Assuming I have an antenna for receiving GPS signals, can I use it also for transmit — to create a repeater in a tunnel (or some place with no reception)?
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3$\begingroup$ Not an answer to your main question, but GPS passive repeaters exist. They basically work by having an active antenna on a roof or above ground, and a passive antenna inside the RF dead zone. In the 90s, we used them to provide signals inside of fire stations for AVL to work instantly and have a faster TTFF. As stated in the answer and comments below, the position is at the passive repeater receiving antenna and not the vehicle - but it keeps the almanac updated, effectively in a hot/standby condition. $\endgroup$– PaulCommented Mar 29, 2016 at 16:11
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$\begingroup$ Hey, @Paul, what do "AVL" and "TTFF" mean? $\endgroup$– Pete NU9WCommented Mar 13, 2019 at 13:15
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1$\begingroup$ @PeteNU9W AVL = automatic vehicle location (in other words, vehicle tracking). TTFF = time to first fix (a common figure of merit for GNSS). $\endgroup$– hobbs - KC2GCommented Mar 13, 2019 at 13:33
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$\begingroup$ Hey, @hobbs-KC2G, what does "GNSS" mean? <g> Remember, you're talking to a general audience here, people who probably aren't in on your jargon. $\endgroup$– Pete NU9WCommented Mar 13, 2019 at 13:36
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2 Answers
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No. Many types of antennas may be unsuitable for transmitting.
A so-called active antenna contains an integrated low-noise amplifier (LNA) for amplifying weak signals. (This is very common for GPS antennas.) This amplifier cannot pass a signal in reverse, and could be damaged by transmit power levels appearing on its output. It is possible to have an automatic switch to bypass the LNA when transmitting, but this feature would not be designed into an antenna meant for receiving.
Some designs of antenna, such as the Beverage antenna, contain resistors or other dissipative elements. If they are used for transmitting, the resistor must be capable of dissipating a certain fraction of the power input to the antenna. This is inefficient since that fraction of power is not transmitted, and requires a resistor which will not be destroyed by the heat.
A receiving antenna may contain a ferrite element, which allows it to be much more efficient when receiving than its size would otherwise permit, but which will saturate at any significant power, becoming nonlinear and creating harmonic distortion (as well as heat).
A transmitter requires impedance matching between the final power amplifier and the antenna. An antenna's impedance varies with frequency, so it has some frequency range over which there is an acceptable match. The same antenna can be used for receiving over a much wider range; antennas used solely for receiving may be deliberately mismatched vs. the intended band to gain other advantages such as being physically small.
An antenna with the wrong impedance at the desired frequency can be matched using a matching network or antenna tuner (adjustable matching network), but this is still less efficient because there will be additional losses.
This is the least significant problem of all those I have listed: with a good antenna tuner, just about any chunk of metal insulated from other objects can be used as an antenna. The problem comes when non-metallic elements are in circuit as in the other cases.
Since you mention GPS in particular:
You cannot repeat a GPS signal usefully. Or rather, if you did, it would cause your GPS receiver to think it is at the position of the original receiving antenna, not its actual location. (This is because GPS operates by analyzing the combination of signals from multiple satellites, and this combination happens in the receiving antenna.)
It would also be likely illegal to operate such a transmitter, because you are transmitting on frequencies you do not have permission to and interfering with normal GPS operation.
answered Mar 8, 2016 at 23:09
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$\begingroup$ +1. Also loopstick antennas, because the ferrite core saturates. $\endgroup$ Commented Mar 9, 2016 at 11:55
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1$\begingroup$ @PhilFrost Worth a mention. I was loosely thinking of that under “antenna with dissipative elements”, but reading a bit on saturation suggests that's not accurate — is it more accurate to expect that the impedance will change drastically and therefore problematically in such a case? $\endgroup$– Kevin Reid AG6YO ♦Commented Mar 9, 2016 at 15:24
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$\begingroup$ I suppose the impedance would change, but I don't think that's the biggest problem. The problem is that the core becomes non-linear. At small powers when the core doesn't saturate, the magnetic domains in the ferrite align with the magnetic field portion of the transmitted signal. The aligned magnetic domains then contribute to the magnetic flux through the coil, increasing the current in the coil by some multiple proportional to the core's permeability. But.. $\endgroup$ Commented Mar 9, 2016 at 22:51
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$\begingroup$ ...on transmit they also align, and they would multiply the magnetic field generated by the transmitter, except that with even very low transmit powers, all the magnetic domains are fully aligned, and they can't become "any more aligned". So the effective permeability drops towards zero. And yeah, the core will get hot, and also this nonlinear behavior will create tons of harmonic distortion. $\endgroup$ Commented Mar 9, 2016 at 22:54
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2$\begingroup$ > it would cause your GPS receiver to think it is at the position of the original receiving antenna — I was going to say "that's not true!" because I was thinking in terms of repeating a single GPS satellite. But by repeating them all, yes! Which unfortunately makes some GPS spoofing attacks much easier (vs. knowing the P(Y) keys) even against e.g. military receivers. $\endgroup$ Commented Mar 10, 2016 at 1:10
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No issue. You can use it on both ends and the loss is almost negligible.
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$\begingroup$ This answer immediately following the previous one is unintended SE poetry. $\endgroup$– user4182Commented Mar 26, 2016 at 22:35