max distance achievable using the following set up ?

Joined: 02 Aug 2003 Posts: 134

Just wonder what sort off range could be achieved in a built up area between say two proper pmr sets operating on 446mhz at say 5 watts output using a decent 70cm ( wavelength not size !! ) mobile antenna and a proper 70cm collinear mounted around 30 feet up.

appreciatte that the above set up would be totally illegal but if we ever did get higher power / allowed better antennas what sort off range would be achievable.

any amateurs who use the nearby 70cm allocation care to comment on the this ??

given a similar set up on say vhf what would win in terms reliable communications using the above set up - vhf - uhf ??

p.s does seem to be a heck off a lot off ex pmr sets retuned to 446mhz going on ebay lately - so cmon who knows the answers to the above ??

Joined: 29 Oct 2005 Posts: 1373

All depends on what terrain exists between the two radios.

For example, there is a weekly club net each Tuesday for the BRATS club (my local AR club). Last night the usual net started up on 145.400MHz and about 7 or 8 locals joined in. At the end of the net a few of them (3-4?) switched up to 433.400 to test out one of the members radios. Guess what? All of a sudden I could only hear 2 out of the 3-4 people present. Medway is very hilly and the different people were all situated on and around different parts of hilly Medway. It's all down to clear line of sight with 70cm. With clear line of sight I can get the Margate repeater from Rochester on 70cm.

Another example:

GB3RE (Maidstone Repeater) very poor reception from my home, about 2 miles away (absolute maximum!).

GB3ER (Chelmsford, Essex Repeater) end stop signal from my home, about 20+ miles away?

The only sure way to know is to try it (you may be pleasantly surprised), but a safe assumption (if an assumption can ever be safe) is to go with VHF for range.

There is a reason that amateurs use VHF more than UHF for reliable local comms.

Jon

Posted: Wed Mar 07, 2007 4:39 pm

The Australians (and NZ) have a 477MHz CB system with 5W, so their experiences will be very valid.

It's safe to say it's enough power to reach the radio horizon... it's just that the radio horizon varies a fair bit depending where you are

Joined: 23 Feb 2006 Posts: 119

Picked up GB3EK (Thanet 70 cms repeater) at Erith ( Almost in London) on my VX-7R Handie!

Line of sight (ish!) straight up the Thames. Must be close to 50 miles.

Joined: 29 Oct 2005 Posts: 1373

The coverage for the Margate (GB3EK) Repeater is quite limited according to the 'projected coverage map', but in reality it can be considerably further.

This website ( http://www.ukrepeater.net/ ) is invaluable for getting to grips with the amateur radio repeater system. Here's the projected coverage map for GB3EK from that website. Not too bad for only 12W output.

Full details here: http://www.g4tkr.demon.co.uk/technical.html

BIG Repeater Map

Joined: 02 Aug 2003 Posts: 134

Thanks guys really do appreciate your advice.

cheers

Joined: 10 Jan 2007 Posts: 35

Getting the antenna above the local roofline is likely to make a noticable difference.

Joined: 29 Aug 2006 Posts: 211 Location: France

I can't remember where but i found some years ago a program to estimate a radio range acording to your location, antena hight and frequency. It gave the same kind of map than above, using topological maps (not including buildings or vegetation).
Maybe someone here can give you a link, or i'll check on my computer when at home.

Joined: 09 Apr 2006 Posts: 194

Okay you want a 70cm vertical as higher gain as you can afford...

and a 20ft ali scaffy bar. set of T & K brackets at the side enough to hold the bar past the gable end, and away you go.

don't think 6ft then 12ft iv been there done that. and all that happens is you want more and more, so go full hog and you wont have to poop yourself (my house is already very very high) and with the antenna mounted at 54foot whats the point in getting a 60ft tower???

RG213 Double screened or WF103 for the coax in a continuous run into your shack, with high quality PL259 or N-type plugs.

Then if you get board, add a rotator, and an antenna switch, and have a 70cm beam mounted under your vertical coli.

Go for a dual band antenna for 2m/70cm or even 6m/2m/70cm for if you ever decide to get your HAM licence.

(saves upgrading)

Joined: 14 Mar 2007 Posts: 8 Location: N.Y.C. U.S.A.

Well using my Icom T-90A or Kenwood THF6A
and using a high gain dual band 2 meter/70cm antenna
i regularly hit a repeater from street level 75 miles away
even with passing over very busy nyc area way out to long island repeator

have used same HT's and tried simplex as far as 10 miles
(me walking in street to friend with base antenna)

Joined: 29 Aug 2006 Posts: 211 Location: France

I finaly found that propagation program.
It's called "Radio mobile" and you can download it free from the author website.
http://www.cplus.org/rmw/english1.html

Joined: 19 Jul 2006 Posts: 108

x-alt wrote:

I finaly found that propagation program.
It's called "Radio mobile" and you can download it free from the author website.
http://www.cplus.org/rmw/english1.html

I've been using that software to produce coverage maps of local repeaters (just for my own amusement) using the 3 arcsecond SRTM elevation data, and its amazing how detailed they are if you produce a cartesian rainbow plot with 1 pixel resolution (the polar plots aren't quite so good) - much more detailed than the maps on ukrepeater.net

It only has the one terrain model (Longley-Rice) but seems to match quite well with reality in my experience.

Joined: 19 Jul 2006 Posts: 108

Here's a coverage prediction for GB3EK I've produced for comparison. I'm not familiar with that repeater so the antenna height may not be correct (and the antenna pattern certainly isn't) so the figures may not be accurate. But still, not bad for free software.

Joined: 29 Oct 2005 Posts: 1373

Quote:

EK was commissioned in 1976 when there were very few 70cms repeaters in the UK. The coverage of East Kent is good from the top of a 14 story block of flats situated in Cliftonville, on the eastern side of Margate. The equipment currently in use for the repeater is based on 2 IC-U2 Icom hand portables controlled by a G3XCB logic board running software customised by the working group. Five cavities are used to provide TX/RX isolation which is further enhanced by the use of separate 3 element yagis for TX and RX. Both the yagis and cavities were constructed by the working group. The transmitter output power is 12 watts ERP.

Source: Kent Repeater Group Website

The prediction isn't bad, it just doesn't take into account the fact that GB3EK uses directional beams!

Jon

Joined: 19 Jul 2006 Posts: 108

2H2UZ wrote:

The prediction isn't bad, it just doesn't take into account the fact that GB3EK uses directional beams!

It is possible to use your own antenna patterns in the Radio Mobile software, I just preferred to use an omni rather than guess the pattern of their antenna (and which way its pointing).

EDIT: It seems that many of the coverage maps on ukrepeater.net have now been updated with *much* higher resolution ones (although sadly not GB3EK). Here's an example;
http://www.ukrepeater.net/repeaters/maps/gb3ev-hi.jpg