Author Topic: LM didn't have enough power.  (Read 1116 times)

Offline ka9q

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Re: LM didn't have enough power.
« Reply #15 on: February 10, 2019, 12:49:24 AM »
That's right, the link budget for video was much more demanding than for voice. The LRV had a medium gain antenna (a white rod) that only had to be pointed somewhere near earth to carry voice while traveling. Video required the astronauts to park and manually sight the high gain antenna (the gold-colored dish) on the earth. The "videos" from the LRV in motion were taken with a 16mm movie film camera.

Apollo 16's high gain antenna on the LM failed, but fortunately they had the LRV for all surface video.

 

Offline Allan F

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Re: LM didn't have enough power.
« Reply #16 on: February 10, 2019, 09:02:05 PM »
Weren't there some intermittent video recorded during LRV traverses? Like when the high gain antenna got close enough to the right direction to connect?
Well, it is like this: The truth doesn't need insults. Insults are the refuge of a darkened mind, a mind that refuses to open and see. Foul language can't outcompete knowledge. And knowledge is the result of education. Education is the result of the wish to know more, not less.

Online raven

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Re: LM didn't have enough power.
« Reply #17 on: February 10, 2019, 09:43:10 PM »
Weren't there some intermittent video recorded during LRV traverses? Like when the high gain antenna got close enough to the right direction to connect?
Yes. It looks like video when you finagle around with a pair of rabbit ears to try and get the TV signal to look its best in a snow storm, but there is video. My Google-fu seems to be failing however.
 Most of the stuff taken with the LRV in motion is with the 16MM DAC though.

Offline ka9q

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Re: LM didn't have enough power.
« Reply #18 on: February 11, 2019, 06:53:26 AM »
I found one of the link budgets I posted years ago. It's still on my website:

http://www.ka9q.net/Apollo/Apollo_Link_Budgets.xls

Offline bknight

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Re: LM didn't have enough power.
« Reply #19 on: February 11, 2019, 08:33:38 AM »
I found one of the link budgets I posted years ago. It's still on my website:

http://www.ka9q.net/Apollo/Apollo_Link_Budgets.xls

2282500000 Hz?  I'm not anything like an electrical, but I did take some electronics courses 50 years ago.  This number seems high, but maybe I was remembering rudimentary black boxes.

Thanks for the sheet.
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Offline Glom

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Re: LM didn't have enough power.
« Reply #20 on: February 11, 2019, 08:35:27 AM »
That's microwave isn't it?

Offline Glom

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Re: LM didn't have enough power.
« Reply #21 on: February 11, 2019, 08:36:59 AM »
No that's nonsense. It's UHF.

Offline VQ

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Re: LM didn't have enough power.
« Reply #22 on: February 11, 2019, 11:10:51 AM »
No that's nonsense. It's UHF.

It's both. Microwave = 300 MHz to 300 GHz; UHF = 300 MHz to 3 GHz.

Offline bknight

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Re: LM didn't have enough power.
« Reply #23 on: February 11, 2019, 11:38:57 AM »
One aspect of the transmissions that ka9q reminded me of during a discussion with hunchbacked was that VHF range is 30-300 MHz and was able to penetrate the VARB.  hunchbacked was claiming that the signals were UHF that could not penetrate the VARB, poor boy indeed.
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Offline apollo16uvc

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Re: LM didn't have enough power.
« Reply #24 on: February 11, 2019, 12:28:31 PM »
The omnis were only for voice, right? The TV had to go through the HGA?

On such topics, may I recommend the Technology Connections YT channel. Really interesting stuff about antique media technology. Useful basis for understanding how Apollo media tech differed.
TV had to go through HGA yes.

I think Omnis were also for telemetry, for the suit for example.
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Offline ka9q

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Re: LM didn't have enough power.
« Reply #25 on: February 11, 2019, 02:59:11 PM »
VHF and UHF have precise definitions; VHF is always 30-300 MHz (10 to 1 meter wavelengths) and UHF is 300-3000 MHz (1 meter to 10 cm wavelengths).

I don't think "microwave" has ever had a precise definition, but I personally tend to think of it as anything above 1 GHz (or maybe 2 GHz). It certainly includes 2450 MHz, since that's where microwave ovens operate. (It's also used for WiFi, and it's also a ham radio band. When the FCC wrote the rules in the early 1980s that enabled the development of WiFi, they thought 2.4 GHz was "garbage" spectrum. We hams and engineers proved them wrong.)

I think "microwave" has traditionally been distinguished by the different technologies and construction techniques used at those frequencies. Below "microwaves" you generate signals with conventional oscillators, amplify them with transistors or grid-type power vacuum tubes, send them around in coaxial cables and radiate them with single or multi-element antennas. But you generate and amplify "microwaves" with a special class of vacuum tubes including magnetrons, klystrons and traveling wave tubes, send them around in waveguides, and radiate them with dishes and other high gain antennas.

These microwave tubes all exploit the fact that at these high frequencies, electrons take significant time to transit the tube compared to the period of the signal.  At lower frequencies you usually ignore this effect and think of the electrons as infinitely fast.

These distinctions have been blurred quite a bit by advancing technology -- which is why "microwave" has no formal definition that I know of. Klystrons and waveguides are common in UHF TV transmitters operating well below 1 GHz, it's now also common to see conventional amplifier designs (and coaxial cables) well into the "microwave" region, and dishes are often used below 1 GHz if they're big enough. Basically, "microwave" techniques can be used at lower frequencies if they're physically large enough, and "conventional" techniques can be used at microwave if they're physically small enough.
« Last Edit: February 11, 2019, 03:14:46 PM by ka9q »

Offline ka9q

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Re: LM didn't have enough power.
« Reply #26 on: February 11, 2019, 04:50:57 PM »
One aspect of the transmissions that ka9q reminded me of during a discussion with hunchbacked was that VHF range is 30-300 MHz and was able to penetrate the VARB.  hunchbacked was claiming that the signals were UHF that could not penetrate the VARB, poor boy indeed.
Up through the low microwave range, the only obstacle to radio signals leaving the earth is the ionosphere, which is much lower than the VARBs. (Atmospheric absorption, especially by water, becomes significant at higher microwave frequencies.) The ionosphere has several layers, some of which form only in the daytime and others that (sometimes) persist through the night. The layer that enables "shortwave" (HF, 3-30 MHz, 100-10 meters) signals to travel the globe is the "F" layer, at roughly the altitude at which the ISS flies. By our standards it's a pretty hard vacuum, nonetheless there are (sometimes) enough free electrons to reflect signals back to the earth.

But not all signals get reflected. Sufficiently high frequency signals will go right through the ionosphere. The highest frequency that will be reflected depends on the free electron density and the angle the signal hits the ionosphere. Signals arriving at oblique angles have a better chance of being reflected than signals coming straight up from the surface. The highest frequency that will be reflected at a 90 degree incidence angle is called the "critical frequency". It depends directly on the free electron density in the ionosphere, which in turn depends on solar activity. Even at the peak of the sunspot cycle the critical frequency is almost never above the top of the HF band at 30 MHz. Sometimes you can get reflections at much higher frequencies from the ionospheric E layer, but they're rare (and much sought after by hams).

I've never heard of radio signals reflecting off the VARBs, but it's an intriguing idea. The VARBs are composed of charged particles just like the ionosphere, so it stands to reason that they might. There are "ionospheric sounders", special purpose radars that beam chirped or pulsed signals to determine the structure of the ionosphere, and I might scan that literature for any mention of it.  It could be that the charged particles, despite being energetic enough to be a radiation hazard, are simply too sparse to compete with the ionosphere.
« Last Edit: February 11, 2019, 04:54:27 PM by ka9q »

Offline ka9q

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Re: LM didn't have enough power.
« Reply #27 on: February 11, 2019, 05:01:53 PM »
2282500000 Hz?
That's 2282.5 MHz or 2.2825 GHz. It's in the low microwave range that is often referred to as "S-band" -- not far below the 2.4 GHz range used by microwave ovens and WiFi. S-band was and is a very heavily used band for space communications. It's split into two parts, with the upper part used for space-to-earth downlinks (like this one) and the lower part for earth-to-space uplinks. There is usually an exact rational ratio between the two link frequencies equal to 240/221. E.g., for a downlink of 2282.5 MHz, the uplink would be 2282.5 * 221/240 = 2101.8 MHz. The spacecraft carries a "phase coherent transponder" that locks its downlink to exactly 240/221 times the uplink so the ground can do extremely accurate Doppler tracking of relative velocity. It was so exquisitely sensitive that during the Apollo 13 emergency the ground asked the crew to stop dumping urine because it was affecting their Doppler measurements. They didn't bother to tell the crew that the need was only temporary, so they kept doing it for the rest of the mission and were running out of space to store the filled bags.

Offline bknight

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Re: LM didn't have enough power.
« Reply #28 on: February 11, 2019, 05:57:27 PM »
One aspect of the transmissions that ka9q reminded me of during a discussion with hunchbacked was that VHF range is 30-300 MHz and was able to penetrate the VARB.  hunchbacked was claiming that the signals were UHF that could not penetrate the VARB, poor boy indeed.
Up through the low microwave range, the only obstacle to radio signals leaving the earth is the ionosphere, which is much lower than the VARBs. (Atmospheric absorption, especially by water, becomes significant at higher microwave frequencies.) The ionosphere has several layers, some of which form only in the daytime and others that (sometimes) persist through the night. The layer that enables "shortwave" (HF, 3-30 MHz, 100-10 meters) signals to travel the globe is the "F" layer, at roughly the altitude at which the ISS flies. By our standards it's a pretty hard vacuum, nonetheless there are (sometimes) enough free electrons to reflect signals back to the earth.

But not all signals get reflected. Sufficiently high frequency signals will go right through the ionosphere. The highest frequency that will be reflected depends on the free electron density and the angle the signal hits the ionosphere. Signals arriving at oblique angles have a better chance of being reflected than signals coming straight up from the surface. The highest frequency that will be reflected at a 90 degree incidence angle is called the "critical frequency". It depends directly on the free electron density in the ionosphere, which in turn depends on solar activity. Even at the peak of the sunspot cycle the critical frequency is almost never above the top of the HF band at 30 MHz. Sometimes you can get reflections at much higher frequencies from the ionospheric E layer, but they're rare (and much sought after by hams).

I've never heard of radio signals reflecting off the VARBs, but it's an intriguing idea. The VARBs are composed of charged particles just like the ionosphere, so it stands to reason that they might. There are "ionospheric sounders", special purpose radars that beam chirped or pulsed signals to determine the structure of the ionosphere, and I might scan that literature for any mention of it.  It could be that the charged particles, despite being energetic enough to be a radiation hazard, are simply too sparse to compete with the ionosphere.

Thanks, I thought I remembered it was the VARB, but ionosphere it is, that he was claiming the signals would not go through, because they were UHF
Truth needs no defense.  Nobody can take those footsteps I made on the surface of the moon away from me.
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Offline ka9q

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Re: LM didn't have enough power.
« Reply #29 on: February 11, 2019, 09:05:13 PM »
Thanks, I thought I remembered it was the VARB, but ionosphere it is, that he was claiming the signals would not go through, because they were UHF
It'd be hard to find a more easily refuted claim, but then again we're dealing with hunchbacked. I hate to say this, but having sparred with the guy for at least a decade now I think there's something wrong with him.

The main GPS signal on 1575.42 MHz is UHF, and it gets through the ionosphere. It is delayed slightly, and this is one of the major remaining error sources. But there's a clever trick to get around this problem. The delay is a well-defined function of the frequency and the total free electron content along the path. So if you have the satellites transmit on a second frequency, you can measure the time offset between the two, compute the total electron content, and then compute the actual delay for either frequency. Until recently, most civilian receivers only used the 1575.42 MHz L1 frequency; there's a second frequency, L2, but it's encrypted and available only to military receivers. But another frequency, L5, has been added to recently launched GPS satellites specifically to give a second frequency to civilian users. I don't know how many receivers implement it, but it will be nice when it's widely supported.

By the way, the extra delay through the ionosphere goes to infinity at the critical frequency I mentioned in my last post. Instead of passing through the ionosphere, it is reflected as from a mirror. Also, metals are dense "electron seas" (lots of mobile electrons, which is why they conduct heat and electricity so well), so their critical frequencies are very high -- well above the visible range. That's why metals are shiny, at least in pure form.

This plasma physics stuff has been very well understood for a long time. I learned it 40+ years ago as an EE senior.

.
« Last Edit: February 11, 2019, 09:10:33 PM by ka9q »