Best Antenna and Rectifier Diode

[45tt]

What must I do to maximize the voltage received from an antenna?
Also, what kind of rectifier diode has the lowest voltage cut?

 

[Mordred]

welcome to the forum, unfortunately your going to need to supply more information of your application. Are we talking an RF antenna, a cell phone antenna, ethernet or some other application. In RF there are considerations that ethernet does not need to worry about as one example

 

[45tt]

RF antenna, I apologize for my lack of information.

 

[Mordred]

ok not much more information, but enough to get the discussion going.

In RF you want an amp that is designed for RF, I recommend looking here

http://www.analog.com/en/rfif-components/rfif-amplifiers/products/index.html

for the antenna, you need low loss 50 ohm coax cable, for short runs use RJ-58. keep the number of connectors at a minimal, do not have excess wiring to the antenna. Each connector has approximately 1/2 bd loss. This is a rule of thumb in RSSI calculations, (connectors are typically less loss).

Another consideration many forget is that you will be transmitting a frequency, you will need to make sure that, that frequency does not interfere with local RF systems. In Canada the frequency assignments are regulated through Industry Canada. I do not know the regulatory RF body in the states.

The antenna itself is designed in a manner to increase gain, Each antenna is designed to operate at a specific frequency range, factors suchas the number of windings, a compared to the inner metal material, or dipole and length oe antenna itself on the whip style antenna's determine its optimal operating frequency. I would recommend looking pre designed antenna's for the frequency your working at.

 

[Baluncore]

I presume you want to maximise the voltage so you can rectify it with the diode to extract the power. The answer is that you need a high impedance antenna, probably with a 300 or 600 ohm termination such as a tuned loop, rhombic, Sterba or Bruce array. You need the biggest possible antenna to intersect the maximum power.

You will get the lowest voltage drop with germanium diodes, but these days a synchronous detector made from MOSFETs will actually do better. Start with silicon diodes, they are cheap and more robust.

It is important that you do not tune to aircraft or shipping navigation beacons as your antenna (when tuned properly) will modify the radiation pattern and so distort the radiated field. You should also be aware that you can get RF burns from such circuits if a powerful transmitter is nearby.

 

[45tt]

Thank you sooo much Baluncore and Morded. I apologize for not being as specific as possible but I have little technical knowledge.

Where would I procure a high impedance antenna?

 

[sophiecentaur]

So far, you do not seem to have mentioned the operating frequency. This is the most important thing to know before selecting or designing an antenna. What is the purpose?
If you are just asking about the best form of diode to use, could it be that you are planning a simple 'crystal set'?
More details of what you actually want to do will get you some much more useful answers.

 

[45tt]

Where to buy high impedance antennas would be sufficient.

 

[the_emi_guy]

The best antennas (delivering most voltage output for a given RF signal strength) are not available from a single company, but are procured via competitive bits with multiple contractors typically involved and cost millions of dollars. http://en.wikipedia.org/wiki/Very_Large_Array

How deep are your pockets? Why not at least share with us what frequency you are considering?

If you are actually planning to spend money and try to build something I suggest you share more about your goals. There are lots of folks here who can help you steer around the multitude of landmines encountered when attempting to build a radio.

Cheers

 

[sophiecentaur]

Where to buy high impedance antennas would be sufficient.

Are you aware of what you are asking here? The impedance of any antenna will depend upon the frequency it is operating on. An 'antenna' could be 2cm or 2km in extent , and anything in between, depending on various factors.
If you want an answer then you will need to ask the correct question.

 

[45tt]

I don't need an a very large array or those big antennas. I just need one to build a rectenna.

 

[sophiecentaur]

What frequency?

 

[Baluncore]

The design of your system requires us knowing what you aim to do. You appear to be reticent to communicate your aims. If you are trying to recover energy transmitted by yourself then you have control of the transmitter frequency and the power radiated. In that case you should maybe first Google “Rectenna” to familiarise yourself with the field and the availability of commercial “off-the-shelf” components.

If you are trying to harvest energy from a local transmitter then you will need to know the frequency of that transmitter. We really cannot help you unless you tell us an approximate frequency. You should also research what laws in your country apply to energy theft. It is a trivial exercise for enforcement officers to locate a significant RF power sink in their antenna radiation pattern. That could prove more expensive to you than your investment in equipment.

Please state your aims.

 

[jim hardy]

45tt - you need this:

http://www.arrl.org/shop/ARRL-Antenna-Book-22nd-Edition

you might find a used copy someplace cheaper. But it's well worth the full price.

 

[sophiecentaur]

45tt - you need this:

http://www.arrl.org/shop/ARRL-Antenna-Book-22nd-Edition

you might find a used copy someplace cheaper. But it's well worth the full price.

No doubt there is a copy available in your favourite local surplus store, eh Jim? (Buried amongst the cables and whatnot.)

 

[jim hardy]

No doubt there is a copy available in your favourite local surplus store, eh Jim? (Buried amongst the cables and whatnot.)

That's where you find them. I have about four of varying antiquity.

btw where do you get those smileys? I pasted 'biggrin' from your post...

 

[dlgoff]

That's where you find them. I have about four of varying antiquity.

btw where do you get those smileys? I pasted 'biggrin' from your post...

Reply with "Go Advanced".

 

[45tt]

I can't divulge the exact details of what I am building, but the frequency is 154.96500.

 

[sophiecentaur]

I can't divulge the exact details of what I am building, but the frequency is 154.96500.

That didn't hurt, did it? You see, we could have been discussing anything from long wave to microwaves. So you are after a 2 metre antenna. Why not buy one off the shelf? You seem to know very little about RF so you have the best chance of a good solution if you get yourself a good Marine VHF antenna. (Quarter wave monopole)
If you want to get clever and build your own, then look on Ham Radio sites. They are chock full of antenna designs that people have actually made at home.

 

[Mordred]

I would advise you check your local frequency regulations. In some areas that frequency has regulated uses.
Unfortunately I live in Canada here its controlled by Industry Canada. For the states I do not know.

 

[Baluncore]

155 MHz is in the VHF, high band, which is allocated world wide to two-way FM with maybe 25kHz bandwidth and about 25W output power. These are not usually continuous transmissions. Unless otherwise specified I will assume the transmitter uses a short a vertical whip antenna. The antenna design can now be refined and the cost reduced by asking a few more questions;
1a. Are you simply trying to detect local activity or do you need to extract power ?
1b. If power then how many watts do you actually need ?
2. Do you know the approximate transmitter power ?
3. Approximate how far from the transmitter will your antenna be ?

 

[Mordred]

I should note that the States are now a mandatory 12.5 khrz bandwidth. I work for a Motorola and Kenwood dealership. We had to purchase patches to all our software. To allow Canadas 25 khrz.
Trust me its a nuisance. Requires a new license key.

The whip for that frequency is made by Sinclair. You can also get mag mounts for it.

 

[Baluncore]

A power receive antenna will almost certainly require significantly more gain than a simple whip.

The energy budget involves the gain and power of the TX antenna, the gain of the RX antenna and the distance between the two. The cheapest solution is to show numerically how it can be done before buying bits that may not be needed.

This is an engineering design problem. There are many systems that will not work, there may actually be some that will work. In order to resolve it, we need specific information for this particular situation. Hence my questions.

 

[sophiecentaur]

@45TT
You ask about a "diode". I suspect you have seen a very simplified 'block diagram' of a system that you think will do what you want. The diode itself us a very small part of any working system, active or passive. I think you will need a lot more specialist knowledge if you are to get anywhere with it. It's not LEGGO, you know.

 

[Baluncore]

Anyone enthusiastic enough to try deserves every possible support. An early success is the best reward and often leads to ongoing interest.

It would be like LEGO if it was designed by someone with over 25 years experience in RF and antenna engineering. Unfortunately that requires specific information which I am finding very hard to extract. But I'm patient.

 

[jim hardy]

To get a handle on the very basics , search on "Crystal Radio". It's nothing more than an antenna and diode.

http://www.crystalradio.net/

Original question, how maximize power, has been answered. Make it big and tune it.

 

[45tt]

1a. Extract power
1b. 2 watts
2a. No
3a. Less than 1 mile

 

[the_emi_guy]

So let's put some numbers to this problem.

I'll assume a generous 10dB gain for both transmitting and receiving antenna.

Effective area of 155MHz 10dBi antenna is 1.5m^2
Thus power density needed at receiving antenna to extract 2W is 1.3W/m^2.

Free space path loss at 155MHz at 1 mile is: 9.16e-9 (-80dB). (As a starting point, of course this is not free space).

Power needed at 10dBi transmitting antenna: 2.2MW (MegaWatts).

You will need either a million dollar transmitter, or a pair or million dollar colossal dish antennas, or both. And god help the guy who walks in between them.

 

[Baluncore]

45tt. I do not like your chances.

We do not know the output power so we will start with some assumptions. The transmitter operates continuously and is the base station for a fleet of mobile vehicle radios. It is most unlikely to radiate more than about 30 watts, as range at that frequency is decided by line-of-sight. The transmit antenna will probably be an omni-directional whip and may be optimised to transmit towards the horizon rather than overhead, but then the power will be reduced as the regulations will specify “effective isotropically radiated power” (EIRP), not the total power. You will be close to the horizon for a 2 metre transmitter that is one mile away.

Now you want to catch (2watt / 30watt) = one fifteenth = 6.67% of the effective power transmitted.
You will therefore need to cover one 15th of the transmitters horizon. At a distance of one mile that will be 1mile * TwoPi / 15 = 0.4186 mile long, and quite high. Since this is much greater than the 2 metre wavelength it will need to take the form of an elliptical wire fence reflector screen with vertical wires spaced approximately every fifth of a wavelength (15 inches). The ellipse should be drawn on the map with the transmit site at one focus and your receive site at the other. Only part of that fence need be built, namely the part behind you when you face the transmitter, but it will need to have an opening of at least half a mile wide facing the transmitter. It's position will need to be accurate to about one tenth of a wavelength (8 inches). That might deliver a maximum of 2 watt to your whip antenna.

You could probably do better by tuning to a longer wavelength, higher power (10kW) broadcast transmitter, even though it is further away. It would also operate continuously.

Remember that there is a nuclear power source that transmits in the optical spectrum. A 10 watt solar panel, (with a small lead acid storage battery to get you through the night), would be a much more economic solution.

 

[45tt]

To end this discussion, is there anywhere that I can look at multiple antennas regardless of the frequency?

 

[Baluncore]

To extract significant power you must tune your antenna and the coupling to the rectifier. The tuning will be frequency dependent so all your frequencies will need to be within about one percent of each other.

There is also the problem of direction. To capture significant energy you need to aim your antenna, or it's reflector. Unfortunately, multiple transmit sites will probably not be in a line and in the same frequency band.

 

[45tt]

Thank you all for your help. This discussion can be closed.

 

[pcbguy]

45tt, designing an antenna is not something that everyone can do. There are so many applications, types and considerations of antenna design that only professionals can design an antenna that can actually transmit some power. There are so many different types of antenna for so many different types of frequency with so many different power output specifications. For anyone to answer this question, it is important that you mention how much gain is required as well.