Here is the HOWTO:---------
|Posted: 11 October 2016 at 4:26pm | IP Logged
If you happen to
have a LPFM transmission running on a relatively inexpensive
transmitter you may inadvertently be interfering with other
transmissions on the FM Band and elsewhere.
The problem occurs
when your transmitter radiates on frequencies other then the one you
have set it to.
This is most
commonly caused by having the audio volume set too high. Your
transmitter begins to radiate on adjacent frequencies above and below
your frequency of operation.
Many operators think
that by winding the volume up they increase their coverage. In fact
winding the volume up increases the interference you create
coverage by winding up the power.
broadcasters have been told they should have a LIMITER in the chain
between the “Desk” and the transmitter. This is good advice as a
Limiter/Compressor can control the peak levels and keep levels
consistent. This device will fail to be of any benefit if the output
to the transmitter is set too high. Thanks to the maths of how this
happens your overloaded transmitter can cause interference to
adjacent transmissions but at frequencies out of hearing range. Only
a few very expensive transmitters have circuitry to limit the maximum
deviation of the transmitter and it's still possible to overload
them. The only really accurate test is a spectrum analyser looking at
your RF output.
The blanketing of
nearby frequencies is a ticketable offence, but because commercial
broadcasters have enough power to overcome these issues and their
listeners don't know how or where to complain you may not get caught.
Affected LPFM broadcasters likewise generally don't have the
technical knowledge or equipment to detect these problems. Anyone who
does know how and where to complain is reluctant to because calling
in the RI's generally results in everyone being given the once over.
In a revenue driven regulatory environment it's easy to write a few
thousand dollars worth of tickets for minor infractions.
Here is a simple way
to set the level going in to your transmitter.
Use a regular
domestic mantel radio.
Tune it to the
National Programme and note the volume level.
Re tune to your
transmission frequency and adjust te audio input to your transmitter
so it sounds about the same level.
Switch back and
forth to confirm.
Your local National
Programme transmission on 101 FM is the strongest full power
transmission available. If yours sounds louder then you are probably
over modulating and for FM over modulation means over deviation. Over
deviation means interference and so it goes.
Get in a vehicle
with a fitted radio and find a park within 1km of your transmitter
site. Tune your station and compare it with all the other local
stations and the National programme. They should all sound the same
although some of the commercial stations will 'sound' louder because
they have processors which cost lots.
On a standard car
radio if your transmission is on 88 you should just hear it on 88.1
and 79.9 but no further away from your centre frequency. If you can
hear it on 88.2 for example you need to back off the audio level into
the transmitter. While you are out there in the car scan the FM band
and see if you can hear your signal anywhere else on the band. If you
can you have a problem which may be a mis matched aerial.
While I have used 88
as an example you need to check the Radio Spectrum website for the
permitted LPFM frequencies in your area. Ideally everyone is 200 away
from everyone else.
In any one area
there is room for 10 LPFM transmissions. Thats a 10km circle based on
your transmission point. In Auckland demand is about 50 times what is
available, in most other urban areas about 20 times what is
Many LPFM stations
are operated by enthusiastic well meaning people who have no real
technical knowledge, and don't get much help from those who do.
system should be tuned to your frequency of operation. Exactly. It
should also be designed for FM Broadcast. Many are too narrow and as
operators try to correct for this they cause problems for others.
You should have a
Cavity Resonator between your Transmitter and the Aerial to stop
spurious transmissions from your transmitter getting to the aerial
and more importantly to stop outside transmissions from getting into
your transmitter. That cell site down the road can send a signal into
your system which is turned into interference that you transmit, and
are liable for. Current RSM policy is to fine first. No
correspondence will be entered into, even if you are perfectly legal,
and had no idea your transmitter was re radiating on some other
The cheaper the
transmitter the more likely it will clause interference.