GSM modem antenna termination - RF query

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

Reply to
rob
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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

Reply to
PeteS

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
Reply to
rick H

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.

Reply to
John Popelish

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

Reply to
PeteS

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

Reply to
Chris Jones

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

Reply to
rob

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

Reply to
rob

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

Reply to
rick H

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