Hi Stefan,
> > The unexpected losses are not just the
> > capacitors (their contribution is probably small). It is also the
> > coaxial cable that I used between the antenna feedpoint and the
> > amplifier.
> How long is that cable? And didn't you use a ferrite transformer to
> match the loop to the amplifier? If it is a normal audio amplifier that
> expects a 4 Ohm load, then maybe you don't need one, but then you should
> place the amp close to the loop, without the coax cable inbetween. If
> the loop is far from the house and you want to stay the amp into the
> shack, then use 2 ferrite transformers to transform up to 50 Ohm and
> then down to the antenna impedance again. I would place the amp in the
> shack and monitor voltage and current there using a scope. Then you see
> their ration (Impedance) and product = power (at resonance). I would
> first build a transformer that matches 4 Ohm to 50 Ohm for example, then
> use the cable, and then a transformer from 50 Ohm to the feedpoint
> impedance of the tuned loop.
Given the antenna has an impedance of about ~4 ohm and the amplifier
produces its maximum output on that impedance, it was tempting not to
do any impedance transformation and to stay at 4 ohm.
I have an idea about impedance transformation and that is to use high
power 70V/100V audio distribution transformers at both ends. I don't
have to go to 50 ohm as I don't use the coaxial cable as a
transmission line (it's only a tiny percentage of the wavelength!), so
I could use a 4 ohm to 400 ohm transformer or similar. I think in
general the impedance transformation ratio goes down as the power
rating goes up and we can live with that. THe audio transformer should
work fine on the higher frequency but that will have to be tested.
Perhaps something like this but not at that price!!
https://www.altronics.com.au/p/m1160-300w-100v-line-pa-transformer/
Another option is to get a PA amplifier of suitable power output that
has a 70V or 100V output. You only need one transformer at the antenna
end to get down to 4 ohm to reduce losses. I have a 50 W amp with such
output but that's not enough power.
> In the end, such a closed loop will not perform as good as a ground
> loop. You will get much better results when connecting two of these
> little ponds beeing some hunderet meter distant. The current will be
> lower and so the capacitors will be less stressed.
I can easily try the ground loop on the 120m long antenna as I do have
ground rods at both ends. It would be interesting to compare the
signal strength in both cases. THat's why I am building a network of
VLF stations by giving away the large ferrite rods to interested
amateurs! They can help doing comparisons like this.
> BTW is your ground very dry? you could even try a dipole on the ground
> for the 630m band. A full size dipole. It would be able to carry much RF
> power :-)
The ground is very rocky and there is almost no soil at the top.
Typical Australian native vegetation is the only thing that can
survive on it. I have thought about the dipole on the ground and I
have the perfect spot for it which is putting the feed point at the
top of the hill therefore making an inverted-V :-) Only concern with
that is the high voltage at the end of the dipole wires and the
possibility of a bush fire ignition. Definitely a project for winter!
> And what about a vertical antenna?
I have tried to tune my ~120 metre wire to VLF in the past with mixed
results. Worth revisiting this. The wire basically starts at my house
and follows the hill resulting in the top of the wire being maybe 20
metres higher than the bottom.
> Oh i see, i would spend several hours per day there building antennas!
;-) It was part of the motivation for buying this property!
73, Dimitris
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