Hi Terry. First of all thank you very much for taking the time to help
out and for chiming in. It is interesting because what you have is what
I wanted to do but did not know how. I wanted to be able to put the
coils at the feedpoint because I have a tower & arm that will be able to
suspend the feedpoint & inductors even if it weighs 5 pounds.
I realize the losses will be much greater here in the center vs out in,
say, the middle of the elements where the current will be less but that
will require even more "L" and will cause a lot of stress & sag on the
wire elements.
So, center loading it is. You helped me with the one big issue I had and
that was placing the gamma coil across the feedpoint. Guy also helped
out there. But using my limited EZNEC program I was not able to get a
proper solution using the model he & I developed.
I am very eager to try out what you did here, and again, thank you very
much !
Oh, now truth be told, the actual antenna will be longer on the
(negative Y) leg. That length will actually be around 185' and will have
a 60' vertical drop wire off the end to add capacity and thus lower the
required "L". And, that far end will actually be a little higher, it
will be at 85'.
Not that it makes any difference on these frequencies hi-hi......70',
85', or 150' they are all kissing the ground. But as mentioned earlier,
I know full well before I even build it and try it that it will not work
well, it is just something I want to do for my own curiosity and
something I can check off the list.
I already have plans for a fairly long Inv-L but my biggest problem will
be an adequate ground system. My cutting tractors have eaten more coax
and #14 wire then you could ever imagine over the last 20 years or so !
I am so tired of crawling under the 6' cutting decks with a pair of
dikes and hacking away at hundreds of turns of twisted mess around 3-4
props hi-hi.
Anyway, I will readjust your model with my longer wire elements and see
what that gets me....and again, thank you very much ! 73
Gedas, W8BYA
Gallery at http://w8bya.com
Light travels faster than sound....
This is why some people appear bright until you hear them speak.
On 3/7/2019 2:18 PM, Terry Conboy wrote:
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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