If receive performance doesn't depend on SWR, then how can I tune my manual HF antenna tuner by band noise?

Question

The popular question "What is the relationship between SWR and receive performance?" asks if a receiver with a well-matched antenna (low SWR) beats a receiver with a poorly-matched antenna, with everything else being equal. The consensus opinion of the answers, weighted according to the votes on the answers, is no; the two receivers should receive about the same.

The idea is that an impedance mismatch leads to power loss, and a weaker signal at the receiver. But losses shouldn't affect receiver performance as long as the signal-to-noise ratio (SNR) is the same, because the receiver's automatic gain control (AGC) feedback circuit can easily add a few more decibels of gain to compensate for any such losses. There is an important caveat to this rule that the received RF noise floor must be above the receiver's noise floor, because otherwise the signal-to-noise ratio will be affected.

This logic makes sense to me, and I can find no flaws in it. But if that is so, then why is it that when I use my manual T-network transmatch (antenna tuner) to tune my ZS6BKW antenna on various HF bands such as 40m and 20m, I can get a fairly good match without transmitting by turning the knobs to maximize the band noise in my headphones? (I say "band noise" because I tune the radio to an unused frequency and then adjust the transmatch for maximum static, but if there were a signal on the frequency then I would adjust the transmatch to make the signal the loudest.) If the consensus answers to the other question are correct, then I would think that turning the knobs would make no difference at all to the volume of sound in the headphones.

My HF rig is an Elecraft K2, which has an excellent receiver. The caveat to the rule shouldn't apply, because the RF noise floor should be well above the receiver's noise floor on 40m and 20m.

Answer 1

Receive power does vary with SWR; but for reception quality we're interested in signal-to-noise ratio. As long as you're getting enough receive power that the noise received from the antenna is greater than the receiver's own noise floor, getting more power from the antenna won't result in any noticeable improvement to your ability to copy. That's why we say things like "gain doesn't matter for receive antennas" or "SWR doesn't matter for receive". It's not an absolute truth, but it's a good starting point. For a typical HF noise floor, you need a lot of negative gain or a lot of mismatch loss before you'll notice any difference.

So why can you tune an antenna by peaking the noise? Because, again, receive power does vary, and because receiver AGC is (deliberately) imperfect, especially at low levels. So when you're just listening to noise, and turning the knobs on the tuner, the audio level will go up and down — not by as many dB as the RF level goes up and down, but by enough to notice. If you do the same thing while tuned to a good signal, you might or might not notice the effect, but in any case after making any necessary volume adjustments to equalize the signal level, the noise level should end up the same as it was before.

Answer 2

The noise you hear in your headphones is coming from the antenna. When you tune the antenna and minimize the SWR, the losses between the antenna and the receiver are minimized, thus more of the noise received by the antenna makes it to the receiver, and you hear more noise in your headphones.

(Assuming of course the receiver is set to a mode where power is proportional to noise, as in AM, SSB, or CW.)

It's also possible that the noise you hear in the headphones comes from thermal sources in the feedline or the receiver, or something other than the electromagnetic field around the antenna. In this case tuning the antenna would not significantly increase the noise volume in the headphones, and improving SWR would indeed increase receive performance. This is however very unlikely on HF, since the electromagnetic noise floor is so high compared to the internal noise of even a very inexpensive receiver.

Answer 3

Missing in this discussion is the fact that optimum power matching does not necessarily correspond with optimum noise matching.

Antenna tuner alignment on band noise is possible when the input impedance of the receiver is 50 Ohms real. Then maximum received noise corresponds to correct antenna tuner alignment.

Answer 4

You can get a good match by maximizing received noise, because that is the point where more RF noise might be transferred from the antenna to your receiver, and thus indicate where the SWR is lower (e.g. less noise is reflected back to the antenna at the frequency of interest).

Tuning to minimize SWR might improve receiver performance by changing the efficiency of RF transfer near the frequency of interest compared with that of other frequency ranges where the SWR is higher. (e.g. some antenna tuner circuit configurations act as bandpass filters). But it can also reduce receiver performance by reducing total usable dynamic range (e.g. if your receiver's front end needs some attenuation of the RF input, rather than gain, to possibly reduce IMD).

Answer 5

As per Phil's answer, the idea that a matched system is most efficient from a system level perspective is undeniable.

However, at VHF and UHF the receiver noise is well above the noise received by the antenna, so then without doubt less mismatch means better system signal to noise ratio.

Also, i imagine that the signal to noise ratio probably changes for active electronic circuits depending on where on their operational characteristic curves the signal levels are sitting, so maybe signal to noise ratio is better at higher gain for a device which doesn't have AGC applied for example. In that case less mismatch could mean better signal to noise ratio.

Apart from all that, it always seems to me that a matched system appears to be more 'sensitive' or responsive regardless of what the overall S/N ratio is, even if it is only because the noise received by the antenna is also amplified along with the desired signals, so from a psychological perspective it makes you more confident that you have a system that's working well.

Answer 6

A possible answer came to me in a flash of insight. It occurred to me that the AGC in my Elecraft K2 does not try to make the volume in the headphones constant wherever I tune the receiver; if the AGC worked that way then the static of an unused frequency would be exactly as loud in the headphones as a contest station transmitting 1,500 W from 10 km away. I'll call that "naïve AGC". Hearing loud static on every unused frequency would be very annoying, so receiver manufacturers don't design their AGCs to work that way. If the AGC did work that way, then I do think I wouldn't notice any volume difference in the headphones when I adjust the knobs of my transmatch.

Instead, my Elecraft K2 is designed so that the AGC only partially boosts the gain for weak signals. Because the AGC action is not as aggressive as "naïve AGC" by design, strong signals are louder in the headphones than weak signals. (That's one of the features that I like about my K2 compared to Yaesu/Icom/Kenwood HF rigs, in which the audio volume of a weak station seems almost as loud as the volume of a strong station.)

Because stronger RF signals are actually louder in my headphones than weaker signals thanks to the partial action of the AGC, adjusting the transmatch for a stronger received signal will result in more audio volume in my headphones. A better impedance match will result in less mismatch loss and therefore a stronger signal. So that's how I can adjust my transmatch fairly well by adjusting the controls so the static of an unused HF frequency is loudest in my headphones.