NASA Marine Active Antenna for reception problems

[alpard]

I got this antenna a few days ago, and installed it.  It works great during the day above say 9 Mhz upwards.
But at nights, and below 7Mhz, it seems to be overloading the radios.  It is saturating the reception at nights with MW signals on HF.

What could be done to this antenna to improve the reception?  Add attenuator 10 - 20 dB?  Feed with less input voltage say 10V instead of 12V?  Add antenna tuner between the radio and antenna?   Add balun?  9:1?  49:1? 4:1?   

Any thoughts?  Tks 73s

[Charlie_Dont_Surf]

First question: I assume you are talking about this?
https://www.nasamarine.com/product/hf-active-antenna/

Second: Does it have a red or black-colored label?

Third:
If black, unplug the power supply. What happens to the overloading below 7 MHz and to reception above 7 MHz?
If red, turn the switch on the back of the antenna off. What happens to the overloading below 7 MHz and to reception above 7 MHz?

The questions I have asked and the actions I have asked you to take relate the built-in amplifier. Turning it off is the first part (and maybe the only part) of the process of removing the overloading. The overloading is occurring either because the amplifier is saturating and creating mixing products or the amplified signal from the antenna is overloading your receiver so answering these questions are important.

[alpard]

Thanks for your reply.   The antenna is used with the Nasa Target HF-3, which is also from the same company.  At the back of the radio, it has red panel, which means that it has built-in amp for the NASA antenna.   When it is used with the built-in amp, the antenna seems to give more overloading at nights on the lower frequency reception below 7 Mhz.   Hence I use the battery powered T-Bias with around 10-11V DC input into the T-bias.

When the NASA antenna is not powered by T-Bias or built-in amp, it becomes totally deaf, and the reception goes silent.  The NASA antenna must be powered by the amp (either by the builtin amp or T-bias) at all times in order to get any reception on the radio.  Hope it makes sense.

[Charlie_Dont_Surf]

Yes, I understand. Clearly you need some amplification to use that active antenna at all, which is what I would expect.

Understand that the overloading, which I will sometimes call "spurious" below, could be coming from the active antenna amplifier if it is overloaded or it could be coming from the receiver "front end", the circuitry before the signal detection and conversion to audio. We're still in the mode of figuring out where the overloading is coming from, so a couple things to try, in recommended order:

1) You can buy or make a fixed or variable attenuator and install that between the receiver and the active antenna. I suggest a 20 dB attenuator as a start, as a nice round number. This should reduce all signals coming into the receiver, spurious or not. You may have to increase the attenuation beyond 20 dB to see things start to happen.

For the purposes of experimentation and to avoid having to buy something, you can try making "a poor man's attenuator" with a thru-hole resistor (i.e., a resistor with wire leads) and maybe some alligator clip leads, placing the resistor in series (i.e., in line) with the output of the active antenna. Use whatever you have available; it doesn't have to be precise, but know that in this case more resistance should mean more attenuation. Try something in the 100 to 500 Ohm range to start.

 If you can kill the overloading at the cost of only slightly impacting received signal strength, then this is the best case and the overloading is likely coming from the receiver "front end". If you haven't already, make the attenuator a permanent fixture and go about your listening. You are done.

If you keep increasing attenuation and get to the point where things are starting to go "deaf" and there are still signs of overloading, even while the overall level of everything is reduced due to the attenuator, and the proportion of overloading to non-spurious seems about the same as before you added the attenuator, then it's likely that the antenna amplifier is the root of the issue. This is because adding the attenuator after the amplifier means that you have done nothing to impact the incoming signals to the active antenna, thus it is spewing out spurious products just as before and the proportion of spurious products to intended signals remains the same.

2) If the the results of 1) above point to the antenna amplifier here's something to try:

If you have a variable power supply (even a switching supply), you can try running the antenna off an even lower voltage than from the battery, (<10 V) and see if that improves the overloading. You can continue to lower the voltage until it either goes away or at least becomes tolerable. (You won't damage anything by doing this.) You will hopefully notice things becoming increasingly "deaf" as you decrease the voltage. Given the results of 1) above, I would expect that the overloading reduces massively as you vary the voltage to the amplifier. This is because reducing the voltage to the amplifier reduces the gain of the amplifier and reduces the tendency to overload.

At some point the amplifier won't work very well but I suspect that won't happen until you arrive at a much lower voltage, maybe 6 Volts, so there will be plenty of room to experiment with. If you like the results at some arbitrarily lower voltage, you can buy or make a fixed voltage linear power supply to power the active antenna at that voltage to replace the variable one, if you so choose. The active antenna does not demand much current, likely less than 1 Ampere.

[RobRich]

There is a chance lowering voltage on the active antenna might actually lower the integrated preamp's signal handling performance. Perhaps not as I suspect the active antenna uses a basic preamp circuit, but I have not seen a schematic, so YMMV. AFAIK, your SDRPlay devices have integrated 4.7v bias tees, which might be worth trying instead of the standalone bias tee.

Otherwise try an AM BCB notch or highpass filter between the bias tee and the receiver(s).

Some popular examples:

https://www.amazon.com/Flamingo-AM-Attenuation-Broadcast-Applications/dp/B0BHXFLB72
https://www.amazon.com/Broadcast-Reject-Filter-RTL-SDR-Blog/dp/B01N9SHS7P
https://www.amazon.com/Distill-Barebones-Broadcast-Bandstop-Applications/dp/B076CVW6LC

Note you might drop out much of 160m as well with these basic MW BCB filters, assuming you have a need for the band.

Naturally there are better filters with sharper filter responses, but plan to homebrew or spend more. I know my PAR BCST-HPF has a rather sharp highpass filter point to keep 160m largely intact, but it is also a 7th-order elliptical filter circuit. Like $70 years ago and probably not in production anymore.

https://www.parelectronics.com/bcst-hpf-specs.php

[Charlie_Dont_Surf]

There is a chance lowering voltage on the active antenna might actually lower the integrated preamp's signal handling performance. Perhaps not as I suspect the active antenna uses a basic preamp circuit, but I have not seen a schematic, so YMMV.

There is that chance that we change the IP2/3 performance and/or there might be a risk of that its headroom is "crushed" by lower voltage, both of which would make things worse. However, that's not enough of a reason to not experiment with it IMO since there are few "knobs" to "turn" here to ameliorate the situation.

Otherwise try an AM BCB notch or highpass filter between the bias tee and the receiver(s).

Assuming the overloading is coming from a strong local mediumwave/AM BCB TX, which may or may not be the case, which is why I went with a broadband attenuation as a first try.