Antenna Power Regulation Using H-Bridge

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Hello

I am responsible for a project that involve in antennas transmission and receiving. I have two loop-antennas and I want to set them up so that they can communicate to each other, as show in the figure. I will call one as a master antenna and one is a slave antenna. The master is the one I will keep at my computer and the slave is the one I will randomly place around the room. I want to master to transmit a status-request signal to the slave and I want the slave to response with a pulse. I am using Arduino board as a micro-controller to send and receive these status-request and response, respectively. In term of the microcontroller side I am very comfortable in recieveing and transmitting status request. What I need help with is how to I initiate that request and how to initiate that response to make the antennas transmit/receive.



The frequency I am working at is LF of which around 500 Hz to 5 Kz and the expected power is 1 KW.

My idea is as follow but I am unsure whether its valid or not.

I will use a H-bridge to deliver the power to the antennas and also I am using the microcontroller to control that H-bridge.

Does that make sense?
Any input is greatly appreciated.

Thanks
L.M
 

Hi,

The military LF antennas have a size of about 100m. I cant imagine that it is possible to transmit 1kW with that low freqency..

But i'm not experienced that stuff.

Klaus
 

KlausST said:
Hi,

The military LF antennas have a size of about 100m. I cant imagine that it is possible to transmit 1kW with that low freqency..

Klaus
Click to expand...

The power is an estimate figure. I am expecting that but that does not mean I can achieve it. The size of the antennas we have here is humongous. I understand what your concerns is, you would need a bigger antennas for bigger power.

Nevertheless, with all that said, do you think that I can just use an H-bridge to deliver power to the antennas?
 

Well possibly,
but how are you going to tune the things, electrically small loops tend to have very narrow bandwidth and enormous circulating currents, so whatever you do needs to cope with lots of current and have very low losses.

You might find some of the ameteur work on the 73KHz band to be of interest.

Actually radiation efficiency tends to rise as the aerials get electrically bigger so you need less input power at higher frequency.

To give you a flavour, one of the US hams is working on 73KHz with a pair of 30m towers supporting a marconi aerial (T shaped centre fed wire fed against earth) brought to resonance with a dustbin sized variometer, 1,500W transmitter output gets him about 1W ERP!

TBH a mag loop down there is a scary (and massively expensive) concept lots of really butch copper, huge amounts of really scary high voltage capacitor and still almost all of your power goes up as heat.

You may wish to investigate 'Class E' power amplifiers for operation down there, 1 transistor rather then 4.

Now the sane way to do this.....
You are only looking to cover a room, correct?
If so, go for an 'induction loop' (A single turn of wire around the periphery of the room fed with a current mode drive of a few amps), pickup is a small coil. This is exactly the technology that used to be commonplace in cinemas for hearing aids with the switch for 'T' mode.
That gets you base to device in the room trivially, going the other way is harder as in that direction the small coil is transmitting and the noise contributions in the two cases are not symetrical.

Actually if I was doing it I would go for just putting in LED lighting and doing biphase modulation on the LED drivers (Simple, effective, fast) and forget about running VLF nearfield comms......

73 M0HCN.
 

Hi,

The half bridge may work..

Maybe you can use class D audio amplifiers. Also with half bridges inside.
Half bridges are digital power stages, with transmitting data you may need sinusoid output.
You will need a lowpass fillder.

Klaus
 


This is a great input, I do appreciated it.
However, I am not just doing this in a room, I am testing in a room however, but the final product is to works in a 2 km range.
 

One group that may have relevant knowhow are the cave radio guys, https://bcra.org.uk/creg/ they usually seem to go for 87KHz using ground current electrodes, but I know tuned loops have been employed.

What data fate are you hoping to manage? VLF tends to be measured in seconds per symbol rather then baud....

73 Dan.
 

Wavelength at 5 kHz is 60 km, so even transmission over a 2 km range could be analyzed as purely magnetical problem which is easy to calculate. Respectively we better speak of magnetical loops or couplers than "antennas".

For a loop with reasonable diameter, e.g. a few meters, we can expect that only an infinitesimally small part of the power feed to the loop will be actually radiated to the far field. Most of the power will be dissipated either in the loop itself or as eddy current losses in the vicinity. Here comes a practical problem that some near metallic objects may absorb a considerable part of the power and heat up.

To prevent from dangerous situations and possible personal injuries, EMC and industrial safety regulations limit the permitted field strength of low frequency magnetical fields. I expect that the intended 1 kW tranmitter will violate this regulations and isn't allowed to be operated in a regular residential or industrial enviroment.
 

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