On 03/07/2011 02:34 PM, G-IANI wrote:
> I am not a radio expert but from what I have been told the ferrite rings are
> needed for impedance matching (ie they are the balum).
The ferrite rings are not for impedance matching but to convert the
asymmetric signal (one center wire against an outer sleeve) in a
symmetrical one (the two legs of the dipole antenna).
Without such a conversion, the electrons can not really see the
difference between the sleeve of the coax cable and the leg of the
dipole antenna. They get confused, turn the wrong way, and radiate off
the cable instead of radiating from the dipole leg.
In this situation, it is more difficult to obtain a good and stable SWR.
Furthermore, as the sleeve of the coax cable starts radiating, you
induce more interferences in the avionics while transmitting, and
receive more noise from within the ship while receiving.
Ferrite rings are a way to "filter" the electrons. Slow moving electrons
can pass, but electrons in the mood for radiating can not, and can only
stay within the antenna. Be warned though that ferrite rings come in all
kind of flavours, if you pick a random ring from a catalog you will most
likely get one that is not efficient on the VHF frequencies we are using.
There are other ways to obtain the same result. You can coil up the coax
immediately behind the antenna connection, or create a so called
"balun". (Google on "balun" and "dipole" to find the formula how to
create such a contraption).
I have ferrite rings near the antenna connection, AND a coil in the tail
of the ship. I believe that this double measures are needed because it
is impossible to route the coax cable sufficiently far away in a 90
degree angle from the antenna, so it picks up some of the radiation from
the antenna again on the other end of the ferrite rings. The coil
further upstream takes care of this, making sure the cable is completely
"dead" between the radio and the tail of the ship.
As a result, I never have had any interference problems between the
transceiver and the rest of the avionics. Even the three strobes, fed
---From a central power supply, have no influence at all at reception.
Frans
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