Wimpie wrote:
El 03-04-14 20:54, escribió:
wrote:
snip
My results (IE3D, now Mentor Graphics Hyperlynx):
Quarter wave radiator over 4 quarter wave radials, no sloping:
impedance at resonance 23 Ohms, Gain at zero elevation: 1.52 dBi
0.625 wave radiator over 4 quarter wave radials, no sloping:
Gain at zero elevation: 1.52 dBi, 2.29 dBi at 20 degr elevation.
And an impedance in the hundreds of Ohms.
From my memory it was closer to 50 Ohms, but for the gain figures
this isn't important.
But it is if you want to attach the antenna to a real radio.
0.5 wave radiator over 4 quarter wave radials, no sloping:
Gain at zero elevation: 2.05 dBi.
And an impedance of about 1,000 Ohms.
Depends strongly on thickness/lambda ratio, therefore I didnt mention
the value, and it isn't important for the gain.
But again, it is if you want to attach the antenna to a real radio.
Quarter wave radiator over 4 quarter wave radials, 45 degrees sloping:
Impedance at resonance 54 Ohms, gain at zero elevation: 1.97 dBi
Quarter wave radiator over 4 quarter wave radials, 85 degrees sloping:
Impedance at resonance 74 Ohms, gain at zero elevation: 2.14 dBi
All in free space, without a mast.
Again, in free space the maximum is ALWAYS at zero elevation.
Except for the 5/8 lambda, as I mentioned.
Yep, but it isn't a GP antenna which by definition has a radiator about
1/4 lambda.
Adding a mast, especially for the sloping case can give large
deviation depending on the CM impedance as seen from the floating
ground. I did simulations and current measurements for my own mast,
but the results cannot be applied to other configurations.
As I stated before, the difference between the configurations is
hardly measurable. Nice to see that the over-rated 5/8 lambda antenna
doens't perform better then the quarter wave antenna (at low elevation
angle).
I wouldn't call an impedance range of 20 Ohms to 1000 Ohms "hardly
measurable". In real life you have to feed the thing.
We were discussing gain....
You may be discussing gain but I am discussing antenna systems which have
gain, bandwidth and impedance and to be usefull have to be practical to
build.
Though the design is more demanding, I prefer the half wave option as
you can use less, sloping, shorter radials without running into common
mode mast current problems.
And requires some sort of feed arrangement to transform 1,000 Ohms into
50 Ohms.
In my opinion, dealing with the added complexity of impedance matching,
which is almost always narrow banded, is not worth a dB or two of gain.
I think I will stick with 5% longer radials at 45 deg and some ferrite at
the feed point.
What someone will use, depends on many factors (not only electrical
ones).
I have some experience with HV, and I know how to design these
circuits without loosing useful bandwidth, so I prefer the half wave.
I like racing and wood pigeons, but not on my antennas! In residential
areas over here, generally people don't like to see lots of aluminum
in the air.
So for my situation it is not the less then 0.5 dB addditional gain
over a classic 4 radial GP, but just the common mode issue together
with visible apearance (I don't want 4 radials). Note that we have
maximum 400WPEP in PA-land, that also makes it easier.
In practicality, you will see little difference between 2, 3, or 4 radials.
You will have common mode currents of some magnitude with ANY GP type
antenna.
You will be hard pressed to notice 1 dB difference in a typical amateur
system.
--
Jim Pennino