Why does the receiver need to be so small? I had to re-read you post because at first I thought that you were discussing the transmitter. Is this receiver for the car end of the link, or the human end of the link? Keyless Entry systems usually have a simple and small transmitter at the human end and a receiver at the car end.

Given that the receiver needs a cable bundle down to the rest of the automobile anyway, why not move the receiver down into the main module and 'change' the cable run into the antenna?

Have you examined current systems? I've seen some that seem to have a range of hundreds of feet.

There are plenty of advanced techniques like spread spectrum and so on that could be applied. This is a fairly mature market with rolling codes and resynchronization features fairly common.

By the way, what is an 'ESA'?

That monopole should be 9 inches tall for optimum results (1/4 wave).

This is a small antenna, you can just about fit a quarter wave shorted patch in your dimensions if the antenna is shaped like an L.

I recently made a 390 Mhz antenna in about 4.5x2x0.060" antenna size using FR4, with your 315 Mhz frequency, you need 5.6 inches or 141 mm length. Using higher dielectric from Rogers corporation, that 141 mm length can be as small as 89 mm.

An important note: this is a tiny antenna, your mount location on the car i.e. the installation will change your results greatly, range factors of 5x by repositioning it will occur. Where is the present one?

kch

He should pay attention to the tradeoff between antenna gain and resultant holes in the coverage pattern. If this antenna is intended for the car end of the link, then an omnidirectional pattern with gain along the horizontal might be the best he could hope for (and even this would assume that he can control the installation).

Sorry I hadn't been able to get back.  First of all, this is for my senior design project at my university and were doing this with the sponsorship of an automotive company.  With that said, we have to keep the same dimensions of their current one, and can only work on the receiver side of the system.  As far as mounting goes, the requirements given by the company was that i should double the range from 30 meters to about 60 meters, and the tests shall be done with the reciever inside the car.  So I guess i could put it in any position I want.  I'm sure that the folks from the automotive company are not actually planning on manufacturing our design, maybe just use ideas or something, so there testing requirements are not too strict.
Thanks Alot

P.S. ESA stands for Electrically Small Antenna

At the outset you stated that you're "...concentrating on the design of the antenna." Combined with the other information just posted, it would probably be most time and effort efficient to revisit that decision.

If the current antenna is already very close to optimal, given the rather restrictive constraints (and it seems like it might be), then there may not be a range doubling available from that approach no matter what you do.

In other words, you can probably examine and model the current antenna and see how far from optimal it is. If you can't find a ready +6dB improvement (and I'll bet that you can't), then there's no point is wasting your time on that approach.

On the other hand, if you widen the approach, then it should be fairly easy to achieve 60m range.  Simply making it easier to mount the antenna in an optimal location, rather than under the seat (if thatis an issue with this system?), could make a huge difference in range. I'm not sure what the overview looks like so I don't know what design approaches would be the obvious to follow-up on.

When talking about Electrically Small Antennas, there can easily be 6 dB difference in gain from a good design to one just thrown together to fit a form factor. i.e. short monopole (air loaded) versus dielectrically loaded monopole.

Actually, you can pick up 20 dB gain by dielectrically loading antennas that are too short.
A 7.5 inch monopole (@315 MHz) would have about 0 dBi gain, 3.75 inch monopole has -10 dBi gain, 1.8 inch monopole has -18 dBi gain.
Loading the 3.75 inch monopole with dielectric of 4+ would pick up 10 dB. This assumes a large ground plane.

The best design would have you mount the antenna to the inside of the roof to form a groundplane using the roof.

Most plastics are 3.5-4.5 dielectric. One low cost option is FR4 (dielectric value is in the name = 4) from McMaster Carr for dielectric loading.

kch

Hey Higgler,

The low gain figures mentioned (-10 or -18 dBi); they must be an expression of loss (inefficiency), right? If so, then what if the short element was loss-free (say it was  superconducting for purposes of discussion)?

Some hams use common 102-inch whips on 3.5 MHz HF band in mobile installations. This is proportionally a 1.1 inch antenna at 315 MHz. The Z is very low, the currents are very high, the wires and connections must be huge to maintain efficiency and not have something melt.

Basically I thought that the issue with short antennas was losses due to low Z and high currents. The loss pulls down the gain figure. If you can sort out the loss, then then gain would rise.

Am I missing anything?

Yea, I wish I could move the antenna outside, that would make my job a lot easier but no they want it in that packaging, I'm guessing somewhere along the fire wall of the vehicle.  The ground plane will be about the size of the package (about 60mm X 45mm).  
What i have done so far is simulate their antenna and different of my designs on EZ-NEC to check for gains.  The best antenna, as far as gain, was a radial antenna.  I'm going to cut a disk out of a copper plate, feed it from the middle and use it as an antenna because according to NEC, this antenna give me a gain of about 10dBi over the antenna i'm trying to outperform.  For those who aren't familiar with EZ-NEC, its just a new version of an antenna simulating software that gives you radiation patterns and gains.

VE1BLL,
I was looking at recent measured gains on a wideband antenna we built and correlating to the 315 Mhz.
I was proposing that if the present antenna was just a short monopole connected to a wire, the gains would be at levels noted.

You can do better with tuning (and dielectric loading), and you can get a small antenna to radiate at one frequency.

Superconducting does help, cuz those currents become huge.

I'm curious what the gain spec is for that ham antenna. 102 inch whip + length of car (16'x12" = 192") = 294" total. That says the car probably radiates more than the whip since it's larger. Really small tuned antenna having a larger ground plane result in alot of radiation from the ground plane. Reduce the ground plane size and the gain and efficiency goes down too.
So your whip plus car is 0.09 wavelengths at 3.5 Mhz. If you compare that to this antenna, 1.1 inch size is 0.03 wavelengths, since the unit is 66mm longest length, that's 0.07 lambda - gets pretty close to your whip dimensions. Probably get similar gains with proper tuning then. I'm curious what the present antenna is, or what it's gain spec. is.

Placing a small antenna in a metal box with windows  will surely put some bumps in your antenna pattern.

kch

10dBi is an absolute gain relative to isotropic, not "over the antenna...".

High gain is unlikely when the antenna is such a small fraction of a wavelength. How can you achieve phase separation (to make gain) if the antenna system is so small that it's all at the same phase?

High gain could be bad if it isn't aimed in the right direction. The user could be anywhere.

I suppose if you can improve efficiency, then you'll improve range. Higgler has a good idea about dielectric loading.

Roy, W7EL, the author of EZnec often hangs out on the rec.radio.amateur.antenna newsgroup ('Usenet' - the original Internet forums).

His 10 dBi was an improvement relative to the other antenna. Can't be +10 dBiL gain, that's more than one wavelength in size or one square meter effective aperture at 315 Mhz.

This is a real wide beamwidth antenna, it is difficult to get good antenna patterns on tiny antennas since the test cables can change things. Your dc power wires from the car will cause changes in your calculated results. Maybe add an equivalent ground wire, or use this as part of the antenna. Hence if the requirement is to measure antenna patterns to match the computer, for a beginner antenna user, this can be puzzling. I'm thinking back on how the same technician with the same antenna got very different antenna patterns some 21 years ago.

Can your computer code calculate antenna patterns with the car in place, i.e. does it have wire code modeling capability. That would show you some interesting results.

kch

"10 dBi": Shouldn't use the 'i' unless you're comparing the antenna-in-question to THE hypothetical isotropic antenna.

Is dBiL (with an L) dBi 'Linear'? Now that you mention it, CP at one end would help to eliminate cross polarization losses, but at the expense of 3dB overall.

In my mind, even though gain equals directivity plus loss ('plus' depending on how you like to do the signs), I like to consider loss as a totally separate issue from directivity.

High directivity can be a bad thing if you don't know how the car is parked relative to from which direction the user is approaching. There's no point having a high directivity antenna aimed out the back window if Bubba is approaching from the side.

Therefore, a "low gain" (common misusage for 'low directivity) antenna may provide better coverage, but that doesn't mean that you can add loss to a high directivity antenna to provide a low (net) gain antenna that will make anyone happy.

Even in SW modeling, it can be worth varying the inputs a bit either way to aid manual detection of bad results. For our work, we varied EVERYTHING a bit in each direction to check for over-sensitivity to the input data. Found many anomolies, exposed several bugs, did a few work-arounds.

The hams with the 'ESA' HF whips typically use every sort of loading (except dielectric, maybe they should try that) to get a higher Z at the base of the whip. But the bandwidth gets squeezed so they often use motorized tuners either within the antenna or in the trunk. They're generally very happy if the antenna system stays cool, as opposed to melting.

Just a side note, but the 4 in FR4 doesn't refer to the dieletric value, it refers to the standards level.  FR = Fire Retardant.  If memory serves, there are levels 1-5 (one of those levels may not exist, kind of like there's no such thing as MPEG-3).  Again, from memory, FR1 would be like that cheap cardboard type material, FR4 is the standard phenolic/glass type stuff everyone is used to.  My memory is rusty, but a Google search will turn up more useful info.

Dan
Owner
http://www.Hi-TecDesigns.com

thanks macgyvers2000, never knew that. I can't say that anymore.
Just a coincidence that the dielectric is 4.

kch