As Jim K9YC says, a Misconception Alert’ here is very important – and part of
the problem is about what we name things.
In the TT Orion 2 manual, as per what seems the general convention in
electronics today, the –ve connection from a mic element is the connection
called ‘MIC GND (mic signal ground – pin 7).
As checked by my ohmmeter, there is no direct connection between the MIC GND at
the rig and the actual rig ground (labelled GND (pin 5).
The good news is the Orion 2 manual says: ‘Keeping the chassis ground and mic
signal ground separated are done to reduce the possibility of inducing stray
hum or RFI into the transmitted signal” and gives a nice diagram to help ensure
you do this.
What I said in the previous email was what I physically did, but the idea and
the end result was to avoid a path for RF current flowing on the shield of the
microphone cable (which runs from the TT Orion 2 to the interface box) into the
electret microphone element.
As K9YC points out, this was achieved by connecting the shield/chassis ground
to the interface box, which kept the RF outside the box. The so called MIC GND
(microphone -ve) travels through the interface box without any connection to
chassis ground.
The CM-500 enclosure is made of plastic and so (of course) offers no shielding
to the electret element. As one of the antennas I now use – a 40m Moxon
Rectangle – has feeders which come into the shack, so I can vary the amplitude
and phase of the current in the reflector for maximum gain/F to B and its
tuning unit currently has no cover, there seems to be a lot more RF in the
shack than there used to be...
Despite this situation, I was grateful (and a little surprised) that doing the
above to the interface worked OK without the need for any further RF
suppression.
Sorry for any confusion caused – and thanks to all who helped.
Vy 73
Steve, VK6VZ
On Fri,4/10/2015 12:44 AM, Steve Ireland wrote:
>> Isolating mic ground from the rig ground, along with making sure the
>> interface aluminium box was connected to the rig ground, stopped the RF from
>> getting into the CM-500 electret element.
>Misconception Alert!
>We are not talking about a connection to Mother Earth. That is NOT part
of a solution to RFI. What we are talking about is avoiding a path for
RF current flowing on the shield that goes THROUGH the box. When we
connect the shield to the chassis, we keep the RF outside the box.
---
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