[AMPS] Re: TopBand: Followup Beverage Box

Tue, 23 Sep 1997 06:01:22 +0000

> From:          philk5pc@connect.net (Phil Clements)
> Date:          Sat, 20 Sep 97 10:03:00 +0000

Hi Phil,

Thanks for the info.

> Hi Tom,The ZJ boxes I have are not broad band. They are either single-band, or
> 1.8-4.0 mhz.
> The tuned circuits in the pre amp are very good, as a 50kw AM station 4 miles
> away
> cannot be heard at all on my EWE.

IMHO, it's better to use a IMD-immune device rather than filter 
the input/ output of a device that by itself offers marginal 
performance. Remember it's usually not the level of ONE signal 
that is the problem, it's the total energy of many signals combined 
that causes most overload problems. The larger the number of signals, 
the less level that can be tolerated from any individual signal 
before the amplifier "falls apart". Even if BC stations are 
eliminated, the accumulation of  signals in any two or three 
MHz wide spectrum can cause problems. 

Add a good filter if necessary, but its best to start with a good 
device. A less than a dollar CATV type transistor will produce a much 
better noise figure and overload long after most receivers. 

The cheaper MMIC's are not wide dynamic range low distortion 
devices, nor are they rugged.

> > A single LT1001 or cheap 2N5109 is much more immune to damage, has
> > better IMD performance, and a better noise figure. MMIC's are simple
> > to use, but that does not make them good devices.
> 
> I have no idea why they chose these devices in such a hostile environment.There
> must be some advantages.

The only advantage I know of is the MMIC has "designed-in" bias and 
feedback circuitry. In other words, it eliminates a few external 
resistors and capacitors and does not require any design effort. The 
disadvantage is cheaper MMIC's have poor IMD performance and 
are particularly poor with  even-order harmonic distortion. You are 
also stuck with the gain and bias settings they designed in the 
device.

Rolling a pre-amp out of discrete components allows the 
designer to add negative feedback to reduce gain, and that 
improves dynamic range and noise figure! Solid State Design for the 
Radio Amateur covers this, as did Ulich Rhode in some of his RF 
Design articles in the late 70's and early 80's.

> In my case, the utilities
> are all underground and even using 9913 coax, there was 60hz pickup in the
> shack mounted pre amp which was amplified along with the signals. By putting
> the pre amp at the EWE and adding a line isolator at the ZJ box output to keep
> the
> 60hz out of the ZJ box, all noise was eliminated.

Other than with pre-amp construction or design shortfalls, 60 
Hz hum should never modulate the signals. That's always a sign of 
poor internal supply line filtering, or a poorly designed input/ 
output system that has unwanted LF AC coupling into a non-linear 
device. It could be caused by core saturation in the input/ 
output transformers too. If AC ground loop currents pass through 
the windings and cause core saturation (or even a permeability shift) 
the AC will cross-modulate  signals inside the transformers 
themselves. MCL transformers saturate very easily with any winding 
current. If the input is dc or LF ac coupled to the primary, or if 
the same is true at the secondary, I'd expect trouble with induced 
hum in some systems.

Any modulation of signals by ac hum means something is NON-linear. 
Non-linear is not good in DX receiving systems, and the line isolator 
is a band-aid for the real problem...the real problem being a 
non-linear device somewhere in the system.

> > Even at the quietest location, a Beverages noise will
> > greatly exceed cable losses. Install the cable properly, using
> > reasonably good feedline (CATV drop line is excellent) and GOOD
> > connections at the connectors, and the preamp might as well be right
> > next to the receiver
> 
> See above....not in ALL cases!

You're right Phil. I should have included the qualification the 
system must not have non-linearity. If the Beverage receive system is 
properly designed and installed and the feedline has less than five 
or ten dB loss, even the quietest location would never require a 
remote amplifier.

73, Tom W8JI
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