Range of tolerated Output voltage from Active Antennas to Receiver

[alpard]

This recently acquired active RX antenna amplifier is AC T-Bias powered which sends 12V DC to the amp.  From the T-Bias box, there is output BNC socket which connects to the radio antenna.  I connected it all up, and powered it on.  When measured the output voltage from the T-Bias box to the radio, it was whopping 3V DC.

Is this right?  I thought the voltage to the radio antenna input was just too high.  Long wires and dipoles and GP antennas with no amplification has 0V when measured.  How is it possible to feed 3V DC into the radio antenna input?  Would it not damage the front end when fed with active antennas with this type of DC voltage?

Just wondering.  What are your thoughts on this?  When the radio was switched on with the active antenna outputting 3V DC, the receptions on MW and SW were overloading with loud MW signals all over.  Had to put on the 30 dB attenuator on, but then the radio becomes deaf. 

[alpard]

My old MLA30+ outputs to the radio antenna about 50 mV.

[ChrisSmolinski]

Active antennas often send DC down the coax to power a remote amplifier circuit. They also typically have a DC blocking capacitor on the bias tee injection circuit to block DC being sent to the receiver. Yours seems to be missing that. You have to wonder what else it is missing.

Many (most?) radios have a capacitor on the RF input, which also would act to block the DC. If yours has that, it should be OK. If it doesn't, then yes, the radio could be damaged.

You could always add your own capacitor inline to the radio connection, to block DC. The value depends on the frequency range, from memory something like 0.1 uF should be OK?

[alpard]

Sure.  Thanks for your great reply.  It has given me some understanding on what is happening with the Active Antenna and also radio antenna input.
I was using the old radios Kenwood R-5000 and R-1000.  They were not too happy being connected to the active antenna pumping out 3V DC.
The reception didn't sound right at all.  But they don't seem to have been damaged in anyway, as they went back to their usual good performance in reception when paired with the longwire and 91 balun.

Yes, I do need to investigate the active antenna further. 

[ChrisSmolinski]

I was using the old radios Kenwood R-5000 and R-1000.  They were not too happy being connected to the active antenna pumping out 3V DC.
The reception didn't sound right at all.  But they don't seem to have been damaged in anyway, as they went back to their usual good performance in reception when paired with the longwire and 91 balun.

It's entirely possible the poor reception was not due to DC voltage on the antenna connection, but the amplified antenna itself. Many (most? nearly all?) of these Pride Of China designs are terrible, generating a higher overall noise floor, and lots of mixing products and other nasties.