G8JNJ

These are typical of RF welding / Drying / Curing processes. Very high power RF sources are also used during some types of semiconductor manufacturing processes.

Another source, is from the cheap Chinese "Radio Frequency Facial Machine" or "wands". These typically produce between 5 to 25 watts, but some are rated at several hundred watts.

All of the above can usually be observed around most of the ISM allocations, and they are a good propagation indicator on the low VHF bands.

Here are a few that should really be in the 40MHz ISM band, most likely from Asia.

When I used to live in an industrial town, we had one in a factory that welded seams on industrial clothing, that operated on 27MHz, and the Interference Investigators were forever chasing the owners, as they often removed safety covers, so that they could run the machine for longer before it overheated. The stray RF signals and harmonics used to interfere with the 2m amateur band and local taxi services over a very large radius. Once you have heard the distinctive warbling, humming, and drifting signals, you can easily spot them again.

Regards,

Martin

It's a common US HFGCS frequency, and they are sending an encoded Emergency Action Message (EAM)

Nothing to worry about (yet) these are very common transmissions.

But I'm pleased your SNR is creeping up :-)

Regards,

Martin

Good luck.

I don't use mine that much, but if you are interested in radio, it's always handy to have an amateur licence.

In the past, it used to be a valuable "pseudo" technical qualification when seeking employment, but those days are long gone.

Have fun,

Martin

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

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

It's an amateur Slow Scan TV SSTV calling frequency.

Listen between approximately 7033 and 7085kHz, and you will hear lots of other strange sounding digital modes.

Regards,

Martin

If you are using an unbalanced antenna, it needs something to provide the "missing" half, that would otherwise be present in a balanced antenna. All simple antennas are basically dipoles. It's just that sometimes you can't figure out where the missing bit is, but it's always there !

The trick with unbalanced antennas is to provide the "missing" half in a controlled manner, and not to let random conductors, which may be polluted with noise from other sources, provide it.

Use a separate "clean" earth at the base of the antenna, and choke off unwanted noise that may be present on the feedline, by means of strategically placed common mode chokes and maybe additional "clean" earth connections along the cable route, as this improves the rejection by working a bit like a potential divider network.

Burying the coax can also help, as this provides both additional shielding and a low impedance path to ground.

Balanced antennas with good common mode rejection are nearly always better performers, and low impedance balanced antennas, such as broadband loops, tend to have intrinsically good common mode rejection characteristics.

Regards,

Martin

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

Yes, I think 1 & 6 connected alone, should be loops A & B

I believe you have 4 & 6 transposed on S1 in your switch diagram

Regards,

Martin