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Author Topic: VHF vs UHF Range  (Read 31374 times)
LB5KE
Member

Posts: 141




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« Reply #30 on: December 21, 2011, 02:58:51 PM »

Regarding vhf vs uhf using directional antennas :
http://www.qsl.net/oz1rh/troposcatter99/troposcatter99.htm
From  OZ1RH 's  study :

Greater range on 70 cm than on 2 m

70 cm may have greater range than 2 m, because:

a. lower noise level in the sky means you can take better advantage of a low-noise preamplifier in your 70 cm receiver

b. greater path loss is compensated by a larger antenna gain, given the same physical dimensions of the antenna

c. more frequent ducting because a smaller duct will do

Why do most amateurs then think that 70 cm has shorter range?

    fewer other amateurs are active
    greater antenna gain => smaller beam width
    difficult to have the same transmitter power output
    in the old days it was more difficult to make a low-noise preamplifier for 70 cm than for 2 m
    greater cable loss
« Last Edit: December 22, 2011, 11:12:08 AM by LB5KE » Logged
K1CJS
Member

Posts: 5855




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« Reply #31 on: December 27, 2011, 07:24:52 AM »

Regarding vhf vs uhf using directional antennas :
http://www.qsl.net/oz1rh/troposcatter99/troposcatter99.htm
From  OZ1RH 's  study :

Greater range on 70 cm than on 2 m

70 cm may have greater range than 2 m, because:

a. lower noise level in the sky means you can take better advantage of a low-noise preamplifier in your 70 cm receiver

b. greater path loss is compensated by a larger antenna gain, given the same physical dimensions of the antenna

c. more frequent ducting because a smaller duct will do

Why do most amateurs then think that 70 cm has shorter range?

    fewer other amateurs are active
    greater antenna gain => smaller beam width
    difficult to have the same transmitter power output
    in the old days it was more difficult to make a low-noise preamplifier for 70 cm than for 2 m
    greater cable loss

I said it once, and I'll repeat it--since others seem to be too eager to ignore the plain facts as stated:

Quote
If you take a simple fifty watt 2 meter repeater--one transmitter, one receiver, and compare it to a simple 50 watt 440 mhz repeater.....with one transmitter and one receiver, hanging similar gain antennas at the same height for both systems, the two meter repeater will win the distance competition hands down over most any terrain--PERIOD.


Referencing to the numbered points in the first quote: 

1.  With NO preamplifier on either the two meter or the 70 cm machine.

2.  NO greater gain antenna for the 70 cm repeater vs. the two meter machine.

3.  Ducting, at best, is an abnormal circumstance--which makes this point moot.

So, in general with all circumstances (receivers, transmitters, antennas, etc.) being equal, the two meter band has greater range than the 70 cm band, just as the ten meter band have greater range than the two meter band. 
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ONAIR
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Posts: 1735




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« Reply #32 on: December 27, 2011, 10:18:14 AM »

       So then, with ALL things being equal, and ZERO help from the ionosphere, ducting, reflections, the wind, etc...   In a point to point ground communication, which (ham) frequency would have the greatest range? (including HF)
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WB6BYU
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Posts: 13008




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« Reply #33 on: December 27, 2011, 06:33:38 PM »

That still depends on a lot of factors, including antenna height, ground conductivity, etc.

Off-hand I'd say 160m ground wave - that will get over hills that will block VHF signals.
Attenuation is fairly constant with distance (for a given ground characteristic) and
higher power makes a big difference in extending the range to overcome the losses.
Losses increase with frequency, so ground wave really isn't very practical on any HF
bands.  (160m, of course, isn't an HF band.  It is the only MF ham band.)  Over salt
water this certainly would give the best range, while across rocky terrain the range
will be significantly less.  From Galveston its probably an easier path to Tampa than
to Abilene, even though the latter is shorter.

But if the antennas are high enough in the air, you can probably get a longer line-of-sight
path and use VHF/UHF.  The White Alice (or "White Elephant") system in Alaska relies on
troposcatter to cover relatively long distances - with enough transmitted power you can
collect enough microvolts at the other end rather reliably.  If your points happen to be
on mountaintops then you can get much further - 100 miles or more isn't uncommon for
repeaters on hills around here but on the flats without the benefit of a 7000' tower it
usually is less than that (especially with 7000' hills between the two stations.)

So you still have to specify further what conditions you are considering before one
can give a definite answer.  For example, if you would allow a pair of 13dB gain yagis
on the VHF path, would you allow the same antenna gain on 160m?
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K1CJS
Member

Posts: 5855




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« Reply #34 on: December 28, 2011, 11:46:41 AM »

      So then, with ALL things being equal, and ZERO help from the ionosphere, ducting, reflections, the wind, etc...   In a point to point ground communication, which (ham) frequency would have the greatest range? (including HF)

In an ideal circumstance, 160 meters should be.  Remember, though that there are other atmospheric, magnetic and natural factors that nobody has any control over that influence radio signals.  Generally speaking, while you can 'work the world' with HF band radio, try doing it with 70 cm.  You simply can't.
« Last Edit: December 28, 2011, 11:52:59 AM by K1CJS » Logged
WB6BYU
Member

Posts: 13008




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« Reply #35 on: December 28, 2011, 12:49:01 PM »

Quote from: K1CJS

Generally speaking, while you can 'work the world' with HF band radio, try doing it with 70 cm.  You simply can't.



But only because the situation was qualified by not allowing reflections.

I was at one end of a QSO from New Zealand to Germany on 70cm.  Others have worked further.
This would be a more common occurrence if more hams were equipped to make use of it.  All it
takes is good antennas, high power, a good receiver, and the moon in the right place.
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LB5KE
Member

Posts: 141




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« Reply #36 on: December 28, 2011, 05:24:17 PM »

So 10 and 160 meter "surface wave" has a longer range than 2 meter? I have never managed to get longer range on HF than on 2 meter, perhaps i haven't tried hard enough.

(By surface wave i mean: waves traveling along the earths curvature, not using special modes like tropo, e1,fi,f2 and so on)
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WB6BYU
Member

Posts: 13008




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« Reply #37 on: December 29, 2011, 08:37:09 AM »

Again, it depends on the ground conditions, power levels, and height above terrain of the VHF
antennas.

And, of course, we're not talking about HF ground wave, were talking about MF.

10m won't do it - the attenuation of the ground wave is too high to get much distance.  Most
local 10m (and 11m CB) propagation is space wave, just like VHF.)

On 80m, somewhere around 50 miles is a practical limit for ground wave propagation.  It
requires vertical polarization, of course, and relatively high power.  Attenuation is lower
with good ground conductivity - that attenuation is the limiting factor, which is why
ranges over salt water are much longer.

On 160m I'd guess you'd get at least twice the range, perhaps more.  Seems to me a
friend used to work 100 to 150 miles using 100W, but it gets difficult to separate the
propagation modes at some point between what is ground wave and what is reflected
from the ionosphere.  (Similarly with VHF:  sometimes you can't say whether a particular
path is purely line-of-sight or has some tropo enhancement.)

And then, again, you have to carefully define the conditions:  does "everything else
being equal" mean you have the same size antenna on 2m and 160m, or the same
antenna gain?


But let's say that the 160m ground wave range is around 100 miles / 160km.  Clearly
there are conditions where VHF won't cover such a path between two points, and
others where it will (when the antennas are on mountain tops, for example.)  You may
easily work such distances using SSB on 6m, but is that really tropo or not?  How do you
know that there isn't also some knife-edge diffraction from the mountain in the middle
of the path, or a reflection from one off to the side?  With just about any mode you
can find one location that can't hear the signal and another at a further distance that
can - so which one determines the "range"?

That is why, to me, the question really doesn't make any sense because there are so
many variables.  If you specify two particular locations, antenna heights, transmitter
power levels, and required path reliability then one could better assess the options.
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