Designing a GSM monopole to dipole transformer - Page 1
Forum section: electronics / projects

author: ElectroWarrior
author_rank: Contributor
timestamp: April 09, 2015, 03:43:35 pm
content: I am currently making a design including a GSM module which has a 50 ohm impedance output for the antenna. However I am interested in using a dipole (73 Ohm img 43Ohm) antenna with this module.I have been searching around for either a dedicated , or discrete balun that can handle this transformation, with no luck.Does anybody have any knowledge on how to design one, or where to look for such a design?Best regardsNikolaj

author: Howardlong
author_rank: Super Contributor
timestamp: April 09, 2015, 04:09:11 pm
content: Quote from: ElectroWarrior on April 09, 2015, 03:43:35 pmAssuming we are talking about a single band, you can use a balun filter, which can be made out of lumped L and C elements for GSM frequencies, although it'll need to be small smd parts, and the inductors should probably be wirewound rather than MLCC to minimise losses.http://www.odyseus.nildram.co.uk/RFMicrowave_Circuits_Files/Balun%20Design.pdfhttp://leleivre.com/rf_LCBalun.htmlhttp://leleivre.com/rf_lcmatch.htmlEssentially it combines an impedance matching function, as you might use when using an LC match on a simple unbalanced only scenario, with the balun function.

author: LaserSteve
author_rank: Super Contributor
timestamp: April 09, 2015, 05:05:37 pm
content: Use a coaxial balun, Made from ordinary coax. Performance will be adequate, but will probably be better then lumped components at GSM frequencies for a novice.Steve

author: Howardlong
author_rank: Super Contributor
timestamp: April 10, 2015, 08:10:36 am
content: Quote from: LaserSteve on April 09, 2015, 05:05:37 pmMeasuring and terminating accurately enough might be a challenge at those frequencies though. I remember making coax power splitters and switchable phasing harnesses out of coax transmission line transformers using RG174/RG178/RG179 for 430MHz many years ago, that was hard enough.

author: LaserSteve
author_rank: Super Contributor
timestamp: April 10, 2015, 01:33:55 pm
content: Unless he's using Dual Band GSM, we could just send him to Minicircuits.com and get it over with.A bit expensive for large volume production, but known to work very well if used properly. They have some to 3.5 Ghz these days, listed under "transformers". This way he can speak to one of their application engineers as well.Coaxial Baluns in modern cheap antennas are often made using a bit of brass tubing and the stripped core of the mini coax. Of course, that takes a bit of learning about controlled impedances and at least a home made RF Bridge. If he downloads HP APPCAD, that will do some of the math for him.If you rip open the right 2.4 Ghz WIFI antenna, you'll see its not too hard to duplicate.Steve

author: Howardlong
author_rank: Super Contributor
timestamp: April 10, 2015, 05:39:38 pm
content: If it were just a one off balun, I'd agree, pick up an off the shelf solution but there's some matching needs doing too to a complex impedance. An off the shelf balun is unlikely to offer the OP the right match.That is why I offered the suggestion of a lumped balun filter, so that the balun and matching can be achieved at the same time. It's a very common technique these days. Five or six carefully chosen 0402/0603 Ls and Cs, tweak the values as necessary on a VNA, and it's done for just a few pennies, and it would be an easily repeatable design without the need for repeated production measurements and it's on a tiny footprint.My concern about a more mechanical approach is in the manufacturing and how much that would cost in terms of time and money to fabricate such a design to tolerance. For a one off, yes I can see a coax solution working, or if it is integrated somehow into the antenna manufacturing itself.

author: ElectroWarrior
author_rank: Contributor
timestamp: April 12, 2015, 11:17:19 pm
content: Hello guys, thank you very much for all the advises, this is amazing!!One thing I forgot to mention is that we are using dual band GSM, specifically 900, and 1800MHz. Space is also a very important factor, there is very little of it. Therefore I believe a discrete component solution would probably be the best .Would it be possible to use two balun filters, as Howardlong presented in parallel to achieve the dual band performance?Best regardsNikolaj

author: KJDS
author_rank: Super Contributor
timestamp: April 13, 2015, 07:51:08 am
content: Do you have a dual band antenna and is it fed from a single point for both bands?

author: LaserSteve
author_rank: Super Contributor
timestamp: April 14, 2015, 04:35:13 am
content: Take a look at Johanson's matching modules. Steve

author: Richard Head
author_rank: Frequent Contributor
timestamp: April 14, 2015, 05:28:05 am
content: Is a 1.8:1 SWR a problem? You will still radiate the same amount of power as long as the PA doesn't pull back. The RF reflected from the mismatched antenna is re-reflected at the PA and begins a second journey down the coax to the antenna so virtually all is radiated eventually.

author: Dago
author_rank: Frequent Contributor
timestamp: April 14, 2015, 06:56:58 am
content: Quote from: Richard Head on April 14, 2015, 05:28:05 amThis is not true, with this logic SWR wouldn't matter at all, anywhere. In reality the reflected power will be dissipated as heat in the PA (or if there is a circulator, then in the circulators terminated port).

author: Howardlong
author_rank: Super Contributor
timestamp: April 14, 2015, 08:17:20 am
content: You mention that the "dipole" is 73+j43 ohms, it's highly unlikely to be this on both bands.So we need to know...Is the source single port, i.e., both bands from one port, or does each band have its own port?Is there only one antenna, or one for each band?What is the impedance of the antenna on each band? Even better, is there a Smith chart or other impedance analysis such as s parameters over the frequencies of interest?If there is only a single source port, then using an off the shelf dual band balun and then a high- and low-pass topology LC networks to do the matching, one for each band.As soon as it becomes dual or multi-band, the complexities (and usually losses) really start mounting up, particularly if it's single port at each end.

author: ElectroWarrior
author_rank: Contributor
timestamp: April 14, 2015, 10:08:14 am
content: Quote from: Howardlong on April 14, 2015, 08:17:20 amThe source is a single port for both bands.Quote from: Howardlong on April 14, 2015, 08:17:20 amThe plan is to use a "vee" dipole antenna with traps, so both bands will be directly connected to the same point - not on separate radiating antenna elementsQuote from: Howardlong on April 14, 2015, 08:17:20 amThe antenna has yet to be designed, this is a initial investigation if it is realistically possible to do a transformation going from monopole to dipole.Thanks alot for all the help!Nikolaj

author: Richard Head
author_rank: Frequent Contributor
timestamp: April 14, 2015, 01:54:10 pm
content: This is not true, with this logic SWR wouldn't matter at all, anywhere. In reality the reflected power will be dissipated as heat in the PA (or if there is a circulator, then in the circulators terminated port).That is in fact true. It's a falacy that the reflected power is dissipated as heat in the PA. It is re-reflected toward the antenna again. If the coax is lossles then no energy is actually lost.The mismatch will cause higher voltages or currents at the PA which is undesirable but with a lossless coax all the power will eventually be radiated.I'm not saying a poor match is an acceptable condition, just that in most cases it doesn't cause the antenna to radiate less power, as long as the PA doesn't pull back to protect itself due to the high VSWR.There is an excellent book "Reflections" by Walter Maxwell that deals with this controversial issue.

author: Dago
author_rank: Frequent Contributor
timestamp: April 14, 2015, 02:50:58 pm
content: Quote from: Richard Head on April 14, 2015, 01:54:10 pmI don't buy this one bit. Assuming the PA is not some magical ideal component it will have a resistive output impedance component, which WILL absorb most of the power. Only a fraction will be reflected back, and even that bit of power is not simply "re-sent power" because it can cancel out depending on the phasing of the reflected and transmitted signal.

author: KJDS
author_rank: Super Contributor
timestamp: April 14, 2015, 05:30:17 pm
content: Quote from: Richard Head on April 14, 2015, 01:54:10 pmAs someone who gets paid a great deal of money to deign RF PAs, I can say that they are complex beasts.What is important to a PA is the load that is presented to its output.Get it right and you'll get a good compromise between output power and distortion, efficiency and gain. Usually you line the load up between the max power and efficiency impedances. Sometimes you'll go for a circuit that focuses more on efficiency and sometimes more for max power. Get it even a little wrong and you can easily lose half the max power and suffer horrible efficiency, causing the device to overheat.So after you've done that in the lab with a 50R load on the output, along comes a poorly matched antenna. If you look at the specs on a typical high power transistor (a few hundred watts) you'll find that even 1.3:1 is poor enough to prevent the specs being met. That's far more important than some being reflected back to the transistor, some of which will be absorbed as heat and some will be reflected back again, possibly in antiphase reducing the total output.

author: Richard Head
author_rank: Frequent Contributor
timestamp: April 15, 2015, 06:19:22 am
content: "---and some will be reflected back again, possibly in antiphase reducing the total output."The fact that it's out of phase is what makes the impedance change. And it changes periodically every 1/4 wavelength along the transmission line.Also, an RF PA designed for a 50R load does not have a 50R output impedance. 50R is the impedance which should be presented to the PA for it to work correctly. If the PA did have a 50R output impedance then it would only ever be able to achieve a 50% efficiency. The fact is the vast majority of power that is reflected from the mismatched load is indeed re-reflected back to the load/antenna for that reason. This can be verified with a dual directional coupler or directional wattmeter. When the load is mismatched it is possible for the forward power to exceed the output power of the source! This is because the re-reflected power is adding to the forward power.