Gedas,
I think you’ve gotten some great advice on this antenna, but I think this is
better as a thought experiment than a real antenna.
I’ve attached my EZNEC model for your consideration (which will probably be
scraped off by the list server). It is a lot simpler than your original
configuration. I just used a single 260 foot straight wire with 99 segments
and placed a pair of 476.8 uH inductors with 3.55 ohms of resistance (Q=400) at
49% and 51% (Config=Ser, Ext Conn=Ser). Then the feed Z is 10.31 - j 20.4
ohms. Of this, the radiation resistance is about 0.115 ohms over perfect
ground with zero loss conductors.
To match this to 50 ohms, I placed an 8.5 uH coil (R=0.0633 ohms, Q=400) at the
50% point on the wire (Config=Ser, Ext Conn=Par). This shunt match (aka
hairpin) coil creates a virtual L-network that steps it up to 50.42 + j 1.14
ohms (SWR = 1.024). Using the “Par” load connection allows you to dispense
with the extra conductors around the feed point in your model.
Note that it is important to use RLC loads in EZNEC rather than R+jX loads.
Otherwise, the reactance will not change with the model frequency and your SWR
plots will show much wider than actual bandwidth.
With these loads, the 2:1 SWR bandwidth will be only 1.2 kHz. Using “average
ground”, the radiation at the zenith is about -9.3 dBi and the signal at 10
degree elevation will be -22 dBi or less. And depending on your actual soil,
these gains may be somewhat optimistic. Note that the loading inductors will
dissipate 68% of your power, which represents a 5 dB loss.
Actually creating the high-Q 477 uH inductors will be fun and their weight may
be an issue at the center of the dipole, too. Tuning will also be challenging
with such a narrow bandwidth.
Let us know if you you actually put this on the air and how it works out.
73, Terry N6RY
> On 2019 Mar 5, at 4:30 PM, Gedas <w8bya@mchsi.com> wrote:
>
> Hi Guy. ok now that I have an initial model built correctly I can proceed
> with my experimentation. You had asked "The next question, what are you
> trying to accomplish with this? You wanting to put this in the air? Coming up
> with the coils will be interesting. 1265 ohms X is a lot of wire turns." The
> simple answer is pure fun mixed with a little science.
>
> In reality I have the property to make a full sized inv-v type dipole for
> this frequency and suspend it from 85' but I realize even this would be a
> pure cloud warmer as the antenna is kissing the ground. Your analogy of the
> 40m antenna is spot on. But I could eliminate the inductors completely if I
> wanted to. I am doing this out of curiosity. I like to see how reality
> differs from different models using different models.
>
> Ok, so getting back to this antenna.....I already have the 160m dipole up in
> the air operating and it is purely curiosity driving me to try this antenna.
> I know with 100% certainty that I will be going with a vertical radiator fed
> against some sort of ground but given I like to play with antennas, in
> particular model them, build them, and actually put them on the air this is
> only for my personal gratification and my own curiosity.
>
> I should note that the values of "R" for the inductors in my model were 100%
> guesses on my part before I did any calculations. That was going to be my
> next question to you. I was wondering if a half-way accurate number for the
> resistive component could be obtained before the inductors built and
> measured. I do own a VNA (actually an ENA) from Agilent that may help me get
> an accurate number but I am wondering how accurate the number would be based
> on the predicted Q from some of the better Inductor Calculator programs out
> there.
>
> I have just run several calculations to make a large inductor using a 4.5"
> dia form and if one can trust the predicted Q at this frequency to be around
> 400 is it safe to initially use a value of 3.2 ohms for R?
>
> I will of course measure it with the VNA to see how close this number is but
> is this the value for R that I would use (R derived from XL/Q) ?
>
> Gedas, W8BYA
>
> Gallery at http://w8bya.com
> Light travels faster than sound....
> This is why some people appear bright until you hear them speak.
>
>
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