The goal is admirable. I did some modeling of inductor shortened BOGS
with EZNEC Pro4 and found it is extremely sensitive to height above
ground. A half an inch change in height can reverse the pattern. Thus,
I consider it an impractical antenna given the nature of most real
ground surfaces, i.e. exactly where is the modeled "perfectly flat
ground of uniform properties". This is in addition to the practical
realization of the needed inductors.
Grant KZ1W
On 11/21/2019 19:09, K4SAV wrote:
Mikek said:
"I'd like to apply the loading to a BOG to slow the VF and make it seems
longer.
ie. make a 80 meter BOG length work on 160 Meters. But then make the
reactance go away for 80 Meters
My actual goal is to have a BOG that covers 500kHz to 4MHz. "
Mikek, I see you haven't given up on this project. Experimentation with
antennas is one of the things I like to do too. Good luck with this. I
won't offer possible alternatives to do this (there are several) but
will consider only your desired approach.
I don't see any switches in your circuit diagram to switch series
inductors, so I assume you are trying to make them variable and have
them operate from 500 kHz to 4 MHz.
There are some difficult tasks ahead to make this work over a frequency
range of 8 to 1.. One is the RF choke. EZNEC simulations show you need
about 1mH or more to get enough impedance at 500 kHz. The problem is
that the self resonance of the choke has to be well above 4 MHz. A
distributed capacitance of the choke of 1.6 pf would cause the choke to
resonant on 4 MHz, so the distributed capacitance must be significantly
less than that. It may be possible to build an inductor like that but
it will have to be an air wound coil using very small wire and well
separated from its surroundings. A ferrite or powder irom core won't
work. Accepting some degradation of the pattern at 500 kHz would
decrease the choke requirements. Other than that, I don't have a
solution for this, other than something complicated, like switching
chokes as a function of frequency.
Those values of inductance were derived from simulations and there is
some possibility the numbers may not agree with experimental results.
Position of the wire carrying the current may affect the pattern if it
is close. Too close (like in the same cable) and the capacitance
between the wires will effectively short out the inductors at RF.
The resistive component of the impedance of the series inductors is
important. Inductor material is important. If the resistance is too
large, it will kill the response of the antenna.
DC blocking caps should present no problems.
I assume you have breadboarded the series variable inductor and
determined the inductance range needed to cover this frequency range.
Mikek said:
"I see certain measurements for the length of a BOG, such as 200ft for
for 160 meters."
You will find various recommendations for BOG lengths from different
people. I didn't trust NEC to give a correct answer for this so I spent
a month comparing a 250 ft BOG to a 366 ft one. I posted my results here:
http://lists.contesting.com/archives/html/Topband/2018-08/msg00073.html
Jerry, K4SAV
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