TV/Radio aerial signal power levels: simple explanation needed.

Hello,

Can anyone explain to me like I’m an idiot, because I’m an idiot, about the signal power levels at the receiver end of a piece of coax?

(I’ve posted this in uk.tech.broadcast and uk.d-i-y)

BACKGROUND

Classic FM, for example, broadcasts from Holme Moss on 101.1 MHz with an output of 250 kW (which I assume is ERP, which I sort of understand as the power focussed in particular direction by it’s transmitting antenna in the same way light can be focussed by glass lenses).

There is attenuation as the signal travels to me (50.31 km away as the crow flies), and this attenuation follows an inverse square law, so 1/2

so:

I assume this is in watts?

And this is the signal strength in free air at the position of my aerial (half wave dipole).

Then there is about 20 metres of coax, which I believe is RG-11, which according to a datasheet I found on the RS website has an attenuation of

I do not know how to work out the power loss through that 20 metres of coax as I do not really understand dB, other than “it’s a comparative level of power”. I’ve no idea what that means.

My radio (which is in the cellar, hence the need for an external aerial) has the following details in the manual:

Sensitivity = 2.5 µV (IHF) (what is IHF?)

S/N 26 dB 2.2 µV

So it needs microvolts but how is this figure related to the watts from the transmitter? Do I need to know the current in the coax?

How is the signal strength measured? Does it need a special meter, or can a multimeter be used?

My radio doesn’t always get a good signal; in some weathers there is a background hiss. I would like to get rid of this. I suppose an amplifier at the aerial itself would be needed?

Thanks in advance for any help, and apologies for the lengthy question.

D.P.

You are not going to get much further with this until you find out what the antenna gain is ...

"feeder loss" (as some people call it) at 100MHz is is going to be much less than 0.748 dB/metre - but you need to find that out as well

How does an antenna gain anything? It's just a piece of metal.

A single dipole has no gain. A yagi does, because it has a reflector element(s) behind the dipole, and director(s) in front of the dipole. These focus if you like extra signal onto the dipole. It's clever stuff because it's totally passive gain.

ERP on the Tx antenna adopts the same principle but in reverse.

High power FM and UHF transmitting antennas typically have a 'gain' of about 10-12dB, so to make the maths easy, if Classic FM has an ERP of

ERPs for FM radio transmissions however are a composite figure for horizontal plane radiation, and vertical plane. Most high power transmitters are mixed polarisation, with equal H and V radiation, so if you have a fixed plane receiving aerial, the ERP you are being 'sent' will be half the quoted value

A single half wave dipole has a gain of around 2.1 dB

Different designs capture different amounts of the radiated signal depending on how much metal and which way its pointing.

If a particular design is twice as good as a dipole then it is said to have a gain of 3dBd. As you say it doesn't amplify it just transfers more of the signal from to the receiver.

Of course some don't perform as well as a dipole so another measure of "gain" is used dBi. This is the improvement compared an isotropic antenna, a theoretical antenna which radiates equally in all directions.

A dipole has a gain of 2.14dBi so to convert dBd to dBi add 2.15 to do the converse subtract 2.14...

There are two ways to measure it. The simplest is to

It extends its noodly tentacles through the electromagnetic universe...

Think of it as a magnifying glass making the image of the sun hot., It's 'just a bit of glass'

I am not sure that is true. A monopole (whip) has no gain

But a dipole is directional

Classic FM will not have any gain - its omnidirectional - 250kW is going up that feeder Probably more

Yes. It has directionality

You are talking complete and utter bollocks I'm afraid

It is not clear what is being asked here. If its a receiver the answer will be

1. 'really very little at all'
2. 'As much as a good antenna and low loss coax can make it'

Transmitter power creates IIRC 'watts per square meter' attenuated by the inverse square law to much less watts per square meter.

An antenna used to receive has two basic properties IIRC:

How much *effective* area it has to suck those watts from an antenna specific area The *gain* it has to extend that effective area in a given direction, to the exclusion of all other directions.

Those watts will be turned into volts using the characteristic impedance of the air which is around the magic 75ohms

Depending on the bandwidth of the receiver, the noise floor of even a perfect amplifier will be that of a 75 ohm resistors thermal noise. Again from extremely rusty memory for an FM signal that was a fraction of a microvolt. I do remember shooting tor <2uV front end noise on a high quality RF input stage,

For quality stereo reception you really wanted 60dB signal to noise so around 2mV at the input socket, That's a decent transmitter less than 50 miles away with a good quality directional aerial

The maths used to determine the effective area of an antenna are enough to put you off your guinness.

If you really want to know:

The maximum effective aperture Aem [m^2] for any half wave antenna can be calculated by multiplying the squared signal wavelength λ [m] with the antenna gain Gi and dividing it by 4π.

Utter bollocks is Turnip's specialist subject.

Isotropic radiator? They're over there next to the monopole magnets.

The characteristic impedance of air is about 377 ohms.

I believe a sphere is as good as it gets

It's not something you'd want anyway, the narrower you can make the beam pattern the stronger the signal where you want it, and the less interference to people who don't want it. At some point, environmentalists will insist we stop leaking RF upwards from the planet...

I've always felt that RF is more an art than a science.