While common mode current may be a bit of a misnomer, it is accurate in that
the effect of "outer shield current" is identical to that of common mode
current in a balanced system because of the "pin 1 problem"
that K9YC has written so extensively about. Since the signal return in most
amateur equipment is not tied to a complete and effective shielded
enclosure, the outer shield current is *indistinguishable* from a true
common mode current as it passes through the unbalanced input/output
circuits of modern equipment.
Again, if this "outer shield current" is sufficient it can appear in in
microphone inputs, be fed back into audio amplifiers or even be impressed
across control and power supply circuits. The results can be anything from
mildly annoying to fatal to the equipment depending on the level of the
common mode signal and the circuit involved.
73,
... Joe, W4TV
### There has not been a Pin 1 problem in pro audio gear in well over a
decade.
Everything is referenced to the chassis. Anybody making pro- audio gear with
Pin 1 problems will be outa business in 15 mins flat. Nobody would buy it.
These days its all servo balanced. Now if you really want to crank the CMRR
specs through the roof, then add jensen transformers at each end.
## yaesu has pin 1 problems. Now that’s real easy to fix. The stock yaesu
mic pre-amps are pure junk anyway, and noisy, plus have the Pin 1 problem.
The easy fix is to just install a new RCA connector onto the rear chassis apron
somewhere.
Then use s 220 uf non polarized panasonic SU series coupling cap from the hot
side of the newly installed rca female jack, directly to the balance modulator
input.
Then use a quality outboard audio pre-amp..with a balanced input, and use a
balanced mic.
Output of the pre-amp is also balanced..which in turn is fed to a jensen audio
xfmr. Xfmr is
wired balanced on the input side, and Un balanced on the output side. A real
short length
of small diam coax is run from xfmr output to newly installed rca input on
xcvr.
## problem solved, and you kill several birds with one stone.
## If you want to see how much outer coax shield current you actually have on
the coax to
your ant, then simply measure it, using a clamp on RF ammeter, and try the
ammeter in several
places along the length of the coax.
## I bond the braid of the coax at the top of the tower, then again at the
base of the tower, then
again at the SPG just inside the basement wall. I use the remote switch box
to bond all the shields to
the galvanized chromolly mast. On the input side to the remote switch box
goes the nema box, with the
4 x turns of 393 coax wrapped around 4 x 2.4 inch OD type 33 cores. Each
yagi gets its own type 43
bead balun, including the 40m yagi + 80m rotary dipole. In an ideal world,
run the coax and all control
lines on the inside of the tower. At the base of the tower, I use 2 ga wire
from each tower leg to its own
8 foot cu rod. 2 ga wire is cadwelded to each rod.
## beware of using a large number of coax wraps around cores. A buddy had
all sorts of parasitic arcing
between the last 2 turns, when using just 8-10 turns of Rg-162 teflon coax
wound onto several 2.4 inch cores.
Jay at array solutions had warned us of that effect. Now that teflon coax
also has a super HV type outer jacket.
It had arced from the last turn to the next inner turn. It was wound solenoid
fashion, very neatly. Now just think
about it. You need a lot of voltage to puncture through TWO jackets. The
answer was more cores, and less turns.
14 turns of RG-58 size coax is gonna result in problems when running high
power under a lot of conditions.
On 4/21/2012 4:26 PM, Gary Schafer wrote:
>
> A better name for what is commonly called "common mode current" on the
> coax outer shield would probably be "outer shield current".
>
> 73
> Gary K4FMX
>
## congrats, I believe you just nailed it.
later... Jim VE7RF
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