On Tue, 29 Jul 2003 22:41:15 -0400, "jeff millar"
<wa1hco@adelphia.net> wrote:
>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.
If you missed what I said about what was bad, read it again. You even
mentioned it yourself in your original post. See my next comment
about severe damage to the radio.
If the capacitors run hot, use more than one in parallel. I have
never run more than 100 watts mobile but at that level, one is enough.
>All mobile radios I know of use the multiple ground technique.
It isn't necessary and it can cause severe damage to the radio in the
case of a loose cable from the battery's negative terminal to the
car's frame. You mentioned this yourself.
>- Icom, Kenwood, etc all recommend two wires direct to the battery.
It's the best way. Using the car's frame is not recommended.
>- Each vendor supplies a cable with fuses in both leads.
Again, the best way.
>- Each vendor supplies a metal mounting bracket to connect the radio to the
>vehicle...
I wouldn't use the word "connect". "Mount" is more appropriate.
There is no need for a connection through the mounting bracket.
>- Vendors don't recommend one way or the other about connecting the radio to
>frame ground. It works either way.
It works either way, but a grounded radio is susceptible to the loose
cable issue mentioned above. Perhaps the vendor would like to sell
you another radio? <joke, sort of>
>- All the VHF and UHF mobile whip antennas I know of have a coax directly
>connected to the vehicle body or frame.
Mag mounts don't, as far as I know.
>Motorola and GE use the same installation techniques for their commercial
>service radios.
>From time to time they probably have damage from the loose cable
issue.
>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.
Again, mag-mounts. On other types of mounts, you can leave the
antenna grounded, just break the coax for DC. As mentioned above,
parallel the capacitors if they run hot.
>I recommended grounding the amp to the frame because there's so much supply
>current running around.
That is exactly the WRONG reason to ground the amp to the car frame.
Confine the current to the two wires. A car body, especially modern
ones with lots of plastic parts, is not to be trusted.
>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.
Precisely the reason why this type of connection should not be done.
You're making my argument for me. :-)
>Bill's "no extra DC ground" approach is simple and clean at DC, but doesn't
>work at RF.
Not true. It works fine. I've used it for years. It has been
presented in QST and endorsed by every one who has tried it. Not one
person I know of has reported problems.
>The RF output coax need a solid connection to the vehicle to
>make a ground plane for the whip.
It needs a low impedance connection at RF but not at DC. A .01 uF
capacitor has an impedance at 144 MHz of .111 ohms. That's low enough
that neither the antenna or radio will ever notice. If you think
that's too much impedance, use a larger value or add some more in
parallel.
>Grounding the Amp body isn't essential,
>but reduces a worry about the DC on the coax shield.
If the only DC connection in the whole system is at the battery, there
is nothing to worry about on the coax shield. There will be no DC
there at all.
>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.
A short in the amplifier that blows the 50 amp amplifier fuse will
certainly blow the lower-amp transceiver fuse. If you're concerned, a
capacitor which breaks the braid will eliminate the concern, just like
in the other cases.
--
73, Bill W7TI
|