Lna dilemma

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dave3825

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Right now, my indoor antenna couples to a catv amp. From the amp, about 15 feet of rg6 is my pc desk. At the desk, the one line gets split between a bcd436, an airspy mini. and 2 cheap china dongles. And I get good distance for an indoor antenna. And I receive most of what I want to hear. I am into it all. Police and fire, fed, mil air, civ air, maritime, trunked dmr with the dongles. Unitrunker weather sat.etc

I want to try an lna that's meant for radio and not the catv amp I have been using. Tho it has done the trick, it's time for something new.

I am torn between lna4all (with bias t) and janilab (with adjustable gain).

I know my airspy should power a bias t lna, but can any harm come to either the 436 or the china dongles if the bias t was enabled on the same run of coax? I am in an apartment and between their rules, and my wife's rules, I can only get away with so much.

I like the lna4all with bias t because when I travel, I go light. And a dongle that can power an lna means less stuff to pack. Plus, here at home it would mean 1 less wall wart sucking up juice all day. But I also like the janilab because its gain can be adjusted by turning the pot. I think I read lna4all gain can be raised or lowered by changing the voltage if not running bias t..

But I guess the main question is, if I enable the bias t on the airspy mini, can anything connected to the coax get fried? Like can the power screw up the china dongles or the 436?


Open to any advice, opinions and comments.

Thanks
 

jonwienke

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If you are using a bias-t to power some device through coax, you must have a direct coax connection between them. No splitters, filters, multicouplers, or anything else. Otherwise power will probably not make it to the intended device, and you do run the risk of frying an imput not intended to handle DC power.
 

dave3825

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So then maybe the bias t won't be a good fit here. Then maybe the powered lna might be the way to go as I cant run 3 more runs of coax under the carpet . It's bad enough that my wife already sees the one and yesterday asked how long one of my antennas was going to be hanging from the curtain rod. Lol


I don't have to power an lna with the airspy bias t when home. Just wanted it for when traveling.


Let me ask this,

Since the airspy, china dongles and scanner are after the splitter, If I used something like this, Bias Tee on ebay with the correct voltage of course, right before the splitter, would the power only travel toward the amp since its marked rf on one side and rf + dc on the other? To me it seems directional, but I am not educated enough with bias t.



Thanks


.
 

prcguy

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Since you are using RG-6 you can buy TV/satellite splitters that have one or all ports DC pass. If you get a two way splitter with only one DC pass port, that will allow the airspy to power the LNA and receive signals on that port and it will also allow another receiver to work on the non DC pass port so nothing bad happens to it.

There are other components made for the satellite TV industry like directional couplers (taps), attenuators and filters that will pass DC and work between the airspy and antenna with DC on the coax.
prcguy
 

dave3825

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Ok, I have one splitter that says dc pass.



If I did it right on the meter, hooking the airspy to the sat port, and enabling the bias t, I got 4.52v on the ant/sat port and got no power on the ant port. So as long as the other dongle or my 436 are on the ant port, no harm can be done?


 

Ubbe

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Your catv amplifier probably have a 3-4dB noise figure and the janilab have 2dB and lna4all have 1dB.
If you go with the lna4all it will be like having an antenna that are twice the size or two stacked ones compared to the catv amp.

You can never regulate the gain by altering the voltage to it. These amps have a 5v regulator onboard but if they didn't you would only change the voltage range outside where the amplifier IC works at its best.

It's super important that you use a catv variable attenuator to adjust the signal level so you do not overload the receivers and get worse reception than before. They don't have power-pass so cannot be inline with the 5v coax. You probably only need 6dB amplification besides the compensation for loss in coax and splitters. You loose 3dB with each split, 1-to-2 will attenuate 3dB and 1-to-5 will loose 12dB.

/Ubbe
 
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