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What range will we get with rx and tx but no antenna? (433MHz)

Discussion in 'Radio and Communications' started by Flyback, Jul 17, 2015.

  1. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    When I operate mobile on 40m (7.1MHz), I cannot have a 1/4λ (10m long) antenna on my car; so I use an inductively-loaded foreshortened antenna that is less than 3m long, driven against the car body (which is much smaller than it needs to be to be an effective ground plane at this frequency). The shortened antenna presents close to the required 50Ω to the transmitter, so it is happy with the "load" that it sees. However, the shortened antenna is quite inefficient, in that it dissipates most of the RF power put into it as IsquaredR losses (heat), and not much RF is radiated. In fact, it is less than 10% efficient, meaning that the far field signal strength is less than 10% of what it would be with a better antenna.

    Antennas scale in frequency, so your 433MHz chip antenna is likely to accept power from your transmitter, but the signal at a distant receiver will be as though the signal was being transmitted at a power level >10db below the actual power into the chip antenna...

    There ain't no such thing as a "small" antenna that works as well as a full-sized antenna... 433MHz data links work best when connected to a 1/2λ dipole (both transmitter and receiver). Both antennas must be either horizontal or vertical. 1/2λ at 433MHz is 0.5*3e8/4.33e8 = 34cm
     
    Last edited: Jul 18, 2015
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  2. Flyback

    Flyback Well-Known Member

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    I think the quarter wave ones are best, but the short ones can do a job
     
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  3. alec_t

    alec_t Well-Known Member Most Helpful Member

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    The physical size of a fractional wavelength antenna is inversely proportional to the dielectric constant of the medium it's working in. Ceramics such as barium titanate (IIRC) can have a high dielectric constant.
     
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  4. dave

    Dave New Member

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  5. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    No, an efficient antenna must be 1/2λ long ; not 1/4λ!
    A proper dipole is center-fed such that there is 1/4λ on either side of the feed-point. When you talk about a 1/4λ mono-pole antenna, it only works when it is driven against an orthogonal flat-plate metallic counterpoise (ground plane) which approaches 1/2λX1/2λ in size.

    In crappy 433Mhz modules, they drive a 1/4λ monopole against the ground pour on a p.c.b. that is a tiny fraction of a 1/2λ, and the 1/4λ monopole is as bad as the aforementioned chip antenna! Take my word for it; I've been doing this stuff for 50years!
     
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  6. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Which solves the feed impedance problem, but does not generate the far field as well as a full-size antenna would. A 50Ω resistor also solves the feed impedance issue, but it turns all of the transmitter power to heat without radiating any of it.
     
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  7. Willen

    Willen Well-Known Member

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    Interesting! Due to lack of knowledge about matching impedance and balancing 1/2λ dipole, I am using 1/4λ monopole in basic devices, however always wish to make a dipole and Yagi-Uda for interesting experiment. Does the 1/4λ monopole also require impedance matching (theoretically)?

    I think yes, because it has almost 20 to 50 ohms impedance I think. Maybe we do not need to Balance the signal for the monopole.

    If we do not need to balance the signal for monopole then the ground plane for the monopole is directly connected the 0V or negative supply (Gnd) of battery.
     
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  8. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Last edited: Jul 21, 2015
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  9. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Unfortunately, the oV connection to the power supply is rarely a 1/2λ x 1/2λ ground plane, so using the DC ground as the counterpoise for a monopole antenna works very poorly...

    OTOH, using two co-linear 1/4λ monopoles back-to-back (it is called a 1/2λ dipole) works just fine...

    The classic monopole antenna is a 1/4λ to 5/8λ vertical tower at your local medium wave AM broadcast station. It stands upright, orthogonal to the dirt under it. Buried in the dirt (sometimes laying on the surface) are somewhere between a few and about a hundred copper 1/4λ wires radiating outward from the base of the tower like the spokes of a wheel. All of the "radials" are connected together at the base of the tower, but isolated from the tower itself, which stands on a huge porcelain insulator. The center-conductor of the coaxial cable that feeds the tower is connected to the tower, and the coax shield is connected to the radials.

    Literally, Amps of RF current flow into the tower along the center conductor. An equal, but opposite phase current flows into the radial ground-plane. If the ground-plane is not present, as advocated by Willen, where is the image current going to flow?
     
    Last edited: Jul 21, 2015
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  10. Willen

    Willen Well-Known Member

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    Hi,
    Did you mean the battery's 0V Gnd supply connected to down rod of dipole or ground plane of monopole works poorly?

    Then only one of the solution is to balance the signal using BalUn transformer, isn't it?

    Also impedance matching is there for complicating the experiment. :)
     
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  11. granddad

    granddad Active Member

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  12. Flyback

    Flyback Well-Known Member

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    looks good, but we cant do all the comms protocol stuff that goes with it, so we have to have a dumb ASK transmitter, then do the bit bashing ourselves.......the RF module is just being used like a digital isolator for us.
     
  13. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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  14. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    No, I mean leaving off half of the dipole (using only a 1/4λ monopole active element) and expecting the (0V) ground-foil on a 3cmX3cm PCB to act as the missing counterpoise is a crappy antenna. Even worse is where the "antenna" is a coiled pcb trace which is a tiny fraction of a λ.
     
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  15. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Do you have a Tx and Rx to hand? A simple experiment would save a lot of speculation :).
     
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  16. analog1

    analog1 Member

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    RF dimmers are available for 12v leds cost $5 st 25% 50% 100% or flush ing or dimming flashing all kinds of way reomtly of course
     
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  17. JimB

    JimB Super Moderator Most Helpful Member

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    Flyback,
    Please tell, what was the outcome of this?
    What solution have you or your organisation used to communicate from the cab of the lorry to the beacon(s) ?

    Enquiring minds would love to know.

    JimB
     
  18. Flyback

    Flyback Well-Known Member

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    one of the replies here made us realise that even complex coding in a ASK 433MHz comm system would suffer interference from others communicating nearby at that frequency.
    We then looked at using the power supply to signal......ie having a boost converter downstream of the battery, and boosting batt to either 24v or 48v depending on console input in lorry cab........24v means do flash pattern x , 48v means do flash pattern y...etc etc....but even that wasn't so great because it means taking the power wires into the cab from the battery, (to the console) and then all the way to the beacons from there............so its all up in the air at the mo.

    At the end of the day, we cant come up with anything better than what already exists...ie , having extra signal wires going to the beacons from the cab.
     
  19. JimB

    JimB Super Moderator Most Helpful Member

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    Back to basics!
    Sometimes the traditional old tried and tested ways are best.

    JimB
     
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  20. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    I am surprised IR remote control wasn't tried.
     
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  21. RadioRon

    RadioRon Well-Known Member

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    Small antennas can work in some situations, but their performance comes at a price. I can explain by giving a bit of background first. The key to getting any antenna to radiate well is to maximize the RF current flow on the antenna. If you can devise a way to force a large amount of RF current to flow on a very short piece of wire (and by that I mean short relative to the wavelength of the RF frequency you are using) even that short piece of wire will perform as good as a full size half wave dipole. However, its not that easy to get a lot of current on that wire. If the wire is open at one end, the current at that point must be zero, and a tiny distance away from that point it must be nearly zero, so it is not possible with a simple very short monopole. One common trick is to add a lot of metal to the "top" of the wire, with the added metal at right angles to the wire. We call this a "top hat" and it works because it gives the RF current someplace to go off the end of the wire. In this case the current in the wire is much larger than zero and it radiates well, even though the wire is short. This is just one example of ways to get a short conductor to radiate well.

    Now, I must mention the price that I alluded to above. One of the things that you have to give up is bandwidth. This is because a short radiator is never a nice handy 50 ohms, and the combination of the matching circuit and the very low radiation resistance of a short radiator results in poor bandwidth. If you are working at only one channel at UHF, well this is not a big problem. Bit it implies that it is very hard to make a short broadband antenna. The other ways of shortening antennas pretty much always force you to give up bandwidth. The other thing that you often have to give up is efficiency. As antennas get shorter, their radiation resistance decreases, which in turn makes all the other ohmic losses in your antenna and matching network more dominant, which in turn reduces efficiency. Less power gets radiated because more gets burned up as heat. In the extreme case, a very short loop, for example, may have radiation resistance of less than a tenth of one ohm, so if your matching network or conductor losses are 1 ohm, your efficiency is going to be poor.

    My own experience with small chip antennas at UHF makes me suggest two facts of life. One is that their bandwidths and efficiencies may be lower than you expect. The second is that they are usually designed to work with a large counterpoise (the metal that carries the "ground" current, what you might call a ground plane even if it is not planar). So, the true radiator is the chip plus its groundplane and in this case it really isn't a small antenna anymore, only one side of the dipole is small. Without the ground plane, these chip antennas don't work as well. However, even despite these facts of life, such a chip might work better than any old scrap of wire, so its still worth trying.

    EDIT: I must apologize for adding these comments so late in this conversation that they seem out of place. I made the mistake of reading only the first page of messages before responding and then later found most of my comments already covered by others on the second page. Oh well, I'll just leave it at that.
     
    Last edited: Jul 30, 2015
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