Put 12VDC across A and B then touch 1 or 2 to 12V + and you should hear a faint click. If you ohm out the SMA connectors you should see it changes when you put 12V to 1 or 2.
Bias-t & LNA
- Thread starter Pape
- Start date
I've used a lot of these in my career and have seen the various ways they can be switched.
Ubbe
Member
How does the latching works, it doesn't have connection between any SMA when the 12VDC are removed from +A and - B ?
/Ubbe
/Ubbe
it is latching in 2 modes and remain latched in place when the power is removed.
Mode 1 is J1 and J4 together and J2 and J3 together
Mode 2 is J1 and J2 together and J3 and J4 together
It is good to add or remove device inline, example a filter.
Mode 1 is J1 and J4 together and J2 and J3 together
Mode 2 is J1 and J2 together and J3 and J4 together
It is good to add or remove device inline, example a filter.
Ubbe
Member
OK, so cannot be used to bridge a preamp without running a second wire, and a third, up the coax as it has no fail safe mode.
Or use a bridge rectifier for the relay and preamp and then a diode in one direction to one control pin and a diode in the other direction to the other control pin and then switch polarity of the 12V at the bias-T.
Will probably need a 470uF reservoir capacitor on the bridge rectifier to hold a steady 12V during the polarity switch.
Probably better to buy a fail safe relay that configures the connection to bypass the preamp at power loss.
/Ubbe
Or use a bridge rectifier for the relay and preamp and then a diode in one direction to one control pin and a diode in the other direction to the other control pin and then switch polarity of the 12V at the bias-T.
Will probably need a 470uF reservoir capacitor on the bridge rectifier to hold a steady 12V during the polarity switch.
Probably better to buy a fail safe relay that configures the connection to bypass the preamp at power loss.
/Ubbe
Ubbe
Member
I believe someone made an antenna switch up at the antennas using a latch relay. I think it involved using a diode rectifier to always power the relay with the correct 12V polarity and then a capacitor in series with the latch control pins that would make a pulse to the control pins when the polarity where switched at the bias-T and then the 12V could be removed from the coax, or still power a preamp.
/Ubbe
/Ubbe
Maybe you can point me in the right direction. i am gonna put up a J pole antenna and i will be using 3 RTL-SDR V4 dongles with sdrtrunk "maybe 4 eventually" but i am confused on what LNA i need and where it should go i'v heard i should put it at the antenna and weatherproof it...just out of curiosity what would happen if i put it at the dongles? Also i have been told i need to ground the coax with one of these https://www.amazon.com/gp/product/B06XDX5PNN/ref=ox_sc_act_title_3?smid=A3LYGL3VCPC7H8&psc=1 . now the LNA's i am looking at are the Lano nooelec bias tee powered and the RTL-SDR Blog Wideband LNA (Bias Tee Powered) which one of these two would you recommend? i'm basically just trying to listen to police trunking and ems . i am brand new to all of this so please if u reply try to keep it simple like really simple lol .but thanks alot!
Ubbe
Member
Then it will works as a multicoupler, like the ones Stridsberg has. You always have to attenuate the signal going to a scanner, so that you have something like 0 to 6dB gain at the most, calculated from the antenna to the scanners antenna jack, or you get overload issues.
A 1-4 splitter has usually a 7db loss and most LNA amplify 20-25dB so you will have to attenuate something like 8dB to 18dB. Receivers and scanners handles strong signals differently, some cannot take any gain and even needs to have a signal attenuated and others can take high signal levels. I use a variable 0-20dB attenuator connected to the input of a splitter and listen to a weak signal in analog mode and adjust the attenuation level to get the lowest possible noise from that particular receiver. Too much signal and a receiver starts to overload and loose the signal in noise and too little signal also gives too much noise. It's just at the proper level where you get the best signal noise ratio, that will be different depending of scanner brand and model and receiver types.
There's specific safety rules for US and we have nothing like that in our country so I cannot comment on it.
They are identical, using the same components. If you search for SPF5189Z you can get the same performing LNA for $3 from Alibaba and those kind of places.
If you only monitor the nearest site and it has a good strong signal then you probably don't need any LNA amplification. If you have a scanner and look at the signal strength from the site you are going to monitor using RTL-SDR and you have 4 or 5 signal bars or if dBm it is -90dBm or stronger like -80dBm then no LNA needed when you use a 1-4 splitter. You can use any 1-4 $5 CATV 75 ohm splitter and buy coax cables that have the correct connectors at each end to match your receivers, F in one end for the splitter and SMA in the other for the RTL-SDR's and any LNA.
If you have weak signals now when using a scanner or RTL-SDR with an indoor antenna then you'll need that LNA if you use the same indoor antenna, or the 1-4 splitter will reduce the signal even more and make it unusable.
A j-pole antenna type are very narrow banded, it receives fine within a small frequency range and attenuates signals outside of it, so will work as a bandpass filter helping the receivers to perform without being overloaded from out of band signals. It's a balanced antenna type and the coax are unbalanced and usually creates issues with the coax being part of the antenna and receives interferences and perform badly when the coax makes the direction pattern shift in unpredictable ways. To proper connect an unbalanced coax to a balanced antenna needs a balun.
A J-pole antenna that you build yourself are tricky to adjust as the point where you attach the coax usually needs to be adjusted to get the best match. Also J-pole are a high impedance antenna that needs it to be for from any metal objects, and as the coax are metal that also will be an issue. It's an antenna for the seasoned user that have dealt a lot with antennas.
/Ubbe
that is alot to take in and i am completely brand new to all of this i have to admitt i didn't understand half of what u said lol
could u maybe dumb down your answer alittle for me lol i'm sorry ... basically i am doing this and can u tell me where i'm wrong. i am putting the j pole on top of my house i will put the LNA at the antenna then i am going to use a 4 way catv splitter and power the lna with one dongle out of the three that has bias tee on "does it just take one dongle to have bias tee on and the othet 2 with it off?" i still don't know if i need it grounded i hear u don't have to ground the j pole antenna . so i'll search more on that
Ubbe
Member
I realized that when I read my answer, I'm sorry about that.
But to simplify I need to know more. Do you have a scanner now or use a SDR dongle and what antenna are you then using and what signal strength do you get while receiving the trunked system and EMS that you want to receive? If you have a portable scanner, can you go up on the roof and see what signal strength you get there?
Are everything that you would like to receive in the same frequency band? Usually trunked system in the 800MHz band are received with good strength but if you have non trunked EMS in the 150MHz or 470MHz then those can be weaker and the antenna needs to be tuned to those band but it will still receive 800MHz if those transmitters have been designed to have better coverage.
Maybe it turns out you can have a dipole antenna in a window made of electrical wires and a CATV 1-4 splitter without any further amplification. If you look in RR's data base for the systems you would like to monitor how far away are you from the nearest site and how high above sea level and average ground are your house?
Have you already concluded that the received signal are not enough and it needs the best possible roof antenna and why have you decided to use the most difficult antenna type to use as your antenna? Grounding antennas to a ground rod are something that I believe has to be done to all installations in US or the insurance will not be valid if you suffer from a lightning strike.
/Ubbe
i have 3 RTL-SDR V4 dongles and i'm using SDRTrunk
everything that i want to receive is in the 150 to 162 band
i have a J pole antenna and i just ordered a catv 1-4 splitter along with wire to connect everything
my signal strength is not the best i'll add some pictures to show u
why do u say the j pole is the most difficult antenna?
this is what i will be doing putting the LNA at the antenna and using the catv splitter to hook 3 dongles to it and power the lna with one dongle with bias tee enabled. does that sound right?
Attachments
Last edited:
No... it doesn't sound right (to me at least).
I drew a picture/ diagram... (I was tempted to say that I had a child draw it, it's rough from the cookie crumbs on my desk).
It sounded like you wanted the "wrong way"
if you hook it up the "wrong way", you could put DC voltages onto the receivers inputs, which is bad...
make sure that the bias t's are hooked up right.
and you'll have to terminate the unused port with a 50 ohm or 75 ohm terminator (super cheap).
I hope this helps.
(any affirmations would be helpful) (ie can I get an amen).
Thanks
Joel
I improperly assumed that you were using an external bias T (even though you said you weren't.)
while looking into the situation further, some dongles have thier OWN bias T that is enabled or disabled in software
SignalsEverywhere: SDR Bias-Tee's and Enabling the V3 Bias Tee
Today's video from Corrosive on the SignalsEverywhere YouTube channel discusses Bias Tee's. He explains what they're used for, and how to enable them on various SDRs. In particular he shows how to use the software to enable the bias tee on our RTL-SDR Blog V3 dongles. A bias tee allows you to...
www.rtl-sdr.com
it sounds like some dongles have thier own blocking capacitors. if yours do, then great. I just recommend terminating the unused port with an appropriate terminator.
Basically everything I said and did up until the last sentence was gibberish given your situation.
Have a great day
Thanks
Joel
Last edited:
i'm sorry but i took that as sarcasm. am i right?
Sorry... No... gibberish is my second language, so I struggle with it.... I started out with a regular radio/bias t/splitter arrangement. Then started editing when I realized there was more to it than that.
Things got out of hand pretty quickly. The drawing I made (and deleted) had lots of squiggly lines though (from cookies crumbs on the desk).
It sounds like you were right the way you wanted it as long as the power provided by the Dongle is enough...
I do recommend terminating any unused ports though.
and I fear any nearby lightning strikes will take out all of your radios.
On a regular receiver / scanner, without built in bias T's, it could be a problem the way you wanted it.
To be clear, I was the one with the gibberish.
Thanks
Joel
Last edited:
no problem lol your all over the place tho haha
Yes... I have an autistic son that sleeps for 20 hours and stays awake for 20 to 30 hours (needing monitoring or he'll destroy the house), so sometimes lack of sleep (and its side effects) can creep up on me, and a sense of humor helps a lot, I couldn't imagine how a person takes things seriously all of the time without having some humor.
Playing with radios and electronics helps a lot, as well as computer games.
Thanks
Joel
i agree. sorry to hear about your son. i'll keep you both in my prayers
Similar threads
BCD436HP/BCD536HP:
bcd436hp help