Hi Chris

Chris Car wrote:
>
> Ian,
>
> There is no single solution. You say it is inelegant to use 2 aerials
> for the separate channels. By channel do you mean Transmit channel and
> Receive channel both operating on the same frequency or are they operating
> on different frequencies. If the latter then you may be able to operate full
> duplex with the receiver being isolated from the transmitter by filtering.

I figured to operate them on the same frequency. Mainly because the
preset modules you can buy are fixed frequency, at least the ones I've
seen, and wouldn't allow FDM. I figured VHF 'cause it's easier to add
on an RF booster amp if extra o/p power is needed, with lumped-value
components - i'm not all that nifty with microstrips and Smith charts.
Plus I have a set of Coilcraft RF inductors to play with. The values
seem to be in the ball park for low-to-mid VHF. I think there's a UK
co. that offers a Tx/Rx pair at around 70MHz?
>
> If you intend operating semi duplex with the transmitter chain and the
> receiver chain operating on the same frequency then the best switching
> method tends to be determined by the transmitter power but other factors
> such as the speed of switching, isolation, operating frequency, losses and
> cost must also be considered before reaching the best compromise for your
> set of conditions.

I'd guess the relay option probably wouldn't lend itself all that well
to fast switching? I'm a high school student so it needs to be
reasonably cheap.

>
> My first reaction is that the schema you are proposing is inadequate, not
> only do you need to open circuit the connection between the aerial and the
> receiver, you also need to short the receiver input to earth (or create the
> effect of a short with a quarter wavelength of coaxial cable). Similarly to
> reduce losses you will probably need to switch the output of the
> transmitter.

Yeah, I'm not totally in love with it either. Any alternatives come to
mind?
>
> What transmitter Power are you intending to use and what frequency ?

For a prototype, I'd just use the o/p power from the module which, I,
think, is 100 mW. Modulation would be ASK. that is, carrier 100%
on/off.

As far as "elegance" is concerned, you want something that looks
"technically sweet", as someone once said. Two separate aerials for
each direction, Tx1->Rx2 and Rx1<-Tx2, wouldn't really qualify.

The project is just intended to see how it can be done. Might use it
for a school project next year (tomorrow!). Basically, I want a master
to send instructions to a slave & the slave to act upon the
instruction, ie close a relay or something, and send back a report on
its current status, say confirmation plus battery condition or
something. Probably, you'd want to use it outdoors to do something
useful. Hence VHF for greater range, diffraction around tree sized
objects etc.

Hey thanks for your advice. Any ideas are welcome.

Regards - Ian

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