Advice on GSM antenna for poor coverage areas

[ulrikp]

I need a PCB style GSM antenna - 900/1800 MHz or quadband - for remote areas with poor network coverage. I need advice on the best available commercial PCB antenna (price could be an issue). Optimally, it should be omnidirectional so the device can be placed in any manner.

I hope someone has experience to share.

Thanks!
-Ulrik

 

[RF_Jim]

I need a PCB style GSM antenna - 900/1800 MHz or quadband - for remote areas with poor network coverage. I need advice on the best available commercial PCB antenna (price could be an issue). Optimally, it should be omnidirectional so the device can be placed in any manner.

I hope someone has experience to share.

Thanks!
-Ulrik

Ulrik,

A PCB (Printed Circuit Board) antenna?

That automatically puts severe 'constraints' on any sort of 'remote area' coverage antenna solution!

In antennas, you have basically 3 parameters to work with - any 2 of which you can choose and the 3rd is 'fixed' (for the choice of the other two parameters).

BES - Bandwidth, Efficiency (gain) and Size: choose any two and the 3rd will be dependent/related to the other two.

If you want small (Size) and some BW you trade off Efficiency (lower gain in this case).

Jim

 

[E Kafeman]

RF_Jim is correct. GSM antenna customers always want to have a bit more bandwidth and higher efficiency in less antenna space.
For GSM is bandwidth given as four frequency ranges which are the most common used, in US GSM850//GSM1900 and GSM900//GSM1800 for EU.
A common requirement is that the antenna should handle all these four bands for cellphones that are sold worldwide.

for remote areas with poor network coverage

That is two different requirements.
1. Remote area - GSM coverage have a technically limit of 35 km and in reality slightly above 30 km, due too the time delay limitations in GSM system. Better antenna does not change that. Minor exceptions exist as a few rare GSM systems can handle twice that distance.
2. Poor network coverage - Weak signal coverage can be improved by a directive antenna and better antenna position to avoid absorbers and deflectors in line of sight between base and hand unit.

Optimally, it should be omnidirectional so the device can be placed in any manner.

Antennas for omnidirectional or even almost spherical coverage are in opposite of #2.
Omnidirectional antennas can be done directive but as they are directive cant they be placed in any manner. A number of stacked dipole antennas can give a radiation pattern peeking against horizon in all directions.
It is a rather big structure if it should be effective and can not be tilted without dropping gain, so placing it in any manner is excluded. Can be built on PCB.

If directional antennas can be accepted (not free placement), can good gain can be achieved even if the antenna is built on PCB but that PCB will be rather big and complex if both GSM900 & GSM1800 (and/or GSM850 & GSM1900) should be covered.
A multi-band PIFA-antenna is rather omnidirectional, comparable with a small monopole. Typical efficiency -2 - -3 dB covering actual frequency bands.
That is the most common type of antenna in GSM cell phones. In phones is actual antenna efficiency a bit reduced due to space and ground limitations.

If it is a special location and situation with weak signal at that place, but distance to base antenna is less then 30 km, find out if it is 900 MHz or 1800 MHz, build a simple Yagi antenna. Can be done on PCB, if that is a requirement.

If distance is less then 30 km can local GSM relay stations be used. Then can cellphone be held in any manner but cheap relays have very short coverage, from centimeters to a few meters.
Remote relay stations are needed for longer distance coverage. High cost and must be built by a Telco with GSM-license.
Satellite phone or other kinds of radio relays are thinkable alternatives to establish phone communication or**broken link removed** in remote locations.

 

[ulrikp]

Thanks for your replies. I know that antenna theory is quite hardcore stuff, and I will not go into designing my own antenna. Here are my requirements
1. Max. 55x13x13mm in size: The size of the enclosure puts restrictions on the antenna.
2. We are within theoretical network coverage, so something that matches the properties of an ordinary Nokia phone antenna should be ok.
3. The product is to be used in western Europe, so I imagine dualband should be ok (900/1800).
4. Omnidirectional (just to narrow down the selection and to keep it easy for the user).

Now, do you have any specific advice for a good antenna? It does not necessarily have to be PCB. The one I currently have is PCB style, and it has shown to not be good enough in remote areas (where a Nokia phone still barely works).

Another question (reveiling my lack of basic understanding): Would it make sense to solder two or three of my current PCB antennas to the same connector and arrange them in different directions?

Thanks!
Ulrik

 

[E Kafeman]

55 mm. Give antenna 15 mm of that space, and remaining 40 mm will be RF-ground. Good performing omnidirectional antenna with 40 mm real ground plane at 900 MHz is not possible.

You can bundle a number of antennas, if they can be placed with a certain distance so that they not load each other. Besides total radiation pattern are also phase and impedance affected so you must know a bit what and how to connect these antennas. Total efficiency will in best case not be degraded compared to a single antenna.

 

[ulrikp]

Thanks! I still want to get a commercial antenna as designing my own is not an option right now.

Based on what you have written, I have decided to change the application (for prototyping, at least). I will go with an external antenna with a cable going out of the enclosure. I am thinking I will get a vertically polarized antenna that stands upright (i.e. omnidirectional). There are many on the market, and they promise anything between 0 and 5 dB gain. I don't necessarily trust these specs, and I also have doubts they are using a proper cable. Do you have any experience or good advice for choosing this type of antenna?

 

[E Kafeman]

External antenna, if monopole it assumes that you can place it on your own ground plane, big enough, such as a car roof. Most simple is then to select a typical antenna for car mount. Some types promises higher gain if of type collinear antenna. No special brand recommended, they are about equal. Main difference can be that some of these antennas are delivered with too long and too high loss coaxial cable. Can cost many dB.
If you prefer something like handheld antenna or when you not have any additional ground plane is a dipole antenna the most simple selection.
**broken link removed**
**broken link removed**
**broken link removed**
These antennas need a certain distance from ground to perform best. Keep coaxial cable as short as possible and do all connections with low RF-loss as it easily can cost you a few dB.

My self, if I need a temporary antenna for testing, I do a dipole antenna by cutting a wire to the right length. Final cutting/bending is done while measuring with a network analyzer but that is not absolutely necessary as I have done a few such antennas and have a feeling for what precision that is needed. It have a gain around 2 dBi if you not do something very wrong. It is possible to make even if you not have done an antenna before if you have basic electronic soldering skills and some RF knowledge.

If communication not is possible due to low signal level do seldom 1 or 2 dB antenna improvement make any difference either. By just changing antenna position with 100 mm can signal level change by 10 dB so that can be a more important factor then the antenna. For that reason, in case of weak signal, to be able to have a reliable communication in general, calculate 8 dB s/n minimum for not dropping connection (GSM) and additional 10 dB signal margin due to local signal variations. In places with a lot of local reflections can much more margins be needed, such as near houses or cars.

 

[ulrikp]

Thanks for the three options. The first one is too expensive to put on this product. The second one seems like a good option. I have seen many of this type, and from what you are saying, it sounds like they are all equally good if the cable is short. A few questions:
1. Is it correct to say that this antenna is directional if it is mounted horizontally, and it is omnidirectional if it is mounted vertically?
2. If #1 is correct, I assume I will be able to get higher gain if I use it as directional antenna and point in the right direction. Correct?
3. "A certain distance above the ground"... The product is about 20cm above the ground in most cases. Do you see this as a big problem for the gain? I would like to avoid making a special arrangement for getting the antenna up higher.

In my application, there will not be a large ground plane (like a car) available. Often, it will be in a remote area with trees nearby (sometimes even in the forest).

Again, thanks for your help!

 

[E Kafeman]

1. Yes, somewhat directional when mounted horizontally. It have two narrow rather poor directions but else is it as horizontally mounted almost as omnidirectional as when mounted vertically. Local environment and what polarization base station antenna use can also affect what antenna orientation that works best. Vertical orientation is in most cases to prefer.
2. Same as 1.
3. Ground, is also the ground you have in the device (PCB) it self. At 20 mm height will effect be minimal if antenna is mounted perpendicular to PCB. If antenna 2 or 3 is mounted parallel to PCB can minor directive effects be seen also at 20 cm distance.
If handheld, can the body or hand affect antenna gain just as much as ground.