Stephen Parry wrote:
"Yes I think I do have alternator noise could you explain what I might
do?"

Often the first sign of diode failure is a whine in the loudspeaker of
your radio which varies in pitch with engine speed.

Alternators have no commutator but use silicon diodes to make d-c from
the a-c they generate. It`s usually 3-phase, using (6) diodes in (2)
groups of 3 each in a full-wave rectifier. These are located inside the
alternator. If any of the diodes fails, at least one of the 3 pahases
quits contributing to the alternator output. Maximum output is reduced.
Replace the diodes. The whine goes away and full output is restored to
the alternator when required.

Stephan also wrote:
"I think we might have been divided by our common language!"

What Americans call a hood, I believe, is what the English call a
bonnet. The effectiveness of a screened enclosure depends on good
connections between its pieces.

In a modern diesel truck there may be noise generating electrical
fuel-injection control, rotating belts which generate static discharge
noise, electrical relays and contactors, and other sources of radio
frequency noise.

Noise containment is one goal of the bonnet. The hinge which holds the
bonnet and the latch which locks it are often inadequate in r-f
continuity. Their electrical contact is not good enough.

Wheels rotate on stationary spindles and axles. Rotation can produce
charge which repeatedly arcs across wheel bearings or other points.
Spiral springs are made with a large diameter on one end to rest on a
stationary part of the wheel assembly. The spring tapers down to a small
diameter or point on its other end. This point makes contact with a
hammer-on cap over the nut which holds the wheel on its spindle. The cap
rotates with the wheel. So we have a conducting connection between the
wheel and the non-rotating parts of the vehicle. Only extreme noise
cases require these springs.

Stephen also questioned what I meant by perfect antenna cables. I once
repaired rafios and my experience is that most noisy automobile
receivers got that way from defective antennas and antenna cables.

The base of the antenna needs a good connection to the vehicle body,
which is the other half of the antenna. The vehicle body is the
connection for the cable shield. The shield needs to be continuous to
carry the signal rather than having the body carrying the signal where
it may be exposed to noise currents.

I wrote:
"Keep the antenna away from and shielded from sources of interference."

When one installs an antenna, it should be placed in a quiet receiving
location if possible. It should not be installed on the same side of a
screen as wires carrying interference. The antenna should be installed
on the outside of the truck cab where it has large exposure to the
signal.

When I said bonding, I did mean a clean earth return but I was not
thinking of battery current. I was thinking of r-f currents, signal and
noise.

Radio frequencies don`t penetrate deeply into metal but travel on the
surface. Large surface area is needed for low r-f impedance. Battery
current uses the entire cross section of the metal.

Wires slung under running boards and the chassis are not much used now.
Other noisy conductors may be nearby Broadcast ground waves are
vertically polarized. It makes sense to use a vertical whip for
reception, though almost any wire works.

Stephen`s portable likely has a loop antenna inside which makes it
somewhat bidirectional in azimuth. A vehicle mey be found in any
position. This makes an antenna which is nondirectional in azimuth
desirable.

Best regards, Richard Harrison, KB5WZI