On 06/11/2013 20:48, Remi Guerner wrote:
> <air.guerner@orange.fr>
> I thought it was as simple as that before I read an
> article in the May 2013 issue of the Light Aviation
> magazine. You must transmit/receive on a given
> frequency in 25kHz mode or in 8.33 mode depending on
> the mode used by the ATC unit for that frequency. Let's
> say the required frequency is 130.025, the radio will
> transmit/receive on 130.025 on 25kHz mode. Now dial
> 130.030, the radio will transmit/receive on the same
> 130.025 but on 8.33kHz mode. Crazy, isnit ? Now
> dial 130.035 and you will transmit/receive on 130.0333
> (130.025+8.33) on 8.33 mode. Dial 130.040 and you will
> get 130.0416 (130.0333+8.33). Next frequency will be
> 130.050, and so on.... Remi
Exactly right.
> [quote] Am I missing something here? Why do you need
> to switch modes to work in the 25 spacing? Does not the
> 8.33 spacing simply provide 2 extra frequencies in the
> gap between the exiting 25 spaced ones?
It is to do with the accuracy of the transmissions. a
25kHz transmitter will span more of the spectrum than a
8.33kkHz transmitter. Thus all the 833 frequencies are
distinct and don't step on and over each other.
If a base station retains the (using the above example)
130.025 frequency, staying in 25kHz mode, then the two
(833) frequencies either side will not be usable within
transmission distance.
Now that I am on, I will give you my take on intercoms.
A proper stand alone intercom will have two microphone
circuits (sometimes four or six) - each independent. Thus
each microphone will have its own circuit.
The problem with using the intercom provided in a radio is
that they usually only have one microphone circuit and
therefore microphones have to be connected in parallel.
This leads to the oft quoted problem that David Clark
headsets are incompatible with other makes.
David Clark have low impedance microphones which if a DC
and a non-DC are connected in parallel, will swamp the
non-DC microphone. Thus leading to the "incompatibility" tag.
In an aircraft with a "proper" (separate) intercom, there
will be no incompatibility issues with DC headsets as each
microphone has a separate circuit.
I know the KX125 I have has only one microphone circuit
available so I have a separate intercom. Other radios may
have more than one mike circuit (and therefore probably
more than one PTT circuit) but I am not aware of them.
I suggest therefore that throwing away your intercom may
not be a good idea with certain radios. Obviously if you
only connect one headset up then the radio intercom is
fine, but with one one headset you don't have anyone to
communicate with through the intercom !!!!!!!
Richard Holder
G-OWWW
aka ANRMAN
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