Two of us trying to explain stuff mostly off the cuffs.
Hosted by Various Hosts on 2017-08-16 is flagged as Clean and is released under a CC-BY-SA license.
Tags: amateur radio, modulation, AM, FM, SSB, doppler, satellite.
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mp3 format. | Comments (1)
This time only the two of us:
- Steve, KD0IJP
- Michael, DL4MGM
A lot of off the cuff technical explanation.
Starting from the comment by David Whitman to the last round table, we talk about frequency, wave length, propagation velocity and their relations. https://en.wikipedia.org/wiki/Wavelength. Approximate wave length values are commonly used to reference to entire "frequency bands", which are the frequency spectrum portions allocated to a certain radio service in proximity. Status of (amateur) frequency allocation may vary with band or country. They may not be "exclusive" and there can be "primary" and "secondary" radio services sharing that spectrum. Secondary services must not interfere with primary ones, while the other way round has to be accepted.
What frequency is most powerful? - It depends! We ramble a bit how different frequencies have different propagation depending on certain factors like daytime, time of year, sun spot cycle. - Please help out with more in depth information here! Join us. There is a varying maximum and a minimum useable frequency for ionospheric propagation. https://en.wikipedia.org/wiki/Maximum_usable_frequency
VHF (globally > 30MHz) and UHF (> 300 MHz) waves have a more line of sight propagation. Steve shortly introduces the concept of repeater stations. Usually at an exposed location, a repeater retransmits the signal that is received at another frequency, thus extending communication range. Participants only need to reach the repeater in order to be able communicate with each other. We hint at additional propagation modes for VHF, like sporadic E-layer propagation, but are not able to go into detail. - Please tell us, if you have experience in those fields!
Some thoughts about RF output power and how it is less important if conditions are right.
Modulation: Putting "information" onto a radio frequency signal. Staring out as a clean "carrier wave", its parameters are modified according to the modulation scheme. We start out and explain the basic concepts of amplitude modulation, AM, where the amplitude of the radio wave is altered by the modulating signal. Then frequency modulation, FM, where the modulation process influences frequency of the output signal. We use voice audio as an example as modulation content, but this can of course be of digital nature. SSB, single side band modulation. It is the standard voice modulation mode for short wave amateur communication. We give a very brief explanation of one possible way of generating it. We discuss how it is more efficient than AM in regard to occupied frequency spectrum use and transmit power.
This leads to ideas how great it would be if someone could record a show about those things, including audio examples. We further digress in how it may be a good idea to single out individual topics separate shows. The "rabbit holes" (tm MrX I think) we fall in while explaining other stuff. Make them available to be simply referenced in later shows and we can concentrate on the topic at hand.
There will be a place to put ideas and draw inspiration for shows here: https://etherpad.net/p/HPR-HAM-TOPICS
Next we pick up the discussion of frequency shift caused by the Doppler effect and its effect on satellite operation. This was triggered by a question in "hpr2216 :: Working AO-85 with my son" (http://hackerpublicradio.org/eps.php?id=2216). https://en.wikipedia.org/wiki/Doppler_effect
We ramble a bit how in the wide field of amateur radio no one can know everything right from the beginning. Take the jump start provided by the knowledge required for the test and go on with learning by doing.
How cool would it be to have shows from "Ham fests" like the "Dayton Hamvention", the "HAM RADIO" or any other event. A brief mention of the "ARRL Fieldday".
We have a mini poll and want to get feed back from the audience, if they would be interested to have some sort of decoding riddle in future shows.
Comment #1 posted on 2017-09-25T19:07:38Z by Josh Huber KF6ZZD
Doppler shift of RF at terrestrial speeds
I liked the explanation of the Doppler effect's effect on radio freqeuencies. At 33:30, it was mentioned that at the speeds that satellites travel, the Doppler effect is noticeable, so much that you may have to adjust your RX frequency. And at the speed of car travel, which is very slow compared to RF propagation that the Doppler shift wouldn't play a role.
I just have one nitpick, which that at car speeds, a measurable Doppler shift of RF signals indeed happens, even at GHz frequencies, and this is exactly how police radar works (commonly using RADAR way up in the 10 GHz or 24 GHz bands). This is totally a nitpick, since we're probably not talking about a shift of more than a few kHz, and very few if any radios can tune in less than 10 kHz increments in UHF anyway.
Enjoyed the show. Cheers.
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