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Originally Posted by WM6
Yes, but huge changing in propagation (like changing wave polarization etc) are in first line characteristic of very high frequency band as UHF not in VLF/ULF band. By local small transmitter we find some others aspect of problem. We are searching for weak source of reflected signal and at the same time we have in vicinity relative strong source of transmitted signal at the same frequency which cover weak target signal and disturb successive search . This is why I repeated that 0.3W is far enough and even too much for successive search. This is why we need as much as possible directive RX antenna to suppress direct TX signal in combination with some tricks and searching skill.
If we compare UHF and ULF frequency band from reflectivity view UHF are better to detect especially small target in air because of its directivity and worse to detect something underground because of his weak penetration ability, on other side ULF are weak on directivity and so worse to detect something in air but way better to detect something underground because of his excellent penetration ability in soil. Theoretically ULF radar can detect invisible objects (objects behind of hills or behinds of horizon) which UHF radar cannot. This is why ULF can reach targets on unusual way and why we speak about importance of RX sensitivity and Rx antenna directivity.
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Hi WM6,
What you say about ULF radar is very interesting. ULF has a very large wavelength (many km) which makes it hard to image any small object from interference/reflection in the way microwave radar can. But the ground penetration is excellent for very large things. If you wanted to use RF to detect things in the ground, then VLF would be more suitable because the wavelength is shorter to allow better resolution of the buried object. As you increase the frequency, the resolution improves but the ground penetration is less. So a treasure hunter is looking for a frequency between the low and high that will have good ground penetration to maybe 1 meter, and good resolution to be able to locate where the anomaly is.
We know geologists use frequencies usually between 10-30KHz to map large rock fractures and ore deposits as well as fault lines. But they cannot find a buried coin 10 cm deep. Their signals are long wavelength that detect up to 25m deep in the ground or more.
But if we raise the frequency to 100 KHz, then the wave becomes shorter and will only penetrate to maybe 10 meters deep. At 250KHz, we expect even less depth, but better resolution of shallow targets.
My question is what is the best frequency you would consider for as treasure hunter who only wants to see to 1-2 meters depth maximum?
From the charts I look at, it seems that this may be very close to the lower AM broadcast band of 550 KHz.
Do you think an AM radio or modified AM radio at below 550 KHz could be used for looking at LF/VLF interference under the ground?
Also, is there a way to use these frequencies as a VLF radar?
Best wishes,
J_P