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Author Topic: New here, and my journey building a SW xmtr  (Read 14474 times)

Offline KaySeeks

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Re: New here, and my journey building a SW xmtr
« Reply #45 on: September 27, 2018, 0137 UTC »
i would tend to believe using highly shielded coax as feedline to the dipole is akin to bringing the TX right at the center fed part of the dipole.

What the TX sees looking into its end of the coax is based upon four things: 1) the coax's characteristic impedance, 2) the loss of the coax, 3) the length (in wavelengths) of the coax 4) the load at the other end of the coax.

You want the TX to see the antenna's inherent impedance? Make the coax length a whole multiple of a half wavelength.

Shielding doesn't have anything to do with it unless it's part of the active part of the coax and affects the characteristic impedance. (There are double-shielded coaxes for low-leakage applications, as an example of coax with shielding that does not affect characteristic impedance.)
« Last Edit: September 27, 2018, 1715 UTC by KaySeeks »
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Offline R4002

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Re: New here, and my journey building a SW xmtr
« Reply #46 on: September 27, 2018, 1548 UTC »
Even with a 50 foot antenna in sloper configuration (lowest point around 5 feet above ground level, highest point maybe 12 feet) NVIS and further distance propagation are both possible at modest power levels.  Propagation conditions (read: luck and timing) also play big parts, so keep on experimenting - you'd be surprised what you can do with low power when the conditions are good.



I would recommend using a frequency closer to 6925 kHz if possible.  The 6800-7000 kHz range is where many people with SDRs recording the entire band look at for activity.  6850-6975 is an even better range to shoot for (if possible).  I know your rig is crystal controlled so if you have crystals for those frequencies they might get you better results in terms of listenership and reports.  6925 kHz is the "home channel" for 43 meters, followed by 6955 kHz.  Just avoid using 6910 kHz and 6915 kHz as these frequencies are used by the U.S. Government for disaster relief operations and other emergency communications. 

« Last Edit: September 27, 2018, 1550 UTC by R4002 »
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Offline TheRelayStation

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Re: New here, and my journey building a SW xmtr
« Reply #47 on: September 27, 2018, 1632 UTC »
Just avoid using 6910 kHz and 6915 kHz as these frequencies are used by the U.S. Government for disaster relief operations and other emergency communications.
and 6855Khz, this is used by the US Military.
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Offline R4002

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Re: New here, and my journey building a SW xmtr
« Reply #48 on: September 27, 2018, 1714 UTC »
^Well said.  6940 kHz and 6945 kHz are often used for Link-11 data transmission by the US military as well.  Check the HFU loggings for Shortwave Pirate [here: https://www.hfunderground.com/board/index.php/board,3.0.html] to see if any other stations are on the frequency you plan to use before transmitting to avoid interfering with another pirate station.  It's also a good idea to check the RocketChat chat room for HFUnderground to ask listeners if there are other stations on the air or utility users on a frequency you might pick.  With low power, its extremely important that you use a clear frequency. 

Which frequencies do you have available to you? 
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Offline fieldstrength1947

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Re: New here, and my journey building a SW xmtr
« Reply #49 on: September 27, 2018, 1829 UTC »
Thanks for the good info on frequencies, guys!

What I did when I ordered the parts from Mouser is I just ordered as many crystals as fit the following criteria:

1. Not within an amateur radio band
2. Not being used by any current shortwave station that reaches North America

But obviously I need to update my criteria...

I don't have anything between 6800 and 7000. And looking back at Mouser it seems they simply don't stock any of those, at least not at the moment.

Anyone know where I could get crystals in that range?

Offline Josh

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Re: New here, and my journey building a SW xmtr
« Reply #50 on: September 27, 2018, 1905 UTC »
Yeah don't believe quite all of the book values and antenna voodoo you read.  An inverted v 20 feet up in the center and legs just off the ground to 6 feet up is only supposed to give NVIS.  Why then have I seen 10 watts cover well over 1000 miles?  Magic resonance I guess.  My first 2-3 watt crap build was heard (poorly) almost the entire N-S distance of the US in the daytime, tossed center wire 20-25 feet high over a limb, edges maybe even touching the ground.

To get tactical nvis coverage the antenna must be low to the ground, as in only a foot or two. Even then, it also depends on the ionosphere, muf, luf, etc.

https://brushbeater.files.wordpress.com/2017/04/1996-fiedler-near-vertical-incidence-skywave-communication-book-worldradio-books-2.pdf
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Offline R4002

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Re: New here, and my journey building a SW xmtr
« Reply #51 on: September 27, 2018, 1910 UTC »
eBay is my first bet -

https://www.ebay.com/itm/FT-243-Radio-Crystal-Oscillator-6-9250-MC-Ch-320-M-P-Co-093-Pins/273025354397?hash=item3f91944a9d:g:VvQAAOSw8RZaW7KU:sc:USPSFirstClass!23235!US!-1 - 6925 kHz FT-243 style

https://www.ebay.com/itm/FT-243-Radio-Crystal-Oscillator-6900-KC-Ch-319-Daughetee-093-Pins/253412122175?hash=item3b008a1e3f:g:USQAAOSwZoZaeO7z:sc:USPSFirstClass!23235!US!-1 - 6900 kHz FT-243 style

https://www.ebay.com/itm/FT-243-Radio-Crystal-6850-KC-Channel-317-Daughetee-Tested-Good/253374341295?hash=item3afe49a0af:g:RBEAAOSwFLBaXrLA:sc:USPSFirstClass!23235!US!-1 - 6850 kHz FT-243 style

https://www.ebay.com/itm/6800-KHz-FT-243-Crystal/183403927542?hash=item2ab3b9d7f6:g:uPkAAOSwheZaoF95 - 6800 kHz FT-243 style

There are hundreds and hundreds of the FT-243 style crystals on eBay.  Of course, if your rig uses a small crystal holder then you'll have to either source different size crystals or make an adapter. 


I'll let other posters chime in when it comes to other sources...

If 6780 kHz is the closest you can get to 6800-7000 kHz then I would go with using that while you look for crystals that are a bit higher in frequency.   

The criteria you list are pretty...open!  Be careful that you're not transmitting in aeronautical bands or other frequencies used by safety of life communications.  6525 kHz to 6765 kHz is the closest HF aircraft band to 6-7 MHz.  Your best bet is to stick to the 6765 kHz to 7000 kHz range (which is allocated as a fixed/mobile band).  6200 kHz to 6525 kHz is the 6 MHz HF-SSB marine mobile band and while it is popular with European pirates and a stray SW broadcaster, there are safety of life communications in that band that you certainly don't want to interfere with either.  The 49 meter broadcast band is 5900 kHz to 6200 kHz and is also popular with European pirates. 

For the time being, it may make the most sense to stick with 6780 kHz thereabouts (don't use 6770 kHz, as that is used by Old Time Radio, a low power pirate station that is on the air basically 24/7) or pick an empty frequency in 49 meters if you have crystals for that.
« Last Edit: September 27, 2018, 1924 UTC by R4002 »
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Offline JimIO

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Offline redhat

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Re: New here, and my journey building a SW xmtr
« Reply #53 on: September 27, 2018, 1957 UTC »
If your transmitter will accept TTL/CMOS drive at 5V, you can use these oscillators programmed by digikey to any frequency between 1-133 MHz.  The cost is nice too.

https://www.digikey.com/product-detail/en/cardinal-components-inc/CPPT1-HT0PT/CPPT1-HT0PT-ND/387755

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Offline fieldstrength1947

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Re: New here, and my journey building a SW xmtr
« Reply #54 on: September 27, 2018, 2137 UTC »
Thanks! I went ahead and got the 6850 kc crystal; hopefully that's a good channel... in the meantime I'll keep using 6780 if I do any more test between now and then.

Offline R4002

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Re: New here, and my journey building a SW xmtr
« Reply #55 on: September 27, 2018, 2201 UTC »
If you're going to go with crystal control, I'd get the 6900 kHz and 6925 kHz crystals as well.  They're cheap and having the ability to move around the band as needed is nice. 
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Offline fieldstrength1947

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Re: New here, and my journey building a SW xmtr
« Reply #56 on: September 28, 2018, 0722 UTC »
Testing again tonight on 6780, in celebration of the new Logic album - playing YSIV on repeat.

Offline JimIO

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Re: New here, and my journey building a SW xmtr
« Reply #57 on: September 28, 2018, 1713 UTC »
I prefer bad words from a girl with a guitar.

https://www.youtube.com/watch?v=dv3QWOueK74

« Last Edit: September 28, 2018, 1747 UTC by JimIO »

Offline fieldstrength1947

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Re: New here, and my journey building a SW xmtr
« Reply #58 on: September 28, 2018, 2313 UTC »
I prefer bad words from a girl with a guitar.

https://www.youtube.com/watch?v=dv3QWOueK74

Hahaha... to each his own. Somehow to me it seems more appropriate in a hip-hop context. Also the legacy of pirate radio broadcasting hip-hop music, but I guess that's also kind of diluted by the mainstreaminess of hip-hop now.

Offline JimIO

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Re: New here, and my journey building a SW xmtr
« Reply #59 on: September 29, 2018, 0120 UTC »

https://www.youtube.com/watch?v=vS-zEH8YmiM

"the legacy of pirate radio broadcasting hip-hop music"

Hip-hop is legacy? Look it up on wikipedia, then look up rock and roll.

 https://www.youtube.com/watch?v=5DGF1tQ8V4o

https://www.youtube.com/watch?v=dAmSILueixA

 

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