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CODAR Article in October QST Magazine

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SCPD

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FYI, for anyone interested... there's a short article (page 82) about CODAR in the October issue of QST Magazine.

Needless to say, CODAR is well-known to most SWL'ers! :twisted:

Apparently, there were reports of CODAR interfering with the 12m and 60m ham bands and the investigation led them to a USC-run CODAR station. It's a short article but it does show an interesting photo of a CODAR transmission antenna. These stations are limited to 40W.

The good news is that the interfering CODAR transmitter has had a frequency change and should no longer interfere with the 12m and 60m ham bands. :cool:

I found the picture that they used in the magazine article:

 
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ka3jjz

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This is an interesting picture - what kind of antenna is this? It appears to be a ground plane with extra verticals, but surely there is more to it than that

??

best regards..Mike
 

SCPD

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Mike here's an article I just read about CODAR antennas. It doesn't offer any real details but it's informative:

Codar Antenna Mount

PS. It looks very much like my Zero Five 10-40 GP vertical except that my antenna has six radials. :)
 

majoco

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Isn't Codar on two frequencies? I guess the vertical and the radials have traps in them at the appropriate lengths.
 

SCPD

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Martin, I think it depends on exactly what they're studying. We have them here on 4 Mhz, 5 Mhz and 12 Mhz and likely in the 16, 18 and 24Mhz ranges too (near the marine bands.)
 
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lep

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At present these sea-state radars are operating with experimental licenses. At the next World Radio Conference in Jan 2012 they are an Agenda item for the member states to see where they might find an allocation. More than likely they will find a home "somewhere", all we can do is urge that it not be in or adjacent to any frequency band we like to use/monitor.

LEP
 

Token

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This is an interesting picture - what kind of antenna is this? It appears to be a ground plane with extra verticals, but surely there is more to it than that

??

best regards..Mike
The CODAR antenna has two possible configurations. Below 25 MHz it is a single vertical element. Form 25 MHz up it consist of both a vertical element and horizontal elements. The picture at the top of this thread is an antenna configured for operation from 25 MHz up. A none technical description of this operation can be found here http://www.codar.com/Manuals/SeaSonde/Docs/GuidesToTheHardware/HW6_Antenna_Pattern_Measure.pdf How the antenna is assembled is found here http://www.codar.com/Manuals/SeaSonde/Docs/GuidesToTheHardware/HW4_Radial_Antenna_Assembly.pdf Keep in mind that guide is a little dated, the new antennas are slightly different, as in only a single antenna for a site.

The selection of frequency depends on what you want to plot. The higher the frequency the smaller the wave size that can be plotted.

The CODAR most commonly uses an IFMCW mode of operation. In this mode it has a linear FM on a long pulse, with occasional short pauses in the pulse. Basically it transmits for 900 microseconds to 2 milliseconds, one chirp across the pulse, and then pauses for a relatively short time, say 500 microseconds, with the frequency sweep continuing during the blank. This pause probably is for no other reason than to reduce duty cycle. It is not long enough in relationship to the pulse to act as a listen time for the radar and since the system is pure Doppler there is no need for a "listen" time as such, the shift from the transmit frequency is the data and the linear FM is the way to move the transmitter away from the received data. There are other modes but this is the most common I have seen. One of the other frequent modes is about a 50% duty cycle.

T!
 
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Hooligan

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Circa mid to late 2000s, I visited a number of HF CODAR sites in California, and the tx antenna has always been of a different design than the rx antenna. Below are the two CODAR antennas at a site in San Francisco. Can't recall the freq of fthe top of my head, but it was either 12 or 14MHz, and I could hear it (weakly) in my car via ground-wave from about 18 miles away, despite a ridgeline, etc. in-between.
 

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Token

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Circa mid to late 2000s, I visited a number of HF CODAR sites in California, and the tx antenna has always been of a different design than the rx antenna. Below are the two CODAR antennas at a site in San Francisco.
The older design was two seperate antennas, one for TX and one for RX. More recent versions are a single antenna.

T!
 
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