Re Monitoring 1540 MHZ.....

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Re Monitoring 1540 MHZ.....

Postby USAFE501 » Wed Dec 04, 2019 12:25 am

Hi Guys,
Just a quick question - seen a few articles regarding Immarsat monitoring using a small GPS type antenna and an SDR type receiver.
It seems that most SDR receivers have the facility to provide voltage to the antenna via the Bias Tee...
...my question is if I am not using an SDR style receiver , but a conventional radio such as AOR 3000, how do I supply the needed voltage to the antenna ?

Does the above make sense ? :?: :?: :?:

Thanks in advance,
USAFE501 :wave:
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Re: Re Monitoring 1540 MHZ.....

Postby Chris P » Wed Dec 04, 2019 5:22 am

Try the following link

https://www.rtl-sdr.com/building-a-diy- ... -bias-tees

commercially available versions are available on ebay for around £20
Regards Chris aka G8FFF nipper or tazmin88
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Re: Re Monitoring 1540 MHZ.....

Postby rozbiff » Wed Dec 04, 2019 9:41 am

You would normally build the DC-block that Chris linked to as part of the circuit (the web page is really for a specific use example where you are applying power direct to a LNA which doesn't itself have a DC block eg. Adam's LNA4alldesign).

Adam has a blog page explaining the circuit and purpose of the power injector (http://lna4all.blogspot.com/2014/11/diy-bias-t.html). You don't need to use smd components or base it on a piece of PCB substrate. You will find numerous other examples on various radio sites as the circuit has been in use since the days of valves ;)

If you are going to buy a 'Bias-T' then you might as well just buy an rtl device with one built in. For example the rtl-sdr.com /nooelec smart devices will only cost you a couple of quid more than circuit in it's own box. Another advantage of the rtl device with one built in is that you don't need to build a regulated power supply or mess about with extra power cables.

If you plan on making one the electrical components will only cost a quid. The SMA (or whatever) connectors, case, power supply and cables will cost much more if you want it to look nice.

As you have access to the base radio workshop why not ask them to make one for you ;)

Note that if you are leaving the 'Bias-T' in the antenna circuit you will need to ensure the power is off if you attach a DC-short-circuited antenna design.
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Re: Re Monitoring 1540 MHZ.....

Postby USAFE501 » Wed Dec 04, 2019 9:02 pm

Good evening,
WoW !!!! :shocker: :shocker:
This forum never fails to surprise me.....thank you so much for the replies , Chris and Rozbiff and all the advice and technical information -it's amazing :thumbup:

So after following the supplied link and google searches etc , Rozbiff mentioned buying an SDR with bias tee already built-in. I think that's the way I'm going to choose,
Just one question , after a few google searches as mentioned earlier , there are numerous SDR options however am I right in thinking not all of them have the bias-tee option ? So is there a preferred SDR that people are using that will suffice ? Don't want to spend lots if I can achieve what I want which is mainly voice traffic up in the 1540 mhz area....seen a GPS style antenna that looks like a big cadburys chocolate chunk for under £15 , so what SDR would you guys start with that has bias tee and will get me started ????

Hope you don't mind me asking , I know its basically me asking everyone else to do the hard work for me , sorry :embarrased: :embarrased: :embarrased: :embarrased:

Look forward to your replies,
USAFE501 :wave: :wave: :wave: :wave:
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Re: Re Monitoring 1540 MHZ.....

Postby rozbiff » Wed Dec 04, 2019 11:25 pm

I'd suggest either of the two I mentioned. Both well supported and popular and both from reputable desgners.
There are cheaper but I'm not sure the few quid saved would be worth the chance of poor quality/false claims about features/performance.

Of course there are also plenty that are much more expensive but for your application a better antenna would be the next upgrade step to take.
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