Page
253
No netwwk needed. Bahidth is about
150
kHz.
The antenna fw this
band
is normally cut of
114
wavelength,
with no
load~ng coil required. The bandwidth
is
about
200
kHz.
A
Typical
Tuning
Procedure
A whip antenna that is properly tuned on
75
meters will
have a high peak of receiver activity for about
25
kHz
around the antenna's resonant frequency. Turn on the
receiver and tune through the band to find the peak of
receiver activity for the
present setting of your antenna.
Then adjust the length of the whip in
114
inch increments
and retune the receiver until the peak of receiver activity is
centered around the frequency at which you normally
operate. The antenna can then be tuned as described in the
following steps. The receiver peaking may not be noticeable
on
bands other than
75
meters.
(
I
1.
Connect an SWR meter in series with the lead to
your antenna.
(
1
2.
Set the SWR meter
to
the "forward" position.
CAUTION: To avoid damage to the power transistors during
the antenna tuning procedure, be sure to observe the duty
cycle restrictions described in the "Operations" section of
this Manual. Better, use
low power output if you can obtain
adequate meter deflection.
(
)
3.
Depress the HI and TUNE buttons and adjust the
LEVEL control for a full-scale meter indication on
the SWR bridge.
(
4.
Switch the SWR meter to the "reverse" position.
Note the SWR reading.
(
1
5.
Switch the SWR meter to the "forward" position.
Then set the transmitter to higher and lower
frequencies, and repeat steps
5
and
6
at each
frequency, until you find the minimum SWR.
(
)
6.
Set
the transmitter to the desired operating
frequency. Then adjust the length of the antenna as
follows:
A. If the point of the lowest
SWR is lower than the
desired operating frequency, shorten the antenna as
described below.
B. If the point of lowest
SWR is higher than the
desired operating frequency, lengthen the antenna
as
described below.
C. Change the antenna length in
114''
increments
and repeat steps
2,
3,
and
4
at each new length
until the minimum
SWR is obtained. The SWR
should be about
1.2
or less at the desired
frequency. NOTE:
It
may be necessary to add a
capacitor at the
base
of the antenna, as described
previously, if you cannot get the
SWR down to
about
1.2.
NOISE SUPPRESSION
To obtain good noise suppression, you must suppress
electrical interference at its source so
it
does not reach the
input of the receiver. Once
it
has been radiated, noise cannot
be
suppressed by bypassing, etc.
Although all automobile ignition systems are similar, there
are numerous
differences- depending on make, model, year
of manufacture, etc. We are therefore unable to offer
specific advise on how to eliminate noise and interference
originating from these systems. However, generally accepted
noise suppression techniques are included here as a guide.
The Heath Company cannot accept responsibility for any
damage resulting from the use of this information.
It
is difficult to determine the source of various
types
of
noise, particularly when several items are contributing to the
noise. Follow the procedure outlined below to isolate and
identify the various items that may be producing the major
noise interference.
In most cases, one source of interference will mask others.
Consequently,
it
will be necessary to suppress the strongest
item first, and then continue with the other steps. Figure
2-9 (fold-out from Page
2521
shows
a
typical ignition system
and the suggested placement of noise suppression
components.
1.
Position the vehicle in an area that is free from other
man-made electrical interference such as power lines.
manufacturing processes, and particularly other
automobiles.
2.
With the Transceiver
on,
run the automobile at
medium speed. Then let up on the gas, turn the
ignition switch off and to the
accessory position, and
allow the vehicle to coast in
neutral. This may not be
possible on cars with automatic transmissions or
power steering. If all noise stops, the major source of
interference is from the ignition system.
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