[Amps] Powering 400w mobile amplifier...

[jeff millar]

Bill objected to my advice but didn't say what was bad about the multiple
ground approach.  And his suggestion to use RF current carrying capacitors
at the 400W power level is questionable.

All mobile radios I know of use the multiple ground technique.

- Icom, Kenwood, etc all recommend two wires direct to the battery.
- Each vendor supplies a cable with fuses in both leads.
- Each vendor supplies a metal mounting bracket to connect the radio to the
vehicle...
- Vendors don't recommend one way or the other about connecting the radio to
frame ground.  It works either way.
- All the VHF and UHF mobile whip antennas I know of have a coax directly
connected to the vehicle body or frame.

Motorola and GE use the same installation techniques for their commercial
service radios.

What kind of mobile whip antenna has an ungrounded shield or uses a dc
blocking cap to ground?...none that I know of.  Besides, I don't think a 0.1
uFd cap would last very long with 400W of VHF energy flowing through it.

I recommended grounding the amp to the frame because there's so much supply
current running around.  A 40W moble can return its ground current through
the antenna coax without even noticing.  I know because the fuse blew in the
return lead of one of my radios and didn't notice for a year or so.  But 50
Amps through the coax to the antenna ground might cause heating and paint
bubbling the the antenna mount point.

Bill's "no extra DC ground" approach is simple and clean at DC, but doesn't
work at RF.  The RF output coax need a solid connection to the vehicle to
make a ground plane for the whip.  Grounding the Amp body isn't essential,
but reduces a worry about the DC on the coax shield.

Lastly, there's another, unmentioned, ground return, namely, through the
input coax back to the transceiver.  I don't know of any way to easily block
the DC on that line.  And it's not a good idea to return 50 Amps through
your transceiver coax if the amplifier return line fuse blows.

jeff, wa1hco

----- Original Message ----- 
From: "Bill Turner" <wrt at dslextreme.com>
To: "jeff millar" <wa1hco at adelphia.net>
Cc: <gerry at customsoftware.tv>; <amps at contesting.com>
Sent: Tuesday, July 29, 2003 7:35 PM
Subject: Re: [Amps] Powering 400w mobile amplifier...


On Mon, 28 Jul 2003 23:14:14 -0400, "jeff millar"
<wa1hco at adelphia.net> wrote:

>Gerry...
>
>Definitely run two wires to the battery.  Also ground the Amp solidly to
the
>car frame at it's mount point.  Also fuse _both_ power wires near the
>battery.  Several scenarios to worry about.
>    1. Battery ground to frame loses connection and you go to start
it...all
>starter motor current goes through Amp ground lead
>    2. Amp ground lead opens, all amp current goes through coax to antenna
>ground

_________________________________________________________

With all due respect, Mr. Millar has given some bad advice here.

First off, there is no need to ground the amp chassis to the car
chassis.  In fact, doing so invites exactly the scenario in #1 above.
The amp needs the two wires to the battery, but that's all.  If you
feel the need to "ground" the amp chassis for RF, do it with a small
disc ceramic capacitor, .01 uF or so.  Make no DC connection.
However, you will find it works fine with no chassis ground connection
at all, either DC or RF.  If the chassis proves to be "hot" for RF,
you have an antenna problem, just like at home.  Fix it the same way.

Secondly, if you are concerned about scenario #1, do not directly
ground the coax at the antenna or any where else.  Any ground you need
should be done with a small capacitor as mentioned above.  This
includes the transceiver as well.  NO DC GROUNDS ANYWHERE EXCEPT AT
THE BATTERY.  If you follow this advice, neither scenario #1 or #2 can
happen.

Some antennas are internally grounded.  In those cases, break the coax
shield and install the .01 uF capacitor to jump the break.  Mag-mount
antennas are not normally grounded, but measure them to be sure.

I have seen pictures of radio equipment which has suffered the fate
mentioned in scenario #1 and it is not a pretty sight.  The radio will
be essentially destroyed.  A couple of cheap capacitors will prevent
it completely.

-- 
73, Bill W7TI