1. the requirement was half duplex not full

2. a directional coupler has nothing to do with what is needed.

3. the suggestion of a 1/4 impedance switcher was a good one, except at vhf
it will be probably be prohibitively large for a small project and hard to
set up without test equipment/knowledge.

4. if you really have to do this then a small RF relay is the easiest
solution for a 'hobby'project, switching time will probably not be an issue
because most low cost modules need a 'stabilisation' time that will exceed
antenna switching time. Isolation is not an issue , use the pic to power
down the tx at rx times.

PIN diodes is the second easiest solution, they are cheap and easy to find
but a bit lossy, prohibitively so if you have not laid up an RF based board
before, but, given you have a PC and an internet connection then you can
surely find all you need to know on that....make that search/layup challenge
part of the project ?

HOWEVER, and in any case, there is no 'free to use' band at vhf for this
application...

the idea of using 70 mhz is not good from a) a licensing viewpoint b) a
noise viewpoint c) obtainable modules d) inefficiency of small (in terms of
wavelength) antennas.

Using other vhf spectrum is not a good idea, most of it is occupied, and not
free to use or licensable for simple one off projects, UHF or SHF will offer
better indoor performance and free to use ('ish) bits of spectrum.


if you want to escape most of the issues you face, ie two modules at each
end, antenna switching, no free frequency to use, no experience in rf
design/layup

then look at using the *transceiver* from NORDIC vlsi, which does it all for
you on 433 mhz (or others) at less cost and complexity than most else
available....the features available are adequate for what appears to be a
school or 'hobby' project.

their web site (http://www.nvlsi.no) has all info available in .PDF format,
they have a board layout that you can just 'use' so no need to worry about
tricky rf pcb layouts ....they also have 'oven ready' artworks for a range
of loop antennas with a fully documented explanation that discusses antenna
efficiency (or rather lack of it !) and design....that in itself is a good
educational project.

The PCB is a bit tight for hand building and also contains a lot of 'VIAS'
which MUST be there...the components are all smt and very small series parts.

If you look first, plan second  and then are still serious....write them a
note explaining your project and see if you can 'beg' a couple of
pcb's......if your project/school/college (?) has a 'budget' then they may
have an evaluation kit and may take a kindly view of an educational
application for one ....OR you can try their distributor(s), the list is on
the website. OR you can ask them for a local customer and write some begging
letters to them to try and scrounge a couple of gash/test boards they may
have lying around.

if not, and you think you can make a double sided pcb by hand to the
necessary tolerances (and you probably can with care) then contact me off
list and I will send you a revised artwork (not yet tested, but will be next
week) I have done which will accomodate hand inserted copper 'via' rivets
(0.8mm) and 1206 size smt parts...*but that will be it*....after you get it
you are on your own, so if you are spending limited resources and not sure
of success without further hand holding, then take that into account.

Graham


-Need *high IP3* AND good N/F ?? try this- http://www.rfham.com

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