Chitika: Cuba

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Beautiful use of a cigar box by Ben, KK6FUT. Ben is working in close proximity to Pete N6QW and has obviously fallen under the influence of Pete's "Build Something With Tubes" field.
Watch out for the high voltage Ben. You aren't in Arduino-land anymore! One hand behind the back OM!

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For the past month or so I have been wondering about a strange echo that I've been hearing on the 31 meter transmissions of Radio China International. I first noticed it on my "Kings Speech" regen receiver. Then I heard it again on my "Off the Shelf" regen.

For a while I thought that what I was hearing was a propagation effect: Perhaps the very strong RCI relay station in Quivican,Cuba was sending north a signal so strong that it was travelling around the earth along the grey line and coming back to me about .133 seconds after the original reception.

This sounded plausible (and it does happen sometimes). But there were reasons for skepticism: Why wasn't anyone else hearing this? Why wasn't the effect showing up on signals from Radio Havana Cuba?

Pete Juliano had suggested that perhaps I was getting signals from TWO different RCI transmitters. I had quickly checked the RCI schedule and didn't see them transmitting on the same frequency at the same time from multiple transmitters, so I kind of put that idea aside. Hey, the round-the-world idea was just more appealing!

But then I remembered something strange about the echo: It seemed to disappear when I tuned close to the center frequency of the RCI signal, but then appeared when I tuned off to one side. Hmm.... That was an important clue.

I've long been wary of regen receivers and for a while suspected that I was dealing with some weird regen effect. Regen and Echo seem to go together, right? Well, as it turns out, no. But I was right about this being an effect of the nature of my receiver...

Last night I was listening to RCI English service at around 0030 UTC on 9570 kHz. Nice clear signal. No echo.

At 0100 UTC the program changed, and the echo started. A very strong echo.

I went to the RCI schedule. Here I found the answer:

At 0030 they were transmitting from their relay station in Cerrik, Albania on 9570 kHz.
At 0010 they switched programs, frequencies and transmitters. At 0100 Cerrik shut down, but the Quivican, Cuba relay came on on 9580 kHz. At the same time the RCI transmitter in Kasi Sabagh in far-off exotic Western China, in Xinjiang, fired up on 9535 kHz. Both transmitters were carrying the RCI English service.

You see, my little regens are not very selective, and the RCI transmissions are quite strong. So if I have my receiver tuned to around 9560 kHz, I'll be hearing BOTH the signal from Cuba AND the signal from Xinjiang. That would explain the echo.

To try to confirm this, last night I fired up my old Hammarlund HQ-100 receiver to see if I could discern the two different signals. I could. And the echo appeared when I tuned BETWEEN the two. You can hear this in the video above.

There is one remaining question here: Is the echo caused by the RADIO path difference between the two transmitters? Or are we just seeing the effect of the programming being transmitted at slightly different times, perhaps with this delay caused by INTERNET latency? Anyone know how RCI gets its signals from its Beijing studio to its distant transmitters? I calculate that the path difference is about 10,000 km. With c at 300,000 km/second, that would yield an echo of only about .03 seconds. The echo we are hearing sounds longer than that, so I suspect we are hearing a difference in studio-transmitter transmission time. What say the SWL RF gurus?

BTW: I think the same phenomenon may explain the echo on Brother Stair's "Overcomer" signal. I see that starting at 2200 UTC he is on BOTH 7570 kHz and 7730 kHz from RMI transmitters in Florida. Perhaps they are not synched up.

I think this is all very cool. Think about it: Here I am, sitting in Virginia in 2014, listening to the Albanian, Cuban, and Xinjiang relay stations of Radio China International on a receiver first built by some guy in England during the 1930s. And I'm trying to figure out if the echo I hear is caused by the limits imposed by the speed of light and the size of the earth, or by the time it takes packets to move through sub-oceanic fiber optic cables.

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Hola amigo Bill:

I was able to pick up the podcast with excellent audio quality.
It is quite true that regenerative receivers are very much in use
even today... for example many if not all of the automobile RF
keys opening and closing the cars doors rely on a superregenerative
receiver circuit !!!

The radio that you copied at the blog works very well indeed
but it would be good idea to include a 5 kilo ohms volume
control.... Very easy to do indeed.

But let me tell you that my favorite regenerative receivers are
the classic ones, using vacuum tubes, and operating them
at voltages not higher than 50 volts... As a matter of fact many
tubes work very well at the 24 volts DC voltage level.
Using the classic Hartley circuit , there is no need for a hard to
find throttle capacitor required by the Armstrong circuit, because
the regeneration control works very well by using a potentiometer
to change the screen grid voltage of the detector.

I agree that using an RF stage ahead of the detector is always
a very good idea.... In my tubes regenerative I use a triode connected
6AK5 clone.... as a grounded grid stage....another 6AK5 clone ( the
6ZHE1P Russian tube ) is the detector and I use another 6AK5 clone
as the first audio amplifier then feeding an audio output pentode
all provided from a very simple basic 70 volts DC power supply.
BTW, using regulated DC on the filaments of the detector stage,
although a luxury by my standards is very helpful to reduce
hum .... 7805 regulator recycled from a bad motherboard, with
one 1N4007 from broken Compact Fluorescent Lightbulb inserted
in series with the regulator ground pin, produces a nice 5.7 volts
regulated DC that with a brand new tube is more than enough... with old
6ZHE1P recycled from Russian TV sets, you add another 1N4007 to obtain
6.4 volts regulated DC....

As said in the podcast, it is very important to do a very good
mechanical engineering job, place the main and bandspread tuning capacitors
away from the front panel, use isolated shafts between the capacitors
and the dial mechanism and make the front panel of a a thick steel
plate if possible.

There is a Dutch Cascode Regenerative radio that several Cuban radio
amateurs have built... it was designed with the amateur bands in mind so
the information about the tuning coils and capacitors lets you
obtain a very excellent bandspread on the ham bands.
I can send you that circuit that uses very common 12AT7-ECC81
and Russian equivalent double triodes.

Keep up the good work amigo and always tell us when the next
podcast is available. BTW it lasted for almost an hour !!!

73 and DX

Your amigo en La Habana, Cuba
Arnie Coro
CO2KK
Host of Dxers Unlimited radio hobby program
Radio Havana Cuba

Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htmOur coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmokeOur Book Store: http://astore.amazon.com/contracross-20

That's Radio Havana Cuba.

Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htmOur coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmokeOur Book Store: http://astore.amazon.com/contracross-20

Chitika

Tube Transmitter in a Cuban Cigar Box

Radio China International Echo Mystery SOLVED!

Regen Receivers in Cuba

Some Progress on the Old Regen

AHMED HASSAN MOHAMMED

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