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?

BH2VZC

In various groups, new members (and even experienced users) often ask about antenna issues for receivers. Here are some opinions from forum members on this topic.

For example, a typical question might be:

"I want to listen to other people communicating using a shortwave radio. Is it possible to use random wires for reception, and will that affect the quality?"
"What kind of antenna is best for SDR?"
And so on.

BG8AMG

I don't think it's necessary. The useful signal is attenuated, and the interfering signals are also attenuated. As long as the signal-to-noise ratio is sufficient, you can still receive it. RBN decoding CW, receivers often use single-band antennas to receive across the entire band, which has very good results.

BG4TRF

I set up two antenna systems, using a non-resonant antenna for reception. The difference in performance between the resonant and non-resonant antennas was very clear.

SuoYuC

The issue seems to be caused by insufficient length. Is it really necessary to increase the wavelength by a factor of 1.5?

BI1OHW

SuoYuC 长度影响的是电长度，电长度正比于天线增益。一般情况下匹配不匹配影响的是能量传输效率。
如果天线是简单的线天线，并且工作在短波、V段等低频，那可以认为这两个维度是正交的，一个好可以抵消另一个差的影响，但最好肯定是两个都好。
如果天线本身结构比较复杂而且频率较高（不能视为集总电路），那么反射会影响天线上的电流分布，反过来影响天线增益和性能。
综上，要达到标称性能肯定是要追求匹配的。不匹配可能因为主瓣偶然对准变得更好，但更可能因为反射和方向图劣化收到更多噪声。

SuoYuC

Oh, and there's also a resistor added at the end, which doesn't emit anything.

yitom

It will get better.

VR2WBZ-BD7PTZ

I'm renting a room. Could you recommend which antenna is best to use with the "Xiao K" radio, and have you seen other ham radio operators using antennas similar to this one (I don't know the name)?
It's not a donut; the iron ring has a diameter of approximately 15 cm.

BloodBlade

When using a pull-out cable, the signal strength is actually higher at the resonant length, rather than with longer cables. I also heard that even when just receiving, if you adjust the antenna frequency after setting it up, the reception quality will be better.

VR2WBZ-BD7PTZ Based on its appearance, it seems to be a small loop antenna with an amplifier. There are many antennas of this shape available on Taobao. You can search using the keywords "active loop antenna" and compare their appearances.

BH2VZC

VR2WBZ-BD7PTZ
元芳 K180, or any BG7YZF active ring.
I don't recommend buying it individually...

BI1LWR

VR2WBZ-BD7PTZ
I bought a similar one and placed it on my balcony to receive SDR signals. It performs better than a simple dipole antenna, and it's directional.
This is a solution for situations where space is limited. If there is more space available, larger antennas are preferable, as they can both receive and transmit signals.

VR2WBZ-BD7PTZ

BH2VZC Is it Taobao?

BH2VZC

Take a swipe at me with a Luo Yang shovel.

When transmitting, a radio (typically with an internal impedance of 50 ohms) should usually be connected to an antenna and feedline that provide proper impedance matching (i.e., characteristic impedance between 50 and 50 ohms). Similarly, for receiving and transmitting, the signal source and receiver sections are simply reversed, with the antenna acting as the signal source in the receiving scenario.

Radios are typically high-impedance receivers, which can be adapted to any length of dipole antenna. Therefore, there is no need to consider impedance matching.
If a radio has a high impedance receiver port (such as the Kenwood TS-2000), you can simply extend the cable directly through this port without needing to match impedances. It seems reasonable to consider common radios as high-impedance receivers.

BH2VZC

BH2VZC
Regarding situations where radio signals are received:
If there is a mismatch in impedance between the antenna and the radio, some of the signal will be reflected back to the antenna and re-transmitted.
If it is possible to dynamically adjust the impedance at the radio end, for example, to vary the impedance between 50 ohms and a short circuit (as shown in the figure), then the signal reflected back from the antenna end would be a CW CQ.

BH2VZC

Meanwhile, amateur radio enthusiasts using a "M" head receiver to listen to broadcasts fall into the following categories:

I can hear the broadcast very clearly (which is great).
I can't hear the radio clearly.
If the weak signal is due to the signal being transmitted from the broadcast to the antenna, and some of the energy is lost due to impedance mismatch when entering the radio end, the signal strength reaching the radio may be lower than the sensitivity. Improving the impedance matching will improve the effect.
If the broadcast signal is strong but reception quality is poor, it could be due to inadequate external shielding on the device, which makes it susceptible to interference from strong signals near the antenna's resonant point. Using an antenna designed for the broadcast band would improve the effect.

It might also be possible to try a "hacky" method of connecting the antenna directly to random wires, using the radio's M-head as the connection point.

BI1XJT

BH2VZC 感觉很鸡肋，如果那个上面有信号，反射的信号势必比上面的信号弱，即便“理论”上有微弱信号被反射回去，也会被埋没在那个强信号/本底噪音中。

BD6MM

Definitely, impedance matching is essential.

BH5UWL

Because a pure SWL receiver is in a passive state, there isn't a direct and obvious measurement to assess the antenna's impact on receiving performance. Therefore, we can only use the principle of reciprocity to infer that if an antenna is adjusted for optimal transmission, it will also perform well in reception.
If it's a VHF/UHF handheld transceiver, which already has transmitting capabilities, then adjusting the antenna will produce immediate results.
However, when using HF, especially with pure SWL (Shortwave Listening) reception, the uncertainty arises due to factors such as: the structure of the transmitting antenna, unclear polarization direction, and complex long-distance multi-path propagation caused by ionospheric skywave reflection. These factors make it impossible to quantitatively compare the received signals.
Personally, I believe that the height at which the receiving antenna is placed (away from urban buildings and low-level electromagnetic smog), the full-wavelength size structure (for capturing weak signals), and directional gain are more important than having the receiving and transmitting antennas have the same structure. Impedance matching is only necessary as a safety condition to prevent damage to the output power component.

BA7NNC-JOE

Anonymous8 Yes, I've experienced that too. I was completely confused for a while before finally posting on the forum to ask for help.

When the antenna resonated at 14 MHz, the inherent noise was too high, making it difficult to receive many signals. Instead, I switched to a non-resonant frequency of 21 MHz, where the inherent noise was lower and I could successfully receive signals.

I tried adjusting the antenna to 21 MHz, but the noise level actually increased. This made it difficult to receive. However, when I switched to 14 MHz, the noise level decreased, and I was able to clearly hear some of the audio that I hadn't been able to pick up at 14 MHz.

Therefore, if I were to simply answer based on whether it's "yes" or "no," then my answer would be "Yes and No." 🤪

BI1XJT

BA7NNC-JOE I believe that many people mistakenly think that "resonance" leads to better reception, but this is only a superficial effect. The change in the length of GP/DP antennas doesn't just affect whether they resonate or not; it also changes their radiation pattern. For example, using a 1/4 wavelength GP and pulling it up to 1/4 wavelength may seem to result in stronger received signals, but this is only because the effective receiving angle has become lower, leading to more noise being received. (Furthermore, a 5/8 wavelength GP is even less resonant, with an even lower receiving angle and impedance that is not 50 ohms, but it still receives DX signals without impedance matching.)

Furthermore, standard GP and DP impedances are not always 50 ohms, and neither are those of QUAD or direct feed systems. However, I have never personally encountered a situation where signals received without an unun/balun were significantly worse.

In conclusion, impedance matching is not necessary for the receiver.

BH5UWL

Press down on the flask to raise the bucket. Different resonant points may result in a better signal-to-noise ratio for a particular signal, or stronger intermodulation/multi-channel/ground noise interference in a specific frequency band. Changes in the resonant point can be due to changes in the length of the antenna structure, changes in the height of the feed point relative to ground, or impedance matching. The resonant point is simply an indicator of the static electrical symmetry of the antenna's structure, not an indicator of transmission efficiency, nor an indicator of propagation and reception effects in the far-field region.

BI1XJT

Another possibility to consider is that if we use a direct feed without an unun (because the wavelength is an even multiple), the impedance will appear extremely high. What would happen when using a direct feed for reception?

Assuming the SWR at that point is 38.2, this indicates a significant impedance mismatch. If you transmit 10W at this SWR, 9W will be reflected back. However, what if it's just receiving? The preamplifier can provide up to 10dB of gain, so if the input is 1(u)W, the preamplifier can amplify it to 10(u)W. Therefore, a feedline connected to the preamplifier can effectively solve the impedance mismatch problem.

As anyone who frequently uses shortwave communication knows, in many cases, the pre-amplifier doesn't need to be turned on at all, and sometimes a pre-amplifier is even used. Therefore, antennas with impedance mismatch can also perform receiving work perfectly well without a pre-amplifier.

BI1OHW

BI1XJT 阻抗不匹配会影响天线的电流分布，进而影响方向图。如果是八木的话旁瓣可能劣化的厉害，导致底噪升高，这种是前放抵抗不了的

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