How to tidy up FM Signal?

[fmuser877]

Hi
I search google but its all the wrong type.
what is the best way to clean up the transmitter's signal before it goes to the aerial thanks?
Only attuatues seem to come up in google.
sorry if my spell is bad.
as some times I hear my signal on another freq over the dial.

[reverend]

It very much depends on what your problem is. Hearing your signal all over the dial could simply be that the receiver is being overloaded. Try moving the receiver further away from the transmitter (a few 100 metres) and if the signals disappear, you didn't have a problem. If they don't disappear, then the transmitter is producing them and no amount of filtering will clean it up. Transmitters sometimes produce these kinds of 'sprogs' if they are connected to a bad load (i.e. not near 50 Ohms) so it may be that your antenna is not correct, or is badly connected.

[fmuser877]

Yes, they seem to go away at the end of the road so hopefully, it's just the radios overloading.

[NTRRadio]

You can tidy a signal up somewhat by building a bandpass filter between the transmitter and the antenna.

[Albert H]

ALL transmitters need a good output lowpass filter, since all PAs are certain to generate harmonic products!

I usually design for the filter to start to attenuate at about 1.2 times the carrier frequency, and use a minimum of 9-poles, so that the signal is seriously attenuated by the time you get to the first harmonic signal (typically about 51 - 59 dB). In critical circumstances - such as higher powers and proximity to other radio equipment - I use "Stub" Filters (or Cavity types) exactly tuned to the harmonic frequencies I want to attenuate.

[radionortheast]

Unless its something you’ve built your transmitter will problalbly have a filter already built into it, adding an extra filter might lead to mismatch. The best thing to do would be get tinysa, then use it with the little telescopic antenna provided, you need to have something to see whats going on. All you have to do is switch it on, as long as you don’t see extra signals between the fundemental signal and the harmonics, you can say its pretty good, basic checking.
Checking in the fm band is best done with a car radio something that dosen’t pick up intermods. If its a transmitter that is afew watts, you can compare what the fm band is like before and after switching on, you should be able to see all other fm signals on the tiny sa, unless you're right next to the aerial.

[Albert H]

Most commercial installations include critically tuned filters at multiples of the output frequency of the transmitter. Up to a few hundred Watts, a basic lowpass filter is usually sufficient - one current commercial design we make uses a pair of FETs in the final (for 500 Watts output in the basic version), and the balanced PA tends to "null" even harmonics, so the filtering becomes less onerous - before the filter, the second harmonic (ie: twice the output frequency) is already -68 dBc (down on the carrier), but the third is only -54 dBc. The lowpass filter reduces all the harmonics to <-84 dBc (worst case) at any frequency, so the harmonic outputs are in the microwatt range!

However, when installing a rig like that close to other transmitting equipment (like in a "Facilities Site"), critically tuned cavity filters are used to prevent the output of my rig getting into (and thereby desensitising) the receivers on the site. It's not uncommon to find dozens of cavity filters in the various down-leads of the receivers and repeaters on these sites! Sometimes, simple proximity effects can cause interference between site users, and a big part of our commissioning procedure involves the use of a Spectrum Analyser, a Network Analyser (and sometimes a SINAD meter) to make sure that the signals aren't mutually interfering with each other. The prevention of interference is the biggest part of the job in some cases.

Sometimes, the only option to avoid causing interference is to site your rig well away from anything else. Remember - even if you have the cleanest rig in the world, intermodulation products, mixing products and other spurious signals can still occur. It's also worth remembering that many bits of domestic electronics can be susceptible to interference - analogue TVs used to be notorious for "patterning" and other interference effects when rigs were run nearby. The advent of digital TV makes visible interference much less likely, but "swamping" the front end of a modern TV receiver can make it unable to receive any signals at all!

One amusing story: Back in the 80s, we ran a little AM transmitter up a tower block in East London into a sloping wire aerial off the top of the block. It was on 576 kHz which was clear at the time (I used a 4608 kHz baud rate generator crystal divided by 8), and despite that it was only about 8½ Watts carrier and 35 Watts peak, it gave a pretty good signal throughout the London area and beyond in the daytime (we turned it off with a time switch at night because it was too weedy a signal to be heard over the European stuff at night). About eight days in, it got raided for causing interference....

We'd forgotten that the old VHS video recorders recorded their colour information at around 580 kHz, so everyone in the block was getting psychedelic video recordings when our rig was on! Whoops!

The next MW rig was higher up the band, and lasted for several months!

[nrgkits.nz]

Was it Eric Gots who did the raid?

[Albert H]

You mean "Gotts". It might have been - he was a complete maniac, and wanted the airwaves "clean" of anything that wasn't the BBC! He even took issue with the legal commercial stations in London!

A friend of mine was in his studio when Gotts came in with Brian Holder. Fortunately, the mic was live at the time, and we had an off-air recording of Eric Gotts saying "Kill it Brian" (he was pointing at the link rig at the time). When it was pointed out that Eric had contravened his beloved Radio Telegraphy Act himself, he was completely freaked out! There were no prosecutions forthcoming for that raid! They did take the link rig in the mistaken belief that it was the actual main rig (which was a couple of miles away, but went off when it lost the link carrier).

The station was back on the next day from another studio.....

[Albert H]

One further thing to consider - Soshin make some neat little Band II ceramic bandpass filters. You can get them from the Far East for ~$0.50 each. They'll handle around a Watt at most, and are really good for tidying up the output of an exciter!