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Subject Topic: LPFM in Hamilton Post ReplyPost New Topic
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traderbob
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Posted: 11 October 2016 at 4:26pm | IP Logged Quote traderbob

The number of LPFM stations in Hamilton has reached the point where there are more transmissions than available frequencies. Not a problem if you have a station in Hamilton East because it was never going to reach Te Rapa. FM stations can occupy the same frequency at a reasonable distance from each other and the 'capture effect' means each has a primary service area. Problem we are observing is that someone in Te Rapa or thereabouts has a signal which is also transmitting on a number of other nearby frequencies. Not the 'same programme' but a series of "carriers" which block out 3 or 4 channels either side of the frequency you are on. Most likely to suffer are other LPFM stations, but you can affect full power station reception in your neighborhood. This is often caused by having the audio input to the transmitter up too far in an attempt to make the transmission sound louder, which it might do a little bit, but generates all these extra carriers which it does a lot.  This is a problem with the cheap made in China transmitters and with plenty of the others. The Tugicom is a good example. Big Expensive FM exciters have systems to restrict the deviation to no more then 75kHz no matter how far you wind up the input gain. Only one LPFM transmitter I have seen has anything like this. The *BIG* guys check everything with a spectrum analyser. So in an effort to clean up the LPFM interference in Hamilton I wrote a 'howto' set your transmission up without tens of thousands of dollars of test gear.  Which I will post in this forum.
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traderbob
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Posted: 11 October 2016 at 4:26pm | IP Logged Quote traderbob

Here is the HOWTO:---------

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 exponentially.


You increase coverage by winding up the power.


Many LPFM 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 available.


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.



Your transmission 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 frequency.


The cheaper the transmitter the more likely it will clause interference.


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traderbob
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Posted: 11 October 2016 at 5:06pm | IP Logged Quote traderbob

One of the other problems I have found on LPFM is that people have aerial systems that are not 'resonant' at the frequency they are transmitting on.
If the Aerial is not tuned to the frequency of operation some of the signal is returned from the aerial to the transmitter and depending on your transmitter the return signal assists the creation of spurious transmissions. Worst offenders are not home built aerials but some commercially sold 'FM Band' systems which are unsuitable for the purpose in every respect.
A correctly configured aerial will maximise your signal and minimise interference.
If you use a folded dipole the feed point is 300 ohms. Your transmitter is 50 ohm so you need something that will provide 6:1 matching. You can do that with a coax transformer but the coax transformer is so narrow band you lose a large amount of your transmission.  You can buy a commercially made folded dipole with a 50 ohm feed on a specific frequency and they do work. Problem is 50% of your signal is wasted. Thats a big deal when you are on 1 watt.
The commercial guys use antenna systems set up for the FM broadcast band which provide 50 ohm matching across the FM band. They don't send signals up, they send them out. You need a lot of height and space to install one of these systems.
Since we have limited space, and height, and are restricted to vertical polarization, the ideal aerials are vertical and have about 200kHz band width [you need 150kHz ]. This guy is the only game in town. https://fmbroadcastantenna.com/dominator.html
There are some copies. The biggest problem is that they are narrow band. For your listener to get the best 'fully quieting' signal they need to get all the deviation. If your signal does not occupy enough space the reception is affected.  Getting it right is a challenge. Too much and you wipe out the neighbors.
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