Transmitter power

Discuss all things relating to the busy London Pirate Radio scene.
Albert H
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Re: Transmitter power

Post by Albert H » Sun Jul 19, 2020 12:24 pm

Sorry XXL. I've only been doing this since the 1960s......

Any VHF signal at any power level always goes to the horizon. It doesn't matter what the power is - whether it's 100mW or 1MW - it still goes the same distance. That may seem counter-intuitive, but it's so.

However - whether it can be heard above the atmospheric (and man-made) noise is determined by the field strength. The field strength is (mostly) determined by the transmitter power and the antenna gain. The higher the field strength, the better the noise is flattened. There's a further effect to be taken into account:

The IF strip in a modern receiver will include a "limiter"* circuit - usually just a simple clipper. If the received signal strength is sufficient, the signal going through the IF (Intermediate Frequency) amplifiers will reach the limiting threshold. This is a sort of "saturation point" - the signal can't get stronger than the limiting point. Limiting in the receiver is what gives rise to the "FM - No Static At All" myth! The signal is then passed through the discriminator circuit to recover the modulation from what is (effectively) a squarewave signal with a complex and highly variable mark / space ratio!

Commercial broadcasting considers that the "range" of a station is determined by how far away from the TX site a calibrated test receiver can reach the "limiting" state. It is assumed that listeners will want a fully limiting (and therefore fully noise-quietening) signal. Reception that isn't fully limiting and noise-quietening is considered to be "fringe" reception and is thought to be unacceptable to most listeners because of the degraded audio quality.....

Increased field strength also improves penetration into buildings. More power is (generally) better! However, the fact remains that all VHF signals go to the horizon that the transmit aerial can "see". The various attempts by broadcasters to use different polarisations have very marginal effects - I've never had time for circular polarisation: I've always found that vertical works better, particularly between buildings.

* Note - the IF limiter in a receiver is nothing like the audio limiter used to prevent over-modulation on the way into a transmitter!
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drumandbasshead010
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Re: Transmitter power

Post by drumandbasshead010 » Mon Jul 20, 2020 5:02 pm

To be fair Albert, while you are technically correct, what matters to the owner of a radio station is how far their signal can travel at listenable level, so that is what is being referred to, when posters here ask about it. Saying that a signal is going into the horizon is all well and good, but what really matters is that the listener can hear it. What's the point in being on in the first place otherwise?
The radio station wants to reach the listeners, therefore this whole idea of the signal being there underneath the noise floor, to the station owner who wants to actually get heard a good distance from their site, is irrelevant. I'm not contradicting what you're saying, but at the end of the day power is not irrelevant to how far a signal travels if that signal cannot be picked up properly at the end of the day. If I ask someone how far their signal is travelling, what I really want to know is how long it'll take before I can't hear the station! What determines that? Height, obviously, but also power!

fmuser877
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Re: Transmitter power

Post by fmuser877 » Mon Jul 20, 2020 6:04 pm

how come you can get french and dutch stuff on FM right by the sea which are way over the horizon in places like Mersea Island and parts of Suffolk & Norfolk?

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teckniqs
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Re: Transmitter power

Post by teckniqs » Mon Jul 20, 2020 7:09 pm

FM signals don't just go as far as the horizon and then stop, I've put a transmitter up and you can only see about 25 (line of sight) miles to the horizon yet the signal has reached well over twice or maybe even three times the distance further, depending on how much power was used. :tup

....Also if a 1mW signal which can only be received a few metres away on a portable or good quality car stereo reaches all the way to the (for example) 25 mile horizon but cannot be heard at all above the noise-floor and the radio cannot detect it then it's not reaching there if no receiving equipment can receive it that far in my opinion.

Albert H
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Re: Transmitter power

Post by Albert H » Tue Jul 21, 2020 3:55 am

You're entirely right. My pal Eamonn and I did the first 144MHz path across the Atlantic from County Mayo in Ireland to Newfoundland. That was only possible because of a freak effect called "tropospheric ducting". It's an effect that happens in periods of high pressure, but can't be considered as useful for "broadcasting" since it's so transient in nature.

You're right - there is some refracted signal that gets beyond the horizon, but it's usually pretty weak compared to the direct signal. You extend the horizon by putting your aerial as high up as possible.

The point that I was making is that all VHF signals travel the same way - it's just that the higher powered ones will crush the atmospheric and man-made noise better than the weaker ones. I make this point because I get annoyed with cretins asking "What's the "range" of a 50 Watt transmitter?". I tell them that it's the same as a 500 Watt transmitter, and their poor little brains explode.

I remember some of Roger's "Hackney 'Mitter-Men" complaining that they'd been ripped off because their new 200 Watt rig wasn't going any further than the 100 Watt one they'd been using before. It wasn't possible to explain to those knife-weilding thugs that there wasn't much practical difference between 100 and 200 Watts and that they'd have to change from 100 to 400 Watts to notice an increase in received signal strength on a hi-fi receiver meter.....

As I explained earlier, broadcast field strength contours are measured with a calibrated receiver and antenna, and the point at which a receiver gets enough signal for the IF strip to just achieve limiting is considered the edge of the primary service area. This is the point at which the receiver won't "hear" noise or other co-channel stations - "capture effect" is seeing to that.

If you want a practical demonstration of the difference in noise floor vs transmitter power, run a Watt into a dipole in mono and go down the road until it starts to degrade. Now switch on a stereo coder, and to get the same useable "range", you'll need to run 4 Watts because of the widened bandwidth of the receiver in stereo mode. The NRG Pro III was great for demonstrating this, because it could be easily switched between 1 and 4 Watts.

Incidentally - there are techniques used in communications that can recover signals from below the noise floor. Granted this isn't for broadcasting, but it's fascinating stuff, and has implications for future deep space communications, data recovery from corrupted hard drives and even receiving radar images in hostile environments. I wrote a monograph on the methods - along with a comprehensive mathematical analysis - about 20 years ago if anyone's interested. There's an abstract in Wireless World - it's an interesting read if your maths is up to it - and I'll put a link up when I can remember when it was published!
"Why is my rig humming?"
"Because it doesn't know the words!"
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