Newbie antenna questions

[sacentre]

I'm a complete beginner so please excuse some dumb questions. As will be obvious, I'm a bit confused.

I bought a couple of these active whip antenna kits (see photo) to try them out on an RTL-SDR Blog v3 dongle. Each one came with a separate module - one labelled "Low Noise Amplifier" and the other, "Bias-T".

I know that both modules are designed to supply DC to the active antenna component via the coax but

a) are they otherwise the same thing and can they be used interchangeably with either antenna?
b) if the modules are meant to do different things, can/should they be used together on the same antenna and if so, how connected?

Thanks

 

[krokus]

I'm a complete beginner so please excuse some dumb questions. As will be obvious, I'm a bit confused.

I bought a couple of these active whip antenna kits (see photo) to try them out on an RTL-SDR Blog v3 dongle. Each one came with a separate module - one labelled "Low Noise Amplifier" and the other, "Bias-T".

I know that both modules are designed to supply DC to the active antenna component via the coax but

a) are they otherwise the same thing and can they be used interchangeably with either antenna?
b) if the modules are meant to do different things, can/should they be used together on the same antenna and if so, how connected?

Thanks

The bias-T is to provide power to the amplifier, and is not an amplifier itself. You may be able to swap one bias-T for the other, but you need to make sure that you are sending the correct voltage, current capability, and polarity, to the amplifier.

 

[sacentre]

Krokus, thanks for your reply but I'm afraid I'm still a bit confused. I just want to understand a bit more about exactly what each one is for. Please excuse more questions.

You're saying a Bias-T is not the same thing as an amplifier. So I have one Bias-T and one LNA as the labels on them indicate which makes sense. But you go on to say that I may be able to "swap one Bias-T for the other". But I only have one, right?

I understand that the job of the Bias-T is to supply DC to the masthead antenna module. But the LNA also has a DC input. Is this to power both the external amplifier (the LNA module) AND the masthead antenna or only the LNA itself?

If the LNA is also a Bias-T, would that make it a better choice rather than just a Bias-T alone?

Thanks for your patience.
Trevor

 

[mmckenna]

Both units with and in and out connector are Bias-T's. Even though it says "LNA Out and LNA In", it's not an LNA. It's just telling you which port goes to what device (and doing it poorly, as you'd expect with Chinese products).

The amplifier is in the antenna module.

 

[sacentre]

Thanks, mmckenna. I saw your post immediately after replying to Krokus above. I kinda assumed from the PCB and components that they were more or less the same thing but these days (at 74) I have less and less confidence in my ability to understand stuff so ask a lot of questions. Quite agree about the lack of clarity with Chinese products. Neither of these kits came with any diagram or instructions. The price we pay for low cost, I guess.

As for the "LNA Out and LNA In", LNA IN goes to the antenna, I take it.

Thanks again.
Trevor

 

[Ubbe]

a) are they otherwise the same thing and can they be used interchangeably with either antenna?

They are the exact same thing. The bias-T modules that you set DC power to are only a coil to isolate the DC from the RF signal and some capacitors to block DC to the receiver and do RF decoupling. Either of those will work with either antenna as the voltage you input will also be the voltage on the coax to the antenna.

The actual antenna and its amplifier works differently in the two antennas. The top one have a DC regulator and probably supply 3.5volt to the amplifier IC. You can then power the bias-T with 5v up to 25v.

The lower antenna use another amplifier IC or transistor that use the voltage from the bias-T as is without any DC regulation. That will require a precise voltage from the bias-T to work in its designed voltage range, that might be 9v-12volt. You have to check documentation or if its some writing on the circuit board that tells what voltage it needs. That design are more prone to self oscillation and similar problems.

The RTL-SDR v3 have its own bias-T that you can enable. I don't remember if it was 3.5v but if it was 5v then you can power the antenna at the top of the picture directly from the RTL-SDR and don't need any separate bias-T module.

/Ubbe

 

[sacentre]

Thanks for the explanation, Ubbe

Neither of these products was supplied with any documentation.

The lower one in the photo has a DC socket and came supplied with a 9v battery clip.
The upper one has pads marked "DC" and "GND" but otherwise no indication of DC voltage. I assume 9V will be ok.

I knew that the SDR dongle has an on-board Bias-T but wasn't sure if it was ok to use that and discard the supplied module. I read somewhere that since it's powered from a USB port, the current is quite low but what that means for me in practice, I'm not sure. For now I'll just use what was supplied and maybe try the on-board one later. I don't think I've seen one of these SA0RDT type whips without a Bias-T module supplied.

What I thought I'd try is connecting the antenna directly to the Bias-T with an SMA male-to-male barrel connector, a second one to connect the SDR and then use a 5 meter active USB cable to the PC. This eliminates the coax. I'll put a ferrite choke on each end of the USB cable. Then I'll tape the whole thing to the end of a pole and stick it out the window. I'm on the second floor of a apartment block so there's no way to put a proper mast up. I'm not holding my breath for brilliant results.

Thanks again.
Trevor

 

[Ubbe]

Then I'll tape the whole thing to the end of a pole and stick it out the window.

Keep in mind that the coax from the antenna are also included in the antennas function and needs to be of a certain length to cover shortwave frequencies.

/Ubbe

 

[sacentre]

Ok, I see. Can you give me a rough estimate of what the length should be for HF in that case? I heard a recommendation from the RTL-SDR Blog that the dongle could be used this way because it was better to have the dongle as close to the top of the antenna as possible. They sell the cables for this purpose in their on-line store. I have very little options for RF grounding in my apartment which makes matters worse.
Trevor

 

[vagrant]

Whuuuut? Ubbe, please elaborate on the coax regarding the "...certain length to cover shortwave frequencies". Furthermore, please be specific with examples of a particular band or frequency range...or did you mean the antenna needs to be a certain length?

Keep in mind that the coax from the antenna are also included in the antennas function and needs to be of a certain length to cover shortwave frequencies.

/Ubbe

 

[Ubbe]

Radio waves consists of a magnetic component and an electrical component and a Mini Whip type of antenna only use the electrical part, the difference of the electrical field between that copper pcb antenna and the coax. I have seen reports that a 10 meter coax are needed to pick up the electrical signal to make an enough voltage difference to the antenna to be able to receive a signal.

Grounding of MiniWhip and other active whip antennas

Fundamentals of the MiniWhip antenna

www.pa3fwm.nl

/Ubbe