70Mhz Band Propagation Characteristic

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Re: 70Mhz Band Propagation Characteristic

Post by Admin on Thu Mar 21, 2013 6:16 pm

Hello TC,

The 70MHz band is about the same as the 144MHz band but with a 6dB improvement in signal strength. The 88-108MHz band is closer but cannot really be compared because the FM band uses +/-75kHz frequency deviation which means that all receiving equipment must have a wide bandwidth and greater noise levels.

I have worked professionally on 68MHz / 74MHz PMR using both AM and NBFM. I rather liked that band because although the antennas were larger, it is still has all the VHF characteristics but seemed to suffer less from fading due to reflections and rayleigh scattering. Probably because of the longer wavelength. The occurence of hopping over the horizon is a little more common but still quite rare.

As a matter of interest I still have some 70MHz PMR dipoles in the oriiginal packaging under the front steps of the house. I keep meaning to use the bits to make myself a more strudy 144MHz Yagi. One day I will have the time to do all these things I want (and one day there will probably be peace on Earth, but I will not put any bets on that).

BR Harry - SM0VPO

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Re: 70Mhz Band Propagation Characteristic

Post by dare4444 on Wed Mar 20, 2013 7:45 pm

Thanks Harry. Have you got any experience with the 70Mhz Band? FM in 88-108 Mhz should have propagation characteristic of 70Mhz I am thinking.. And radio mobile is really accurate in predicting the coverage.. cheap but reliable software for the ham bands. Tc

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Re: 70Mhz Band Propagation Characteristic

Post by Admin on Tue Mar 19, 2013 6:56 pm

There are several ways you can estimate the coverage.

1 - quick'n dirty Cool
Assume you have enough power to hit the horizon. Square-root of antenna height above ground multiplied by 4.128km. Does not work on hilly ground and not very accurate when it comes to built-up areas. But if you have kine-of-sight to the horizon then this tells you roughly have far the horizon would be if the Earth did not have sticky-up bits.

2 - take of your socks 'cos you need to calculate Neutral
Use the basic formula for path loss. If your broadcast receivers have a sensitivity of 10uV then this is roughly -90dBm. To get a good signal you also need a fade margin of 10dB (fixed receivers) to 20dB (portable receives) and so you must provide a signal level of around -70dBm.

20Log(frequency in MHz) + 20Log(distance). To this you add a constant of about 30dB (if km) or 34dB (if miles).

From this you will get a good idea as to the transmitter power you need. Use the formula backwards (transpose it) to get the distance.

If you have a transmitter of 1W (+30dBm) at 100MHz and want, say 30km.

20 x Log(frequency) = 20 x Log(100) = 20 x 2 = 40

20 x Log(distance) = 20 x Log(39) = 20 x 1.5 = 30

40 + 30 + 30 = 100dB

This means that a 1-Watt transmitter with a line-of-sight path will "only just" give you 10uV at 30km distance. If the receiver antenna is rubbish (and domestic FM systems usually are) then you can take away a few dB.

If you have a good antenna then you can add a few dB. But +/-10dB is nothing. A tree in the way or the aditive reflection from the building across the road will give you this.

For more information see http://www.sm0vpo.com/data/pathlos.htm

The danger is that for domestic use the average musical wireless box varies a LOT from one manufacturer to another. A B&O FM tuner had a RX sensitivity of about -115dBm, which is about 10 times better than your average box. A cheap $2 handhelf AM/FM box may only have a sensitivity of -65dBm. Thats a 50dB (100000 x power 300 x voltage) difference.

For general use, use 10uV for domestic receives and and take away 20dB fade/noise margin to get a quiet signal. The cheap radio you can ignore.

Interestingly, at 144MHz NBFM it is possible to get a useable signal at -125dBm and with 1W (-30dBm) have a good 155dB acceptable loss. With a 10dB gain Yagi you can accept 165dB path loss. This corresponds to 50,000km with just 1 Watt of power and a 0dB gain antenna at the receiver.

It is not the power that determines the range, but how you use it.

Ok, that is my soapbox lecture for the day. I tend to get carried away when someone askes me "what is the range of a transmitter", or how can I calculate the range of a transmitter?".

Have I (eventually) answered your question?

BR Harry - SM0VPO

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70Mhz Band Propagation Characteristic

Post by dare4444 on Tue Mar 19, 2013 3:41 pm

I am using Radio Mobile http://www.cplus.org/rmw/rmonline.html to predict my FM broadcast band transmitter coverage. However, it doesn't take 88-108 Mhz frequencies so I had to put in 144 Mhz. What about 70Mhz band? A fm transmitter broadcasting at 90.1 Mhz will be similar to a 70Mhz transmitter in range right?


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