Topband: Preamp shielding

[Tom Rauch]

> preamp circuit and locating it at the antenna. It would be more convenient
> to place it in the shack, but doing so will slightly degrade the system
> noise figure, due to the feedline loss ahead of the preamp. The feedline
is
> RG-58C/U, about 125 feet in length.

Feedline loss, and with that system noise degradation, should be
immeasurable with the feedline at either end of  the line since line loss is
less than .7dB compared to a noise figure that certainly must be well over
5dB for the rest of the system. If you don't have good enough noise figure
with the amp at the house, you won't notice a change moving it to the
antenna. Subtle changes in amplifier design and matching transformer design
would have a much larger effect than 125 feet of transmission line, assuming
you make proper connections.

> A 7-pole Chebyshev high pass filter will be placed in the circuit, before
> the preamp.

Watch the loss there, which mostly relates to inductor Q and component and
design tolerances.
Are you sure you need such a large filter before the amp??

>My question is in regard to the shielding of the preamp. I have
> an aluminum Bud box, and a painted steel box by Hammond. Either can be
used
> for the enclosure. What is the most effective box material from a
shielding
> standpoint? Does it matter?

Shielding, once it is a several skin depths thick, can be any material.
Either box would be the same at HF. It is the connection and length of
"floating" seams and how you bring cables into the box that generally
dominates effectiveness. A good rule is to have less than 2ft of shield seam
length per 5MHz. Shielding channel 2 TV would require seam ground points
less than 2 inches apart, and of course at each corner. You can see by that
unless you are shielding VHF energy, it isn't very critical what you do.

On low frequencies like lower HF, you don't even need a shield with good
board layouts if the amplifier and filter mount over and close to
groundplane or have a groundplane on the PC layout! The exception is if you
mount the amplifier very close to a strong source of noise (like a switching
supply with strong HF harmonics).

73 Tom