Spiral Antenna and Baluns 1

[mrkenneth]

I am planning to use a spiral antenna with an RF Detector (for cellphones) using Analog Device's AD8314. The antenna currently looks like this:

http://s92795644.onlinehome.us/scipho/spiral.jpg

(reduced to 25% of actual size)

http://s92795644.onlinehome.us/scipho/spiral2.jpg

(actual size at 600 dpi)
It is supposed to operate from 700 MHz to over 10 GHz, but I have not run any simulation on it (because I do not have the software and the know-hows. )

I would like to connect the antenna directly to the AD8314 detector circuit using a microstrip (without a cable), but I think I will need a 2:1 or 4:1 balun. I am using MCalc (

http://mcalc.sourceforge.net/#calc)

to calculate the microstrip dimensions. But what does the electrical length mean?
How would I make a balun for this? I have read a few guides for designing baluns, but all seem quite large. (In fact, larger than the AD8314 board that I have designed...) And the baluns are for connecting to a coaxial cable.

Sorry for all the questions. I am new to all this. Any comments appreciated!

 

[Comcokid]

The electrical length input for the mcalc is used to calculate items such as the velocity-of-propagation, phase delay, or to use a short microstrip stub to add capacitance or inductance to tune a circuit at a frequency. For these calculations you need to know the length since the RF will travel slower than the "free-space" speed of light.

I don't know what the feedpoint impedance of your antenna is, but the AD8314 has a 300 to 1K ohm impedance with some reactance. The input impedance of the AD8314 is set to 50 Ohms with an external resistor (53.2 Ohm). Optionally, the matching impedance can be set for the frequency of interest. This is all covered in the data sheet.

It may be possible for you to interface the AD8314 directly to the antenna with no more than a matching network without using an intermediate 50 Ohm microstrip or coax. You may also be able to avoid need of a balum. Use a microstrip dimensioned at whatever chareristic impedance you need (within limits).

 

[mrkenneth]

Sorry, I have forgot to mention the impedance of the spiral antenna. It is supposed to be 188 ohms, but I have not (and cannot due to the lack of equipment) verified it. Assuming the antenna is actually 188 ohms, then the microstrip would be less than 5 mils, or not possible with my current setup or even outsourcing the board. (My current PCB fabricater can only go down to 10 mils. If I use a 2:1 balun, then the microstrip can be around 30 mils.

Does the microstrip have to be a specific length to the spiral antenna, or should it be as short as possible?

I would like the antenna and the AD8314 to be as broadband as possible (from 800 MHz to over 2500 MHz), so I will just be using the 52.3 ohm resistor and not the reactive match.

Since the spiral antenna is balanced (am I right here?), and the microstrip to the AD8314 is unbalanced, would not using the balun lead to any unpredictable losses or more loss? Almost all of the resources regarding baluns are for transmitting antennas, so would the same apply to receiving antennas (balanced and unbalanced lines)?

Thank you in advance!

 

[Higgler]

Why use a spiral? Do you need circular polarization?
Best antenna options for your bandwidth if you want to research antennas that aren't spirals:

"notch",
"bicone",
"monocone"
"flattened monocone"

type antennas.

If size isn't important, look up monocone and just make that, it's the friendliest antenna type to make.

If you want to build one quickly, buy a metal funnel, find one 5 to 8 inches tall. Have a ground plane 12" diameter or larger, drill some holes and mount an sma connector on the ground plane (

http://www.Pasternack.com,

see part number PE4000 for example). Attach the connector center conductor to the funnel's small end.
Connectorize your circuit and use a cable to the antenna.

If you need to be on one circuit board, then a printed circuit flat monocone is the way to go, or a notch antenna.
kch

 

[mrkenneth]

Circular polarization is preferred, since I would like to measure radiation polarized both vertically and horizontally. I know there will be loss, but it should be better than receiving something horizontally polarized using a vertical antenna.

Size is important too, since I would like to have eight antennas arranged so they form the perimeter of the RF meter. I am even complaining about the size of the spiral. Of course, if it can be printed onto a copper clad board, it would be so much better since I can just let the printer do the work instead of my trying to measure the length, width, etc.

I will research the antennas that you mentioned some more.

Also, I would like to connect the RF detector circuit directly to the antenna, instead of using a SMA connector, due to the cost. The cheapest I can find is around $4 USD, and sixteen of them would cost $64 USD! This is in addition to the cables.
I am wondering whether I can just run a feedthrough capacitor from the sealed RF detector to the antenna. And also whether I need a balun for this purpose. I am guessing so because I would need some impedance matching.


Here is a rough sketch of what I am thinking:

How the eight antennas will be laid out.

How I would like to connect the RF detector circuit to the antenna.


Any comments appreciated!

 

[Higgler]

If you planned to detect cell phone (1/2 watt, +27 dBm) radiation with this chip (spec's below), your range is very short. 900 mhz range is 300 feet with zero dBi gain antennas on both the cell phone and your unit. At 1800 Mhz that's 150 feet range for bare minimum sensitivity (-45 dBm) with a perfect match to your antenna.

Normally 20 dB Signal to Noise ratio is needed to compare directions, hence the detection range for +27 dBm transmit power to -25 dBm receive sensitivity is about 50 feet range. I hope you weren't planning on a mile range or more. Cell phone towers could be detected from 4x the values listed above since they transmit closer to 10 watts.

Chip Spec's.
Features Complete RF Detector/Controller Function
Typical Range –58 dBV to –13 dBV –45 dBm to 0 dBm re 50 O
Frequency Response from 100 MHz to 2.7 GHz
Rapid Response: 70 ns to a 10 dB Step.

Based on your sketch, I'd still go with notch antennas oriented at 45 degrees between horizontal and vertical.

Wide open detectors aren't good for long range unless they're looking to detect high power radars.

 

[mrkenneth]

Actually, I was only going to put the cellphone less than a foot from the antenna. The cellphone is supposed to go into the hole in the first diagram. The device is used to measure the radiation pattern of the phone.

Sorry for the misunderstanding.

When I used a dipole with a AD8361 detector before, the signal was only around -40dBm, so I guess I need a better antenna and lower losses this time.

 

[Higgler]

I get it. I've tried things similar to this. You have both sensitivy and antenna bounce problems. You need to duplicate a standard antenna chamber with lower cost stuff. I think you need an amplifier and filter before your detector for better sensitivity and some microwave absorber to make better antenna measurements.

The energy from the surrounding antennas will bounce around and hurt your results greatly. If you need results of antenna patterns within +/- 1 or 2 dB then you won't get that with your setup. Energy will bounce off your antennas and really hurt your results.
Antenna chambers to test this frequency use at least 12 inch absorber and that's a little too thin for really good results. I suggest one antenna with one detector, put your phone on a rotating pedestal device, add foam atop it and locate the antenna/detector about ?6" to 24" away. If you have a big room you need only a little absorber, otherwise be on the roof of a building or buy some absorber from Cumingcorp.com to place around the phone. Absorber comes in 24"x24"xT" thick, and is about $60 per unit for 12" absorber, you'd need at least 10 pieces.

Is this for a commercial company project or are just curious how your phone is doing?
kch

 

[mrkenneth]

Yes! Exactly!

I know I will get all the nasty reflections, but I heard that fibreglass (the FR4 subtrate that I plan on using) is pretty transparent to RF. Hopefully the reflections will be less if the antenna is close enough to the cellphone. I don't really need the results to be all that accurate, only to give an indication of whether the cellphone companies are trying to kill us by focusing all the microwaves into our heads. :-D

I don't think I will need an amplifier for the AD8314, since I think the IC already has 5 amplifier stages. I agree an amplifier would be needed if I use the diode detectors.

I have heard of these RF absorbers before, but they are just too expensive for me. Are the triangular foam in anechoic chambers made from this RF absorber?

As for the rotating device, I have thought about that too. But it would require more manual labour and take more time... The output power from the cellphone can also vary. If I decide on the rotating method, would it be better to let the receiving antenna move around instead, since the phone may adjust its power while it is moving around.

I don't think a company would hire such an incompetant person for making this thing. At least somebody who knows about antennas and baluns. This is just a personal ambitious project that I am going to use as a trash can later. :-D

 

[Higgler]

I see,
The FR4 may be transparent, but put wires on it and everything changes. Radar signal reflect off metal wires on antennas more that the FR4 that holds them.
Move the single detector around the phone, maybe add a string to set the distance.
Skip the absorber, but get some foam from a packing box to set the experiment on. Maybe make a circle to move the detector/antenna around it maybe.

You're really trying a "nearfield" experiment if you want to see the effect on someone's head due to cell phone heating.
I worked on a Medical experiment for 3 years to heat human beings with RF energy to kill cancer cells in the body. The cell antenna will heat the human much differently being close to it (L<1") than your 6" or 12" away experiment will show.
Standard dipole antennas "don't radiate off the ends in the far field" because the current goes to zero at the ends, but in the near field it heats tissue as much as the center of the antenna.
Taking data like you plan will show you some info, but it'll be a bit deceptive if you think you try to show the absorption in a human head. You'll be able to slightly compare different cell phones.

There is a special antenna chamber built for measuring cell phones with a chair and a person to sit on. The chair rotates and antennas like you describe are set in a circle similar to your concept. So a nearly full circle of data is taken at all the antenna frequencies. I couldn't find it quickly on the web, but I've seen photo's of it.

good luck
kch

 

[mrkenneth]

I will be making the antennas out of PCB, so the antenna arms will be 1-ounce copper. So I guess it will reflect quite a lot. What type antenna do you think I should use in this case?

I will only be comparing different cellphones and seeing whether I can modify their antennas to direct radiation away from the front of the phone.

I won't be trying to add the human head and whatnot in there. That's complicated enough to fry my brain.

The packing foam I can find.

I have seen the fake heads filled with a water and sugar solution with a cellphone strapped on. Is that what you are talking about?

How did the results of your experiment go? I am really interested! Do you have a report or something that I can read?

Thank you!

 

[Comcokid]

On the antenna and detector, If you use something like the AD8313 instead of the AD8314, you then have a differential input instead of a single-ended. This chip again has a high input impedance which you set with an external resistor. You might be able to go directly into it without an intermediate line it you put it almost directly on the back of the antenna.

There are several AD83XX devices with different features. Not all are listed on the Analog Devices site under the 'log amps' category.

 

[Higgler]

If you want to see how much the energy radiates into the person, instead of antenna patterns, you could just probe the area around the phone.
Maybe you could place a phantom head with probes in it and measure the result from each probe. The probe being close to the cell phone will pick up more signal and you can have a weaker antenna. Since you're really at just the transmit frequency of the cell phone, make probes for just that frequency.
There's alot of work in this area reported in the Antennas and Propagation Society monthly papers. Money spent of improving antenna design for cell phones is huge.

On the medical front, we made some progress when our funding was embezzled/stolen by a famous doctor in LA's Cedar's Sinai hospital. We built an antenna that could heat a 1" diameter section of flesh and leave the outer section unheated. Our next step needed expensive dielectric from Trans-Tech and we couldn't afford it, so we stopped work.

 

[mrkenneth]

Comcokid, unfortunately the AD8313 is much more expensive than the AD8314. (~$25 vs. $15 CAD) I am thinking about using Maxim's MAX4003, which is equivalent to the AD8314, but only costs around $3 to $4. This will make a significant difference if I build eight of these. I have also tried the AD8361, but the error was too high when the signal input is less than -20 dBm. The AD8314/MAX4003 can go down to around -45 dBm.

Higgler, is this the website for the Antennas and Propagation Society?

http://www.ieeeaps.org/

I haven't had time to look through the website yet. Are the monthly papers published online? I would very much like to read them.

Sorry you had to stop work on your project. The results can be groundbreaking. Who provided the funding?

 

[Higgler]

The medical funding was a guy named Ed Halimi who died last year. I'd mention the famous doctor from LA, but it was only Ed's word that he stole/embezzled the stuff.

On the APS stuff, the best deal you can find is through IEEE, they sell a $100 compilation on disk (DVD is only 2 disks, CD is 19 disks, so get the DVD version) from 1952 to year 2000 for all the papers. I haven't tried their website specifically, but used the IEEE one, which is the parent of the APS organization. If you know any students in Electrical Engineering, they can search in their college library for free and look up all the papers on cell phone antennas. You'll probably find 25-100 of them.

Here is the IEEE Xplore that I log into;

http://ieeexplore.ieee.org/Xplore/Toclogin.jsp?url=/login.html?url=/lpdocs/epic03/VSearch.htm

The archived data can be ordered from this website;

http://aps.ee.olemiss.edu/cdrom.htm

though you may have to be an IEEE member to order it.

In a quick search, I found 37 articles. Send me an email at khiggins@toyon.com and I'll forward the titles of them to you, here is a sample of some of the articles:

9 A comparative study of the biological effects of various mobile phone and wireless LAN antennas
Yioultsis, T.V.; Kosmanis, T.I.; Kosmidou, E.P.; Zygiridis, T.T.; Kantartzis, N.V.; Xenos, T.D.; Tsiboukis, T.D.;
Magnetics, IEEE Transactions on , Volume: 38 , Issue: 2 , March 2002

10 Cell phone testing and fundamental scientific research
Lin, J.C.;
Antennas and Propagation Magazine, IEEE , Volume: 43 , Issue: 4 , Aug. 2001

11 Specific absorption rates (SARs) induced in head tissues by microwave radiation from cell phones
Lin, J.C.;
Antennas and Propagation Magazine, IEEE , Volume: 42 , Issue: 5 , Oct. 2000

12 Research on health effects from cell phone radiation
Lin, J.C.;
Antennas and Propagation Magazine, IEEE , Volume: 42 , Issue: 4 , Aug. 2000
Pages:142 - 144

kch.