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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

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

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

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

Don't buy the Bonito loop.

Have a look at the Loop Antennas IO group.

https://groups.io/g/loopantennas

You will find some familiar names on there, and one or two, are offering ready built loop amplifier PCB's based on Wellbrook and LZ1AQ's designs.

My personal favourites are versions of the LZ1AQ loop amplifier. But for a bit more money, you could buy a much more versatile ready built LZ1AQ active antenna amplifier, that facilitates both switched active loop and dipole modes. This is highly effective and still costs a fraction of the Bonito (which is based around a cheap video amplifier chip).

https://active-antenna.eu/

Chavdar's other webpages are a goldmine of information and highly recommended too.

https://www.lz1aq.signacor.com/docs/lz1aq-topical-article-index.php

I hope this provides some alternative options.

Martin