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Do receiver antennas need matching or not?
Jeff Liebermann wrote:
Ghosts are a big problem where I live. Or multipath and the resulting dropouts the case of DTV broadcasts. That's why I like highly directional designs (both horizontally as well as vertically). Other than that, I can get all of our local stations with rabbit ears and a UHF loop. I live too far away from the local digital TV xmitters to get reliable reception. My rule of thumb is that if OTA analog TV reception is marginal, digital TV will be worse. Highly directional antennas are the right way to eliminate ghosts (reflections). However, I keep running into problems with f/b (front to back) ratio problems, where the ghosts are reflected from behind the antenna. That's where the lower gain, but higher f/b ratio antennas, such as a barbeque grill backed bowtie array, makes more sense. My preference is to use single channel narrowband yagi's for maximum gain, but that gets really ugly as one per channel is required. Actually, with modern high quality receivers (which may not include the $40 DTV to NTSC converters), they use a rake receiver to coherently combine the multiple paths, so it's very likely that you'd get a better signal with digital (i.e. the receiver would lock and you'd get ANY output) than with analog in a multipathy, but not faint signal environment. One of the design goals for DTV was to be multipath immune. Note that in Europe, they actually transmit the identical signal from multiple transmitter sites, guaranteeing multiple arriving signals with different timing. Different modulation scheme, but the same multipath issues. The same has been proposed for use in the US However, with DTV, signal strength is more of an issue,mostly because there isn't a "degraded mode" like there is with analog. Most viewers are willing to tolerate remarkably degraded signals on an analog channel, so folks are used to being able to receive a TV signal well beyond the nominal service boundary. With digital, either you got it or you don't. Some gain in the antenna helps.. |
Do receiver antennas need matching or not?
On Mar 19, 11:44 am, Jim Lux wrote:
Jeff Liebermann wrote: Ghosts are a big problem where I live. Or multipath and the resulting dropouts the case of DTV broadcasts. That's why I like highly directional designs (both horizontally as well as vertically). Other than that, I can get all of our local stations with rabbit ears and a UHF loop. I live too far away from the local digital TV xmitters to get reliable reception. My rule of thumb is that if OTA analog TV reception is marginal, digital TV will be worse. Highly directional antennas are the right way to eliminate ghosts (reflections). However, I keep running into problems with f/b (front to back) ratio problems, where the ghosts are reflected from behind the antenna. That's where the lower gain, but higher f/b ratio antennas, such as a barbeque grill backed bowtie array, makes more sense. My preference is to use single channel narrowband yagi's for maximum gain, but that gets really ugly as one per channel is required. Actually, with modern high quality receivers (which may not include the $40 DTV to NTSC converters), they use a rake receiver to coherently combine the multiple paths, so it's very likely that you'd get a better signal with digital (i.e. the receiver would lock and you'd get ANY output) than with analog in a multipathy, but not faint signal environment. One of the design goals for DTV was to be multipath immune. Note that in Europe, they actually transmit the identical signal from multiple transmitter sites, guaranteeing multiple arriving signals with different timing. Different modulation scheme, but the same multipath issues. The same has been proposed for use in the US However, with DTV, signal strength is more of an issue,mostly because there isn't a "degraded mode" like there is with analog. Most viewers are willing to tolerate remarkably degraded signals on an analog channel, so folks are used to being able to receive a TV signal well beyond the nominal service boundary. With digital, either you got it or you don't. Some gain in the antenna helps.. I am hoping that because of the narrow filter requirements of ham radio a fast acring multipath antenna switch would allow the dominant polarisation to over rule all otherssince the ear can only accept a few samples at a time. Art |
Do receiver antennas need matching or not?
Not sure about the higher frequencies but in the HF band we absolutely need
an antenna coupler that matches the impedance to the selected frequency. If a coupler fails we can barely throw a signal a few miles whereas when the coupler does it's job we can bounce a signal off of the ionosphere at night for a few thousand miles. Claude Montreal "billcalley" wrote in message ... Hi All, I always hear that antennas have to be matched to their radio, but in receivers (such as FM and shortwave radios) I see mostly long random length antennas used, and these antennas -- be they a telescoping whip or a long wire out a window -- are used over some really wide bandwidths. How is this possible if an impedance match must always be maintained for radios? And since there cannot be a good match over such wide bandwidths with any (typical) wire antenna, what is the downside to using these completely unmatched long antennas for receivers? (Poor gain patterns with lots of nulls? Lower sensitivity due to bad noise figure or gain match for any LNA or frontend amp? Degraded overall antenna gain)? Thanks; I'm very confused on this subject! -Bill |
Do receiver antennas need matching or not?
Not sure about the higher frequencies but in the HF band we absolutely need
an antenna coupler that matches the impedance to the selected frequency. If a coupler fails we can barely throw a signal a few miles whereas when the coupler does it's job we can bounce a signal off of the ionosphere at night for a few thousand miles. That's certainly true at the transmitting end. A good impedance match is needed in order to enable the transmitter to deliver power effectively into the antenna, from whence it can be radiated. It's rather less true at the receiving end, at least in the lower- frequency HF bands. In these bands, the ability to receive a usable signal is often dominated by the amount of natural and man-made noise in the band, and not by the receiver's own self-generated noise. Even with a serious impedance mismatch between the antenna and the receiver, enough signal reaches the receiver front-end to overcome the receiver's own internal noise. If you happen to live in an area which is blessed by a very low background-noise level (e.g. out in the country, away from power lines) and you're DXing in the HF bands, then a good impedance-matched antenna will let your receiver take best advantage of the low noise level. If you're SWLing in a city, surrounded by power lines and electric motors and neon lights and computers, the background noise level is going to be much higher, and the weak distant stations will be drowned out by the noise anyhow... and an inefficiently-matched antenna such as a whip or longwire will give you enough signal to hear the stations which are _not_ drowned out by the noise. As an example - if your receiver has decent sensitivity, and a low internal noise level, you may find that you can hear a signal with decent audio quality all the way down to S0 or below (if there's no noise obscuring it). You then find that with an impedance-matched antenna the band's background noise is S6 or so. Assuming that you can make use of a signal which is somewhat below the broadband noise level, let's say that you decide you can copy stations whose own individual signals are S5 or better, and that lower-level signals are blanketed by the noise. At this point, you realize that you can use an antenna which is 5 S-points (nominally, 30 dB) less efficient, and still receive the same set of stations. With a less efficient antenna, the stations' signals will be weaker... but so will the external band noise, by the same ratio, and thus the signal-to-noise ratio of each station will remain unchanged. As an example, my ARES/RACES group has a multiband HF setup in our city's police and fire admin building, which is downtown near the main commercial-and-restaurant street and the light-rail system. We have a trap-dipole antenna strung up above the building's roof. On the 80-meter band, the broadband noise level across the whole band is rarely less than S9! We could probably receive the same set of stations using an antenna consisting of two coat-hangers and some damp string! -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
Do receiver antennas need matching or not?
On Mar 17, 4:06*am, Jeroen Belleman wrote:
Mark wrote: 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. 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. 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- Hide quoted text - - Show quoted text - good point thanks Mark |
Do receiver antennas need matching or not?
Joel Koltner wrote: This implies only that the antenna/receiver *matching* is good enough... yes? *** No, it says nothing about the matching. It only says that the signals coming down the antenna from the cosmos are greater than the receiver noise. If the antenna is matched to the receiver, whatever is picked up will be more efficiently fed into the receiver, resulting in a still higher level. If the antenna is not matched, well, there may be a heck of a lot of both noise and signal, and even unmatched the results are strong enough to override the rx noise..... One caveat , tho, ... in some conditionsm, a matched receiver input results in a higher receiver noise level.... not much, but enough for purists to argue the point incessantly :))))) . (I'm thinking that you would still sometimes prefer a highly directional antenna over just a dipole even though both increase the background noise. I.e., in both cases the antenna matching is good enough, but without the directionality the antenna itself might not be good enough to eliminate interference, overloading, etc. from sources other than the one you're interested in.) **** Of course, and a good point..... I was only talking about desired signal and atmospheric noise.... If there is a coherent interferer, then that's a whole 'nuther thang...... :))) Andy W4OAH |
Do receiver antennas need matching or not?
On Sat, 15 Mar 2008 04:11:21 -0700 (PDT), billcalley
wrote: Hi All, I always hear that antennas have to be matched to their radio, but in receivers (such as FM and shortwave radios) I see mostly long random length antennas used, and these antennas -- be they a telescoping whip or a long wire out a window -- are used over some really wide bandwidths. How is this possible if an impedance match must always be maintained for radios? And since there cannot be a good match over such wide bandwidths with any (typical) wire antenna, what is the downside to using these completely unmatched long antennas for receivers? (Poor gain patterns with lots of nulls? Lower sensitivity due to bad noise figure or gain match for any LNA or frontend amp? Degraded overall antenna gain)? Thanks; I'm very confused on this subject! -Bill In a short answer to your question, NO! The finals won't burn out as there are none and you won't be feeding radiation into your room. However, as you probably noticed from the FM radio with the extendable antenna, you sometimes get a better signal when you extend the antenna nd move it to the right location. A good (and matched) antenna may allow you to receive signals you could not have received with a random length antenna. Most radios that I have seen usually have a matching network built in to match the antenna that comes with the radio. In most cases they aren't that elaborate, just a small coil and a trimmer capacitor. My father learned that as great as his 50 foot long copper wire worked for listening to his favorite short wave broadcasts, the signals improved immensely when I installed an fan-dipole for his three favorite bands. Did he 'need' that antenna? no, he could hear his stations without it, but the signal strength was improved and he was able to pick up more stations. Dad also ordered an antenna tuner kit, assembled it and attached it to his 50 foot wire. He found a great improvement in signal strength using the tuner, almost equal to the multi-band dipole I installed. Signal to noise ratio are not synonymous with gain. One can have a lower-gain antenna with high s/n that outperforms a higher-gain antenna with low s/n. When it gets critical, the s/n can be the determining factor as to whether you receive intelligent communication or not. You are probably digging for technical, theoretical information more than practical, but just in case... If you look in stereo magazines, you seldom see radio ads bragging about how much better they receive than the competition. If you look in Ham magazines, that's a very important feature. Most consumers are looking for stereos that play music well and they listen to local stations. Most ham operators want to pull that weak signal out of the noise to make the contact. Just some thoughts. Hope this is helpful to someone. Buck N4PGW -- 73 for now Buck, N4PGW www.lumpuckeroo.com "Small - broadband - efficient: pick any two." |
Do receiver antennas need matching or not?
I did not see all the replies nor the original post but in the world of
radio frequency (RF) match yes this would be an ideal situation since maximum energy transfer occur when Imepdance of the antenna (Zant) = Imedance of the Receiver Antenna Port (Zrx_port). All this talk about noise while important to minimum detectable signal more greatly influenced by the internal Noise Figure (NF) of the receiver (RX). In my line of work, which is Radar engineering, we use a standard temperature T = 270 Kelvin to model the noise originating by natural extgernal sources of which the Sun is the biggest contributor. The short end answer on HF you will probably not notice a big difference as long as the antenna is close between a 1/4 to 1/2 wavelength long [e.g. wavelength = Velocity of Light (C) / Frequency of Operation (Fo)]. However, if you use one of those collaspable whips found on the portable shortwave receivers you will. This is because the anyenna impedance is a lot less than the usual 50 Ohm impedance of the RX antenna port (e.g. Zant Zrx_port ). You can match very short antennas with antenna tuners to make them transfer efficently to the RX antenna port but now the nasty parameter of effective antenna aperature (square feet or meters) reduces it caoture ability (think of catching a baseball with and without a glove, the probability of catching the ball is higher with a glove due to it larger capture area). Well that is enough to put someone to ZZZzzzzzzzzzz. Hi Hi 73, Homer J on all lower and upper channels Thane-Fer |
Do receiver antennas need matching or not?
"Homer J" wrote in message
.. . All this talk about noise while important to minimum detectable signal more greatly influenced by the internal Noise Figure (NF) of the receiver (RX). My understanding is that this is not the biggest influence at HF -- there's so much atmospheric noise down there that even with a pretty poor receiver (noise figure-wise) the MDS is usually just about the same as with a much better receiver. In my line of work, which is Radar engineering, we use a standard temperature T = 270 Kelvin to model the noise originating by natural extgernal sources of which the Sun is the biggest contributor. Have you seen the graph in, e.g., Krauss's antenna or EM book? T=270 is a poor model at many frequencies. (Granted, if you're doing narrowband designs, it'll just be some offset error that's probably not too much worse than, say, +/-3dB.) However, if you use one of those collaspable whips found on the portable shortwave receivers you will. This is because the anyenna impedance is a lot less than the usual 50 Ohm impedance of the RX antenna port (e.g. Zant Zrx_port ). You can match very short antennas with antenna tuners to make them transfer efficently to the RX antenna port but now the nasty parameter of effective antenna aperature (square feet or meters) reduces it caoture ability From watching this thread I get the impression that -- at least on HF again -- the (lack of) capture area is the much bigger problem than the mismatch is. ---Joel |
Do receiver antennas need matching or not?
On Mar 30, 6:54 am, "Homer J" wrote:
I did not see all the replies nor the original post but in the world of radio frequency (RF) match yes this would be an ideal situation since maximum energy transfer occur when Imepdance of the antenna (Zant) = Imedance of the Receiver Antenna Port (Zrx_port). All this talk about noise while important to minimum detectable signal more greatly influenced by the internal Noise Figure (NF) of the receiver (RX). In my line of work, which is Radar engineering, we use a standard temperature T = 270 Kelvin to model the noise originating by natural extgernal sources of which the Sun is the biggest contributor. The short end answer on HF you will probably not notice a big difference as long as the antenna is close between a 1/4 to 1/2 wavelength long [e.g. wavelength = Velocity of Light (C) / Frequency of Operation (Fo)]. However, if you use one of those collaspable whips found on the portable shortwave receivers you will. This is because the anyenna impedance is a lot less than the usual 50 Ohm impedance of the RX antenna port (e.g. Zant Zrx_port ). You can match very short antennas with antenna tuners to make them transfer efficently to the RX antenna port but now the nasty parameter of effective antenna aperature (square feet or meters) reduces it caoture ability (think of catching a baseball with and without a glove, the probability of catching the ball is higher with a glove due to it larger capture area). Well that is enough to put someone to ZZZzzzzzzzzzz. Hi Hi 73, Homer J on all lower and upper channels Thane-Fer I never knew that "aperature" could be measured in square feet or metres! Is there a book that describes it in such a way ? Art |
Do receiver antennas need matching or not?
Joel Koltner wrote:
"Homer J" wrote in message .. . All this talk about noise while important to minimum detectable signal more greatly influenced by the internal Noise Figure (NF) of the receiver (RX). My understanding is that this is not the biggest influence at HF -- there's so much atmospheric noise down there that even with a pretty poor receiver (noise figure-wise) the MDS is usually just about the same as with a much better receiver. In my line of work, which is Radar engineering, we use a standard temperature T = 270 Kelvin to model the noise originating by natural extgernal sources of which the Sun is the biggest contributor. Have you seen the graph in, e.g., Krauss's antenna or EM book? T=270 is a poor model at many frequencies. (Granted, if you're doing narrowband designs, it'll just be some offset error that's probably not too much worse than, say, +/-3dB.) However, if you use one of those collaspable whips found on the portable shortwave receivers you will. This is because the anyenna impedance is a lot less than the usual 50 Ohm impedance of the RX antenna port (e.g. Zant Zrx_port ). You can match very short antennas with antenna tuners to make them transfer efficently to the RX antenna port but now the nasty parameter of effective antenna aperature (square feet or meters) reduces it caoture ability From watching this thread I get the impression that -- at least on HF again -- the (lack of) capture area is the much bigger problem than the mismatch is. ---Joel yes |
Do receiver antennas need matching or not?
On Mar 15, 9:11*am, billcalley wrote:
Hi All, * *I always hear that antennas have to be matched to their radio, but in receivers (such as FM and shortwave radios) I see mostly long random length antennas used, and these antennas -- be they a telescoping whip or a long wire out a window -- are used over some really wide bandwidths. *How is this possible if an impedance match must always be maintained for radios? *And since there cannot be a good match over such wide bandwidths with any (typical) wire antenna, what is the downside to using these completely unmatched long antennas for receivers? *(Poor gain patterns with lots of nulls? *Lower sensitivity due to bad noise figure or gain match for any LNA or frontend amp? Degraded overall antenna gain)? Thanks; I'm very confused on this subject! -Bill What has not been mentioned much is that AM band broadcast receivers and FM band receivers are designed to to tune over a fairly wide band of frequencies; so very difficult to build antenna that will mtach at all those different 'wavelengths'. For example; the broadcast band (North America) is roughly 550 kilohertz (that's 545 metres wavelength) to about 1.7 megahertz (about 176 metres). That's 3:1 ratio! On FM, 88 to 108 megahertz (3.4 to 2.8 metres) the ratio is less but still cnsiderable at 1.2:1 So again very difficult to design and build an 'all frequencies' antenna. For stations designed to receive only one frequncy the antennae can be constructed for that only; hence the matching can be as optimum as possible. |
Do receiver antennas need matching or not?
In article
, terryS wrote: For example; the broadcast band (North America) is roughly 550 kilohertz (that's 545 metres wavelength) to about 1.7 megahertz (about 176 metres). That's 3:1 ratio! HOWEVER, if you will look at the Antenna Design for a AM Broadcast Receiver, using a Ferrite Loop Antenna, you WILL notice that the Loop IS Tuned to the specific part of the band that the receiver is being tuned to, by Linked Ganged Variable Capacitors, in the Receivers Frontend. These designs have been around for MANY years, (1940's anyway) and are a very mature technology. |
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