by Daniel A. Grunberg -- Kensington, Maryland U.S.A I wrote the following explanation about why not to assume that an antenna tuner is necessary, or even useful for good reception. Some of the discussion, that followed the posting of my article on rec.radio.shortwave, follows the article here too. The fact is that an antenna tuner won't help you much, unless the level of the noise brought from your antenna to your receiver is significantly lower than the level of the noise that your receiver generates, internally. IMHO, that won't be true almost all of the time. If you wish, you can perform a simple, practical test (see the end of this article) and decide for yourself. IMHO, an antenna tuner might provide some MARGINAL help in the reduction or elimination of interference from signals well off the desired signal's frequency, but it will NOT increase the signal to noise ratio on the frequency to which it is tuned. Therefore, unless, for example, you're having trouble with interference from MW stations screwing-up your reception on 6 MHz; or you are lucky enough to have an antenna in a VERY ELECTRICALLY QUIET LOCATION; an antenna tuner won't help you at all. Here I quote from page 582 of the 1974 Edition (the old one I have readily at hand) of _The_Radio_Amateur's_Handbook_: "Most hf receivers are sensitive enough that exact matching is not necessary." Whether you use an antenna tuner or not, some of the signal will get to the receiver. Any signal or for that matter any noise voltage, that is present at the input to the receiver will be amplified, so long as the signal or noise frequency is within the passband of the amplifier, and the combined signals and noises present do not exceed the amplifier's dynamic range (i.e. they do not overload the amplifier). Modern receiver front-ends are sensitive and quiet, and if a signal has a good enough signal to noise ratio it will be heard. Using an antenna tuner doesn't help because when you "peak" the tuner to increase the signal strength of a signal by a few db, you also "peak" the noise level at and near that frequency by the same number of dB. The signal to noise ratio in dB for a signal strength of S dB and a noise level of N dB is given by: [Signal to Noise Ratio] = S - N If you increase both S and N by the same number of dB, S - N doesn't change. While it is true that an antenna tuner will help a little to reduce or eliminate off-frequency signals, there is a better way to do that. If, for example, you're having trouble with interference from MW stations screwing-up your reception on 6 MHz, an antenna tuner might help to reduce the interference A LITTLE. But an antenna tuner is designed as an impedance matching device for transmitting, and only incidentally would be a (relatively poor) off-frequency signal receiving filter. A preselector (preferably an un-amplified one, or one that can be operated with its amplification switched off) is a device specifically designed as an off-frequency receiving filter, and does a MUCH better job of it. Bottom line: IMHO, unless you have convinced yourself that your location is VERY QUIET (RF-wise), don't use an antenna tuner. If and ONLY IF you're having a problem with out-of-band signals, consider getting a preselector. Otherwise, if you want more signal, consider improving the antenna. ---------------------------------------- SIMPLE PRACTICAL TEST 1. With your antenna connected, tune your receiver to an unused frequency, near a frequency that you might want to receive. 2. Disconnect your antenna from the receiver. 3. Using as SHORT a wire as you can, RELIABLY short circuit your receiver's antenna connection point to your receiver's ground connection point. 4. Turn up the audio gain, and the radio-frequency gain (if your receiver has one). Set your receiver's local/DX switch to DX (if your receiver has one). Verify that the RECEIVER NOISE you hear is nothing but noise (or possibly no noise at all). 5. Remove the short circuit and reconnect the antenna to the receiver. Verify that the TOTAL NOISE you hear is nothing but noise (or possibly no noise at all). 6. If you judge that the total noise heard with the antenna is NOT audibly louder than the receiver noise heard with the receiver's antenna connection short circuited to ground, then PERHAPS an antenna tuner MIGHT make a difference, and you MIGHT want to try one. ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Subject: Re: NEW TO SHORTWAVE From: ce369@FreeNet.Carleton.CA (Daniel Grunberg) Date: 1997/06/19 Dave Rickmers (=firstname.lastname@example.org) writes: | On Tue, 17 Jun 1997 12:48:33 -0400, "Christopher A. King" | wrote: ||Daniel Grunberg wrote: ||| "Christopher A. King" writes: ||| | A passive series-resonant tuner (MFJ-956, Grove Mini-Tuner) ||| |may improve matching and signal transfer from about 5MHz and ||| |below, ||| As has been pointed out on this Newsgroup before, although the ||| better matching and will increase the signal transfer, the ||| matching of naturally occurring noise at and near the tuned ||| frequency also will be improved by the same factor that the ||| signal is improved by. The result is that there will be NO ||| IMPROVEMENT in signal to noise ratio, and therefore no ||| increase in signal readability. Don't waste your money merely ||| for the sake of matching the antenna to the receiver, it just ||| ain't useful. ||I suppose that we could get into a real nitpicky argument about ||how the front end of the receiver has a signal-to-noise ratio of ||its own, and that by providing a stronger signal at the antenna ||input you are improving matters at this point irregardless of ||whether or not you're improving the relationship between signal ||and noise as it exists on the antenna. But since it's clear that ||you already have you're mind made up on the issue, I won't ||bother. | Yeah, Dan. What about the a.v.c. threshold? If you are below | that threshold you have: signal + noise from the antenna AND | circuit noise from the i.f. (and r.f., if applicable) amplifiers | running full gain. So (correct me if I'm wrong) what you're saying here is that the antenna tuner will increase the very-weak signal + noise from the antenna to lower the gain of the receiver so you can hear the not-quite-so-very-weak signal+noise from the antenna. That probably would make sense in a low-ambient-noise laboratory environment. On SW frequencies, where the naturally-occurring, ambient noise level on and near the frequency to which your tuner is tuned is not low compared to the internal noise of the receiver, the ambient noise is what's keeping you from hearing the signal in the first place, with your "amplifiers running at full gain." How does the tuner alter the situation? Try this. Remove your tuner, and connect your antenna directly to your receiver. Find an unused SW frequency, and note (or better yet measure) the noise level you receive. Put the tuner back, and match the antenna to your receiver. Note (or better yet measure) the noise level you receive. Wasn't there more noise with the tuner? If the tuner were to multiply the naturally-occurring, ambient noise level on and near the frequency to which your tuner is tuned, by the same factor that it also multiplied a signal on the frequency to which your tuner is tuned, how would the tuner have improved anything? | Chris: A TUNER which corrects mismatched impedances will | increase the signal. A passive 50 Ohm in/out preselector (e.g. | MFJ-956) will not. I'm not sure what you're saying here. Surely you don't mean to say that the tuner, made up only of coils and capacitors, is an active device. Surely you don't mean to say that a preselector, which I think (correct me if I'm wrong) has a built in amplifier, is a passive device. Assuming that the tuner or preselector is used, as a tunable band-pass filter to attenuate interference that is well off the tuned frequency, and assuming that the preselector is run with it's amplifiers (if it has any) switched off, then the choice of a tuner or a preselector might be based on price and performance. An antenna tuner may be cheaper or more readily available than a preselector. OTOH, a preselector, because it probably has a narrower passband than antenna tuner, might work if the antenna tuner didn't. If the cheaper solution works, go with it. If not, try the more expensive solution. +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ in a posting to rec.radio.shortwave Subject: Random wire/balun/coax/ATU question Date: Wed Aug 28 11:37:18 1996 "Bruce Wilson" <email@example.com> said: |Let's say a random wire is connected to a balun (like the MLB), |and then the coax from the balun connects to an antenna tuner |then to the radio. What is tuned when the ATU knobs are |adjusted? The wire or the coax? In other words, does the |ATU match the coax impedance to the receiver input impedance, |or is the entire system, from wire to the ATU, tuned to match |the receiver? I've looked through many books and can't find |an answer. Maybe experimental evidence will answer the question. A receiver is matched to an antenna, AT A GIVEN FREQUENCY, when the receiver's input circuit, the tuner, the two transmission lines, and the antenna are in resonance at THAT FREQUENCY, and the antenna looks down the line to the receiver and "sees" a resistance that is equal to the resistive component of the receiver's antenna input impedance. This is called complex conjugate matching. For the purposes of the following discussion, frequency means any or all radio frequencies below 30 MHz, that are within the tuning ranges of both the receiver and the antenna tuner. For the purposes of the following discussion, antennas are fed with coax. A SW receiver's designer probably has tried to present a known, fixed, almost-purely-resistive load at the receiver's antenna connector. In a modern SW receiver, usually the antenna input circuit is unbalanced (i.e. one side is grounded). It is likely that the designer of a modern SW receiver has tried to make the impedance at the antenna connector equal to either 52 or 72 Ohms. (52 Ohms or 72 Ohms are the characteristic impedances of several commonly-available, coaxial cables.) If an antenna tuner is used (and I DON'T believe its use generally is justified), typically the tuner will be located quite close to the SW receiver, in order to facilitate tuning. If the cable run between the SW receiver and the tuner is short (perhaps a meter or shorter), and the characteristic impedance of the coax used corresponds to the rated input impedance of the receiver, mismatches between the tuner and the receiver can be neglected. Therefore, practically speaking, the purpose of the tuner is to match the impedance of the antenna, AS IT IS TRANSFORMED BY THE TRANSMISSION LINE between the antenna and the tuner, to the impedance of the receiver. Transmission lines act as impedance transformation devices, but not necessarily as simple transformers). For a given frequency, if the impedance of the antenna is purely resistive and it equals the impedance of the transmission line, then regardless of the length of the transmission line between the antenna and the tuner, the impedance presented to the tuner will be the impedance of the antenna. On the other hand, for a given frequency, regardless of the antenna's impedance, if the transmission line is an integer multiple (0, 1, 2, ... ) of half-waves long, the impedance presented to the tuner will be the impedance of the antenna regardless of its amplitude and or whether or not it is purely resistive. Since we wish to receive signals at many frequencies, we also must consider the much more common classes of cases, where neither is the antenna impedance equal to the characteristic impedance of the line, nor is the line length an integer multiple of half-waves. As the frequency varies, a fixed-length transmission line, whose length beyond an integral multiple of half waves varies as a function of frequency, will transform the impedance of an antenna whose impedance also varies as a function of frequency. As the frequency varies, the amplitude of the impedance presented to the tuner by the transmission line may be large or small. As the frequency varies, the impedance presented to the tuner by the transmission line may become resistive, highly reactive (either inductive or capacitive), or some combination of resistive and reactive. The simple answer to your question is that a receiving antenna tuner must tune the combination of the antenna and the transmission line from the antenna, in a way that varies complicatedly with frequency.
This article was last updated on 12 August 1997.If you have any questions, feel free to Email me firstname.lastname@example.org . I'll do my best to confuse you completely (:-). (Comments or corrections also are welcome.)