Designing a vertically polarized monopole antenna for wideband applications

[npk_may]

Designing a vertically polarized monopole antenna for multiband applications

Hey,
I am trying to figure out a suitable monopole antenna to do the following:
1. To be used with a custom made spectrum analyzer to simultaneously scan the frequencies: 25-80 MHz and 116-174 MHz.
2. To achieve external noise limited sensitivity around 50 MHz and 150 MHz. I used the galactic noise model for this purpose.

I used an equivalent circuit model and 4NEC2 to obtain the minimum detectable signal measurements for different antennae dimensions. However, I couldn't find a monopole antenna dimension which would let it attain the required sensitivity in both the bands.

Please help me out. Are there monopole antennae which show good sensitivity in both these bands? I am looking for an antenna preferably less than a meter in length.

Thank you

 

[DeboraHarry]

1 m is a very small fraction of a wavelength at 25 MHz. Physically short antennas are going to be narrow band. If you Google it, you will find references to theoretical limits on the bandwidths of antennas.

The way to increase the bandwidth of most antennas is to increase their volume. Making a monopole with a larger diameter will increase the bandwidth. Sorry, I don't have any real solutions to your problem, but hopefully that will give you something to think about.

 

[biff44]

Yep, an "electrically small" is not going to be as efficient as one that is closer to a quarter wave in length. Not much you can do about that. And you need 2 different frequencies, so the antenna will be more complicated because of that.

I would recommend looking at fractal and other small antenna designs--some progress has been made in that field.

Just as important as the antenna size is the ground plane size. If you have a ground plane that is too small, that will also adversely impact the radiation resistance. Is this a handheld application? Maybe you can design the case so that the user's body becomes a defacto ground plane extension when he is holding the unit.

Rich

 

[npk_may]

Thank you for those valuable comments. I have to design this as part of a graduate level project. I am required to use a vertically polarized monopole antenna. This is for a portable radio frequency spectrum analyzer. The box is going to be made of a polymer with a special coating inside so as to reduce the effect of the antenna's radiation on the components inside.

So, yet another doubt of mine is how to simulate the effect of such a box on the antenna. 4NEC2 does not let me simulate the effects of an antenna radiating over a non-conducting surface such as that of the polymer box. If there is a way to simulate this, how can I do it? Also, what should I expect from such a simulation? I apologize for this barrage of questions, but I have pondered over this for far too long.

 

[E Kafeman]

It is a bit pointless to have a monopole antenna that is longer then its ground. If it is a short ground, is that what mainly limit antenna efficiency. For handheld equipment is however also the human body a very lossy part of ground plane, especially in this frequency range. Optimizing impedance matching for a ground plane in constant motion is not easy. Dynamic matching is possible but complicated and losses are still very unpredictable. As it is a RX only antenna, use as long monopole as possible and connect it to a LNA with input impedance higher then 500 Ohm. That is a commons solution for cellphones which use headset cable as FM radio antenna. Whatever solution, forget about sensitivity in range of a full size dipole.
As it is a custom made analyzer, do I assume that it not is too difficult to get a specific control voltage for each frequency? Then had I selected a narrow band tuned dipole. A such solution requires a control voltage mapped against actual frequency and a varactor net as matching circuit. For this solution will resulting readings be much less depending of how and who that is holding the unit.
Similar tunable solutions for real small antennas in this frequency range: **broken link removed**.
If possible is something like this a simplerr solution to tune, and also results in higher antenna efficiency:
**broken link removed**
It is a reference antenna, Schwarzbeck UHA9105. Heavy solid metal and expensive. Two wires in a plastic tube will also work.

 

[DeboraHarry]

So, yet another doubt of mine is how to simulate the effect of such a box on the antenna. 4NEC2 does not let me simulate the effects of an antenna radiating over a non-conducting surface such as that of the polymer box. If there is a way to simulate this, how can I do it? Also, what should I expect from such a simulation? I apologize for this barrage of questions, but I have pondered over this for far too long.

If your university has a license for HFSS or similar tools, that would be possible to model the complete setup in that. But HFSS is expensive, so your uni might not have a license. Also, it is not simple to use, so you would need to spend quite a bit of time learning to use it.

 

[zorro]

Re: Designing a vertically polarized monopole antenna for multiband applications

Hey,
I am trying to figure out a suitable monopole antenna to do the following:
1. To be used with a custom made spectrum analyzer to simultaneously scan the frequencies: 25-80 MHz and 116-174 MHz.
2. To achieve external noise limited sensitivity around 50 MHz and 150 MHz. I used the galactic noise model for this purpose.

I'm sorry, but I don't understand the second requirement. Nevertheless, I guess that it means some high sensitivity, what means that the antenna must have high gain.
A monopole is not suited neither for wide band (requirement 1) nor for high gain (requirement 2?). Even worse if you want small size.
As pointed out by other members, you will need increased size in order to get enough gain and bandwidth.

I reccommend a paper and a book, both by R. C. Hansen, related to the subject:

Fundamental limitations in antennas (Proceedings of the IEEE, 1981)
Electrically Small, Superdirective, and Superconducting Antennas (Wiley, 2006)

Regards

Z