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PICList Thread
'[PICLIST] [EE] How to duplex a VHF aerial?'
2000\12\29@182228 by Ian Hynes

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PICers,

Just casting about for ideas here ....

I want to build an Rx/Tx master/slave system using a PIC, 16F84
probably, and a pair of RF Rx/Tx modules that are available these
days. But it seems grossly inelegant to use two aerials for separate
channels.

One idea that occurred to me was :-


\|/ Aerial
|________________|Switch|_____|Rx    |_______|Decoder|____|   |
                 |      |     |module|       |145026?|    |   |
                 --------                                 |PIC|
                     |________|Tx    |_______|Encoder|____|   |
                              |module|       |145027?|    |   |

OK, the switch would have to pass a VHF signal in either direction,
depending on whether the PIC was sending or receiving. You'd only get
half-duplex with this arrangement, but that's OK. You'd be
sending\receiving binary data, not analogue but that's also OK. You
MUST keep the Rx module's input isolated when the Tx module is Tx-ing.
Obviously. You'd control the switch with a PORTA pin, probably.

So what about the RF switch? Any ideas for that come to mind?

Many thanks for ideas!

Regards - Ian Hynes

PS: May I wish PICers everywhere all the best for the New Year?

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2000\12\29@183906 by David VanHorn

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>
>So what about the RF switch? Any ideas for that come to mind?


PIN diodes are commonly used for this, at power levels of milliwatts to
many watts.

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2000\12\29@203248 by Peter Grey

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At 10:19 AM 12/30/00 +1100, you wrote:

RF Microdevices have a T/R switch - RF2436 in a SC-70 package.

Good luck,

{Quote hidden}

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2000\12\30@025355 by Chris Carr

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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.

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.

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.

What transmitter Power are you intending to use and what frequency ?

Regards

Chris

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2000\12\30@073718 by Ray Gardiner

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A small relay with low series inductance is probably the easiest, next
choice would be
a pin diode.  Coaxial relays are expensive. (Depends on your application.)

If you use a relay then the normally closed contact would be the receive side.

Some extra hints, if you are connecting to an external antenna, put some
thought
into lightning protection. Transient suppression and a resistor to ground.


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2000\12\30@120200 by Peter L. Peres

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Use a transmatch (directional coupler). This is the same thing that is
used to measure SWR. Needs no switching and will isolate 22 dB or better
probably. This may not be enough for duplex on the same channel, but it
may be enough if the channels are some distance apart.

Peter

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2000\12\31@045345 by Graham

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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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2000\12\31@111155 by David VanHorn

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

A properly designed hybrid would work as well here, as at any other frequency.


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

Do a search on "radio murs"



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