# Antenna with Distributed Positive Resistance

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#### micael

##### Member
Hi,
Does anyone know if it is feasible, instead of using high resistance wire in the antenna (quite a practical problem!), to use a large number of lumped resistors such that an antenna would act like a perfectly uniform wire of the same total resistance?

I need an antenna with internal resistance >7kOhm so i was wondering if it is possible to use 7 resistors 1kOhm each to built 1/4 monopole antenna with more than 7kOhm internal resistance

#### blueline_308

##### Member
I would love to know why. I'm just curious like that.

#### micael

##### Member
Power received on antenna Pr= 0.084mW
-->assume internal resistance of the receiving antenna Rs= 9kOhm
=> Vin=sqrt(Rs*Pr)=0.87V

i really need Vin to be more than 0.87V and the only way to do that is to have very large antenna resistance or increase the gain of the receiving antenna....

#### blueline_308

##### Member
Ok...if you say so. I will follow this one, 'cause it sounds interesting. I was talking antennas today with a wireless carrier engineer. Very interesting how the commercial antenna people get this stuff to work as well as it does. Good luck in your quest. Keep us informed as to how well it works.

73

N

#### N_Jay

##### Guest
micael said:
Hi,
Does anyone know if it is feasible, instead of using high resistance wire in the antenna (quite a practical problem!), to use a large number of lumped resistors such that an antenna would act like a perfectly uniform wire of the same total resistance?

I need an antenna with internal resistance >7kOhm so i was wondering if it is possible to use 7 resistors 1kOhm each to built 1/4 monopole antenna with more than 7kOhm internal resistance

Sounds like you have misunderstood some basic antenna design concepts.

N

#### N_Jay

##### Guest
micael said:
Power received on antenna Pr= 0.084mW
-->assume internal resistance of the receiving antenna Rs= 9kOhm
=> Vin=sqrt(Rs*Pr)=0.87V

i really need Vin to be more than 0.87V and the only way to do that is to have very large antenna resistance or increase the gain of the receiving antenna....

Continues to sound so.

#### micael

##### Member
Ok perhaps is a little bit confusing...
Using Berkeley Mica2 motes as an example, the transmission power is 10dBm (10mW). The antenna gain varies depending on the length, size and shape of the antenna.choose a typical 5/8-wavelength monopole antenna . According to the manufacturer(Chipcon,Application Note AN003), the gain of the antenna is 8.2dB. Considering environmental issues, choose 8dB as the transmitting antenna&#8217;s gain and 6dB as the receiving antenna&#8217;s gain. They correspond to numerical values of 6.31 and 3.98 ,for Gs and Gr respectively. The wavelength for 433MHz is 0.69m.With the distance, D, assigned a value of 3 meters, the received power is found to be 0.084mW.
The voltage at the receiving antenna Vin=sqrt(0.084mW*Rs)
Rs is the internal resistance of the receiving antenna which if assumed to be 9Kohm, then
Vin=0.87V
So what i am trying to figure out here is how i can actually get an antenna with > 9Kohm intenal resistance working.

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#### N_Jay

##### Guest
micael said:
Ok perhaps is a little bit confusing...
Using Berkeley Mica2 motes as an example, the transmission power is 10dBm (10mW). The antenna gain varies depending on the length, size and shape of the antenna.choose a typical 5/8-wavelength monopole antenna . According to the manufacturer(Chipcon,Application Note ), the gain of the antenna is 8.2dB. Considering environmental issues, choose 8dB as the transmitting antenna’s gain and 6dB as the receiving antenna’s gain. They correspond to numerical values of 6.31 and 3.98 ,for Gs and Gr respectively. The wavelength for 433MHz is 0.69m.With the distance, D, assigned a value of 3 meters, the received power is found to be 0.084mW.
The voltage at the receiving antenna Vin=sqrt(0.084mW*Rs)
Rs is the internal resistance of the receiving antenna which if assumed to be 9Kohm, then
Vin=0.87V
So what i am trying to figure out here is how i can actually get an antenna with > 9Kohm intenal resistance working.

1) Antennas do NOT have an internal resistance, they have a characteristic impedance.
2) The impedance is related to the design and will vary by frequency
3) 9K ohms is unusual for an antenna but cold be designed in required
4) If you are using any transmission line then you want to match the impedance of the antenna to the transmission line, not the receiver as you seem to be attempting.
5) At the receiver you can match the transmission line (and antenna) to whatever impedance you want with an RF transformer using one of several different designs.
6) Why do you want 0.87V?

My guess is you are trying to design an RF circuit using your understanding of DC theory?

#### jhooten

##### Member
There is a name for a group of resistors connected to the input of are RF device, Dummy Load. Resistors do not make good antenna elements. They convert electical energy to heat reducing the usable signal to the device.

Antenna gain does not multiply the amount of rf energy. Antenna gain concentrates the available rf energy in the desired direction by changing the radiation pattern.

To get more signal you have only three choices. In order of desirability:
1. Reduce system losses (better feedline, better connectors, less resistance, proper antenna orientation, proper grounding, ect.).
2. Increase gain ( directional antenna).
3. Amplification (LNA at the receiver, more power at the transmitter)

And the question still begs an answer, what are you trying to accomplish?

#### gcgrotz

##### Member
micael:

Your calculation does not include freespace path loss for which you must know frequency and distance. This is subtracted from antenna gain (plus feedline losses) and transmitter power and will calculate signal strength at the receiver.

If you figure Vin of 0.87 volts at the receiver, it will most likely create its own set of problems in the form of overload, intermod, and distortion.

Most good commercial receivers have a sensitivity in the range of -110 to -125dbm or better. Thats db referenced to 1 milliwatt. You're talking tenths of a microvolt here, not 870 MILLI-volts!

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#### N_Jay

##### Guest
:lol: :twisted: This thread is going to get real interesting as people start providing partial answers (both right and wrong) based on their assumptions (most without even using all the information in his posts):twisted: :lol:

#### micael

##### Member

Well i need relatively high input voltage from the receiving antenna so that using Charge pump approach i could get about 5V which us enough to trigger PIC18F452 to wake-up from the sleep mode.

The attached file shows such a design. It uses a charge pump to step up the input voltage to a suitable level, and stores the electrical energy on a capacitor Cse.
The diodes in the schematic are zero-bias Schottky diodes. When the voltage across Cse
increases beyond a certain level, it generates an increasing edge at Vout, and triggers a wake-up interrupt.

I use PSPICE to simulate this design and supposing that the wakeup radio signal excites an alternating current of 0.1V amplitude the Vout from the pump cicuit increases steadily to 0.6V in 1ms.
So if i get an antenna that could give more than 0.87V at 433MHz i could get 5V from the pump circuit.

#### Attachments

• pump.jpg
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#### N_Jay

##### Guest
micael said:
Well i need relatively high input voltage from the receiving antenna so that using Charge pump approach i could get about 5V which us enough to trigger PIC18F452 to wake-up from the sleep mode.

The attached file shows such a design. It uses a charge pump to step up the input voltage to a suitable level, and stores the electrical energy on a capacitor Cse.
The diodes in the schematic are zero-bias Schottky diodes. When the voltage across Cse
increases beyond a certain level, it generates an increasing edge at Vout, and triggers a wake-up interrupt.

I use PSPICE to simulate this design and supposing that the wakeup radio signal excites an alternating current of 0.1V amplitude the Vout from the pump cicuit increases steadily to 0.6V in 1ms.
So if i get an antenna that could give more than 0.87V at 433MHz i could get 5V from the pump circuit.

I would count on matching to about 50 Ohms and design a detector and and trigger circuit to use what you get.

Is this a hobby project, a class project, or a work project?

#### gcgrotz

##### Member
I'm out... I don't want to be around anything that will put .87v at 433MHz into any kind of receive antenna. That would probably exceed some kind of exposure limit. just a guess. Some engineer type can run that calculation.

Isn't 433 MHz where all the door openers work? Just get a door opener remote and receiver from some catalog and you're done. Not as challenging but a lot safer.

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#### N_Jay

##### Guest
gcgrotz said:
I'm out... I don't want to be around anything that will put .87v at 433MHz into any kind of receive antenna. That would probably exceed some kind of exposure limit. just a guess. Some engineer type can run that calculation.

Isn't 433 MHz where all the door openers work? Just get a door opener remote and receiver from some catalog and you're done. Not as challenging but a lot safer.

That level is way below the level of concern.

As for the garage door opener, it seems like he is using that type of equipment, but wants the receiver to be asleep till a signal is present.

He needs to find a more elegant way to wake it up.

#### Al42

##### Member
micael said:
Ok perhaps is a little bit confusing...
choose a typical 5/8-wavelength monopole antenna . According to the manufacturer(Chipcon,Application Note AN003), the gain of the antenna is 8.2dB.
8.2dbWhat? dbWetNoodle? Nah - a 5/8 monopole wouldn't have that much gain over even a dry noodle.

A 5/8 monopole has 3dbd gain at best. (Gain is referenced to something - db is referenceless - dbd is referenced to a dipole, dbi is referenced to a [theoretical] isotropic antenna.)

You DON'T design the antenna to give the received signal wanted, you design a reasonable transmitter and a reasonable receiver to work with reasonable antennas and reasonable path losses. Then, if you have enough signal you go with the design. Some paths just aren't reasonable. (450 MHz in one hop from NY to LA, for instance. It could be done - at the expense of the lives of anyone living near the transmit site, and most of the real estate.)

Cowthief ???

#### ki5sr

##### Member
Okay...let me make the best sense of this I can...and see if we can get something useful.

micael said:
Ok perhaps is a little bit confusing...
Using Berkeley Mica2 motes as an example, the transmission power is 10dBm (10mW).

Okay...never heard of these until just now, but I found the spec sheet on the MICA2 MPR410CB data transceiver. 433 MHz transceiver for short-range data links, right?
RF power is programmable from -20 to +10dBw.

With you so far.

micael said:
The antenna gain varies depending on the length, size and shape of the antenna.choose a typical 5/8-wavelength monopole antenna . According to the manufacturer(Chipcon,Application Note AN003), the gain of the antenna is 8.2dB.

Chipcon Application Note AN003 that I've tracked down is a paper on antenna design theory for Short Range Devices (SRDs).

It does discuss some theoretical directivities of some antenna designs, including a 5/8-wavelength monopole, which it says has a theoretical directivity of 8.2dBi, or 6 dBd (that last d being a dipole in free space).

There is a good deal of discussion in that paper on efficiency of a tuned radiator (an antenna), where efficiency is expressed as a ratio of radiation resistance to dissipation resistance. [Radiation resistance being the load apparent to a generator contributed by the energy used to create the electromagnetic field emissions, while dissipation resistance is the apparent resistance to a generator contributed by heating/resistance losses in the antenna that is not being used to radiate a field.]

Okay...that's the part I'm roughly following along with.

micael said:
Considering environmental issues, choose 8dB as the transmitting antenna’s gain and 6dB as the receiving antenna’s gain. They correspond to numerical values of 6.31 and 3.98 ,for Gs and Gr respectively. The wavelength for 433MHz is 0.69m.With the distance, D, assigned a value of 3 meters, the received power is found to be 0.084mW.

Here's where you lose me, I'm afraid. I'm not sure what equation you're referencing here, or how the resulting calculation is derived.

There seem to be some references to another paper or source here. I can't locate these variables in the technical note, nor an equation that would derive to these terms.

An equation relating terms like these would have to reference a specific system under specific conditions, such as orientation of the antennas, the environmental issues you mention, and antennas of known efficiency (which we haven't addressed here).

It seems as though you're using an equation here and treating directivity gain as a circuit gain or loss, and then using some derivation of typical field propagation as a measure of induced current and voltage into a receiving element of some specified efficiency.

(I'm assuming that because the units you derive aren't correct for expressing field strength, which would be expressed as power per area or Gauss (volts/unit lenth)--somebody check me on this, okay?)

micael said:
The voltage at the receiving antenna Vin=sqrt(0.084mW*Rs)
Rs is the internal resistance of the receiving antenna which if assumed to be 9Kohm, then
Vin=0.87V
So what i am trying to figure out here is how i can actually get an antenna with > 9Kohm intenal resistance working.

It sounds like you've made the conclusion that the expression of radiation resistances or dissipation resistances yeild an inherent "internal resistance" to an antenna, and that is somehow a property you could manipulate or design by adding simple resistance to the antenna.

Adding resistance in the way you describe will not increase the received power in any circumstance.

I'm afraid that's just not how it works. If you were to add internal resistance to the antenna in some fashion, it would add to the dissipation resistance of the system, which would lower the efficiency of the system considerably. There's not really a way to increase radiation resistance--which is really just a way to describe the energy losses from an antenna system in creating the field it's radiating.

Any added resistance, whether it's at the bottom of the antenna or distributed throughout the antenna, will reduce the voltage and power presented to the receiver.

And we'll not even go into how the characteristic impedance of a reactive circuit is changed when we add resistance.

Does that help?

N

#### N_Jay

##### Guest
Al42 said:
8.2dbWhat? dbWetNoodle? Nah - a 5/8 monopole wouldn't have that much gain over even a dry noodle.

A 5/8 monopole has 3dbd gain at best. (Gain is referenced to something - db is referenceless - dbd is referenced to a dipole, dbi is referenced to a [theoretical] isotropic antenna.)

Have you bothered to track down the technical information he was refering to?

#### gcgrotz

##### Member
Note to ki5sr:

Well said sir!

Note to N_Jay:

I get it now, he wants zero internal power consumption in the receiver during standby and the actual received signal would provide power to turn it on. I guess this could work in theory. After all, they're planning to beam microwaves from space to generate power on earth. That seems scary to me.

Anyway - a great discussion!

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