Antenna for SW CHU time. 3.33, 7.850 & 14.67 Mhz

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mummer
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Joined: Mon Feb 27, 2017 4:53 pm

Antenna for SW CHU time. 3.33, 7.850 & 14.67 Mhz

Post by mummer » Mon Feb 27, 2017 7:11 pm

I am very new to SDR, but find them fascinating. Thoughts of my first project would be the following.

Decode the CHU time signal from 3.33, 7.850 or 14.67 Mhz and use it for a local time source.

- Use a SDR with a linux backend system. It could be a Pi or something like that.

- Have the system auto tune to one of the 3 frequency. Keep rotating until one is found.

- Once one of the stations is found then take the audio and decoded the embedded 300 baud Bell 103 signal.

- I will need to use some type of local clocking, RTC to carry me over when no signal is available.

- Goal is to utilize the data to sync my local clock and run it as a NTP server locally.

(I know a GPS would be much easier, but this is an application that would allow me to use a SDR and sort of old school for reliable time source)

My first question is why type of antenna would I use for this? type, size, length?

Would a SDR dongle be good enough or should I use something else?

Do I need an up converter? Could I use the raw mode and have the Pi decoded it still?

Open to suggestions and any guidance will be appreciated.

V.

rtlsdrblog
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Re: Antenna for SW CHU time. 3.33, 7.850 & 14.67 Mhz

Post by rtlsdrblog » Tue Feb 28, 2017 12:30 am

Sounds like an interesting project.

For an antenna, it depends on your area and how strong the signal is, but any HF antenna should suffice if you know that the signal is there. First try with a simple random wire antenna (any length of wire over 5m long) strung up high and outside. If you don't get a signal with that you can try other antennas like a dipole, or loop.

An HF capable SDR, or upconverter + SDR will be required. The RTL-SDR V3 can receive HF without upconverters, but if you have strong broadcast AM interference you may need filtering. An upconverter may work better, but its up to your testing and environment+antenna on whether its worth it or not.

vmummer
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Joined: Tue Jan 31, 2017 2:37 am

Re: Antenna for SW CHU time. 3.33, 7.850 & 14.67 Mhz

Post by vmummer » Tue Feb 28, 2017 9:22 pm

Thanks rtlsdrblog,

Can you suggest a AM Filter? Thinking it may be better in the long run regardless which SDR I work with.


V.

rtlsdrblog
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Re: Antenna for SW CHU time. 3.33, 7.850 & 14.67 Mhz

Post by rtlsdrblog » Wed Mar 01, 2017 2:33 am

We've just started selling one here www.rtl-sdr.com/store.

DB Gain
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Joined: Tue Dec 20, 2016 8:20 pm

Re: Antenna for SW CHU time. 3.33, 7.850 & 14.67 Mhz

Post by DB Gain » Wed Mar 01, 2017 5:19 pm

CHU transmits in usb plus carrier so receiving in usb mode would be best, with appropriate filter, adjusting a 350Hz filter around the X25 signal would optimise performance. It sounds like an interesting project but with all the variables in app code, sdr latency, pci timer, and system timer, it may be futile unless you are only looking for a second or two of accuracy. But who knows, maybe it'll work out. Also, the packet signal is only present at certain times. If in range, using WWVB or WWV would likely result in a better clock, their sigs always have a bcd time signal.

"A binary-coded decimal time code began testing in 1960, and became permanent in 1961. This "NASA time code" was modulated onto a 1000 Hz audio tone at 100 Hz, sounding somewhat like a monotonous repeated "baaga-bong".[2] The code was also described as sounding like a "buzz-saw". On July 1, 1971, the time code's broadcast was changed to the present 100 Hz subcarrier, which is inaudible when using a normal radio (but can be heard using headphones or recorded using a chart recorder).[4]"

Any decent sdr (set to 100Hz or 50Hz bw) can pick off the 100Hz bcd sig right next to the carrier off any WWV sig, it's also a good example of how an sdr is far superior to analog receivers in this case.

marcds
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Joined: Mon Sep 18, 2017 2:07 pm

Re: Antenna for SW CHU time. 3.33, 7.850 & 14.67 Mhz

Post by marcds » Sat Nov 04, 2017 10:13 am

Hello Mummer
if you have enough space I suggest an OCF dipole for 80-40-20 meters
Such antenna tune well on even frequencies, you should have a good
performance on these three bands

There's many sites arund with such design

snn47
Posts: 153
Joined: Tue Dec 27, 2016 11:00 pm

Re: Antenna for SW CHU time. 3.33, 7.850 & 14.67 Mhz

Post by snn47 » Sat Nov 04, 2017 8:16 pm

Rather than adding filter, for your purposes the antenna should tuneable to provide the selectivity instead in order to relieve the Rx from strong unwanted signals. While coils on ferrites, like the ones found in AM-Rx, would be smaller they most likely won't work above 2 MHz.

That leaves Magnetic (Loop) Antennas using coils of one or more turns. The thick tubing of 1 turn Magnetic Loop Antennas you find in many HAM radio designs are intended for use with transmitters.

A lot of information can be found e.g. here https://translate.google.com/translate? ... edit-text=
http://www.fading.de/rahmenantenne.php

Rahmenantenne means frame antenna, which again is another designation for multi turn Magnetic (Loop) Antenna.

snn47
Posts: 153
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Re: Antenna for SW CHU time. 3.33, 7.850 & 14.67 Mhz

Post by snn47 » Sat Nov 04, 2017 8:20 pm

If you live in Europe there is e.g. DCF 77(.5 kHz), which is used by self synchonizing clocks. I assume that other regions have similar low frequency references and equipment that uses them, that you can use as basis for your trials.

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