Loop antenna glued on back of cow-Gain question 2

[searsjames]

Gentlemen,

I am developing a tracking system operating at 433 MHz that utilizes a loop antenna glued in the horizontal plane to the back side of a cow.

The diameter is 1/4 wave and it will be spaced 1/4 inch from the skin. The height above ground will be 4.5 feet.

According to

http://www.chipcon.com

application note AN003, the impedance of flesh is 38-57 ohms (page 15). Pages 5-8 give a lot of information on loop theory but dont help me solve the following problem.

I need to predict propagation. What would be the relative gain of this loop in the horizontal plane compared to a standard 1/4 wave whip and ground plane?

I cannot use a whip in this application.
Thanks very much for your help.

Jim

 

[VEBill]

Reminds me of the old physics joke, "Assuming a spherical cow with a radius of 1m..."

I've actually seen a finite-element model of a cow (seriously), but I can't find the link. It was in relation to studies of the absorption of RF as relates to safety of RF. Probably overkill for this application to conduct a complete NEC4 analysis. Given the application - tracking large, slow moving animals that are presumably located in a known area, it shouldn't be difficult to achieve a more than adequate RF margin (low duty cycle).

By the way, have you considered a simpler to install, and more comfortable to the cow, collar?

Are you using GPS ?

 

[searsjames]

Thanks,
Collars snag in ranch environments and do not place the transmitter in the physical position we need it. GPS is not used for cost and coin battery life issues.

ANOTHER QUESTION. How will the beam pattern change? Will the horizontal donut flare upward or downward? Will nulling of the electric near field by the cow be sufficient to make the pattern horizontally symetric?

 

[VEBill]

'Udderly' ridiculous... ;-)

I don't have sufficient related experience to answer your question.

You might want to investigate the cell phone safety issue because those researchers have done a lot of work with antennas near heads. You might find some related info.

Another alternative is to use an up-to-date NEC to model your antenna near ground (where the ground parameters are adjusted to match your hip-of-beef).

Also, you could just try several antennas. Perhaps the link margin is such that you don't even need to worry about optimised RF performance.

What kind of range ('Home, home on the range...' - sorry) are you planning? What sort of peak power and duty cycle?

 

[searsjames]

>= 1 mile over varied range conditions to 30 foot receiver height, 10-15 mw, .005%

Emperical testing is always the best way to be sure. I plan on it. Just getting a head start.

Jim

 

[logbook]

1 mile seems an awfully long way for a low power un-licensed radio transmitter. We did a low power radio link at 418MHz (another low power “unregulated” band) and the range was more like 50 metres.

 

[Higgler]

I'd expect a gain of -20 dBi or worse for the antenna and it will vary alot with angle. I'd suggest changing to a patch antenna, using Rogers 3010 dielectric of 10 makes the antenna just over 4 inches long x 3 inches wide x 0.060" thick. That material is cheap and bends easily, so you can place it on like a saddle and it will radiate off both sides easily. I'd suggest if you can use a large piece of Rogers dielectric, say 6x12 inches in size even though the antenna is 2x4 inches. Still plan on roughly -5 dBi gain. It's difficult to get much gain on a small antenna.
Kevin.
PS: Patches are easy to make, and change if water sits directly on it, so you'll need to cover it with 1/4 to 1/2 inch thick material. I made a very similar antenna for an aircraft last year, same frequency coincidentally.

 

[BrianR]

If you use narrow band modulation you might make it. As a rough guide, the free space path loss is 90dB at 1 mile @ 450MHz, so with TX = -10dBm , TX antenna = -10dBi and RX antenna = +10dBi and RX coax = -10dB loss, there's -110dBm at the RX. Not much but possible. You'll almost certainly need space diversity RX antennas and the vertical spacing would be 7.5 ft to start with for the conditions you mentioned.

 

[searsjames]

64khz FSK modulation, TX=0dbm, there is no coax loss, and -103dbm at the reciever. Hope to get 0 to -5 dbi at the TX antenna, am analysing now...that is the main question I have for this forun....

The propagation models i have checked say things should work. Antenna is the question...

What would the configuration of the Patch be?

Thanks Jim

 

[Higgler]

searsjames, send me an email at khiggins@toyon.com and I'll make you a powerpoint drawing of the patch.
Let me know if you have equipment to tune the patch if you place it in a cavity.
The configuration would be cow, aluminum foil 12" square or so, foam spacer, patch, foam spacer, plastic waterproofing cover(ziplock bag). The transmitter can sit in the foam spacer just to the side of the patch. Antenna weight should be less than one pound.
Patches are narrow band and bandwidth is proportional to their thickness, so it'll need to be tunable to get to the right frequency.
Kevin

 

[BrianR]

I just looked back at my last post and I gave TX = -10dBm, it is actually +10dBm so that improves things. I doubt you'll achieve the TX antenna gain you hope for because of the variables in mounting, orientation and other unmentionable things that cows do. Higglers antenna sounds like a good one but I expect a cow will reject a 12" square of non-breathing aluminium on its back.

 

[Higgler]

Thoughts on tracking. IF tracking doesn't have to be continuous and you assume the cow moves occassionally in a circle, you could make an small array that radiates to the port and starboard only with much higher gain. The antenna would be shaped like the letter H and draped over the cow like a saddle. The spacing between the two vertical parts of the H could be anything from 10 to 30 inches, larger spacing for more gain. It would be a 4 element patch with phasing to create nulls upward and downward, hence it radiates broadside. You could end up with peak gains above +5 dBi for vertical polarization. Gain would roll off 20 dB at the front and tail directions.
kch.

 

[searsjames]

The cow will have an intrinsic impedance of about 50 ohms though I dont know what that means as far as minimizing coverage. I would love to get a diagram of any suggestions.

The cow skin does need to breath.

W1VT of the ARRL has graciously turned be on to EZNET so now I can analyze simple antenna structures over the simulated elevated back of a cow.

One question I have with this program is that I have data that the intrinsic impedance of a cow is probably 50 ohms. However EZNET requires logal ground characteristics to be entered in S/m and dieletric constant

I do have access to a friend with a network analyser. And prototyping is easy if I volunteer to be the Cow!

Thanks,
Jim

 

[VEBill]

Just like RC airplanes (etc.), having a null off the back end of the mobile platform is not a good thing. Tracking is needed most when the cow is heading away from 'home base' (assuming an unbounded environment). Any null should be aimed forward as that presents a problem that will eventually solve itself as the range decreases.

Another point is that the receiver antenna can have quite high gain if it is installed to one side of the 'range'. Also, a vertical array can add futher gain by compressing the pattern along the horizontal. If the receiving antenna is installed in a very high location, then the cosecant-squared pattern would be useful. All in all, there is an opportunity for substantial antenna gain, perhaps as much as about +13dBi, at the receiving end by optimally designing the receiving antenna and carefully selecting its location. Worth including...

PS: Hopefully, we won't be finding any bits of electronics in our hamburger...

 

[photistor]

In your original message you asked to predict propagation. If you don't have simulation software you can make a measurement by using watermellons or cantalope to simulate the cow (no joke). If you don't have access to an antenna range you can use a spectrum analyzer to make a path measurement at various angles to get a feel for the coverage.

 

[searsjames]

OK I am intrigued. It is really unwise to get a cow and a $7000 network analyser within pissing distance.

How do you know that a watermellon is a viable substitute?

BTW I was on my hands and knees saying moo yesterday.

Jim

 

[Higgler]

I would think that a human is a closer, friendlier substitute, than a cow, or a melon, or use a well trained dog. A VSWR measurement comparison will give you a hint of comparison. But the antenna pattern is going to be different when it's surroundings change.
Kevin.