Author Topic: RF Frequency : How low can you go  (Read 2434 times)

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Offline airman00Topic starter

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RF Frequency : How low can you go
« on: March 08, 2009, 08:24:30 PM »
There is a rule with RF , the lower the frequency the less interference you have from obstacles.

My question is as follows : how low of a frequency can I go to, for minimum interference from obstacles?

EDIT: I also mean a frequency at which it is not extremely expensive to transmit data , I don't need high data rates ( even 200 bps is fine)

Anyone here used these super low frequencies?

And is there a significant difference ( relating to obstacle interference) between 100 khz and 200 khz?

Thanks in advance
« Last Edit: March 08, 2009, 08:34:23 PM by airman00 »
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Offline paulstreats

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Re: RF Frequency : How low can you go
« Reply #1 on: March 08, 2009, 08:49:03 PM »
theres a sample chapter of this book that says that between 5khz and 300khz is classed in the VLF band

http://books.google.co.uk/books?id=ivdWei8jb-cC&pg=PA317&lpg=PA317&dq=100+khz+radio+power&source=bl&ots=Fwt1vOzyOR&sig=6XsLKkVfnFwApVgju7xJd7LKVD0&hl=en&ei=3oK0SczUJeKJjAecvYjfBQ&sa=X&oi=book_result&resnum=2&ct=result

The trouble with low frequencies is that you often need a big aerial and a lot of power(for distance)

Offline Asellith

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Re: RF Frequency : How low can you go
« Reply #2 on: March 08, 2009, 09:41:16 PM »
Lower frequencies = BIG antennas

To get effective power transmission it is best to hav at least a 1/4 wave length antenna. It's late and I'm tired or I would look up the calculation for that...

Also on those freqs without a Ham radio license you have to watch out for the FCC. If you interfere with a commercial application that can have bad consequences. Keep in mind these low frequencies can travel really far. Even a single watt of transmission power can go really far. The problem comes when you try to get your data rate up and you need a good signal to noise ratio. You could transmit micro watts of power but the noise would eat the signal. So the higher you go the larger the distance and the better chance someone will get interference.
Jonathan Bowen
CorSec Engineering
www.corseceng.com

Offline paulstreats

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Re: RF Frequency : How low can you go
« Reply #3 on: March 09, 2009, 06:47:30 AM »
There are a few commercial systems that run on 0.100w power that transmit low frequency around your home, for broadcasting mp3's etc... but they arent designed to reach further than 100 meters or so

Offline 4by4

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Re: RF Frequency : How low can you go
« Reply #4 on: March 09, 2009, 09:58:37 AM »
There should not be a problem with obstacles for any frequency up to UHF. What kind of obstacles are you thinking of?

I'm not sure what you mean by frequencies that are less expensive. Are you planning to build a radio? It is easier to build and test circuitry operating at 100 kHz than at 10 MHz. But it's even easier to buy something, and there are a lot of off-the-shelf things available that are pretty cheap.
 

Offline Admin

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Re: RF Frequency : How low can you go
« Reply #5 on: March 25, 2009, 12:35:34 AM »
lower frequency = lower baud rate

Offline lemontree

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Re: RF Frequency : How low can you go
« Reply #6 on: March 30, 2009, 11:04:34 PM »
As an alternative you can actually use magnetic induction to transmit data at  low frequency.  That will work up to a few meters. Magnetic induction transmitters were once a popular hobby project.  I did often run a loop of wire around a room linked to a high power audio amplifier when I was a kid.  Strangely enough there is zero information on the internet about them now.

If you really want to transmit EM waves at low frequency (at low power 1 W< and low efficiency 1%<)  you can use a ferrite rod antenna.
« Last Edit: March 30, 2009, 11:17:15 PM by lemontree »

Offline kd5kfl

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Re: RF Frequency : How low can you go
« Reply #7 on: March 31, 2009, 03:47:45 PM »
Quote
To get effective power transmission it is best to have at least a 1/4 wave length antenna. It's late and I'm tired or I would look up the calculation for that...

300/frequency in megahertz = wavelength in meters

1/4 wavelength is for a vertical dipole over a good ground. ground acts as the other half of the antenna.

most effective antenna is 1/2 wavelength long.

Offline lemontree

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Re: RF Frequency : How low can you go
« Reply #8 on: March 31, 2009, 07:48:37 PM »
Magnetic loop antennas can be small than 1/4 wavelength but require special construction techniques, in particular a very low ESR tuning capacitor. 
http://www.standpipe.com/w2bri/ 
http://www.magneticloopantenna.com/

Offline Soeren

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Re: RF Frequency : How low can you go
« Reply #9 on: April 01, 2009, 12:14:39 PM »
Hi,

As an alternative you can actually use magnetic induction to transmit data at  low frequency.  That will work up to a few meters.
They can easily be made to go 50m, 100m or more if needs be, but you might get in trouble if you interfere with an inductive loop used by a hard of hearing person.


Magnetic induction transmitters were once a popular hobby project.  I did often run a loop of wire around a room linked to a high power audio amplifier when I was a kid.  Strangely enough there is zero information on the internet about them now.
There is quite a bit of info out there, although not as much in the hobby department.
They're not very well suited to data transmission either, since just about "anything" will be interference.

If you really want to transmit EM waves at low frequency (at low power 1 W< and low efficiency 1%<)  you can use a ferrite rod antenna.
They tend to be very directive (too directive for something moving).
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline lemontree

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Re: RF Frequency : How low can you go
« Reply #10 on: April 04, 2009, 10:46:18 PM »
Talking about magnetic fields and such has anyone had any luck using the giant magnetoresistance sensor from an old hard-drive?
 

Offline bionic-man

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Re: RF Frequency : How low can you go
« Reply #11 on: April 04, 2009, 10:51:37 PM »
is that a sensor that detects how much magnetic resistance there is?

Offline lemontree

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Re: RF Frequency : How low can you go
« Reply #12 on: April 04, 2009, 11:08:22 PM »
Well here:
http://en.wikipedia.org/wiki/Giant_magnetoresistance
It just changes resistance when subjected to a magnetic field. 

 


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