Coverage formula UHF Vs. VHF

[kb0rpj]

Is there a standard formula for converting VHF expected coverage to VHF.

In other words, if I have a VHF 2 way radio system and I changed it to UHF, it would likely reduce coverage by 5% 50% 75% ?

 

[nd5y]

I have never seen or heard of one.
You can use propagation prediction software on specific base sites and mobile configurations on different bands and see the difference but there are too many variables for one simple formula.

 

[N4KVE]

Years ago I did a few non scientific tests with a few friends. Test 1, my friend, & I were both in our cars parked in front of our jobs. We were a few miles apart, & went to a simplex freq. VHF was full quieting, & a little bacon frying on UHF. Another time a test with 2 dual band HT’s a mile apart, again on simplex. VHF had a little bacon frying, & UHF had more. So I’d “guesstimate” UHF gets 80% of the range VHF does.

 

[rescuecomm]

My experience with UHF/VHF relates with SAR operations in the late 1980s and early 1990s. Rescue/FD could cover the state park on VHF simplex 5 watt portables as long as one didn't go over the ridge dividing east/west. Park rangers and county sheriff were on UHF. If they went off their repeaters, they disappeared radio wise. To be fair, most UHF portables were running stubby duck antennas which reduce range.

N4KVE is on with his tests with equal radios and antennas. About 70 to 80 percent depending on terrain.

On replacing systems, UHF base/repeaters can use antennas with twice the gain in the same space as a VHF with the same physical size. But the coax loss will be higher on UHF so that's a factor also.

 

[mmckenna]

Is there a standard formula for converting VHF expected coverage to VHF.

In other words, if I have a VHF 2 way radio system and I changed it to UHF, it would likely reduce coverage by 5% 50% 75% ?

It really depends on the application.

I run a few VHF systems alongside some UHF and 800MHz systems. There are situations where 800MHz absolutely works better than VHF. Shorter wavelengths/higher frequencies can do a better job penetrating buildings, reflecting off surfaces, etc. Often in an urban environment, UHF will easily outperform VHF.

Get out in the woods, and VHF can perform better.

Good commercial grade propagation modeling tools will show this and take things like buildings and ground clutter into account. It gets expensive and complex, but it'll map it out for you pretty well. I'd question any percentage number as a flat rule of thumb. Just too many variables involved.

 

[AF1UD]

Does UHF digital help at all compared to analog? I.e. NXDN/DMR?

 

[mmckenna]

Does UHF digital help at all compared to analog? I.e. NXDN/DMR?

I've found 6.25KHz NXDN to reach farther than 12.5KHz analog. That was on 800MHz. I'd imagine that the incremental improvement would be similar between bands.

 

[AM909]

The FSPL formula says theoretical path loss is 9.5 dB worse, UHF vs. VHF. However, better antenna efficiency on UHF than VHF (antenna length and ground plane efficiency on HTs especially), and better building penetration probably help offset this. Noise floor and receiver performance can be significant, too, especially since most of the spurious garbage from switchers, lighting, etc. seems to be worse at lower frequencies.

 

[DVINTHEHOUSEMAN]

In my personal experience, VHF works great everywhere else except concrete jungles. I've gotten about a mile and a half of intelligible analog speech out of my VHF radios in a thickly forested area whereas UHF will maybe get you a mile in the best circumstances. That said, in the city, or in really any modern industrial or commercial building, VHF will only work if both radios are near each other. I've tried using them in a Walmart to communicate outside and it was just full of static. I'm sure UHF would've worked perfect in that situation.

There isn't really a formula since you have to take into consideration antenna type and length, output power, alignment, terrain, body loss, etc.

 

[dlwtrunked]

Traveling in Virginia mountain areas, there have been places where VHF works better and places where UHF works better. This was from a car with a dual band antenna to a repeater with a dual band antenna. There are factors of free space loss, antenna gains, terrain and other obstructions. There is no standard reliable formula taking *all* these into account as some are hard/impassible to reasonably model. (Note: I have worked, as a mathematician, in modeling other similar phenomena.)

 

[EWC_BDN]

To me, VHF portables are pretty poor. If you're using portables the UHF ones just do it better.

Mobiles is a different beast. you can use a long antenna on a mobile with 3dB gain no problem. you take that 50 inch long antenna off the mobile and coil it up to an 8 inch one on the portable.

Had a farmer on an ATV say his VHF wasn't working well enough to talk to guys in the same field. We gave him a little antenna to mount on his handlebars and use that one his portable. He was very pleased and said he hadn't found the end of the coverage yet.

VHF also has a MASSIVE issue with modern vehicles. Modern Farm equipment puts off all sorts of RF emissions that kill you range. you get similar problems with LED work lights. To get a VHF radio to work, sometimes. You can generate a signal weak signal over the air to a vehicle with the key off. Turn the key on. you're weak signal is now gone. I've had to raise it by 20dB in some cases to get the same weak signal on the radio.

VHF can work great in ideal situations. it's lower loss in free space does give you a better range in theory, but if I was putting something new in and had no reason to use to VHF? i would avoid it. You'll need more sites on a large system, but you'll have fewer issues. especially when you have cities and towns.

Like around me there's a state wide system. Every city has to build it's own system because the wide area one doesn't work in their city.

where i live the statewide system is 700Mhz. nobody needs to make their own. All the cities can use it just fine. (it replaced an 800mhz system so the number of sites was probably similar if not the same)

 

[EWC_BDN]

I've found 6.25KHz NXDN to reach farther than 12.5KHz analog. That was on 800MHz. I'd imagine that the incremental improvement would be similar between bands.

Found the same to be true. on a UHF system. the customer needed more data for something so they went to 12.5. no big deal right? wrong, now there's coverage issues in new places.

 

[RFI-EMI-GUY]

The increased noise floor of VHF along with poor portable radio antenna performance places UHF at the advantage.

 

[speedway_navigator]

To me, VHF portables are pretty poor. If you're using portables the UHF ones just do it better.

Probably due to the crappy ground plane and stubby antenna. I told some non technical folks the ideal 800 MHz portable antenna would have a ground plane about the size of a CD at the antenna base.