Multiband patch antenna with high directivity

[mnhtapu]

Hi, I have just joined this site. I hope to get a lot of help from this site.
I want to design a multiband patch antenna which can support GSM900,DCS1800, PCS1900, Wi-fi, bluetooth, WLAN. I intended to use it as a panel antenna in home connected externally to cellular phone and laptop.
I also want to increase its directivity without making the antenna larger.
For creating multibands(GSM900,DCS1800, PCS1900, Wi-fi, bluetooth, WLAN) one of my egineer friends suggested me to design the patch antenna like following:
1) Make the length of patch larger than its width
2)Instead of having a reflecting ground plane equal size of substrate, reduce the ground plane size to a narrow strip.
For enhanced directivity he told me to create an array of patches. But I didn't like this idea as it will make my antenna bigger. There are a lot experts in this site. Kindly suggest me some ideas for increasing the directivity without making the antenna larger.
Any sort of relevant ideas will be greatly appreciated.
Thanks in advance

 

[mnhtapu]

very strange. I expected at least 1 expert who could reply me. alright...there's none.

 

[Fast1eddie]

Although I know little of newer antenna technology, I am thinking you need to look at Fractal Antennas, have read some interesting papers on them.

 

[Token]

I want to design a multiband patch antenna which can support GSM900,DCS1800, PCS1900, Wi-fi, bluetooth, WLAN. I intended to use it as a panel antenna in home connected externally to cellular phone and laptop.

First, it would help to know your technical background just a bit. What you are asking is quite a task and frankly will take some effort and know-how. No one is just going to be able to tell you “follow these plans and you are done”.

The bandwidth you want to cover is quite wide, below 900 MHz to over 5000 MHz. Certainly possible, but not all that easy.

I also want to increase its directivity without making the antenna larger.

Essentially you are asking a near impossibility. You might include exactly what you mean by “directivity” and also include what you are using as a baseline, “increase” implies a comparison to something else.

By “directivity” you could mean either higher gain or narrower beam (basically the same issues unless the original design has substantial spurious lobes). Both of these can ONLY be achieved by increasing the aperture of the antenna, that is to say the size. In theory you could increase the efficiency of the antenna without increasing size thus gaining in both, but most well designed antennas are already over the 50% point, so even if you could somehow increase it (and people have been working on that for years) you would not gain much.

For enhanced directivity he told me to create an array of patches. But I didn't like this idea as it will make my antenna bigger.

There is a reason he told you that…it is what will work. There are limitations in physics, and physics drives electronics, TANSTAAFL.

T!

 

[mnhtapu]

Thanks @Token for your informative reply. It helped me a lot. At first I really got frustrated as no one replied me. You unveiled some of the challenges of designing that antenna. I need to check those in detail.
If I cannot achieve wide multibands, that's ok. But I need to work on directivity, I mean the ratio of radiation intensity at a particular direction to the intensity averaged over all direction.
If you know any well known paper about that, I would love to check that.
Thanks again.

 

[Token]

A good place to start would be with the basics.

"Antenna Engineering Handbook", Richard Johnson and Henry Jasik. I believe it is currently in its 4th edition.

"Antenna Theory, Analysis and Design", Constantine Balanis. I believe it is currently in its 3rd edition.

"Modern Antenna Design", Thomas Milligan. The most recent I have is the 2nd edition, I do not know if a newer is in print.

T!
Mohave Desert, California, USA

 

[prcguy]

I also think a patch approach would be a technical challenge and arrays of patches for several bands would be needed to cover the frequency range with gain and directivity.

Why not look into a log periodic type? You can even buy new ones that cover 900MHz to 2.6Ghz with about 6dBi gain for $12.
prcguy

 

[jackj]

I was going to suggest the same as prcguy, a log periodic. It will give you gain, reasonably good directionality and, at those frequencies, a fairly small size. There are computer programs available that allow you to input the low and high frequencies you want it to cover, desired gain and feed point impedance. The program will then give you the element lengths and spacings. If you keep your feed point impedance at 200 ohms then a 4 to 1 balum will match 50 ohm coax (300 ohm for 75 ohm coax). You might have a problem finding a balum that will work over that broad of a frequency spread though.

 

[prcguy]

I would just buy this proven design sold on Ebay for $12. It has a 50ohm feed.
900-2600 MHz PCB Log Periodic Antenna - eBay (item 300483697590 end time Dec-19-10 18:58:50 PST)
prcguy

I was going to suggest the same as prcguy, a log periodic. It will give you gain, reasonably good directionality and, at those frequencies, a fairly small size. There are computer programs available that allow you to input the low and high frequencies you want it to cover, desired gain and feed point impedance. The program will then give you the element lengths and spacings. If you keep your feed point impedance at 200 ohms then a 4 to 1 balum will match 50 ohm coax (300 ohm for 75 ohm coax). You might have a problem finding a balum that will work over that broad of a frequency spread though.

 

[jackj]

prcguy, the log periodic antenna has a balanced feed point. You can't feed it with an unbalanced feed line and expect to maintain the gain and radiation pattern. The listing doesn't say that it comes with a balum and the picture doesn't show one. Also, the OP said he wanted coverage from 800 Mhz to 5 Ghz, this one only goes to 2.4 Ghz.

 

[prcguy]

The particular antenna on Ebay is 50ohm and takes a direct coax feed or SMA chassis connector via holes drilled in the board.

I agree there will be some common mode currents on the feedline but it doesn't seen to cause any noticeable problems. I have several of the same antenna plus many other log periodics that are 50ohm direct coax feed including a few calibrated EMI test antennas. Only one of those has factory installed ferrite beads on the feedline as it runs through the boom to suppress common mode currents.

If the OP needs to 5GHz there are other PC board types that show up occasionally on Epay. I have a 1 to 10GHz version on Duroid board that was not too expensive.
prcguy

prcguy, the log periodic antenna has a balanced feed point. You can't feed it with an unbalanced feed line and expect to maintain the gain and radiation pattern. The listing doesn't say that it comes with a balum and the picture doesn't show one. Also, the OP said he wanted coverage from 800 Mhz to 5 Ghz, this one only goes to 2.4 Ghz.

 

[Token]

My initial reasoning for not suggesting a log periodic antenna to the OP is because he was specifically comparing patch antennas, and he wanted “to increase its directivity”. This also led to my question of what he was using as a baseline or starting point. It sounds like he is already starting with a patch antenna as a baseline.

If so, most patch antennas have at least 6 to 9 dBi of gain, and many have around 14 or 15 dBi. Already at the level of, or significantly higher than, is possible with a log periodic. The problem, of course, is the comparatively narrow RBW (relative bandwidth) of the patch design to the wide band application he is asking for.

The services he is interested in; GSM900, possibly as wide as 870-960 MHz, depending on exactly which GSM900 he wants/needs (P, E, R, or T). DCS1800, 1710-1880 MHz. PCS1900, 1850-1990 MHz. Wi-Fi, frequency ranges in 900, 2400, 3700, and 5000 MHz areas depending on exactly which he actually needs, 2401-2495 and 5170-5835 MHz are the two most heavily used in many places, and likely his primary targets. Bluetooth, falls in the 2400-2485 range, so might be covered under the ability coupled to Wi-Fi. WLAN, really just another way of saying Wi-FI when you get right down to it, the most often used freq ranges again being the 2401-2495 MHz and the 5170-5835 MHz ranges.

Until the OP responds with a bit more detail there are too many unknowns here. However, a couple of generalizations:

The frequencies covered appear to be 870-960, 1710-1990, 2401-2495, and 5170-5835. I am ignoring the 3650-3700 MHz region of 802.11a as it is only used in the US and lightly used even there. Also, as he did not mention GSM850 I assume he had a specific region in mind, not including Central or South America.

Getting this much bandwidth out of a single patch antenna is not possible, naturally. Not even close. So, an array of antennas would be the most likely course of action. Even so, some of the bands themselves are rather wide, requiring RBW’s of 20% or more, also not possible with single patch antennas as far as I know. As a rough, off the shoulder, guestimate an array of 6 or more (and my bet is more, like 10 to 12) patch antennas would be required to meet these needs, and to do so some effort would still need to be put into widening the bandwidth of the antennas. Possibly optimizing for RX vs TX, accepting some compromises in efficiency to gain bandwidth.

If he is interested in only a specific GSM900 system, vs all of them, that might, depending on which system and channel block, reduce the array count by one.

So, real specifications and intended application would help a lot.

It could be that a simple log periodic and its low gain is all that is needed. Possibly there is a structural limitation that will not allow one and that is why the OP was not looking at them, or an array of yagis for higher gain, in the first place.

T!
Mohave Desert, California, USA

 

[prcguy]

Token,
When you say most patch antennas have 6 to 9dBi gain, that's for an array of them fed with a corporate feed and optimum spacing to maximize the main lobe. A typical single patch is not very much gain.
prcguy

 

[Token]

Token,
When you say most patch antennas have 6 to 9dBi gain, that's for an array of them fed with a corporate feed and optimum spacing to maximize the main lobe. A typical single patch is not very much gain.
prcguy

We are talking about a microstrip patch antenna here, correct? Essentially a square flat plate over a ground? Fundamental operating mode is TM10?

For a single example of this type of antenna the radiating slots (edges, if you will, the fringe fields) over the ground plane gives directionality, good for near 3 dBi. The two primary slots each have a gain near 3 dBi. And the two slots combined add another 3 dBi. None of these theoretical numbers are ever fully achieved in the real world, so the gain is between 6 to 9 dBi, normally closer to the low end.

T!