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Author Topic: 2 meter range over average terrain  (Read 1003 times)
WB4ILP
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Posts: 38




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« on: August 28, 2004, 08:19:22 PM »

I have a VX-150 and a friend who is considering getting one.  He lives about 2 miles away as the crow flys but not line of sight.  The terrain is hilly and I can see the low hill he is behind from my back yard.  Could we expect to be able to communicate HT to HT at this distance under these circumstances? I have been able to communicate with a non-ham friend on FRS on the other side of a more prominent hill but at a shorter distance of about 1 mile.
Thanks for any comments!
Jim/WB4ILP
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K1CJS
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Posts: 6055




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« Reply #1 on: August 28, 2004, 09:13:25 PM »

At 2 miles, you shouldn't have any severe problems.  Remember, the average FRS radio has a transmitter in the milliwatt range.  The 150 has what?  5 watts?  The radios you and your friend have should have no real problem at all.
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KR4BD
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Posts: 236




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« Reply #2 on: August 28, 2004, 09:14:06 PM »

I think you are just going to have to try communicating on this path to see if it reliably works for you.  At VHF/UHF frequencies you have so many variables that affect propagation (hills, distance, height of antennas, gain of antennas, transmitting power, sensitivity of the receiver, buildings and even the leaves on the trees!).  I have had very reliable coverage on two meters for distances of 50-75 miles with a 2 watt 2 meter HT from mountain tops while I was unable to talk two blocks in a major urban area with the same radio due to all the intermod, buildings, etc.  

Tom, KR4BD
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KZ1X
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Posts: 3229




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« Reply #3 on: August 28, 2004, 10:05:25 PM »

I doubt it will be reliable.  Let's look at the math.

While the rig is capable of producing 5 watts or so RF, the antenna is the problem.  Referenced to a dipole, it probably has -12 to -16 dB gain.  So, compared to a reference dipole, your effective radiated power is at least 10 dB below 5 watts, or 1/2 watt -- at most.  It's probably more like 250 mW ERP.

Let's assume, to make the math easy, that you are having a good day, and your ERP is one watt, or, in a 50 ohm world (not 316 ohms; I digress) +30 dBm.  It isn't, but let's go with that.

Now, the path loss between the two stations is probably on the order of 100 dB, +/- the phase of the moon, the size of the refrigerator door, the siding on the neighbor's house, etc.  We really don't know, but this is a darned good guess.

So, the signal on the other end of the link is -70 dBm.  Then the other HT antenna knocks that down at leat another 10 dB, so, you buddy's receiver has to be able to hear -80 dBm.  Well, that's 0.02 millivolts at 50 ohms.  So, using these numbers, it ought to work.  

Ought to, I say.  Trouble is, if any of these numbers are off (they are), or you experience some kind of either interference or unanticipated attenuation (you will) the path goes to heck, really quickly.

Now, if you simply added a more efficient antenna, you'd probably have no trouble at all.  Probably.

You gotta do the math.



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K3CLT
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Posts: 55




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« Reply #4 on: August 28, 2004, 10:13:21 PM »

Just get the radios. If they work, no problem. If they don't, then make them work. Its ham radio. If you look at some of these answers, you won't want to buy the radios.
KISS
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N6AJR
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Posts: 9921




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« Reply #5 on: August 28, 2004, 11:01:09 PM »

put a homade 2 meter beam on top of the hill, run cox to the shack and see.. if you stand up there and talk an antenna in the same spot will work.... play with it.. thats half the fun,
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KB1LKR
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Posts: 1898




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« Reply #6 on: August 29, 2004, 12:27:22 AM »

Comparing to FRS sited earlier, they are 500mW w/ same or worse antenna. You could borrow a pair of FRS's and try them, if they work you are golden, if not, you're still not sure, but it may be problamatic. Assuming only equal antennas you have a 10dB or a 1.67 S-unit advantage.

Get'm and if you have trouble, get a longer antenna(s), say a 15-19" 1/4 wave, and/or add a 19" rat tail to the outer shell of the antenna connector(s).

You don't have a repeater available to both of you, or just prefer to stay simplex?
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WB2WIK
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Posts: 20636




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« Reply #7 on: August 30, 2004, 12:24:10 PM »

There's never any guarantee about this, but if your friend is on the opposite side of a "low hill" (how high is low?), it may not work well using HTs and stock flex antennas from ground level.

2 miles isn't very far, and over a flat field for two hand-helds *outdoors*, will almost always work fine.  But now take the HT's *indoors,* and you lose some signal.  Now add a hill between them, and you lose more.  But climb up the stairs to the second level of one of the homes, and you probably pick up some signal -- providing the house has an upper floor!  Too many variables to accurately predict anything.

If this is "house to house" and you want 100% reliable communications, can't one (or both) of you install a real antenna to connect to, instead of relying on the flexible antennas?  With a single outdoor, rooftop antenna, this path's workability is almost guaranteed.

WB2WIK/6
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W9PMZ
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« Reply #8 on: August 30, 2004, 01:18:35 PM »

Do the math,

Pr(dBm) = Pt(dBm) + Gt(dB) + Gr(dB) - 20*log[r](km) - 20*log[f]MHz - 32.44

assume Pr = .2uV or -101dBm, a guess of what a good signal maybe at the receiver.
Pt = 2W or 33dBm
Gt = -10dB (gain of transmit antenna, rubber duck)
Gr = -10dB (gain of receive antenna, rubber duck)
20*log(3) = 9.54
20*log(144) = 43.16

so Pr = 33 + (-10) + (-10) - 43.16 - 32.44
   Pr = -62.59dBm

since Pr is -62.50dBm which is well above -101dBm you should not have much problems.  Of course this is based on line of sight, but there will be enough scattering for your path to work.

73,

Carl - W9PMZ
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WB2WIK
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Posts: 20636




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« Reply #9 on: August 30, 2004, 05:45:09 PM »

Formulas are great, but it's impossible to predict much in this case.  Open field attenuation over earth is one thing; in free space, it's another; and when you're working with walls in the way (indoors in one home to indoors in another home), anything goes.  

Walls aren't transparent, nor are they severe obstacles, but exactly what they are is very difficult to predict, as is the "hill" discussed.  If antennas can be freely moved about to avoid nulls from out-of-phase reflected signals, I think the 2 mile path will be okay.  But if they must be "fixed," and happen to land in a null for the frequency used, 2 miles could be an impossible distance for low power at 2m over an obstructed path.

Then again, I once had a 24/7 communications path with a friend of mine when we were both in fifth grade, using Campbells' Tomato Soup cans and cotton string, over a nearly 2-block path!  It was 24/7 until it rained, anyway.  When the string got wet it didn't work as well.  After several days one nosy neighbor lady noticed the string running over her garage and had her husband pull it down with a long garden rake.

So much for secure communications!

WB2WIK/6
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W9PMZ
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« Reply #10 on: August 30, 2004, 07:23:43 PM »

Exactly, formulas are great and a good place to start when you model problems.  

Most good engineers start with models and adapt the models to find solutions to the problem.  I have yet to meet a good engineer to walk away and say it can't be solved.

73,

Carl - W9PMZ
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WB2WIK
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« Reply #11 on: August 30, 2004, 07:47:08 PM »

Carl, I agree with you entirely.

And being a good engineering student, when my evil neighbor broke our string, I tried replacing it with much stronger fishing line...

Eventually, we both got our Novice tickets and could chat on 40m CW any time we wanted to.

Necessity's a Mother!

73

Steve WB2WIK/6
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KZ1X
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Posts: 3229




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« Reply #12 on: August 30, 2004, 09:59:18 PM »

Carl's math is good, but for the newbie, it might be easier to define terms.

It's also good to point out the concept of margin.  Foe example, in Carl's numbers, he leaves a 38.5 dB margin.  At first glance, this looks healthy (and my previous post showed a similar conclusion).  But, Carl's antenna figures are (by my experience) generous by 12 dB total, right off.  

So the 38.5 dB margin is now 26.5 dB, or, equal to the loss from one set of aluminum siding on one building.  Pfft, goes the squelch.  Or not.  We really don't know.

There simply isn't enough information offered to be certain about this; and, scattering on this path at 146 MHz may hurt as much as help if one or both of the stations are indoors.

As has been said, just one decent antenna upgrade, and this should be a no-brainer.

Frequncy argument:  There's a very good reason police and others went to UHF years ago, besides crowding from the taxicabs and pagers.  Our own 446 MHz band is a much better choice for this kind of indoor-oriented operating.  Assuming one can operate from a fixed location indoors, it's a quick and simple matter to rig a 2W UHF HT to a 5/8 over 1/4 wave colinear with a short set of radials, on each end, and have an effectively DFQ intercom, no matter what.  

This is precisely what I use for Red Cross shelter setups, where ARES relays headcounts and other stats to the EOC during shelter activations and the like.  I use a portable extending mic stand (nice, heavy non-tilt base) as the support.  The top 1' is a stiff steel mic gooseneck, so I can 'tilt' the antenna out of a null if needed.
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W9PMZ
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« Reply #13 on: August 31, 2004, 06:23:43 AM »

Steve -

You indicated that perhaps I was 10dB off, is this in the gain / loss of the rubber ducks?  I had no real idea what the gain of a rubber duck is, but I just guessed it wouldn't be good.

In the exercise there maybe 20dB to 30dB of margin based on open terrain.  But there is alway scattering to a certain degree.  And you are correct that enough information isin't given, indoors, outdoors, siding etc.  

But, making the calculation tells us one important item, that the distance and frequency involved has a possible margin for a communications channel.

73,

Carl
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KC8VWM
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Posts: 3121




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« Reply #14 on: September 04, 2004, 02:40:02 AM »

I have communicated using 2 - 5 watt HT's on simplex over a distance of approx. 10 miles while camping one year.

It was a fun experiment.. Your milage may vary.
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