Do receiver antennas need matching or not?

Bill, That's one of those "sort of" kind of questions. It depends a lot on the particular receiver and what frequency range you plan to listen to, and then on the amount of space you have for an antenna. In general, receivers that listen to the lower frequency bands, HF for example, are more sensitive than those that are used for the higher bands (VHF, UHF, SHF). That means that to hear a typical signal a specifically designed antenna having the exact output impedance as the receiver's input impedance isn't required. Almost anything will work to some extent. The receiver just isn't that 'picky', since it typically has more 'hearing' ability than required. There's a 'catch' though. Reducing the amount of signal losses in that 'whatever' antenna is going to make the resulting signal getting to the receiver that much stronger. Something always nice to have, but there are practical limits. If that 'whatever' antenna meets your requirements, then it's as 'good' as anything, sort of. If it doesn't, then making that antenna less 'lossy' is also nice. That "less 'lossy'" thingy also includes making it directional, longer, shorter, higher, whatever, to increase the amount of signal getting to the receiver. So, looking at it from that point of view, the antenna ought'a be reasonably 'close' to what will typically 'work' well on the received frequency. Huge range in that 'close' quality and the definition of what 'works' means. There is no 'perfect', 'do everything', antenna. Just too many factors involved. 'Higher' and 'longer' tends to 'work' better than 'lower' and 'shorter', in general. Which, like any generalization, is never always true. That's the sort of 'long', half-assed, technical answer. The 'quick-n-dirty' answer is, no, they don't have to be 'matched' to the receiver. Which says nothing about transmitters. - 'Doc

[all puns intended]

Correct. Those are downsides. The upside is convenience and simplicity. It's sub-optimal; but it works!

The best transfer of energy is achieved when the antenna impedance matches the input circuit impedance of the receiver. This should be the case for the entire band of the desired received signal. It is not always feasible to do this and it is often not necessary. Today's receivers have very high gain and excellent selectivity. They need very small signal strengths to operate and excess signal is attenuated. So a piece of wire used as an antenna in an area where the signal strength is large will not do any worse than a perfectly matched antenna. In an area where the signal strength is weak (like Mars) matching of the antenna to the front end of the receiver is desired. There are other considerations for transmitters.

Tom

The AGC circuit in receivers has enough dynamic range to compensate for low amplitude signals from a mismatched HF antenna. The S/N ratio on HF is mostly external to the receiver.

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73, Cecil  http://www.w5dxp.com

More correctly, it's _easier_ to make an HF receiver with a good noise figure, and harder to do so as the frequency goes up.

However, atmospheric noise goes _down_ as the frequency goes up. So for weak-signal work a receiver designer has a lot of motivation to make really quiet front ends on VHF and higher equipment. Basically if the radio is cheap (i.e. if it's for consumer use) then the front end may as well be made of wood. If the radio is used for long-distance communication (i.e. microwave links, space communication, some military or amateur radio) then designers will go to great lengths to get the noise figure down.

-- snip --

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
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In areas where it matters receive antennas are matched, and are something other than random wire. This is why you can go to Radio Shack and buy TV antennas* -- they're designed** to be both directional and a good match over the broad frequency ranges of TV signals.

Get into amateur radio or military communications and you'll find many different permutations of directional, matched antennas on receive.

Note: For many LNA designs, the best signal/noise ratio occurs at an impedance that is close to, but not really, a perfect conjugate match. The signal is coupled to the amplifier best at the conjugate match impedance, but sometimes the noise is enhanced even more.

• or could -- does Rat Shack still carry antennas?

** kinda -- they're really designed to _look_ like they'd make a good antenna, but they're better than rabbit ears.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

That brings up an intersting question I never did get a good answer to...

It is my assertion that an LNA that is physically at room temperature (290K) can have a noise figure no better than 3 dB (i.e. its effective noise temperature is 290K) IF it is also conjugatly matched i.e. looks like 50 Ohms.

Yes you can make the noise figure better than 3 dB, but then you must either cool the device or MISMATCH it to the line.

In other words if it looks like 50 Ohms and it is physically at 290 K, it's effective noise temperature must also be 290K.

How could it be otherwise?

Comments plese.

Mark

OH NO! now you have done it! i hope cecil doesn't see this or you have just openend another endless energy sloshing around thread! what does happen if the antenna isn't matched to the radio? where does the mismatch energy go???

Hi Bill,

It is not always needed if the signal is strong enough. If the signal is not strong enough, then you can obtain considerable gain through tuning.

Tuning also brings other advantages by rejecting signals that could depress your radio's sensitivity.

73's Richard Clark, KB7QHC

Your neighbours ;-) Less of the energy is "taken" from the received EM field.

Joop

I once cranked out an NEC2 model of a Radio Shock TV antenna to see what it really did over the 54-890MHz range. It was fairly horrible. There were actually a few frequencies where the impedance was close to

300 ohms. There were also a few frequencies where it actually had some gain. At some frequenies, it had more gain in the reverse direction than forward. As an example of a "directional and a good match" antenna, that typical Radio Shock TV antenna doth truly suck.

I'll see if I can find the model. Unfortunately, it may have been on a hard disk that crashed a few years ago.

Not having a good match between the antenna and LNA has several effects. The mismatch will affect the system noise figure thus reducing sensitivity. Some LNA's are not unconditionally stable and will oscillate when presented with a weird source impedance. At HF frequencies, the atmospheric noise level is above the receiver input noise level, so considerable mismatching can be tolerated. About about 20MHz, this is no longer the case, and a match is required.

Antennas are also affected by their load impedance. A highly directional yagi antenna pattern can easily be ruined by mismatched coax or LNA input impedance. For the antenna to work as advertised, it has to see the rated load.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

A good deal of it is re-radiated by the antenna.

Cheers,

Phil Hobbs

This means that the atmospheric noise, in the frequency range that the receiver is tuned to, is greater than the internal receiver noise....

It also means that any signal that is to be received that exceeds the atmospheric noise, will be heard......

Unless you are using some signal processing that can detect signals below the atmospheric noise level, this is a very good rule of thumb....

Consider an airborne LORAN antenna, used on aircraft, to receive

100 khz signals. It works out that around 22 inches is the length where the atmospheric noise exceeds the general receiver noise for most receivers. Making the antenna longer will pick up more desired signal, but also more atmospheric noise, in the same ratio.....so the SNR doesn't get much better. Note that a matched antenna for 100khz will be many many hundreds of feet long.... but is never used either in boats or airplanes, since a "matched" antenna serves no advantage to sensitivity ( SNR)....

These are rules of thumb, and useful approximations, but, in effect, you don't need a great antenna unless you are trying to receive a weak signal....or have a method to increase the SNR by signal processing.

Andy in Eureka, Texas W4OAH

For a receive antenna, you don't need to worry as nearly as much about an impedance match as you do with a transmitter. You are dealing with much higher gain on the input side as you are on the output side.

That beer in your hand was also cooled in a 300K ambient. How is that possible? (Hint: the fluctuation-dissipation theorem only applies to systems in thermodynamic equilibrium. The moment you turn on the power, that assumption is violated, just as it is in your domestic refrigerator.)

An ordinary room-temperature diode has a noise temperature of 150K (Tambient/2) as you can show in about 3 lines of algebra, starting from the diode equation and the shot noise and Johnson noise formulas.

Cheers,

Phil Hobbs

Cheers,

Phil Hobbs

Well, I 'm not an expert, but it seems that with a transmitting antenna, the idea is to transfer as much power as possible to increase efficiency, and so the antenna needs to be closely matched to the output of the transmitter for best results. But the receiving antenna is a different problem, since no power from the antenna is needed to drive the receiver, and so who cares about the match? The idea with the receiving antenna is to get the most voltage and highest S/N ratio with no load. The input to the receiver should be buffered with a high impedance FET amplifier, or some such, so the receiver draws almost no power from the antenna. This leaves you free to design the antenna and input tuning circuit for the highest Q and lowest noise figure without worrying about impedance match.

Just my opinion.

-Bill

Agreed. However, the short 18" antenna is commonly used for handheld and aircraft Loran receivers. However marine Loran antennas are typically 8ft long.

That's not the only reason that Loran antennas are rather short. If the antenna were longer, the impedance would increase, causing it to pickup more percipitation static, atmospheric noise, and 60Hz harmonics. A longer antenna would also not be as narrow band and low impedance as a short (loaded) antenna. The relatively narrow bandwidth is helpful for eliminating broadcast, beacon band, and other forms of interference.

Incidentally, that's also one reason why some remote Loran systems have a pre-amp that really burns some watts. It needs to handle the out of band overload and stay linear. If the antenna were made longer, the amplifier would need to handle proportionately more power (and probably melt). Some details in the patent at:

The "background" section is worth reading. The other reason for the amplifier is to give the antenna system a 50 ohm output impedance so that cheap coax can be used.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

Bill;

I haven't looked at the back of a radio in a long time, but those that I have looked at usually had a trimmer capacitor that helped tune the antenna to the radio input. This is the impedance match you are looking at.

Dave WD9BDZ

I make amplifiers with 50 Ohms input impedance and 300pV/rtHz input- referred noise. By your assertion, that should have been impossible. The trick is that the input impedance obtained by feedback: The cooled resistor trick. It works.

Jeroen Belleman