that can get to the overdriven stage.   A cheap receiver is getting more of
the thunderstorm energy on the frequency you're trying to copy via this
'reciprocal mixing' than a quality one.

(Worse yet, crash energy at 3820 and 3840 yields crash signals at 3800 and
3860 ... and this is true for EVERY pair of frequencies ... 3805 and 3810,
3806 and 3812 ... t'storm noise conditions are an excellent test of front
end bandwidth and linearity!)

Some very good receivers with low noise 1st mixers use no RF amplifier at
all.   (The SS 1R with its beam tube mixer is one example but some triode
mixers can be used in the same way.)   The noise of a quiet mixer will be
much below the atmospheric noise on all the HF bands.   Other receivers use
an RF stage with a gain of less than one:  Its only job is to serve as an
AGC controlled attenuator to help protect the mixer from overload.

Part of 'not too much amplification' is a combination of AGC and manual gain
control that will let you reduce the amplification when you're copying a
moderate to strong signal in the presence of extreme noise or unwanted
signals.   

3.  Put as much selectivity as possible, as close to the antenna as
possible.   It's easier to build a good receiver with single conversion and
a crystal filter at 1700 kcs in the mixer plate circuit than to get equal
performance with dual conversion and sharp filtering in the 2nd IF (100 kcs
or 85 kcs ...) because the dual conversion set has at least one and often
two more stages ahead of the filtering.

AND use the highest Q coils you can in the RF and Mixer grid circuits.
High Q here may only cut the bandwidth seen at the mixer from 500 kcs to 150
kcs, but every bit helps!

Here we come to the answer to the original question:  Other things being
equal the 150 pf / 650 uH combination will be the better choice because it's
likely to do a better job limiting the bandwidth at the RF amplifier.   That
is IF you use loose enough coupling to the antenna that you get only enough
signal to be sure of overcoming receiver noise in the RF amp.

Just as for a transmitter, the antenna coil in a receiver is loaded by the
antenna.   Take a coil with a Q of 200, couple it tightly to a resonant
antenna, and you're prob'ly getting an effective Q around 10-30.   Yes, with
the much greater bandwidth that the lower Q implies.   In receivers that do
this, the details of the antenna coil hardly matter -- you could about wind
it with nichrome wire on a wet toilet paper tube and get equal performance.

To minimize this effect disconnect the antenna and adjust your receiver so
you can hear the internal noise of the RF amp.  (Confirm that the noise you
hear comes from the RF amp by detuning the mixer coil; if the receiver
doesn't go quiet you have another problem!)  Then connect the signal source
and set the antenna coupling to the point where the weakest signals just
hide the receiver noise.   This is most easily done with a good signal
generator but with patience you can do okay with an antenna.

On HBR-type designs this is easy to do by tweaking the spacing between the
antenna winding and the grid winding on the antenna coil.

4.  Particularly in the RF, mixer, and 1st IF sockets use tubes that are
good at handling large signals.   The 6EH7 is about the best RF/IF tube out
there, although the 6BA6 is pretty good.   However this isn't as simple as
swapping a better tube for a worse one:  All the AGC controlled stages need
to have similar AGC characteristics, otherwise one of them will be cut off
while the others are still amplifying and this can lead to serious
distortion.   Generally speaking this means that all AGC controlled tubes --
usually the RF and IF amps -- should be the same.

Walt 
KJ4KV