各种群里总有萌新 / 甚至老腿在问接收机的天线问题，来看看本坛坛友意见。最典型的提问例如： - “我想用短波电台听别人通联，能不能接随机线接收，会影响效果吗？” - ”SDR 用什么样的天线比较好 “ 等等

个人感觉能做阻抗匹配最好还是做一下，虽然没有发射电路不谐振带来的负面影响那么大，阻抗不匹配信号反射衰减，信号质量会下降，信号反射的部分还会造成信号失真，影响接收质量

我觉得不用。有用信号被衰减了，干扰信号也被衰减了，只要信噪比够，一样能接收到，RBN解码cw,接收机经常是用单波段的天线去全段接收解码，效果也非常好

搭两套天线，用不谐振的天线接收，对比下谐振的天线效果就很明显了

鉴定为不够长导致的，拉个1.5倍波长还有这么多事吗

哦后面再串个电阻反正不发射

借个楼，请问小k收音机的天线搭配什么天线比较好，看过别的ham用了个像这个样子的天线（不知道名字）[upl-image-preview url=https://storage.hamcq.cn/2024-05-07/1715081756-101918-img-1633.png] 不是甜甜圈，铁环直径15cm左右

用拉杆试的话，确实是谐振长度下信号强度更高，而不是越长接收效果越好。而且也听说即使是纯接收，有条件的话用天调调过之后接收效果也更好。 @"VR2WBZ-BD7PTZ"#p17192 看造型是某种带放大器的小环天线？在淘宝有很多这个形状的，可以用“有源环天线”这三个作为关键词搜一下，对比一下造型看看。

@"VR2WBZ-BD7PTZ"#p17192 元芳K180，或者什么BG7YZF的有源小环个人不建议买...

@"VR2WBZ-BD7PTZ"#p17192 我买了一个类似的，放在阳台上接SDR玩，接收比拉杆天线好一些，并且是有方向性的。属于空间有限的情况下的一种解决方案，要是有空间的话，还是大型天线好的多，能听能发。

Technical discussion without prizes: Should the receiver antenna consider impedance matching?

BI1XJT

BH2VZC It feels like a useless exercise. If there's a signal above, the reflected signal will inevitably be weaker than the original signal. Even if a very weak signal is reflected back, it would likely get lost in the strong signal or background noise.

BD3PSX

BI1XJT However, this does not affect the ability to reflect ( ), but it is very weak.

BI1XJT

BD3PSX This doesn't prevent me from using a radio to pick up signals from all over the world, even if I only hear signals as weak as -300 dBm. 😈

BI1OHW

SuoYuC The length affects the electrical length, which is proportional to the antenna gain. In general, mismatches affect the efficiency of energy transfer.
If the antenna is a simple wire antenna and operates at low frequencies such as shortwave or V-band, then these two dimensions can be considered orthogonal. A good performance in one dimension can offset the negative impact of the other, but it's always best to have both performing well.
If the antenna itself has a complex structure and operates at a high frequency (and cannot be treated as a lumped circuit), then reflections will affect the current distribution on the antenna, which in turn affects the antenna's gain and performance.
In summary, achieving the specified performance definitely requires matching. Mismatch can occur due to the main lobe aligning better by chance, but it is more likely that reflections and beam degradation will introduce more noise.

BI1OHW

BI1XJT Impedance mismatch will affect the current distribution of the antenna, which in turn affects the radiation pattern. If it's a Yagi-Uda antenna, the side lobes may be significantly degraded, leading to increased noise floor. This is something that a low-noise amplifier (LNA) cannot overcome.

BI1XJT

BI1OHW I find this counterintuitive. Does it mean that a single Yagi antenna, when connected to a 50-ohm receiver and a 75-ohm receiver, will produce different radiation patterns? This seems difficult to accept; the radiation pattern of an antenna should be determined solely by its structure. Alternatively, even if there is a slight signal reflection that alters the voltage distribution, it shouldn't prevent the antenna from receiving signals based on its own electrical structure. From the perspective of the receiver, the efficiency would simply be slightly reduced.

BG5TOX

BI1OHW I had a similar idea with you a long time ago (namely, "reflection affects the current distribution on the antenna"), but I later found that it wasn't true.

BH5UWL

Reflection, main lobe, side lobe, and gain are terms used in transmitting. It's important not to confuse transmitting and receiving states simply by using these terms.
The transmission is like a faucet dispensing water. The transmitter outputs a clean, single-frequency sine wave current to the antenna, with minimal stray radiation. When discussing impedance, we're talking about reflections. This refers to the amount of energy lost during transmission relative to the power of the transmitting source and the transmission frequency, as well as the number of redundant frequencies produced in unexpected directions.
Receiving is like catching rainwater in a basin. Any stray energy falling from the sky must be caught together, and then filtered using a fine net to extract the target. The antenna is passive; when discussing impedance and reflection, where does the induced current on the conductor reflect back to during reception? What impact does this have on the receiver if it reflects back into the sky? How can we distinguish between noise that is a product of the transmission line of the antenna, and a mixed product of external environmental interference or intentional interference? Is it a normal part of the background noise? If the antenna is omnidirectional and its structure length is determined based on the expected resonant frequency, will it sense and reflect electromagnetic waves from all directions and unexpected frequencies? What impact would this have on the receiver?
Therefore, I cautiously believe that discussing antenna receiving performance based solely on transmission parameters in an uncontrolled and unknown state is lacking a proper benchmark.

BI1XJT

BH5UWL Radio waves are alternating current (AC), and AC has the characteristic that it is present whether being transmitted or received. The only difference between transmission and reception is the phase. Surely, there's no difference between the neutral wire and the hot wire? I think that concepts like reflection, main lobe, gain, etc., apply to both transmission and reception.

BI1OHW

BI1XJT If you carefully consider the single-element antenna, its reflection can be equivalent to changing the amplitude and phase at the port, resulting in a constant value. However, if you use an antenna array with transmission lines to control the phase difference, it may not work well because standing waves are present in the feed line, which causes the input/output of each element to no longer have the same amplitude, and the radiation pattern will directly split.
However, mismatches in input and output will also affect the RF circuit itself. For example, if the load impedance is not matched to the antenna, it may self-oscillate. If the source impedance is not matched, the noise figure will deteriorate. Therefore, proper matching is still essential.

BH5UWL

Based on the discussions and corrections from colleagues above, I realized that the core issue of this post is the impact of antenna impedance mismatch on a mineral radio receiver.

BG5TOX

Impedance matching is impedance matching. Resonance is resonance. Resonance does not necessarily imply matching, and matching does not necessarily imply resonance.

BI1OHW

BG5TOX Yes, after careful consideration, I realized that reflected current only changes the amplitude and phase at the antenna port; the antenna itself remains unchanged.

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