Paul,
There will be a benefit for you, so if it is done easily, do it! When it can be
done easily, why can't you bring up all the L as high as possible?
Imagine the E-field lines that come out of the antenna wire. You can imagine
that by an analogical example:
A garden hose, in the form of the antenna wire construction, with many holes in
it. Imagine you have a compressor on the bottom and connect the hose there. The
water pressure is equivalent to the voltage, the compressor has the function to
increase the pressure; the coil has the function to increase the voltage. The
hose diameter is assumed to be very big against the hole diameter so the
pressure is almost the same at each part of the hose (even at the end). Your
goal is to bring as much water (water [liters]is here equivalent to electrical
charge [As](absolute value)) as possible through the vertical part of the hose
without loosing to much on the way to the top. This is because, in the
electrical case, you want to fed as much current as possible through the
radiation resistance to radiate the power I^2*Rrad. The water (charge) that
comes out of the hose in the vertical part before reaching the maximum vertical
height is not crossing the upper part of the radiation resistance and thus the
ERP is smaller. So, it is the best to place the compressor/coil as high as
possible since if the pressure/voltage in the vertical section is smaller (like
before the compressor/coil) there is not so much water pressed through the
holes in the vertical section!
And if you increase the horizontal section of the construction, there are more
holes in sum above the compressor and thus more water is pressed through them
at a given pressure. So, the more holes the more water, or charge (=current per
cycle) in the vertical part.
This analogical model can explain even more but surely there are also things
that cannot be compared (like a AC water flow ;-); no closed circuit... ). But
for a first imagination it is useful in my point of view ;-)
If you place all the L on the tower it will be optimal. Then, you could use a
simple wire that connects to your earth system without the need for a distance
to other metallic parts, since there is no HV on this wire any more! That makes
it easier even more!
You can inject the power by applying a isolated primary on the coil and connect
it to your PA in the shack by a loudspeaker cable. Or you need a transformer
and inject it on the bottom of the wire between earth and lower part of the
coil. That may be even a litte easier...
A later step will be to reduce the earth resistance but you can do that after
starting to come up on the band since there will be no changes to the coil,
maybe just to the output ratio of the PA transformer. Which PA will you use?
Best 73 and much fun!
Stefan/DK7FC
________________________________
Von: [email protected] im Auftrag von Paul A. Cianciolo
Gesendet: So 04.04.2010 22:28
An: [email protected]
Betreff: LF: RE: W1VLF 9 Khz Antenna update Putting inductance near the top.
Stefan
On page 60 and 61 of the Low Frequency Experimenters Handbook Peter Dodd RSGB
there is an example putting a coil near the top of the antenna
In this case the transmitter is on 137 Khz and a of a portion (1/2) of the "L"
needed to resonate was moved near the top. According to measurements by Rik
Strobbe ON7YD an increase of 4 to 5 db was measured.
Do you think it would be worth the effort to put 1/4 of the total "L" needed
near the top
I could do that very easily.
PaulC
W1VLF
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