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Optimum frequency for maximum mobile direct simplex coverage?

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g8tzl2004
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Optimum frequency for maximum mobile direct simplex coverage?

Postby g8tzl2004 » Sun Aug 20, 2017 10:14 am

Anybody have any views which frequency (ie Low Band/ Mid Band / VHF/UHF) will give the maximum direct mobile coverage without any skip conditions using an identical "equivalent" vertical dipole or quarter wave mobile antenna cut to the correct frequency?

I think I once read that 86 MHz was the optimal?

Low band is great for skip DX but I'm always disappointed with 29 and 50 MHz direct coverage...maybe its due to poor/inefficient ground planes on vehicles...I guess a vehicle roof is not big enough for an efficient ground plane at 29 and 50 MHz whereas its just right at 146 MHz.

Its a pity the US does not have 70 MHz...I think this is probably the optimal frequency for ham mobile operation.

For handheld operation, the large quarter wave antenna size and massively inefficient 29 and 50 MHz helicals and lack of any ground planes makes 146 and 223 MHz optimal with 446 MHz good for inside buildings and 900 / 1297 MHz good for tunnels!!!

Any views?

KI4LTX
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Re: Optimum frequency for maximum mobile direct simplex coverage?

Postby KI4LTX » Sun Aug 20, 2017 1:29 pm

I am not sure what kind of terrain you are dealing with (it does matter a lot), but here in the flat-lands of Southwest Florida, I find that 6 meters performs the best in both vehicle to vehicle and fixed station to vehicle propagation. My wife and I are both amateurs, and have 6m, 2m, and 70cm FM available in our vehicles. We are using Larson NMO2/70 and Larson NMO50 antennas mounted on the roof of our vehicles. 6m gives us a noticeable edge (2-3 miles minimum ) over 2 meters, and quite an edge over 70cm. to be fair, our transceivers are rated slightly less on 70cm, but since this is essentially line of site communications, I am not sure how much the output power has to do with it. I believe that foliage gives us more attenuation at 70cm. We usually use 2m, but every time we are on the fringe of that connection, we can switch to 6m for a much clearer signal.

I did some early experimentation with 10m FM, but local electrical noise reduced the effectiveness so much that all other bands gave us better coverage.

Hope this helps!

John
AA4TX

g8tzl2004
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Re: Optimum frequency for maximum mobile direct simplex coverage?

Postby g8tzl2004 » Mon Aug 21, 2017 5:11 am

Interesting comments - thanks

Is the Larson MNO50 also a 5/8 on 2m? ie it is a loaded 1/4 wave and not a full length 1/4 wave?

KI4LTX
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Re: Optimum frequency for maximum mobile direct simplex coverage?

Postby KI4LTX » Mon Aug 21, 2017 8:02 am

g8tzl2004 wrote:Is the Larson MNO50 also a 5/8 on 2m? ie it is a loaded 1/4 wave and not a full length 1/4 wave?


I am not sure. I have always run two antennas, one dual band for 2/70, one for 6 meters. Currently 10+ years on the current setup. I never tried to load the NMO50 on 2 meters.

John
AA4TX

jry
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Re: Optimum frequency for maximum mobile direct simplex coverage?

Postby jry » Mon Aug 21, 2017 11:00 am

lower the better ...10 Meters .

Antenna's get to be a little more interesting to optimize.

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motorola_otaku
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Re: Optimum frequency for maximum mobile direct simplex coverage?

Postby motorola_otaku » Mon Aug 21, 2017 1:18 pm

NMO50 is a loaded quarter-wave, as are most (if not all) of the other low band base load antennas.

FM is the real limiting factor when it comes to low band mobile communications. In a noise-free environment SSB on 10 meters will talk hundreds of miles with 100W and a quarter-wave antenna.
When exposed to Rapid Fire Growth or Thermal Emergency, two things are lost:
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g8tzl2004
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Re: Optimum frequency for maximum mobile direct simplex coverage?

Postby g8tzl2004 » Mon Aug 21, 2017 4:16 pm

I wonder how DMR ( or whatever audio digital mode) would perform using a 2.4KHz filter (like with SSB)?

As you narrow the bandwidth, radio range increases....so CW works even better using a 500Hz filter.

I wonder if its possible to use a digital "voice" mode with a super narrow 2.4KHz BW?

You can easily notice a 3dB improvement in RX sensitivity when you switch from 25/20 to 12.5KHz spacing.....you can even notice the 3dB RX sensitivity improvement using an FT817 when listening to a weak FM signal and switching from wide to narrow filters (15KHz to 6 KHz filters I think)!!!

KI4LTX
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Re: Optimum frequency for maximum mobile direct simplex coverage?

Postby KI4LTX » Mon Aug 21, 2017 5:34 pm

jry wrote:lower the better ...10 Meters .


That might be true if you ignore the ambient noise floor. But it is also true that at the lower frequencies, the more QRM you have (and it is getting worse). The old GE noise blanker used to take care of a fair amount of this, but today's transceivers (FM, that is) pale in comparison, so the end to end connection suffers. So, for the bands I have available to me, and for FM, 6 meters performs the best for our terrain.

We have worked a bit with 2 meter SSB, and when stationary in the vehicles, this has given us the best distance, but in a mobile environment, noise gets in the mix, and attention to the transceiver is tiresome. FM is definitely better for driving.

As for digital, I have no experience below 2 meters. I wonder if multi-path issues might raise their ugly head on the lower frequencies.

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motorola_otaku
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Re: Optimum frequency for maximum mobile direct simplex coverage?

Postby motorola_otaku » Thu Aug 24, 2017 7:53 am

KI4LTX wrote:As for digital, I have no experience below 2 meters. I wonder if multi-path issues might raise their ugly head on the lower frequencies.

There's a Yaesu Fusion calling freq on 10 meters. I've heard it used before during Field Day... it sounds surprisingly good.
When exposed to Rapid Fire Growth or Thermal Emergency, two things are lost:
1. Ability to use fine motor skills
2. Presence of mind

g8tzl2004
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Re: Optimum frequency for maximum mobile direct simplex coverage?

Postby g8tzl2004 » Thu Aug 24, 2017 9:29 am

Will they ever have "digital SSB" which uses a very narrow 2KHz BW and sounds like DMR but has the same flat condition VHF coverage range as normal SSB ie 100 miles vs 25 miles??

Why does a VHF SSB signal travel so far when its all meant to be line of sight? I guess horizontal antennas help? I guess its also to do with the minimum disernable signal (the narrower the BW the better) and refraction?

Jim202
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Re: Optimum frequency for maximum mobile direct simplex coverage?

Postby Jim202 » Thu Aug 24, 2017 11:36 am

I have been a long time user of 6 meter FM. I have always used the MNOQ base with the longest whip that Larson sold. Not exactly sure, but think it was like 52 or 59 inches long. Been about 20 years since I purchased them. But the range was great. In southern New Hampshire we were able to get between 20 to 30 miles most of the time. I was impressed. You can't get that with 2 meters without using a repeater.

Have to tell you that we were running 100 watt transmitter on 6 meters in the vehicles. Have used a number of different radio models over the years. Like Master II, GE Delta S, Motorola Syntor X9000 and the likes. They all seemed to perform about the same. As the radios changed, so did the features they provided.

Problem with 6 meters today is that the local noise floor has come way up due to the high use of computers, poor shielding on computers, cable TV leakage and radiation from computers, Ethernet devices like hubs, switches, gateways and the interfacing to WIFI devices. Even some LED lighting is generating enough noise to kill 6 meter use in some areas.

Many of the well used 6 meter repeaters are gone. The noise floor has killed most of the 6 meter repeaters I use to use even 5 to 10 years ago.

Jim

g8tzl2004
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Re: Optimum frequency for maximum mobile direct simplex coverage?

Postby g8tzl2004 » Thu Aug 24, 2017 1:34 pm

All because the Chinese manufacturers of these QRM devices can't be bothered installing 10 cents worth of suppression components? Often you can see the location on the PCB where the suppression components should be but to save 10 cents they are excluded once FCC approval is obtained for the device?? More should be done to stop all this QRM!!

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Astro Spectra
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Re: Optimum frequency for maximum mobile direct simplex coverage?

Postby Astro Spectra » Fri Aug 25, 2017 1:04 am

In mixed urban and rural I find 100W P25 into a quarter wave in the centre of the roof delivers superb simplex comms. I'm a fan of 10m but even at 100W it's not panacea. I have access to 70 MHz (I'm outside the USA) but that's limited to 25W and I find the noise limits what you can do there.

I've tested some digital modes on 80, 10, and 6M and while it freaky to see P25 working over 600 miles on 80m it hardly a convenient mobile option.

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motorola_otaku
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Re: Optimum frequency for maximum mobile direct simplex coverage?

Postby motorola_otaku » Tue Sep 05, 2017 7:54 am

Astro Spectra wrote:I've tested some digital modes on 80, 10, and 6M and while it freaky to see P25 working over 600 miles on 80m it hardly a convenient mobile option.

Service monitor modulator/exciter, or something more exotic?
When exposed to Rapid Fire Growth or Thermal Emergency, two things are lost:
1. Ability to use fine motor skills
2. Presence of mind

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Astro Spectra
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Re: Optimum frequency for maximum mobile direct simplex coverage?

Postby Astro Spectra » Tue Sep 05, 2017 11:30 am

XTL2500 with up/down converter based on Syntor for 10m and 6m, the HF up/down converter is home brew but uses the relatively exotic TN-612 AN/TRC-179 Regency Net HF pre/post selector. The other end of the 80m link was made with a Codan HF radio and a HP signal generator.

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syntor9k
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Re: Optimum frequency for maximum mobile direct simplex coverage?

Postby syntor9k » Sun Oct 01, 2017 7:22 pm

Around here in the Bay Area/ Monterey Bay Area the noise floor for 10-6m is quite high and even on 2m/ 70cm, its bad. It seems to be broad band interference. Sometimes it does not register on the radio S-Meter, but is heard on the repeaters signal. S9+40-60db and there is static on it?

Anyway, I think its 2m; 5/8th wave roof top mounted antenna and 100W is most likely to do the best considering that, a gain antenna is possible, the roof is close enough to a big enough ground plane, in theory less noise (RFI/ EMI), and the wave length is less likely to get caught up in things to attenuate it.


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