外国天文科普现状：雷达天文学简史

外国天文科普现状：雷达天文学简史发射和接收器也都是军队已有的。他们选择了埃德温·H·阿姆斯特朗（Edwin H. Armstrong，跟登月那人没亲属关系）为通讯兵部队设计的晶体发射和接收器，因为晶体能够提供他们需要的稳定性 (7)。他们使用了112兆赫的频率，但发射功率只有3000瓦，与今天的行星雷达相比就跟一个电动玩具差不多 (Curtis par. 5)。回波被接收到时会有视觉和听觉的双重提示：一个9英寸的阴极射线管和一个180赫兹的嘟嘟声 (Butrica 7)。虽然有军方的财力支持，但他们并没有打算设计什么专用的仪器设备。天线是从SCR-271雷达上拆下来的。这个32偶极天线被架在一个三十米高塔上，这个高塔只能横向转动，因为他们很难弄到更好的东西。这意味着他们只能在月出和月落时进行实验 (7)。德威特从小就喜欢玩无线电。1921年，他成为了一名业余无线电操作员，那时他才十五岁，第二年他就建立了纳什维尔的第一个电

雷达天文学简史（一）：两位开山鼻祖，各有独门绝技

在科学探索的道路上，有时会出现两位先行者，各自独立做出开创性的成果，例如牛顿和莱布尼茨各自创立了微积分，达尔文和华莱士各自提出进化论等等。雷达天文学的滥觞时期也是有两位开路先锋，他们是美国的草莽英雄小约翰·H·德威特（John H. DeWitt Jr.）和匈牙利的名门俊杰佐尔坦·拉霍斯·贝（Zoltán Lajos Bay）。

德威特与黛安娜计划

雷达天文学起步于二战结束时，最早是军方组织的。在那个年代，军方不但有钱，还有大量战争时生产的多余雷达。第一个雷达天文学计划名为"黛安娜计划"，领导者是小约翰·H·德威特。

图一：小约翰·H·德威特。

德威特从小就喜欢玩无线电。1921年，他成为了一名业余无线电操作员，那时他才十五岁，第二年他就建立了纳什维尔的第一个电台 (Curtis par. 2)。

1940年，德威特在WSM电台做总工程师。下班时间，除了用自制的射电望远镜收听银河系的电磁噪音以外，他还经常胡思乱想。一天，他突发奇想，打算用从月球上反射回来的无线电波来研究大气层。那年5月20号一个月黑风高的夜晚，他用电台的80瓦发射机尝试用月球反射138兆赫（波长2米）的无线电波，但由于接收机不够灵敏而失败。1942年，他跳槽到贝尔实验室为海军设计雷达，之后便入伍，加入美国陆军通信兵部队 (Butrica 6-7)。在部队中，他还研制了针对迫击炮的反炮兵雷达 (Curtis par. 2)。

参军后，他一直对自己的计划念念不忘。1945年，战争一结束他便开始行动。作为埃文斯信号实验室（Evans Signal Laboratory）的主管 (Butrica 7)，德威特九月份就组织起了团队（估计之前就一直在蠢蠢欲动），在新泽西州蒙矛斯堡（Fort Monmouth）开始研究。这个计划的名字叫"黛安娜计划"，以罗马神话中的月亮女神黛安娜命名 (7)。

虽然有军方的财力支持，但他们并没有打算设计什么专用的仪器设备。天线是从SCR-271雷达上拆下来的。这个32偶极天线被架在一个三十米高塔上，这个高塔只能横向转动，因为他们很难弄到更好的东西。这意味着他们只能在月出和月落时进行实验 (7)。

发射和接收器也都是军队已有的。他们选择了埃德温·H·阿姆斯特朗（Edwin H. Armstrong，跟登月那人没亲属关系）为通讯兵部队设计的晶体发射和接收器，因为晶体能够提供他们需要的稳定性 (7)。他们使用了112兆赫的频率，但发射功率只有3000瓦，与今天的行星雷达相比就跟一个电动玩具差不多 (Curtis par. 5)。回波被接收到时会有视觉和听觉的双重提示：一个9英寸的阴极射线管和一个180赫兹的嘟嘟声 (Butrica 7)。

图二：SCR-271雷达的"弹簧床"天线。

多普勒效应使得返回与发射信号之间的频率差距能高达300赫兹 (Butrica 7)，但德威特的团队依然选择使用窄带宽接收器。德威特后来回忆道："我们意识到从月球返回的信号会非常弱，所以我们必须使用一个带宽很窄的接收器，这样才能将杂音降到可以忍受的地步……我们每次都必须将接收器调到一个与发射信号稍稍不同的频道——这是地球自转以及月球的视向速度造成的多普勒效应导致的" (7)。稍后，我们就会看到其他团队不使用窄带宽的结果。

经过持续的努力以及崩溃（仪器崩溃，人也崩溃），1946年1月10日早上11点48分，德威特的团队开始向刚刚升起的月球传送信号 (9)。

在无尽的白噪音中，他们等待的每一秒钟都仿佛是一个永恒。

终于，在11点58分，一个清晰的响声伴随着示波器一个微小的波峰出现。他们成功了。12点09分，实验结束。从新泽西州到月球走一个来回，无线电信号整整花了2.5秒 (9)。讽刺的是，德威特当时不在场。后来他说："当时我在贝尔马吃午饭以及在药店拿一些像香烟之类的东西（1952年戒烟了，感谢上帝）" (9)。

（这说明不戒烟的话容易错过历史性时刻。是不是这个道理？）

战争部一直等到24号才正式宣布这次实验的成功。在此之前，德威特的团队与研究开发主管范·杜森将军（General Van Deusen）有了点争执。范·杜森将军坚持要在外人确认之后才发布这次成功，以避免闹乌龙。于是两名辐射实验室的科学家和范·杜森将军一起见证了一次月出实验 (9)。

一切准备就绪，仪器状态良好。在德威特的得力干将，金·斯托多拉的指示下，他们开始向月球发射信号 (9)。如果没有差错，他们三秒钟后就将收到从月球返回的回波。

然而，什么都没有发生。

图三：德威特后来表示："你们可以想象，我当时感觉要死了" (Butrica 9)。

然后，奇迹就发生了。一辆大货车经过，回波立马就被检测到了。几乎所有人都立即欢呼起来，只有范·杜森将军尽量装出一副很高兴的样子 (9)。这是雷达天文学以及地-月-地通讯（Earth-Moon-Earth communication）的起源 (Curtis par. 7)。

后来，德威特在通讯兵部队领导人的要求下改去做导弹预警雷达了。因为没有可用于测试的导弹，月球便成了替代品。几年后，通信兵部队又为黛安娜计划建造了一个新的系统，包括一个15米直径的天线和108兆赫的发射器。这套系统继续进行月球回声研究并参与追踪阿波罗计划的飞船 (Butrica 9)。

流离失所的佐尔坦·贝

在德威特完成他的实验后不到一个月，一个匈牙利团队也成功地完成了这一壮举。 它的领导者叫做佐尔坦·拉霍斯·贝。

图四：佐尔坦·拉霍斯·贝。

德威特的最高学历只有本科，但贝可是以优异的成绩在1926年从布达佩斯大学拿到物理博士学位，是学霸中的学霸。在柏林的几个大学和研究所工作了一段时间后，贝终于成为了匈牙利的塞格德大学（University of Szeged）的理论物理学主席（我猜可能相当于物理系主任）。之后又受邀到通斯拉姆公司（Tungsram，当年是全球第三大灯泡生产商），成为了那里的工业实验室负责人。(Butrica 10)

二战末期，贝对电离层很感兴趣。根据当时的假说，短波无线电波能够穿过电离层，而不是像大部分的无线电波一样被电离层反射。然而，还没有人验证过这一假说 (Bay 1)。这时，贝就想到用月球作为一个反射面，看看短波无线电波能不能穿过电离层并被反射回来。

贝使用的仪器参数与德威特非常相似。他们都使用了3000瓦功率的发射器，频率也几乎一样 (Bay 3; Curtis par. 5)。他们也面对着同一个问题：噪音。

由于从月球反射回来的信号实在太弱，声噪比就会十分夸张。根据贝的计算，如果不做任何改善，声噪比η=3.9·10-4 (Bay 4)，也就是说，噪音是声音的三万九千倍！这就好像要在飞机起飞的噪音下听清跑道上一只蚊子的嗡嗡声。

德威特的解决方案是缩窄带宽，这样会接收到更少的噪音 (DeWitt 233)（在无线电中，带宽是指信号的频率范围。由于天气、目标材质或仪器精度等问题，信号频率可能会出现偏差。为了确保接收到信号，接收器便会接收更大的频率范围）。贝也知道这一点。在他的计算中，如果缩窄带宽的话，声噪比η能一下子提高到3.6（也就是说声音比噪音要大3.6倍）(Bay 5)，听到信号简直轻而易举。

然而，德威特能够缩窄带宽是因为他在发射和接收器中都使用了晶体，这能够很有效地保证信号频率的稳定 (DeWitt 233)。但是，贝以及他的团队没有这种设备。

你以为贝会放弃？不存在的！贝可是名优秀的物理学家，而且动手能力极强。他用一种天才的办法解决了这一问题，而他的解决方案在很多领域一直沿用至今。

图五：贝的装置。

贝使用了几个库伦表（又称电量计）。有电流通过时，这些库伦表会将自己管中的水电解，在图五所示的小管子里累积氢气，这些氢气就表示通过这一表的电荷量。这些库伦表被连接在一个旋转开关上，这开关就会轮流给每个库伦表通电，3秒轮回一次。这些电就是接收器接收到的无线电信号(Butrica 10-11)。

由于极低的声噪比，每个库伦表接收到的电荷量几乎一样，主要都是噪声。然而，由于信号的发射是周期性的，信号每次返回时都会通到同一个库伦表上。久而久之，这个库伦表中就会记录下更多的电荷，贝的团队就可以从这里得知他们的确是收到从月球上反射回来的信号了 (Butrica 11; Harrison par. 1)。

这是一个天才的主意。一直到现在，贝的这种方法依然是雷达天文学中的一个重要手段，是之后雷达探测金星和其他天体的坚实基础 (Butrica 12)。然而，再好的主意也需要实施的机会。二战时期，匈牙利被纳粹德国占领。到战争末期，德国节节败退时，布达佩斯不断地被盟军轰炸，之后又发生了激烈的布达佩斯包围战，贝被迫随着通斯拉姆公司的实验室撤离到郊外 (11)。

直到二战结束，贝才得以实施他的计划。通斯拉姆的实验室太吵，干扰严重，所以贝的团队只能在黄昏和晚上进行实验（不知道发不发加班费）(11)。如果没有战争的干扰，贝很有可能在德威特之前成为第一个用月球反射无线电波的人。

德威特和贝都是雷达天文学的先行者。与他们同时期还有许多别的团队也在使用雷达进行别的研究，如研究流星对大气的电离 (12-13)。然而，当时他们并没有掀起什么波澜。根据德威特的成果，战争部预言以后会有"对月球和其他行星的精确地图绘制、对电离层的测量和分析、以及从地面用无线电控制在平流层以上环绕地球的太空船和喷气或火箭发动机控制的导弹" (9)。然而，《新闻周刊》（对，就叫这个，英文是Newsweek）称这简直是凡尔纳的幻想 (9)。雷达天文学真正起飞还要等到1958年后。

我们下次再见。

——刘腾骏

参考资料：

Bay Z. Reflection of microwaves from the Moon. Hungarica Acta Physica 1 1–22 (1947). https://doi.org/10.1007/BF03161123.

Butrica Andrew J. “To See the Unseen—A History of Planetary Radar Astronomy.” NASA History Office 1996 https://history.nasa.gov/SP-4218/sp4218.htm.

Curtis Anthony R. “Space&Beyond: Moonbounce Advances the State of the Radio Art.” ARRLWeb: Space&Beyond: Moonbounce Advances the State of the Radio Art 21 Jan. 2002 web.archive.org/web/20071025022111/www2.arrl.org/news/features/2002/01/21/1/.

DeWitt John H and E. K. Stodola. “Detection of Radio Signals Reflected from the Moon.” Proceedings of the IRE vol. 37 no. 3 Mar. 1949 pp. 229–242. doi:10.1109/jrproc.1949.231276.

Harrison Todd. “RETROTECHTACULAR: [ZOLTÁN BAY’S] MOON BOUNCE COULOMETER SIGNAL AMPLIFIER.” Hackaday 19 Nov. 2013 hackaday.com/2013/11/19/retrotechtacular-zoltan-bays-moon-bounce-coulometer-signal-amplifier/.

图片：

图一：Mofenson Jack. “Lt. Colonel John H. DeWitt Jr. in Charge of the Project a Modified Version of the SCR-271 Early-Warning Radar Used at Pearl Harbor. DeWitt Is the Former Chief Engineer of WSM.” Radar Echoes From the Moon Belmar New Jersey Jan. 1946 web.archive.org/web/20081029000712/http://www.eagle.ca/~harry/ba/eme/index.htm.

图二：Butrica Andrew J. “To See the Unseen—A History of Planetary Radar Astronomy.” NASA History Office 1996 https://history.nasa.gov/SP-4218/sp4218.htm.

图三：AI001. "社会性死亡"是什么梗 小编和大家一样对这个梗充满了好奇心. 17qq.com/qqtouxiang/1999376_p2.html.

图四："File:Zoltán Bay (1900-1992) Hungarian physicist.jpg." Wikimedia Commons the free media repository. 31 Oct 2020 05:47 UTC. 6 Jan 2021 14:26 <https://commons.wikimedia.org/w/index.php?title=File:Zoltán_Bay_(1900-1992)_Hungarian_physicist.jpg&oldid=508227919>.

图五：Bay Z. Reflection of microwaves from the Moon. Hungarica Acta Physica 1 1–22 (1947). https://doi.org/10.1007/BF03161123.

The history of Radar Astronomy (I): De la Terre à la Lune trajet et retour direct en 2.5 secondes

On the path of scientific discovery we occasionally see not one but a pair of pioneers each independently producing groundbreaking results. We saw Newton and Leibniz independently developing calculus while Darwin and Wallace each separately created their theory of evolution. In the early years of radar astronomy we also saw a pair of trailblazers. One was the American engineer and astronomer John H. DeWitt Jr. and the other was Hungarian physicist Zoltán Lajos Bay.

DeWitt and Project Diana

Radar astronomy can trace its origins back to the end of the Second World War. It was organised by the military because the military not only had adequate funding but also large amounts of surplus radars produced during the war. The first radar astronomy project was named "Project Diana". Its leader was John H. DeWitt Jr.

Fig. 1: John H. DeWitt Jr.

DeWitt had been playing with the radio since childhood. In 1921 he became an amateur radio operator at the age of fifteen. He built Nashville's first radio station the next year (Curtis par. 2).

In 1940 DeWitt was the chief engineer for the radio station WSM. In his free time apart from listening to radio noise from the Milky Way using a radio telescope he made himself he also came up with lots of wild ideas. One day DeWitt had a new idea: using radio waves reflected off the moon to investigate the atmosphere. That same year on the 20th of May he used the radio station's 80-watt transmitter to reflect a 138 megahertz (2-metre wavelength) off the moon but his receiver wasn't sensitive enough. In 1942 DeWitt joined Bell Labs to design radars for the navy. He was later commissioned in the United States Signal Corps (Butrica 6-7). While there he designed counter-battery radar for mortars (Curtis par. 2).

DeWitt never forgot his plan. When the war ended in 1945 he took action. As the director of the Evans Signal Laboratory (Butrica 7) DeWitt assembled a team by September (he probably had been secretly plotting for a while) and began research at Fort Monmouth in New Jersey. The project was called "Project Diana" named after the Roman moon goddess Diana (7).

Although they had military backing DeWitt's team did not use any purpose-built equipment. Their aerial was from an SCR-271 radar. This 32-dipole antenna was mounted on a 30-metre tower. The tower could only turn sideways as it was too difficult to acquire anything better. This meant that they could only experiment at moonrise and moonset (7).

The military had ready-made transmitters and receivers. They chose a crystal transmitter and receiver which Edwin H. Armstrong designed for the Signal Corps as the crystal provides the stability they needed (7). They chose a frequency of 112 megahertz but their transmitting power was only 3000 watts toys compared to today's planetary radars (Curtis par. 5). Reception of signals was signified by both visual and audial notifications: a 9-inch cathode ray tube and a 180-hertz beep (Butrica 7).

Fig. 2: SCR-271 radar's "spring bed" antenna.

The Doppler effect caused differences in frequency between the transmitted and received signal to be as much as 300 hertz (Butrica 7) but DeWitt's team still decided to use a narrow bandwidth receiver. DeWitt later recalled: "We realized that the moon echoes would be very weak so we had to use a very narrow receiver bandwidth to reduce thermal noise to tolerable levels… We had to tune the receiver each time for a slightly different frequency from that sent out because of the Doppler shift due to the earth's rotation and the radial velocity of the moon at the time" (7). We shall soon see the result of other groups not using a narrow bandwidth in such experiments.

After continuous effort and breakdowns (possibly both technical and mental) at 11:48 a.m. on the 10th of January 1946 DeWitt's team began transmitting signals to the rising moon (9).

Sitting in the endless sea of white noise every second they waited was an eternity.

Finally at 11:58 a crisp beep accompanied by a minuscule but distinct peak appeared. They succeeded. At 12:09 the experiment ended. A round trip from New Jersey to the moon and back took the radio signal 2.5 seconds (9). Ironically DeWitt wasn't present. He later said: "I was over in Belmar having lunch and picking up some items like cigarettes at the drug store (stopped smoking 1952 thank God)" (9).

(This shows that if you don't quit smoking you might miss historic events. Get it?)

The War Department withheld the news of success until the 24th. Before that DeWitt and his team had a bit of trouble with the head of R&D General Van Deusen. General Van Deusen insisted that the results should be checked by outsiders before being announced so two scientists from the Radiation Laboratory and General Van Deusen observed a moonrise experiment (9).

Everything was ready. Under the instruction of DeWitt's best employee King Stodola (who is not the king of anywhere except the realm of science) the team began transmitting signals to the moon (9). If nothing went wrong they should start to hear echoes after about 3 seconds.

However nothing happened.

DeWitt later said: "You can imagine that at this point I was dying" (Butrica 9).

Then a miracle. A big truck passed by and the echoes immediately popped up. Almost everyone cheered; only General Van Deusen tried his best to look pleased (9). This was the very beginning of radar astronomy and Earth-Moon-Earth communication (Curtis par. 7).

Later DeWitt was ordered to develop missile warning radars. Because there weren't any missiles available for testing the moon became a substitute. Several years later the Signal Corps built a new system for Project Diana including a 15-metre diameter antenna and a 108 megahertz transmitter. This system continued to conduct moon echo research and took part in tracking the Apollo spacecraft (Butrica 9).

Zoltán Bay: the scientist on the move

Less than a month after DeWitt concluded his experiment a Hungarian team replicated his results. The team's leader was Zoltán Lajos Bay.

Fig. 3: Zoltán Lajos Bay.

DeWitt only had a bachelor's degree but Bay graduated with highest honours from the University of Budapest with a PhD in physics in 1926. After working at several universities and research institutes in Berlin Bay finally became the Chair of Theoretical Physics at the University of Szeged. Later he was invited by Tungsram (the then third-largest producer of lightbulbs in the world) to head Tungsram's industrial laboratory (Butrica 10).

In the last stages of World War Two Bay became interested in the ionosphere. According to hypothesis short wave radio waves should be able to penetrate the ionosphere unlike most radio waves which were reflected. However no one had been able to verify the hypothesis (Bay 1). Bay realised that he could use the moon as a reflector to see if short wave radio waves could pass through the ionosphere and be reflected.

The equipment Bay used was very similar to those used by DeWitt. They both used a 3000-watt transmitter and almost the same frequency (Bay 3; Curtis par. 5). They also faced the same problem: noise.

Because the echoes returning from the moon were too weak the signal-to-noise ratio was terrible. According to Bay's calculations if nothing was done to improve the ratio it would be equal to 3.9·10^-4 (Bay 4). In other words the noise would be thirty-nine thousand times louder than the echo! This is like trying to hear the buzzing of a mosquito on a runway where a plane is taking-off.

DeWitt's solution was to narrow the bandwidth so that they received less noise (DeWitt 233) (in signal processing bandwidth refers to the range of frequencies a signal contains. Due to issues such as the weather target material or equipment precision the signal's frequency may shift. To ensure the reception of the signal the receiver usually tries to receive a larger number of bandwidths). Bay also knew this. In his calculations he found that if he narrowed the bandwidth the signal-to-noise ratio would instantly rise to 3.6 (or in other words the signal would be 3.6 times louder than the noise) (Bay 5). Hearing the echoes would be a piece of cake.

However the reason DeWitt was able to narrow his bandwidth was that he used crystals in both his transmitter and receiver which ensured the stability of the signal (DeWitt 233). But Bay and his team did not have such equipment.

You thought Bay would give up? Of course not! Bay was a brilliant physicist with an equally brilliant hands-on ability. He created a genius solution one which was used to this day.

Fig. 4: Bay's apparatus.

Bay used several coulometers. When a current passed through these coulometers electrolysed the water in their tube causing hydrogen to accumulate in the small tubes shown in Fig. 4. The hydrogen indicated the amount of electric charge which passed through the coulometer. These coulometers were connected to a rotating switch. The switch passed electric current alternately into each of the coulometers finishing every sweep in 3 seconds. The current came from the radio signals received by the receiver (Butrica 10-11).

Due to the extremely low signal-to-noise ratio every coulometer received almost the same amount of electric charge mostly noise. However because the signals were sent periodically the received echo will pass through the same coulometer every time. After a while this coulometer will record more electric charge allowing Bay's team to know that they had received signals from the moon (Butrica 11; Harrison par. 1).

This was a stroke of genius. Even today Bay's method is still an important tool in radar astronomy. It was the foundation for the radar imaging of Venus and other planets (Butrica 12). Nevertheless even the most brilliant of ideas need a chance to be applied. During World War Two Hungary was occupied by Nazi Germany. In the last stages of the war while Germany was on the retreat Budapest was constantly bombed by the Allies. Later the fierce Siege of Budapest occurred. Bay was forced to move into the countryside with Tungsram's industrial laboratory (11).

Only until the war ended when Bay was able to carry out his plan. Tungsram's laboratory was too noisy. The interference was so bad that Bay's team had to do measurements at dusk or at night (I wonder if they received overtime pay) (11). Without the war Bay could have become the first person to bounce radio waves off the moon.

DeWitt and Bay were both pioneers in radar astronomy. Many other teams also used radar in research like investigating the ionisation of the atmosphere by meteors (12-13). That said there was no revolution around the world. According to DeWitt's results the War Department predicted that there would be "the accurate topographical mapping of the Moon and planets measurement and analysis of the ionosphere and radio control from Earth of 'space ships' and 'jet or rocket-controlled missiles circling the Earth above the stratosphere'" (9). However Newsweek called these predictions "worthy of Jules Verne" (9). Radar astronomy would only take off after 1958.

See you next time.

——Tengjun Liu

Citations:

Bay Z. Reflection of microwaves from the Moon. Hungarica Acta Physica 1 1–22 (1947). https://doi.org/10.1007/BF03161123.

Butrica Andrew J. “To See the Unseen—A History of Planetary Radar Astronomy.” NASA History Office 1996 https://history.nasa.gov/SP-4218/sp4218.htm.

Curtis Anthony R. “Space&Beyond: Moonbounce Advances the State of the Radio Art.” ARRLWeb: Space&Beyond: Moonbounce Advances the State of the Radio Art 21 Jan. 2002 web.archive.org/web/20071025022111/www2.arrl.org/news/features/2002/01/21/1/.

DeWitt John H and E. K. Stodola. “Detection of Radio Signals Reflected from the Moon.” Proceedings of the IRE vol. 37 no. 3 Mar. 1949 pp. 229–242. doi:10.1109/jrproc.1949.231276.

Harrison Todd. “RETROTECHTACULAR: [ZOLTÁN BAY’S] MOON BOUNCE COULOMETER SIGNAL AMPLIFIER.” Hackaday 19 Nov. 2013 hackaday.com/2013/11/19/retrotechtacular-zoltan-bays-moon-bounce-coulometer-signal-amplifier/.

Images:

Fig. 1: Mofenson Jack. “Lt. Colonel John H. DeWitt Jr. in Charge of the Project a Modified Version of the SCR-271 Early-Warning Radar Used at Pearl Harbor. DeWitt Is the Former Chief Engineer of WSM.” Radar Echoes From the Moon Belmar New Jersey Jan. 1946 web.archive.org/web/20081029000712/http://www.eagle.ca/~harry/ba/eme/index.htm.

Fig. 2: Butrica Andrew J. “To See the Unseen—A History of Planetary Radar Astronomy.” NASA History Office 1996 https://history.nasa.gov/SP-4218/sp4218.htm.

Fig. 3: "File:Zoltán Bay (1900-1992) Hungarian physicist.jpg." Wikimedia Commons the free media repository. 31 Oct 2020 05:47 UTC. 6 Jan 2021 14:26 <https://commons.wikimedia.org/w/index.php?title=File:Zoltán_Bay_(1900-1992)_Hungarian_physicist.jpg&oldid=508227919>.

Fig. 4: Bay Z. Reflection of microwaves from the Moon. Hungarica Acta Physica 1 1–22 (1947). https://doi.org/10.1007/BF03161123.