[OZAPRS] 6M Radio

[Glen English VK1XX]

Hi Marcus,

Yeah VHF hi band is a great utility/ compromise. I'm a big believer for
UHF around town. At 450 MHz, building penetration is often better (due
to the openings being larger compared to the wavelength) , and for same
reasons , portables work in cars, and lower noise levels around town
make UHF great for local stuff with line of site, or fairly shallow
diffractions (IE small hills, low angles). UHF signals are often higher
than VHF around town because UHF needs less obstruction clearance than
VHF to achieve the same signal attenuation.... But as soon as you go
over a big hill or two big hills, the diffraction losses are higher and
VHF wins... Disadvantages of 450 MHz even around town are the high
levels of the reflections leading to more severe effects of long delay
multipath. UHF will generally produce levels with greater standard
deviations. So where does 6m mobile fit in ? Maybe between two vehicles
requiring simplex communications in difficult terrain.

g




On 25/11/2017 2:04 PM, Marcus B wrote:
> Glen,
>
> This is a great explanation comparing 6m to 2m, no matter how much I
> looked with the help of Google I couldn't find a good comparison of
> the two (as well as 10, 70 & 23). And I suppose the affects are
> amplified in either direction from those bands.
>
> It makes a lot more sense now why 2m is most often the go to utility
> band. I was always confused as to why 6m wasn't used a lot more in
> Australia.
>
> Marcus.
>
> On Sat., 18 Nov. 2017, 15:18 Glen English VK1XX,
> <glenlist at pacificmedia.com.au <mailto:glenlist at pacificmedia.com.au>>
> wrote:
>
>     Hi Robert
>
>     my 2$ worth : a primer on 6m performance and what to expect. it is
>     certainly a curiosity and there is much to be learned from playing
>     about
>     with it.
>
>     1) mobile performance on 6m is in the toilet, unless you work very
>     hard
>     on your vehicle antenna. This is due to low height above ground
>     producing near antenna fresnel blockage, higher radiation angles and
>     higher ground losses. 
>     so you need power to overcome antenna efficiency -  100W.
>     But most HF/6m radios do 100W .
>
>     2) if comparing to 2m, 6m has only advantage in very hilly, and
>     multiple
>     hilly terrain, many hills in the way. 
>
>     2.5) 6m also has a advantage with regard to heavy foliage forest
>     attenuation. (Less)
>
>     3) 6m has less picket fencing, and the short delay multipath (due to
>     nearby scatterers) is not as deep as the reflections are not as strong
>     or coherent.- advantage.
>
>     4) galactic noise (alone) is about 10dB more than at 2m. The only
>     'advantage' of this that sloppier systems will work 'cause they
>     are not
>     as affected by coax losses and high recv noise figures.
>
>     5) 6m base station antennas need to be ~ 1.7x the height  of a 2m
>     antenna to get the same takeoff.  People make the mistake of NOT
>     putting
>     the lowest frequency antenna at the top of the mast.
>
>     So, if the base stations are ideal, and there are multiple
>     obstructions
>     in a path, 6m signals will be stronger than 2m on long , difficult
>     paths
>     that are diffraction paths (not troposcatter) paths.  The signals are
>     stronger but the noise floor of 6m masks the signals, so you got to be
>     prepared to work at narrower bandwidths to take advantage of it. Once
>     the path becomes a troposcatter path, you might well be better off
>     on 2m.
>
>     So, apart from the curiosity which is 6m FM, I think that 300 baud 6m
>     SSB HF packet would be a better utility, because it has a roughly 10dB
>     advantage on 1200 bps AFSK FM . maybe 13 dB depending on the demod...
>
>     Mobile footprints of 6m repeaters are generally much smaller than 2m
>     repeaters at same sites. This is due to alot of things but many of the
>     above.
>
>     Troposcatter is in the toilet, but ionscatter is of interest
>     (900-1500km).
>
>     -glen
>
>
>
>     Having said all that,
>     On 18/11/2017 2:48 PM, Robert wrote:
>     > Hi All.
>     >
>     > Just thinking about playing around with 6M APRS.
>     >
>     > So my questions are.
>     >
>     > What is the freq that is used in Aust.
>     > What would be a good radio to use, or modify to use on 6m
>     > What TNC details/config is needed.
>     >
>     > Thanks in Advance.
>     >
>     > Robert
>     > VK2KGV
>     >
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