Propogation of different frequencies in different area for man portable communic - Page 1
Forum section: electronics / rf-microwave

author: CopperCone
author_rank: Super Contributor
timestamp: May 03, 2018, 04:55:29 pm
content: So i am interested in things like 2 way radios and walki talkis etc.What are the best frequency choices for person to person communications in different enviroments? I mean for voice and maybe slowdata.Does the military basically hog all the best frequencies?How much variation is there in frequency choice in different enviroments like. -heavy jungle-light forest-tundra/desert/plains-open water-mountains-valley enviroments-canyon enviroments (i.e. grand canyon like structures)Does the foilage and presence of ground humidity (moist soil, ice, snow, water) matter much?Or is itmuch more dependant on topography/elevation?To limit the discussion the transmit antenna should not be more then 3 feet longI dont care in this theoretical discussion for band allocation unless ithas some kind of meaning in physics.Pretend we are sending out scout teams to explore another earth like planet.

author: metrologist
author_rank: Super Contributor
timestamp: May 03, 2018, 05:10:58 pm
content: You are going to be limited by band regulation. In the US, I think you are looking at MURS, CB, FRS, GMRS, and Ham radio bands.Higher frequencies, such as UHF FRS/GMRS bands, foliage and other structures will attenuate the RF signal. Lower frequencies, such as HF CB/Ham bands have longer ground propagation and can benefit from "skywave" atmospheric reflection.I think 80 meters and lower provide fairly reliable ground wave propagation over long distance, specially at night.I remember PSK31 was a popular digital mode.I like 2meter ham band for the repeater network where I am, and APRS is a lot of fun (check out APRS.fi). VHF can benefit from tropospheric propagation - where the radio wave can reflect lower in the atmosphere due to differences in air temperature (air inversion zones).UHF Ham/FRS/GMRS are also reliable HH bands, but shorter line of sight range.I noticed that you did not refer to Boy scouts. It's a bit sad in that regard, what is happening there (OT)...I would give scouts FRS radios. maybe a 2-mile line of sight range, over water. Or about 1 mile max in the woods.GMRS is on the same band and can use more power and external antennas - so a 50W base with antenna 50 feet up will give a good range, probably up to 20 mile max radius, or longer if using directional antenna, to a mobile HT. Remember the foliage though.Without restriction, I might opt for the 6 meter ham band. Antenna's are a bit larger, but still manageable, and the wave will propagate better through the woods and terrain. Lower bands have antenna issues for you.Oh, and SSB is probably a more efficient voice mode for you.

author: CJay
author_rank: Super Contributor
timestamp: May 03, 2018, 05:45:39 pm
content: This might help:http://www.voacap.com/p2p/index.html

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: May 03, 2018, 09:20:02 pm
content: An amateur licence gives a wide range of MF, HF, VHF and UHF frequencies. So, no, the military do not hog the best frequencies.Antenna efficiency is a major limitation given your 3 foot maximum length. A loop may also be a possibility.Some examples of pedestrian mobile antennas are described at:http://home.alphalink.com.au/~parkerp/projects/projped.htmYour antennas will be more efficient at higher frequencies as they are a greater proportion of their wavelengths.But working against this is the characteristics of frequencies and local noise. Very roughly lower frequencies can penetrate better but are nosier in urban areas.An efficient mode such as SSB or slow speed data can do better than FM or wideband data.My video channel has a lot of pedestrian mobile communication with handheld antennas. (https://www.youtube.com/results?search_query=pedestrian+mobile+vk3ye)Of greatest interest are probably the videos featuring contacts on 28, 50 and 144 MHz.

author: David Hess
author_rank: Super Contributor
timestamp: May 04, 2018, 12:47:08 am
content: Physical considerations like antenna size argue for 2 meters and above but that is not a problem since this covers the best bands anyway.In the open somewhere between 2 meters and 70 centimeters (144 MHz to 450 MHz) works out the best in my experience. Inside buildings, 33 centimeters (800 to 900 MHz) and shorter have an advantage because the wavelength is small enough to propagate within the building and escape.

author: mark03
author_rank: Frequent Contributor
timestamp: May 04, 2018, 04:17:11 am
content: You are ruling out infrastructure, correct? I mean things like repeaters.For somewhat wider coverage than is possible on VHF/UHF, "near-vertical incidence" HF can be used. You design an antenna to send most of the power upward, instead of towards the horizon like hams usually prefer. Then use a frequency low enough (40 meter band is popular) so the ionosphere still bends most of the energy back down to earth. Not sure this is practical with a 1m antenna, though.

author: Wirehead
author_rank: Regular Contributor
timestamp: May 04, 2018, 07:25:48 am
content: 80m works a treat for this.. you can get reliable NVIS communication.. Altough with that antenna length limit, you'll have to get creative..

author: vk3yedotcom
author_rank: Frequent Contributor
timestamp: May 04, 2018, 11:20:23 am
content: At this phase of the solar cycle 40m isn't even good for NVIS (especially at night). Even 80m can have times when it's not effective.The 1m antenna size is a big challenge - even if conditions did support NVIS it would be very difficult to do it due to low antenna efficiency. But you might get a few km in groundwave. Less if in an urban area with interference. More if over water.

author: CopperCone
author_rank: Super Contributor
timestamp: May 04, 2018, 01:43:03 pm
content: Yea when i was imagining this question i fifured 3 feet because its the biggest thing you can really put on a backpack. Probobly difficult to traverse heavy jungle with a 3 foot long antenna but its possible.And yes i was imagining without repeaters or anything other then two communications units that are being carried by a single individual.I think the military standard for this is the SINGARS system, but its good to know the particulars. Im sure its possible to do better.The downards ionosphere bounce is really interesting.Not really what i had in mind but with how advanced drones are its probobly possible to make a quadcopter that deploys a long antenna for a small time period and tointegrate it all into a fairly portable lightweight radio set. Could be a lifesaver.So long its not near a tree i think it would be pretty easy to make automated. Just have a bullseye on the ground that the drone stays near withcheap camera.

author: metrologist
author_rank: Super Contributor
timestamp: May 04, 2018, 01:47:32 pm
content: Well, then would you consider temporary strung wire antennas?

author: CopperCone
author_rank: Super Contributor
timestamp: May 04, 2018, 01:49:53 pm
content: Nah i think this long antenna should go into a seperate thread about deployable antennas.. Maybe not if people stay focused and it does not swamp my original question.I would vonsider the deployable antenna to be a backup or supplement to this system.And im not building anything this is just for fun and research

author: Wirehead
author_rank: Regular Contributor
timestamp: May 04, 2018, 04:04:07 pm
content: 4.3ft gets you an MFJ-1899T.. Basically a coil with minimal whip.. should work for 80m groundwave/some NVIS

author: David Hess
author_rank: Super Contributor
timestamp: May 04, 2018, 06:01:57 pm
content: Quote from: CopperCone on May 04, 2018, 01:43:03 pmUsing a drone opens up another possibility that we occasionally used to extend the reach of VHF and UHF radios.Dual band radios including handy-talkies usually include a cross band repeat function. Occasionally we placed one in an ammo can with a 12 volt 7 amp hour battery and BNC feedthrough for an external antenna and deployed it at a high location. But some dual band HTs are small and light enough now that they could be deployed as part of a drone.

author: borjam
author_rank: Unknown
timestamp: May 10, 2018, 02:01:53 pm
content: Quote from: CopperCone on May 04, 2018, 01:43:03 pmOr you can use a balloon