lower limits on antenna size for practical reciving of frequency - Page 1
Forum section: electronics / rf-microwave

author: CopperCone
author_rank: Super Contributor
timestamp: August 15, 2017, 09:46:03 pm
content: There are alot of different antennas out there, but are there practical limits to antenna size for signal reception?Fpr instance, if one square centimeter is available, what is the lowest frequency that can be practically received from a far field transmitter?

author: retrolefty
author_rank: Super Contributor
timestamp: August 15, 2017, 11:00:49 pm
content: I believe this can be a very complex subject and maybe no quick simple answer. Antennas can have different 'electrical' length Vs physical length. I seem to recall that Fractal antennas can be very physically small for the wave length being handled.Fractal antennas:https://en.wikipedia.org/wiki/Fractal_antenna

author: cdev
author_rank: Super Contributor
timestamp: August 15, 2017, 11:19:56 pm
content: Obviously, the shorter the wavelength, the better the antenna you can fit in a given space. But people manage to fit even LW antennas in very small spaces, as shown by tiny WWVB and DCF77clock modules that use a ferrite rod/capacitor tuned circuit (magnetic loop) and longwave time signals to maintain their synchronization.Quote from: CopperCone onTodayat 15:46:03

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 16, 2017, 05:43:01 am
content: If the Q factor is unlimited, then the frequency is DC.The question is therefore underconstrainedTim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 16, 2017, 05:47:38 am
content: Quote from: retrolefty on August 15, 2017, 11:00:49 pmFractals, or more specifically, the property of geometric self-similarity, is about wide bandwidth. The bandwidth can be much wider than the lower cutoff frequency (i.e., more than an octave) without taking up much if any additional space, compared to, say, a dipole resonant at the same cutoff frequency.Tim

author: CopperCone
author_rank: Super Contributor
timestamp: August 16, 2017, 07:48:20 am
content: I mean in terms of practical implementation. Kind of like how keithleys low level measurements handbook has a 'near theoretical limits' graph.Like, I guess I am more interested in the capabilities of physically small receivers. Spy devices. Low power consumption. I guess its a factor of how small you can make a low noise amplifier/filter.Not stuff that requires 100lbs of RF equipment and liquid nitrogen to resolve.

author: dmills
author_rank: Super Contributor
timestamp: August 16, 2017, 10:23:51 am
content: What bandwidth do you need? What field strength is the transmitter developing at your required listening position, and hence what antenna efficiency and bandwidth are required?These are the drivers for this sort of discussion.

author: CopperCone
author_rank: Super Contributor
timestamp: August 16, 2017, 06:21:06 pm
content: Quote from: dmills on August 16, 2017, 10:23:51 amAh I don't really want to design anything. I just wanted some kind of reference graph (perhaps to put in a publication/handbook about bug detecting/covert espionage devices). Touchy subject but being able to detect these things, probably made by someone alot richer then you, is invaluable towards having peace of mind (and if countermeasures are widely available/known such a device is less likely to be used due to paranoia of getting caught).Like, if you are looking at a small object, and you only have a spectrum analyzer and antenna handy, what frequency ranges should you look at given the circumstances. (now its fairly common/cheap and useful to own this equipment, compared to buying specialty bug detection gear.. spectrum analyzers are well proliferated). If something is chirping/using occasional transmission, its very useful to know where to look.Also of course, the peaceful application would be for a design engineer, or systems engineer, industrial designer, product designer etc, that has a vision of what his design will look like, and wants to write a specification for it... giving them a bit of rule of thumb knowledge about RF, so they know that yea., in this doodad im imagining its only practical to use frequencies between X and Y. ( i noticed alot of people really look at form factor/size/industrial design when it comes to selling a product.... the last thing you want is to get through a bunch of design work to find out that some specification is not capable of being met well with your existing thing that looks really appealing, customers liked, etc). A bit of 'rf engineering for the common man' kinda

author: G0HZU
author_rank: Super Contributor
timestamp: August 16, 2017, 06:48:11 pm
content: QuoteUmm... Not sure a spectrum analyser will always help. A few years ago I designed an active sniffer/detector that works down to just a few kHz with a monopole antenna that is just 5cm long. It's very good at picking up data leaking from electronic devices, eg equipment displays or keypad presses or low speed data leaking from a cable across a room. It's also brilliant at finding noise from battery chargers or noisy lights or other EMC problems inside a workroom because it can be used as a 'wand' to find interference very quickly.It uses an active interface to the monopole with an 'inverted' capacitor and works up to about 1MHz but it is at its best below 100kHz. You won't be able to detect this device armed only with a spectrum analyserSee below for a screenshot when I last used it. I used it to look at BBC Radio 4 on 198kHz and fed the raw output to a spectrum analyser. It does quite well even up at 198kHz as you can see with a very good signal to noise performance despite the tiny 5cm whip antenna. But it works best for local reception of electronic signals down to maybe 1kHz. Stuff like this is trivial to build but I'm not going to give instructions how to make one on here for obvious reasons.

author: CopperCone
author_rank: Super Contributor
timestamp: August 16, 2017, 07:34:11 pm
content: Your choice. I personally see it as making the world a better place because people have better tools to make more ideal things but suit yourself. It also helps physicists and whatnot, prevents people from having to reinvent the wheel, etc.I heard stories of gyrator circuits being used for LF antennas for this purpose. to prevent the need for massive inductorsimo, what you are doing is analogous to refusing to post information about EMI probes

author: 4CX35000
author_rank: Regular Contributor
timestamp: August 16, 2017, 08:20:45 pm
content: Quote from: CopperCone on August 15, 2017, 09:46:03 pmReception tends not be too much of a issue at low frequencies. Most receivers are probably using a ferrite rod antenna with a variable capacitor for tuning and a simple RF amplifier before the first stage IF mixer. Most portable receivers of the 150 KHz to 280 KHz Long Wave broadcast band in Europe have used this model for years.Lower frequencies than the European Long Wave band tend be carrier based transmissions were the carrier is turned on/off (CW), therefore bandwidth is less critical and therefore tuning the antenna is easier using fixed components. Can even go the route of using a random length of wire with a high gain amplifier stage and a simple filter.Many of the cheap radio watches which receive the 60 KHz MSF clock transmissions use a simple FET amplifier with the antenna being some type of coil inside.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 16, 2017, 11:43:43 pm
content: Quote from: T3sl4co1l on August 16, 2017, 05:47:38 amFractal antennas are pretty much "snake oil".There are other types of wideband antennas which do the same job without all the "woo-woo".Returning to the OP's question, small antennas are always tradeoffs in some form or another.Either loss of efficiency, or bandwidth, & very often power handling ability.Receive antennas can be very small if the received signal strength is high, as the niceties of design can be avoided-----just stick a bit of wire or rod, or whatever up, & live with the inherent losses.