FYI....cumulative discussion...bottom posted...please keep discussion
intact
for future antenna builders who want to know about this subject. All
antenna guru's
are invited to post.
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On Sat, 16 Feb 2019 09:17:25 -0800 jimlux <jimlux@earthlink.net> writes:
> On 2/15/19 3:37 PM, Richard (Rick) Karlquist wrote:
> >
> >
> > On 2/15/2019 2:01 PM, n0tt1@juno.com wrote:
> >
> >>
> >> The question is, if a builder wants to make "loads" for bench
> testing
> >> (to simulate the actual antenna feed point impedances), how is
> the
> >> negative
> >> load resistance simulated? If there is, say an impedance of -27
> + j3.0
> >> Ohms,
> >> does the builder just ignore the negative sign of the resistive
> part of
> >> the load
> >> and use a 27 Ohm resistor in series with the required amount of
> inductive
> >>
> >> reactance or what?
> >>
> >> Thanks,
> >> Charlie, N0TT
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Rick wrote:
> > No, you definitely can't ignore the sign of the resistance.
> >
> > See this construction article about how to realize a negative RF
> > resistance:
> >
> > http://www.vk6fh.com/vk6fh/lambda_diode.htm
> >
> > VK6FH shows how to use this to make a "Tunnel Dipper"
> > without the tunnel diode, but you can repurpose.
> >
> > Rick N6RK
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Jim wrote:
> I'm not sure you can build a negative resistance antenna simulator
> with
> that.. it's negative "dynamic resistance" (small increase in voltage
> is
> small decrease in current), but still positive overall. I suppose
> you
> can AC couple it, and then your circuit has to work at RF.
>
> Another approach is use an amplifier, but that has all sorts of
> complexities too - one could probably get it to work in steady state
> at
> a single frequency.
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Jim and Rick,
I studied the info from the link Rick provided....not sure I
can entirely relate the article's negative resistance with the negative
resistance at some antenna
elements. More....
At first thought, I think all I need to do is to provide RF from the test
oscillator at the
proper phase and voltage and then feed that back into the
reflector/network. In
this case, the test osc provides a relative 3 volts and the reflector
needs the
same phase but 1/3 the relative voltage. So, an L-network or 9:1
transformer
could be used. But does that seem like the right thing to do?...I don't
think so.
The problem is, that it would do nothing. It would simply put the same
voltage with
the same phase in parallel with the dir/refl feeds.
As an illustration, take a look at ON4UN's LBDXing book, 5th edition,
page 11-110, figure 11-161 at the top.
The Director and Reflector are in phase and fed with 1/3 the relative
array
input voltage via a 9:1 transformer. I calculate the complex impedance
of -48.2 - j48.3 Ohms at
test point "m" in the drawing....that's with the impedances shown at the
input
of the "lossless" 3/4-wave lines that feed the two paralleled elements.
Any suggestions/input about this subject is welcome!
73,
Charlie, N0TT
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