High power monopole Antenna
What is the difference between two antennas (material or physical ) working at same operating frequency but at different power level, one at 1 watt and the other at 50 watt.
I want do design a monopole antenna for GSM900 which can handle 50watt of power.
What are the design rules for high power antenna in HFSS.
I modified the monopole antenna for 900MHZ ( given in h..p://www.edaboard.com/attachments/28384d1197871738-projects_1070.zip) and use the dimensions of RG142 coax cable.
My question is that can i expect that it will radiate power from 5watt to 50watt.
Hi Adnan
i am not sure what kind of modifications you have integrated into your model of monopole.
we all know from the equation of radiated power of a monopole is of the order(eta*((current0)^2)/4*pi) or equivalently similar to(.5*radiation resistance*current0^2). so following the equations the only way to increase the radiated power is to either increase the radiation resistance or the current carrying capabilities of the monopole.
a way of increasing the radiation resistance is to use biconical monopole. who resistance values can go upto 600 ohms.other than that i am not sure about a specific current capabilities of a material which can be used to contruct a monopole.
hope i was helpful.as soon as i come across anything relevant,i will make sure to post it.until then
Have fun
Regards
Rotmanlens.
Basically, an antenna is a linear component (so power level is not of importance). It means when you double the power, the radiated E an H field will increase by sqrt(2). HFSS will treat your antenna as linear system
Problem with high power is heat generation, intermodulation and arcing/corona effects.
For well terminated coaxial cable, you can find the max. power versus frequency. Same diameter cable can have different power ratings (as Teflon can stand much higher temperature then polyethylene). In case of bad VSWR in coaxial feed lines, you have to reduce the power.
For connectors it is mostly difficult to find the power rating. Plastics in regions of high E may melt, if so, you may use plastic with less dielectric absorption (for example PC, PP instead of PA or PVC).
Arcing and corona. At increasing power, there will be regions with high field strength (due to voltage maximum in combination with structures with sharp edges). If E goes to around 3kV/mm, the air will breakdown locally and will give rise to a bluish glow around some parts of the structure. You may observe this in the dark.
At increasing power, full breakdown may occur. If due to heat material evaporated, the breakdown may remain, even when the power is reduced.
A quarter wave monopole has its higher current at the feed point, so there heat loss may come into play, you can assess that yourself by calculating the ac resistance (you need to know the skindepth).
At the top, the voltage is highest. Thin structures will lead to relative high voltage at the top of the antenna. As thin structures have small radius, E will be higher given same voltage. So antennas that have to accept high power are mostly thick and have rounded edges. If you increase power and VSWR changes in a step like manner, you can be almost sure to have corona discharge or full breakdown (or you have some insertion locking or instability in your PA!).
Corona discharge is a non-linear effect like bad transition in connectors. Both may give rise to spurious response in case of multi-carrier systems (sharing the same antenna).
Half wave monopoles need special attention as there you have high impedance at the feed point (so large E). Impedance can vary from several hundreds Ohms to kOhm range (depending on diameter/lambda ratio). A thin whip may have for example 1 kOhm input impedance. 50W will result in 300Vp. A small clearance to ground in combination with a sharp edge may result in corona and eventually breakdown of the dielectric. Risk on corona discharge you can reduce by applying sufficient thickness of insulating material. The thickness should be such that at the air-insulator interface the field strength is below say 2kV/mm in the air. Note that breakdown voltage in air reduces with increasing height.
You can find formulas for assessing E close to structures on websites for Tesla coil aficionados.
Normally, power hanling capability is limited by heating due to losses in the antenna, connector or cable. This depends of average power.
The other limitation is dielectric breakdown. This depends of peak power.
The same antenna designed for 1 W works at 50 W if those limits are respected.
Regards
Z