Dave and Hardy,
These problems don't seem to occur as often as they did in past decades. I
am not sure why, but maybe the power distribution system in cars has
gotten better. In any event, Hardy is mostly correct with what he says,
but a few clarifications are in order.
Yes, the sound being heard is the variable AC frequency of the guy's
alternator as he drives at different speeds and with varying electrical
loads. The simplest cure, in most cases, is to take the power feed of the
radio directly to the car battery, or as close as possible. It does do a
wonderful job as a filter. Note: If the car has a power distribution
block located a short distance from the battery, make the connection
there. That's safer than going straight to the car battery itself. Always
locate a fuse right near the connection so that in the event the "hot"
lead to the radio gets pinched or cut and shorts to ground, the fuse can
blow and prevent a fire.
Now, about filtering. This is one situation in which you need
differential mode filtering, not common mode. Years ago, you could buy
simple kits at Radio Snack and even some amateur radio dealers that
consisted of an iron core inductor (choke), a fairly large electrolytic
cap, and some lugs, wire, and so forth. These kits worked well for
(legal) CBs, tape players, and low power (10 watt or less) mobile ham
radios, as the inductor was not rated for enough current to supply a 25 or
50 watt radio. The kits were useful because most of these add-on devices
were usually connected to power feeds under the dash board, frequently at
the fuse block down there. Those power leads were not well decoupled
because of the lengths of wires between the battery and fuse block, not to
mention the various noisy loads, such as blower motors.
It is not always easy to run a pair of 12 or 10 gauge wires from the rig
to the battery, but it is the most effective way to avoid problems such as
Dave's buddy has. Look for unused plugged holes in the firewall or around
the door frame areas. Beware of drilling holes, because many vehicles
have a double firewall; you might have a clear spot on one side but end up
drilling right into some component or wiring on the other side. Plugged
holes (with a rubber or plastic insert) go all the way through and are
meant as access ways for options not used on the particular vehicle.
A final note: Dave, if your buddy has already run his power leads to the
vehicle battery and he still has the alternator whine, that could be a
sign that either his battery is not in the best shape or that the vehicle
has electrical distribution and/or charging problems that need to be
checked out. It can be normal to hear some whine when a vehicle is idling
(as at a stop light), the electrical loads are high (lights on, A/C and
blower on, etc.), and the rig is running full power (50 watts or
whatever). The whine should go away if the engine speed is increased, the
rig reduced in power level, or some other electrical loads reduced.
However, the level of whine on a rig that is direct connected should not
be so loud as to have level of whine noise equal to mic audio level. I
have only heard those levels of whine when the radio was plugged into a
cigarette lighter socket or tied into under dash wiring on a random basis.
Good luck.
73, Dale
WA9ENA
"Hardy Landskov" <n7rt@cox.net>
Sent by: rfi-bounces@contesting.com
10/29/2008 10:48 AM
To
"David Jordan" <wa3gin@comcast.net>, <rfi@contesting.com>
cc
Subject
Re: [RFI] Kenwood TM-D700
Not odd at all. The alternator is varying the 12 VDC and frequency
modulating the transmitter. Look at the 12VDC line with a scope and you
will
see what's happening. I believe a big common mode choke is in order or try
running the TM-D700 power cables directly to the battery as the car
battery
looks like a 1 Farad capacitor. Sounds like the TM-D700 is not decoupled
from the 12 volts very well.
GL N7RT
----- Original Message -----
From: "David Jordan" <wa3gin@comcast.net>
To: <rfi@contesting.com>
Sent: Wednesday, October 29, 2008 8:00 AM
Subject: [RFI] Kenwood TM-D700
> Hi folks,
>
>
>
> This one is a bit odd. My buddy has a Ford F-150 and installed in the
> truck
> is his TM-D700 dual band VHD/UHF radio. We chat during our drive into
work
> in the mornings.
>
> Almost without fail when he is transmitting at least once during the 30
> min.
> drive I will hear what sounds like a siren on his audio Whrrrrrrrrrrr
> starting at a high pitch and slowly going down in audio tone freq. He
has
> grounded the radio, etc., yet the weird noise continues to occur. Last
> week
> I installed a TM-D700 in my Ford Sport Trac and guess what - now he says
I
> have the same odd occurrence of the erratic whine syndrome.
>
>
>
> My radio is also grounded extensively as I run 1KW on 75 and 40m.
>
>
>
> Thoughts and recommendations welcome.
>
>
>
> 73,
>
> Dave
>
> Wa3gin
>
> _______________________________________________
> RFI mailing list
> RFI@contesting.com
> http://lists.contesting.com/mailman/listinfo/rfi
_______________________________________________
RFI mailing list
RFI@contesting.com
http://lists.contesting.com/mailman/listinfo/rfi
_______________________________________________
RFI mailing list
RFI@contesting.com
http://lists.contesting.com/mailman/listinfo/rfi
|