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GSM modem antenna termination - RF query
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Rick H
electronics forum beginner


Joined: 19 Jul 2005
Posts: 18

PostPosted: Wed Jul 12, 2006 1:33 pm    Post subject: Re: GSM modem antenna termination - RF query Reply with quote

rob <not_valid@all.com.au> wrote:
Quote:

"PeteS" <PeterSmith1954@googlemail.com> wrote in message

The amusing thing is, the better the signal, the lower the transmit
power.

In place, do a series of AT+CSQ commands (signal strength at the
receiver). I say do a series because the cell system signal strengths
can easily vary by 20dB or more (I see it regularly) so you need to
capture a representative set of data for each configuration.

Note that CSQ is related to receiver signal strength, not transmitted
power.

Cheers

PeteS


Thanks for the comments guys, we do check the rcv signal strength. As you
said it does move about quite a bit. We test the units on air as we don't
have any RF test gear. We think/know the modem self regulate its output
power depending on the efficacy of the antenna termination. I'm not sure if
this is related to or the same thing as setting o/p power in response to
feedback from the tower.

Hi, Rob.

GSM handsets/modules aren't free to self-regulate their own output
power - they always set the transmit power to the level dictated by the
network. The network can look at the signal received from your
transmitter and decide whether it needs more power or can accept less
power from you.
The "self-regulation" you talk about is probably the module's
power-control loop, which is a circuit on the module to regulate the power
to the level required by the network, but this is just a feedback loop,
and has nothing to do with an autonomous decision made by the module as
to what power to transmit at.

Rick
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rob
electronics forum beginner


Joined: 07 Jun 2006
Posts: 12

PostPosted: Wed Jul 12, 2006 12:02 pm    Post subject: Re: GSM modem antenna termination - RF query Reply with quote

"PeteS" <PeterSmith1954@googlemail.com> wrote in message
news:1152638742.854707.156870@m73g2000cwd.googlegroups.com...
Quote:
rick H wrote:
rob <not_valid@all.com.au> wrote:
We've got some Intercel GSM / GPRS modems in one of our products.

The modems have a couple of solder pads for connecting to the antenna
via
coax. We've been playing around with different termination routes and
seeing
different power consumptions and were wondering how this may relate to
the
matching of the antenna connection. We are using a good quality Belden
RG-178 cable.

We are terminating as recommended in the manuf. data sheets and it all
works
ok however we find the variation in power consumption, estimated at ~
50%
interesting.

Our layout is:

----------
|-------------------------|====\
modem |- - - - - - - - - - - - - - -|------===============
|------------------------ |====/
----------

| coax 12 -->80mm | SMA | 30mm stubby antenna



Is varying the interconnecting coax length between say 12mm and 80mm
likely
to significantly change antenna matching - radiated pwr / rcv signal??

Any advice for some RF dummies is welcome!


It may be nothing to do with the load; are you testing your GSM modem and
antenna into a test-set or on-air? If you're not using a test-set, in
which the transmitted power is under your control, then the GSM module's
transmit power is under the control of the network, and you will indeed
see the current consumption go up and down depending on what power level
the network has instructed your GSM module to transmit at.

--
Rick

The amusing thing is, the better the signal, the lower the transmit
power.

In place, do a series of AT+CSQ commands (signal strength at the
receiver). I say do a series because the cell system signal strengths
can easily vary by 20dB or more (I see it regularly) so you need to
capture a representative set of data for each configuration.

Note that CSQ is related to receiver signal strength, not transmitted
power.

Cheers

PeteS


Thanks for the comments guys, we do check the rcv signal strength. As you
said it does move about quite a bit. We test the units on air as we don't
have any RF test gear. We think/know the modem self regulate its output
power depending on the efficacy of the antenna termination. I'm not sure if
this is related to or the same thing as setting o/p power in response to
feedback from the tower.
regards
rob
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rob
electronics forum beginner


Joined: 07 Jun 2006
Posts: 12

PostPosted: Wed Jul 12, 2006 11:42 am    Post subject: Re: GSM modem antenna termination - RF query Reply with quote

"Chris Jones" <lugnut808@nospam.yahoo.com> wrote in message
news:12b8c0nkdn83bc7@corp.supernews.com...
Quote:
rob wrote:

We've got some Intercel GSM / GPRS modems in one of our products.

The modems have a couple of solder pads for connecting to the antenna via
coax. We've been playing around with different termination routes and
seeing different power consumptions and were wondering how this may
relate
to the matching of the antenna connection. We are using a good quality
Belden RG-178 cable.

We are terminating as recommended in the manuf. data sheets and it all
works ok however we find the variation in power consumption, estimated at
~ 50% interesting.

Our layout is:

----------
|-------------------------|====\
modem |- - - - - - - - - - - - - - -|------===============
|------------------------ |====/
----------

| coax 12 -->80mm | SMA | 30mm stubby antenna



Is varying the interconnecting coax length between say 12mm and 80mm
likely to significantly change antenna matching - radiated pwr / rcv
signal??

Any advice for some RF dummies is welcome!

thanks
rob

Do you have a network analyser? If you are making your own antenna then
you
should check it on a network analyser. Perhaps you can borrow one or
visit
someone who has one. Note that things near the antenna will affect the
matching. It may not be easy to make an antenna that is matched well in
both the 900 and 1800 bands (or US equivalent if that's where you are).
Perhaps you would be better off buying the antenna but I don't know what
they cost in small quantity.

Chris


Thanks Chris, we are using an off the shelf antenna. Unfortunately we don't
have access to any radiocomms test gear (or the expertise to use it).
cheers
rob
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Chris Jones
electronics forum Guru Wannabe


Joined: 06 May 2005
Posts: 278

PostPosted: Tue Jul 11, 2006 11:14 pm    Post subject: Re: GSM modem antenna termination - RF query Reply with quote

rob wrote:

Quote:
We've got some Intercel GSM / GPRS modems in one of our products.

The modems have a couple of solder pads for connecting to the antenna via
coax. We've been playing around with different termination routes and
seeing different power consumptions and were wondering how this may relate
to the matching of the antenna connection. We are using a good quality
Belden RG-178 cable.

We are terminating as recommended in the manuf. data sheets and it all
works ok however we find the variation in power consumption, estimated at
~ 50% interesting.

Our layout is:

----------
|-------------------------|====\
modem |- - - - - - - - - - - - - - -|------===============
|------------------------ |====/
----------

| coax 12 -->80mm | SMA | 30mm stubby antenna



Is varying the interconnecting coax length between say 12mm and 80mm
likely to significantly change antenna matching - radiated pwr / rcv
signal??

Any advice for some RF dummies is welcome!

thanks
rob

Do you have a network analyser? If you are making your own antenna then you
should check it on a network analyser. Perhaps you can borrow one or visit
someone who has one. Note that things near the antenna will affect the
matching. It may not be easy to make an antenna that is matched well in
both the 900 and 1800 bands (or US equivalent if that's where you are).
Perhaps you would be better off buying the antenna but I don't know what
they cost in small quantity.

Chris
Back to top
PeterSmith1954@googlemail
electronics forum addict


Joined: 04 Jul 2006
Posts: 79

PostPosted: Tue Jul 11, 2006 5:25 pm    Post subject: Re: GSM modem antenna termination - RF query Reply with quote

rick H wrote:
Quote:
rob <not_valid@all.com.au> wrote:
We've got some Intercel GSM / GPRS modems in one of our products.

The modems have a couple of solder pads for connecting to the antenna via
coax. We've been playing around with different termination routes and seeing
different power consumptions and were wondering how this may relate to the
matching of the antenna connection. We are using a good quality Belden
RG-178 cable.

We are terminating as recommended in the manuf. data sheets and it all works
ok however we find the variation in power consumption, estimated at ~ 50%
interesting.

Our layout is:

----------
|-------------------------|====\
modem |- - - - - - - - - - - - - - -|------===============
|------------------------ |====/
----------

| coax 12 -->80mm | SMA | 30mm stubby antenna



Is varying the interconnecting coax length between say 12mm and 80mm likely
to significantly change antenna matching - radiated pwr / rcv signal??

Any advice for some RF dummies is welcome!


It may be nothing to do with the load; are you testing your GSM modem and
antenna into a test-set or on-air? If you're not using a test-set, in
which the transmitted power is under your control, then the GSM module's
transmit power is under the control of the network, and you will indeed
see the current consumption go up and down depending on what power level
the network has instructed your GSM module to transmit at.

--
Rick

The amusing thing is, the better the signal, the lower the transmit
power.

In place, do a series of AT+CSQ commands (signal strength at the
receiver). I say do a series because the cell system signal strengths
can easily vary by 20dB or more (I see it regularly) so you need to
capture a representative set of data for each configuration.

Note that CSQ is related to receiver signal strength, not transmitted
power.

Cheers

PeteS
Back to top
John Popelish
electronics forum Guru


Joined: 29 Apr 2005
Posts: 1601

PostPosted: Tue Jul 11, 2006 4:05 pm    Post subject: Re: GSM modem antenna termination - RF query Reply with quote

rob wrote:
Quote:
We've got some Intercel GSM / GPRS modems in one of our products.

The modems have a couple of solder pads for connecting to the antenna via
coax. We've been playing around with different termination routes and seeing
different power consumptions and were wondering how this may relate to the
matching of the antenna connection. We are using a good quality Belden
RG-178 cable.

We are terminating as recommended in the manuf. data sheets and it all works
ok however we find the variation in power consumption, estimated at ~ 50%
interesting.

Our layout is:

----------
|-------------------------|====\
modem |- - - - - - - - - - - - - - -|------===============
|------------------------ |====/
----------

| coax 12 -->80mm | SMA | 30mm stubby antenna



Is varying the interconnecting coax length between say 12mm and 80mm likely
to significantly change antenna matching - radiated pwr / rcv signal??

Any advice for some RF dummies is welcome!

I'm not sure how it involves your question, but here is a comment:

This sort of antenna (center conductor extended out of coax) does not
limit the external RF current to only the exposed center conductor.
There will also be external RF currents along the outside of the coax
shield (as if it were part of a dipole fed from the point where the
stub exits) as the internal coax shield current escapes and makes its
way to the outside of the shield. This makes the length and shape of
the coax (and the rest of the modem and its interconnecting wiring)
part of the radiating structure.

You could almost eliminate this effect if you added a half wave
diameter disk to he SMA connector or a few 1/4 wave radials.

Or, if you can find a ferrite that is effective at this frequency, add
a bead 1/4 wave back from the SMA connector to bounce the external
shield current back to the connector at about a resonant length, so
that this current uses only an intended part of the coax to radiate.
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Rick H
electronics forum beginner


Joined: 19 Jul 2005
Posts: 18

PostPosted: Tue Jul 11, 2006 3:56 pm    Post subject: Re: GSM modem antenna termination - RF query Reply with quote

rob <not_valid@all.com.au> wrote:
Quote:
We've got some Intercel GSM / GPRS modems in one of our products.

The modems have a couple of solder pads for connecting to the antenna via
coax. We've been playing around with different termination routes and seeing
different power consumptions and were wondering how this may relate to the
matching of the antenna connection. We are using a good quality Belden
RG-178 cable.

We are terminating as recommended in the manuf. data sheets and it all works
ok however we find the variation in power consumption, estimated at ~ 50%
interesting.

Our layout is:

----------
|-------------------------|====\
modem |- - - - - - - - - - - - - - -|------===============
|------------------------ |====/
----------

| coax 12 -->80mm | SMA | 30mm stubby antenna



Is varying the interconnecting coax length between say 12mm and 80mm likely
to significantly change antenna matching - radiated pwr / rcv signal??

Any advice for some RF dummies is welcome!


It may be nothing to do with the load; are you testing your GSM modem and
antenna into a test-set or on-air? If you're not using a test-set, in
which the transmitted power is under your control, then the GSM module's
transmit power is under the control of the network, and you will indeed
see the current consumption go up and down depending on what power level
the network has instructed your GSM module to transmit at.

--
Rick
Back to top
PeterSmith1954@googlemail
electronics forum addict


Joined: 04 Jul 2006
Posts: 79

PostPosted: Tue Jul 11, 2006 12:43 pm    Post subject: Re: GSM modem antenna termination - RF query Reply with quote

rob wrote:
Quote:
We've got some Intercel GSM / GPRS modems in one of our products.

The modems have a couple of solder pads for connecting to the antenna via
coax. We've been playing around with different termination routes and seeing
different power consumptions and were wondering how this may relate to the
matching of the antenna connection. We are using a good quality Belden
RG-178 cable.

We are terminating as recommended in the manuf. data sheets and it all works
ok however we find the variation in power consumption, estimated at ~ 50%
interesting.

Our layout is:

----------
|-------------------------|====\
modem |- - - - - - - - - - - - - - -|------===============
|------------------------ |====/
----------

| coax 12 -->80mm | SMA | 30mm stubby antenna



Is varying the interconnecting coax length between say 12mm and 80mm likely
to significantly change antenna matching - radiated pwr / rcv signal??

Any advice for some RF dummies is welcome!

thanks
rob


Rob wrote:
Is varying the interconnecting coax length between say 12mm and 80mm
likely
to significantly change antenna matching - radiated pwr / rcv signal??

Certainly, if the RF track from the modem is not impedance controlled.
If it's not impedance controlled, then you'll set up a nice big
mismatch, and the effective gain/loss will depend on the number of 1/4
wavelengths from the coax termination point (plus some unknown offset
for the actual electrical length of your tracks).

In addition, RG178 is rather lossy, but the distance you are talking
about would not give you the ratio you are seeing.

The most probable cause is mismatch on the tracks and the coax
connection, imo.

Cheers

PeteS
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rob
electronics forum beginner


Joined: 07 Jun 2006
Posts: 12

PostPosted: Tue Jul 11, 2006 12:16 pm    Post subject: GSM modem antenna termination - RF query Reply with quote

We've got some Intercel GSM / GPRS modems in one of our products.

The modems have a couple of solder pads for connecting to the antenna via
coax. We've been playing around with different termination routes and seeing
different power consumptions and were wondering how this may relate to the
matching of the antenna connection. We are using a good quality Belden
RG-178 cable.

We are terminating as recommended in the manuf. data sheets and it all works
ok however we find the variation in power consumption, estimated at ~ 50%
interesting.

Our layout is:

----------
|-------------------------|====\
modem |- - - - - - - - - - - - - - -|------===============
|------------------------ |====/
----------

| coax 12 -->80mm | SMA | 30mm stubby antenna



Is varying the interconnecting coax length between say 12mm and 80mm likely
to significantly change antenna matching - radiated pwr / rcv signal??

Any advice for some RF dummies is welcome!

thanks
rob
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