How far with VHF?

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Metradio
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Re: How far with VHF?

Post by Metradio » 10 Mar 2018, 17:19

I take it you are not one for snap decisions? rule of thumb estimations? shopping without a shopping list? or taking a dump without any paper?

Mike
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paulears
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Re: How far with VHF?

Post by paulears » 10 Mar 2018, 18:38

Oh no - I do that all the time, apart fro. the last one - what gets me is the point in a rule of thumb that doesn't make sense.

isn't it like those crazy claims on radio boxes - 8 mile range, or 10 mile range? The range of my repeater here is no more than 5 miles - I know this because that's what it says on my licence application. In practice, in one direction I can get into it ten miles away, but in the other it's pushing it to do 3!

I'd never heard this LOS+⅓ thing. I just cant' get my head around how it works at all?

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Re: How far with VHF?

Post by VintageTin » 10 Mar 2018, 19:21

paulears wrote:
10 Mar 2018, 15:38
exactly - so what is line of sight, when you can't see it? A mile? 10 Miles? You can't have a formula to work to with an unknown variable?
Wikipedia helps.

d=3.57 x Sq root of h
where d is in kilometres and h is height above ground level in metres. The constant 3.57 has units of km/m½.

Examples:

For an observer standing on the ground with h = 1.70 metres (5 ft 7 in), the horizon is at a distance of 4.7 kilometres (2.9 mi).
For an observer standing on the ground with h = 2 metres (6 ft 7 in), the horizon is at a distance of 5 kilometres (3.1 mi).
For an observer standing on a hill or tower of 100 metres (330 ft) in height, the horizon is at a distance of 36 kilometres (22 mi).
For an observer standing at the top of the Burj Khalifa (828 metres (2,717 ft) in height), the horizon is at a distance of 103 kilometres (64 mi).
For an observer atop Mount Everest (8,848 metres (29,029 ft) in altitude), the horizon is at a distance of 336 kilometres (209 mi).
Malcolm

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Metradio
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Re: How far with VHF?

Post by Metradio » 10 Mar 2018, 20:14

With the aid of an OS map you can look around the terrain and visualise what your line of sight is, how far away you are from distant hills etc - if you can see them you can work them (line of sight), now dependent on the terrain past the hills determins how far past the hills the RF will travel, at best you will get another 1/3 further, if your target is in a valley you could be out of luck..
If your line of sight is impeded by large or close buildings, you know what your theoretical max RF range would be, but without RF modelling software you are down do suck it and see guesswork.. It is well known that VHF signal travel well over open(ish) ground but run into trouble inside or in the RF shadows of buildings. UHF travels well in and around buildings but runs out of steam over long RF paths..
Sorry I can't explain it any clearer, but it is like trying to teach a blind man to ride a bicycle - he needs to take a few things for granted without seeing them..

Mike
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Re: How far with VHF?

Post by paulears » 10 Mar 2018, 23:11

I'm going to give up guys - I'm very aware of how to calculate theoretical distances from point to point, and am quite content that I can derive a reasonably accurate path loss from point A to a known repeater location and a known repeater height. However - with a repeater, in the middle of say, a desert with no hills - if the workable distance to the repeater is for example 10 miles, then the maximum distance to any other users is 2 x 10m. If line of sight is a finite distance, then are we actually saying that path loss in free space is really line of sight multiplied by 1.33?? That seems to have no solid fact base. Indeed, following through Metradio's analogy, if from ground level to the repeater is uphill, then the next possible receive location would be something flying many.many miles away, or even in orbit - as in the satellites people regularly communicate through. Nobody has yet explained where this magic ⅓ comes from. Is is fact or fallacy? Possibly just coincidence? I cannot take this LOS+⅓ as a fact without a physics explanation. It may well be practically correct sometimes - but as people are saying - of course this doesn't work if you are in a valley, or in a spaceship - so that's hardly a rule to rely on. The accepted maths for the horizon is fine - but where is this extra magic ⅓ coming from? (and why?)

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Re: How far with VHF?

Post by Cornelius » 11 Mar 2018, 04:21

I'm guessing that the "+1/3" Metradio are talking about, applied to them in their specific situation - standing beside the truck, gazing to the horizon with their heads at approx. 1.7 meters, and calculate for the antenna, positioned on the roof of the truck (or a short mast) - 3 meters off the ground. Which would account for the +1/3 extra LOS distance... :)

But that's just me, guessing... :mrgreen:

As VintageTin said; h= 1.7 meters; horizon = 4.7 km.
And from the same formula; h=3 meters; horizon = 6.18 km - which is approx. 1/3 further in regard to LOS from 1.7 meter above ground... ;)

So there is no magic +1/3 - it just fits if one place the antenna a bit over you own head... ;)

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