[ARC5] BC-230 mic

[Jim Wiley]

One thing to consider when thinking about using a switching power supply 
sold for computer use (of any type, not just a laptop "charger") is that 
many of these devices, once opened for inspection,  have "missing" 
components on their PC boards.

The missing parts invariably turn out to be those very same components 
that would have been needed to suppress RFI and other types of 
electrical noise that appears on both the DC output and "feeding back" 
to the AC input, where it is radiated from the homeowner's AC wiring.

One must question whether or not these components were engineered and 
installed to allow the power supply to pass FCC and other RFI 
requirements, but left out of production units to reduce manufacturing 
costs.

Just sayin'


PS:  Note my  comment on the coax issue, interspersed below


- Jim, KL7CC


Neil ZL1ANM wrote:


>> The conventional "wisdom" is that these laptop power supplies, being of a
> switching design,
>> create a lot of hash and RF noise.
> <snip>
>
> Also, we have become fixated with 50 ohm unbalanced transmission line to
> the antenna. It's my theory that the braid of the coax picks up radiated hash from appliances, so that hash voltage exists between the braid and the inner conductor. Balanced feed should eliminate most of that.

Not so.  If it is good quality coaxial cable, the outside of the shield 
should be totally isolated from the inside of the shield.  RF 
(supposedly) does not penetrate the shield.

This is the reason that a ferrite choke (or some other means) is used to 
decouple the currents flowing on the outside of the shield. If the coax 
shield is connected directly to one side of a "balanced" antenna, such 
as a dipole, unwanted currents _will_ flow on the outside of the 
shield.   Some antenna designs, such as the gamma match, get around this 
problem by using the matching system itself as its own balanced to 
unbalanced transition.

All other things being equal, if the load is correctly matched to the 
source  (and feedline), changing the length of the feedline should not 
change the SWR.  If it does, then either there is a mismatch somewhere, 
or antenna currents are flowing on both inside _and_ the outside of the 
coax shield.

- KL7CC
> <snip>
>
> 73 de Neil ZL1ANM
> _