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Author Topic: VHF vs UHF Range  (Read 33980 times)
KE4VBG
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Posts: 220




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« on: December 25, 2010, 07:10:30 PM »

If you have a 2 repeaters with the same gain antennas and identical power output at the same location, one on VHF and the other on UHF, which would allow 2 mobile units to communicate at a greater distance in a rural setting given hilly terrain.

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KM3F
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« Reply #1 on: December 26, 2010, 02:46:44 PM »

Assuming  your referring to 6 and 2 meters at the same power, the 6 meter usually has some sleight advantage at a distance but not by much.
Often depends on the Mobile antenna used and what a base station uses and where they are.
One item often not thought much about is the average balance between the repeaters ERP power to get out and it's ability to receive that is often determined by the power the mobile or base is outputting.
Often a Mobile can hear the repeater but can't get back with a solid signal.
Repeaters often use 100 watts to a gain antenna that a mobile does not enjoy.
My fellow ham just 1/2 mile away has  6 and 2 meter repeaters at his home, so know what it does on both bands over the same terrain at a distance, as I travel.
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WB6BYU
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« Reply #2 on: December 26, 2010, 03:09:12 PM »

It depends on the siting of the repeater and the nature of the hilly terrain.  Generally I'd say that 2m has
an edge (or 6m in low, rounded hills) while 440 is better in urban areas where there are more flat surfaces
to reflect off of.  But it will depend on the type of ground cover on the hills (different absorbtion rates
on different frequencies) and that may change between winter and summer.

Generally speaking the lower VHF bands are better at diffracting over a hill and UHF is better for reflecting
around it.  The best way to find out is to put a transmitter for each band on top of the hill where you are
planning to install the repeater and have someone drive around the coverage area recording the signal
strengths.
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KD5KFL
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« Reply #3 on: December 27, 2010, 08:20:36 PM »

if memory serves VHF is 30 - 300 mHz and UHF is 300 up. which would make the question about 2M versus 70CM
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AF6D
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« Reply #4 on: December 27, 2010, 09:22:30 PM »

Yes, UHF is above 300MHz but the question is fair. The edge that UHF has over VHF is building penetration. But that edge is lost with path loss. VHF, according to recent research, yields about a 20% improvement over UHF. Theory and practice sometimes don't agree.

As one that lives in the mountains, uses 6 meters and 2 meters, and as an avid CHP listener, I can speak to the differences of 6 meters versus 2 meters. In the mountains 6 meters wins and not by a slight margin. As stated by theory there is a 20 times increase in range at 6 meters over 2 meters. A 100 watt 6 meter repeater into a unity gain antenna will usually out-perform a 100 watt 2 meter repeater into a 6dB gain antenna. But nothing comes for free. 6 meters will also become unusable with effects of the sunspot cycle(s) and atmospheric noise. I love to hear police dispatchers come up with different reasons for the noise. They haven't got a clue.  Cheesy

One reason that the 6 meter repeaters works better especially in the mountains is refraction, but the other is the unity gain antenna. It's pattern drops down into the canyons better than a gain antenna even at 6 meters. Although a Diamond 5/8 over 5/8 wave vertical does exhibit a true 6dB gain (we tested by decreasing the power 1/2 at a time and noting the S meter,) it worked against me up here in Big Bear.

I originally mounted the antenna about 3 feet above my roof and could hear and talk into the 6 Shooters Network as soon as I raised the antenna up to 20 feet I could no longer hit the repeaters. I was clearly knife edging in and out of the canyons that the increase in altitude affected. The same occurred on 2 meters in that I could hit some machines at low antenna height while not others. UHF was next to impossible. For SAR operations many of us bought the Yaesu VX-7 and added the longer 6 meter antenna for good HT simplex coverage.
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K1CJS
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« Reply #5 on: December 29, 2010, 04:49:56 AM »

Generally speaking, VHF has the edge in distance over UHF.  You can get much better distance coverage with a 2 meter rig than you ever would with a comparible 70 cm rig.
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VA3WXM
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« Reply #6 on: December 29, 2010, 06:15:52 AM »

Path loss is greater with UHF so all else being equal, VHF would win out.
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WX7G
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« Reply #7 on: December 29, 2010, 11:54:14 AM »

Given the same antenna gain the UHF receive antenna has less capture area by the ratio of the two frequencies. Another way to put it is the UHF antenna has a higher AF (Antenna Factor). Comparing 144 MHz to 432 MHz, and given the same antenna gain, the AF of the 432 MHz antenna is higher by a factor of 3, or 9.5 dB.

What does this mean for maximum communication distance? For a free-space path the 144 MHz range is 3X the 432 MHz range.

When free-space path loss is quoted the path is dipole-to-dipole. The higher the frequency, the less capture area that receive dipole has. It's not that there is more loss at a higher frequency through a vacuum, it's that there is less receive antenna capture area.
« Last Edit: December 29, 2010, 11:56:53 AM by DAVE CUTHBERT » Logged
WB6BYU
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« Reply #8 on: December 29, 2010, 12:02:01 PM »

So for antennas of the same physical size at both ends of the path, the 440 antennas should have
more gain (assuming they are properly designed) and that should make up for the difference in path loss.
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WX7G
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« Reply #9 on: December 29, 2010, 04:46:59 PM »

Yes, for VHF and UHF receive antennas having the same capture area the received signal power will be the same.
« Last Edit: December 30, 2010, 01:10:24 AM by DAVE CUTHBERT » Logged
AF6D
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« Reply #10 on: December 29, 2010, 08:57:41 PM »

But that's just it. In a repeater installation on a mountaintop or tall tower one limits the gain as necessary. It is common for a repeater antenna to be no more than 6dB. 9dB from a high altitude has one of a doughnut hole! Since in this example both antennas are assumed to be the same in gain the 2 meter wins. I know a ham up here at 6,300 feet that paid $1k to have tree climbers mount a 9dB VHF stick at the top of 120 foot tree and was most unhappy that he couldn't talk local.
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K1CJS
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« Reply #11 on: December 30, 2010, 05:56:35 AM »

Yes, for VHF and UHF receive antennas having the same capture area the received signal power will be the same.

Come on, Dave, it isn't that simple--and you should know it!
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WX7G
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« Reply #12 on: December 30, 2010, 07:30:06 AM »

For a free-space path, which is the condition listed in my post yesterday at 11 AM, it is that simple.

Now back to the original question which is communication distance for VHF vs. UHF given the same antenna gain and a repeater-to-mobile path. The answer is VHF. Given the same receive antenna capture area , or AF (Antenna Factor), the difference in communication distance between VHF and UHF is less.
« Last Edit: December 30, 2010, 07:36:37 AM by DAVE CUTHBERT » Logged
K1CJS
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« Reply #13 on: January 08, 2011, 05:20:21 AM »

For a free-space path, which is the condition listed in my post yesterday at 11 AM, it is that simple.

A 'free-space' path is an ideal situation--something that exists only on paper.

Quote
Now back to the original question which is communication distance for VHF vs. UHF given the same antenna gain and a repeater-to-mobile path. The answer is VHF. Given the same receive antenna capture area , or AF (Antenna Factor), the difference in communication distance between VHF and UHF is less.

Which is what the majority of the people here have already said, even before you posted.  Just as the HF bands have greater range than the VHF bands, the VHF bands have greater range than the UHF bands--in the real world.

I'm not arguing with your analysis, Dave, I'm just saying that actuality trumps mathematical 'perfection'.  Remember--mathematics PROVED that a honeybee can't fly, but it still does.
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KE4VBG
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Posts: 220




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« Reply #14 on: January 10, 2011, 08:53:28 PM »

Thanks guys for all the input on my question. This started during a conversation I had with a local Deputy Sheriff about coverage of the repeater they use. They went from 155mhz to 465mhz and they can't cover the county very good anymore. I guess that's why the THP still uses 42.42mhz.
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