HT004a / LNA / Anyone used one?
So I thought I would give one of these a try:
https://www.aliexpress.com/item/1005007009575980.html
It arrived today, and it seems to overload the Kiwi something terrible with whistling and crazy waterfall patterns.
I turned the RF attenuator to 20db and it still overloaded. Shame.
Comments
Looking at the AliExpress pictures, it would seem to use a Qorvo TQP3M903 700 - 6000 MHz Ultra Low Noise, High Linearity Low Noise Amplifier
https://www.qorvo.com/products/p/TQP3M9037
Although many devices of this type can operate outside their specified parameters, it is difficult to tell how well this may work, without testing one in a specific application.
In this case, I suspect the IMD performance degrades outside the specified frequency range, and may not be adequate at short wave frequencies. I have used similar devices as the basis of an active antenna, but they easily overload if connected to anything other than a very small antenna element.
It's tricky to find a cheap commercial amplifier that has adequate performance to use ahead of the KiWi. I build my own design of RF distribution amplifier, and it was quite an undertaking to achieve decent IMD performance on the LF bands, whilst still having a good noise figure on the HF bands. In the end I used two separate amplifier chains, with diplexing filters to split the frequency ranges, but it is still barely adequate when connected to a decent antenna.
Regards,
Martin
I was a little fearful of blowing the front end initially given the noise it produced and the 30+ db of crap across the bands.
I have a similar amplifier (see link) on my Kiwi 1 with an endfed. But I still have a 16 dB attenuator after the amplifier, because the 30 dB overdrive the Kiwi ADC.
The quality is reasonably good - I've had worse amplifiers.
Regards, Steffen
That amplifier uses the same Qorvo TQP3M903 device, so I'd assume it's the same circuit too.
As before, the IMD performance isn't really sufficient for use ahead of a KiWi.
SV1AFN produces some good amplifiers that work well.
Although no longer a standard product, on request, he can also produce active splitters using push-pull pairs of FET's that work well. They have been successfully used at the Weston KiWi cluster, where we previously had severe problems with strong medium wave signals from nearby transmitters.
Turn Island Systems also produce a filtered pre-amp, using a Mini-Circuits device. Although the IMD specification is not too good, the use of a shelving and low pass filter ahead of it, minimises the performance issues.
I have no commercial interest in these companies, and do not specifically recommend them. I have simply highlighted them because of their characteristics, which may make them useful to KiWi owners.
Regards,
Martin
Martin, many thanks for the recommendation.
/gets credit card out.............
Before anyone gets their credit card (or other method of payment) out. Make sure you actually need a pre-amp.
In most cases, you will not, they are useful in specific situations, but unfortunately for the majority of KiWi owners, local noise and interference will be the limiting factor, and no amount of pre-amplification or filtering, can improve or remove, what is present, and intertwined with the desired signals.
One test, I suggest, is to make a note of your receiver noise floor with a screened 50 ohm load connected in place of your antenna. Then connect your antenna, and measure the noise floor again. Typically, do this on the HF bands, in a quiet chunk of spectrum, somewhere between 25 to 30MHz.
If the difference in noise floor readings is less than 6dB, you may benefit from using a better antenna, or in some cases a pre-amplifier.
If it is 6dB or more, then your system is already noise limited by your antenna and local environment, and this may be the best you can achieve. Any further improvement can only be brought about by using a better antenna, or by reducing the amount of noise present by some other method.
If the increase in noise is 10dB or more, then it is likely that you have a lot of noise and interference present, and this will be severely limiting reception. This may be because you are using a pre-amplifier ahead of your receiver, which is probably providing too much gain, and it will not further improve the sensitivity of your receiver, but it will be degrading the overall Dynamic Range, susceptibility to overload, and generation of spurious IMD products.
Unfortunately, because of the way that natural and galactic noise is distributed across the spectrum from 100kHz to 30MHz, the noise floor at 1MHz can be 20 to 30dB higher at 1MHz with respect to 30MHz. If you add a pre-amp to improve performance on the HF bands, the likelihood is that it will be providing excessive gain on the LF bands.
Many amateur band transceivers have different attenuator and pre-amplifier settings, that are automatically changed when a different frequency range is chosen. For example, on 1.8MHz a 10dB attenuator may be applied, whereas on 28MHz a 20dB pre-amplifier may be used.
When using a KiWi, which provides continuous coverage from 0 to 30MHz, we can't use this method. So the best alternative is to use a frequency / amplitude equaliser, or shelf filter, that provides a gradually rising amount of gain with frequency (actually an increasing amount of attenuation as the frequency decreases). By doing this, we can try to match the filter characteristic, with changes in the natural noise floor, so that we only provide as much gain as necessary at any given frequency.
In a practical installation, we also have to take into account the frequency response of the antenna, and it's gain characteristics. But this usually only requires a slight modification to the filter response.
This is a link to an old article entitled "Noise equalization in h.f. receiving systems" written by D. C. Bunday, a UK GCHQ engineer, way back in 1977. It explains the concepts very clearly, along with practical circuits. Although the notes refer to problems associated with Analogue receive systems, they are equally applicable to modern day SDR's and digital Short Wave receivers.
https://www.worldradiohistory.com/UK/British-Institution-of-Radio-Engineers/1977/TREE-1977-05.pdf
Regards,
Martin
Martin writes:
"If it is 6dB or more, then your system is already noise limited by your antenna and local environment, and this may be the best you can achieve. Any further improvement can only be brought about by using a better antenna, or by reducing the amount of noise present by some other method."
I don't necessarily agree. Yes, the system may be noise limited but the source may be common mode noise that you can eliminate or mitigate. On many, many WSPR systems I've studied, a majority if the databases are to be believed, the limitation is severe but NOT due to noise coming from the antenna itself. These limitations can be extreme, easily 20-30 dB. They appear as noise present within the antenna when in fact they are not. Nearby 'good' systems can perform vastly better as the databases show.
A preamplifier actually located at the antenna which eliminates transmission line can in some cases push the effects of downstream sources of degradation lower, relative to the desired signal and SNR actually present within what was thought to be the antenna. It moves the system in the direction of making the "antenna be the antenna" and away from "the antenna and all the cruft, CM current, ground loops etc" being the effective "antenna".
After all these external degradations have been sufficiently reduced or removed, there is still the potential for near-field coupling to the perceived antenna. This can be either 'magnetic' or 'electric' but varies as a higher power of distance than does the far-field, DX propagated signal and SNR that is the goal. This is critical since there then exists an option to make small location/polarization changes which may have great benefit in further SNR improvement.
So while I agree that blindly throwing a preamplifier onto a system is not a wise choice, when one examines a particular system to determine the causes of SNR limitation, sometimes, even if for the wrong reasons, a preamplifier might improve performance even when the added noise mentioned is sufficient to overcome the Kiwi or other SDR floor.
Hi Glenn, I don't disagree, which is why I stated "Any further improvement can only be brought about by using a better antenna, or by reducing the amount of noise present by some other method."
If for example you have a problem with noise induced into the transmission line, amplifying and balancing at the antenna, then attenuating at the receiver, can reduce the noise contribution. This is only one example, but as you have said "sometimes, even if for the wrong reasons, a preamplifier might improve performance".
The point I was trying to make is that by just adding extra gain, it usually doesn't make things any better. It requires considered thought to do it correctly.
Regards,
Martin
Martin
The differentiating point I was trying to make is about 'local environment' You wrote that further improvement can only be improved by using a better antenna. I was trying to make clear that there can be and usually are multiple causes for increased noise in a local environment. Some of these are actually arriving by way of the perceived antenna due to near-field coupling but a second type come in after that in the form of CM currents and other kinds of systemic ingress.
A preamplifier at the antenna may actually help reduce this second type and create an overall improvement. This improvement would not be due to an antenna change, at least not what was thought to be the antenna. In fact, in that case "the antenna was not the antenna" ! The system had multiple but separable sources of unwanted ingress.
It is a widely held belief that the shield on single ended/coaxial cable prevents ingress from occurring there. This is only true for TEM systems where there is zero common mode current, unwanted current flowing on the outside of the coax. When such current does flow on the outside, then imbalance at either end converts some of that current to differential current in the desired TEM component.
Because the systems we use often deal with such small levels - down near the thermal noise limit of KTB, it can require far more system balance to sufficiently reject unwanted currents than is achievable with passive hardware. So-called 'broadband baluns' are often only able to achieve 20-30 dB of balance at best. This can be many ten's of dB less than is necessary to force the currents due to unwanted conversion below the incoming noise floor.
Unwanted current traveling through the groundplane of the KiwiSDR, for example, is converted "only" 80 dB down to current through the center conductor of the coax and injected into the ~50 ohm input impedance of the preamp. That CM ==> differential conversion can and does easily damage reception SNR, often without the knowledge of the user who may think "I hooked up the coax and the noise floor came up, I guess it came in from my antenna" when in fact the majority of it did not.
Our goal is to transfer the DX (far-field) propagated SNR present within the radiation resistance of our antennas to the detector of our receiving system. Without understanding and identifying the multiple mechanisms by which this can be degraded and which are active in a particular system, there really are no simple answers to improving a [poorly behaving] receiving system. As you suggest, blindly adding a preamplifier may make no improvement at all and may only make things worse due to IMD, internal noise or additional conversion from CM ==> differential QRN.
Without understanding that "our antennas are not what we may think they are" and "coax shield does not prevent unwanted ingress" the techniques we might use to improve our results likely will not deliver the improvement that is possible.
I find this an interesting (almost) paradox.
On one hand, we have the KiwiSDR, designed to be an all-in-one, simple to use out-of-the-box solution, and on the other, the antenna, which needs to be carefully considered, with real science applied to ensure the best experience from your Kiwi.
My understanding of RF is enough for me to scrape through the hobby, yet I am willing to throw money at boosting the signal on my passive antenna with little to no other consideration, other than suck it and see. Hence, the reason for this post - almost as a warning to others.
Myself, and I would assume a number of others whom lurk within the confines of this forum, deeply value the knowledge and expertise of both Martin and Glenn, and with healthy conversations, keep the spirit of the hobby alive and well, while helping the rest of us mere mortals put the credit cards down and await an objective outcome.
That said, I'm OK spending a little here and there, learning and keeping the people who supply these devices in business - even if it's a bit misguided.
Glenn with respect, I also said " or by reducing the amount of noise present by some other method."
This was intended to encompass other factors, such as those you have mentioned. The key thing was that the local noise, introduced by whatever means, had become the limiting factor, and this needed to be addressed, to facilitate further improvements in SNR.
Just increasing signal levels by adding a pre-amp will not achieve this, unless it unwittingly disrupts some other mechanism causing noise ingress.
We are reading from the same page.
Regards,
Martin
OK. 'unwittingly disrupts" and "for the wrong reasons" can speak to the same mechanism(s). But the added preamp itself can be that method.