Guess I'm thinkin' outside of the box again..
Guess I'm thinkin' outside of the box again..
How important is it to have a vertically polarized antenna in the AM Band with our limitations? How much "polarity" does a legal Part-15 antenna really have? I can't imagine the pattern has much structure, being as short as it is..
After the reads on the Isotron being basically an LC tank circuit, why couldn't a circuit (antenna) be constructed horizontally on the ground, using the basic Isotron theory? The plates could be made large and low frequencys could be a possibility! The distance between the "plates" wouldn't be as important as the height. (I think) It could pass the muster, who knows..
I'm imagining using an Sstran in the center of the beast.. Solar powered.. STL connected..
Thoughts? Comments?
Your description sounds like an antenna I read about some time ago called the Super C, made by Gap.
It consists of a large cube made of wire mesh, on a post, over a large wire mesh screen. The size is about 3 or 4 feet on the cube and about the same for the mesh screen below it.
There is a match box or loading coil between them on the post.
The idea is the large surface area produces a large capacitive component resulting in a low capacitive reactance. As such the loading coil is greately reduced which in turn reduces losses typically encountered.
This was designed for the Amateur bands to allow a small antenna for the HF bands.
Even a short antenna produces a doughnut shaped pattern with the antenna being the axle of the doughnut. Laying this on its side produces two radiation lobes with nulls off the ends of the antenna.
Being very close to the earth (wavelength wise) means most of the half doughnut radiation will be absorbed when with the antenna vertical it is radiated.
At least this is the picture from theory.
Neil
Thanks, MRAM!
I imagine this would work for MW if it was a little bigger.. Neat!
The GAP looks like a humane animal trap I have. I never thought of using it as an antenna.
The picture of the GAP up on a pole looks like a future bird nest.
It is best to use vertical polarization at the frequencies used for AM broadcasting, because the ground reflection from the antenna reinforces the radiation on a direct path.
With horizontal polarization the ground reflection is out of phase with the direct wave, and tends to cancel the signal at/near the surface of the earth.
This is the reason why all modern, licensed AM broadcast stations use vertical polarization, exclusively -- which means that virtually all consumer type indoor radios and auto radios have antennas that respond mostly to vertical polarization.
Those receivers would tend to reject horizontally polarized waves.
I have to wonder how much "polarity" a 10 ft. antenna really does produce compared to a 1/4 or 1/2 wave antenna at these frequencies, even over a good ground field. I see it as being similar to a spark gap antenna producing electromagnetic interference at a controlled frequency. It wouldn't matter about polarity as long as the amplitude of the "static" is as high as possible. An unshielded, tuned tank circuit sure makes sense, looking at it this way..
Just my thoughts..
I have to wonder how much "polarity" a 10 ft. antenna really does produce...
Good question. Below is a calculation of the theoretical fields for horizontal and vertical polarization from a Part 15 vertical AM antenna.
It radiates no useful field in horizontal polarization.
A portable AM radio using a loopstick antenna will show a distinct reduction of received signal (increase in noise) when the radio is rotated 90 degrees around its horizontal axis from its normal orientation, especially on weak AM signals
Sorry that a monospace font isn't possible for posts on this board, so the numbers run together, and the table takes time to figure out.
EZNEC ver. 5.0
3m Vertical Radiator 6/23/2012 8:04:52 AM
--------------- FAR FIELD PATTERN DATA ---------------
Frequency = 1.7 MHz
Fields in mV/m for 1 kW at 1 km
Azimuth Pattern, Elevation angle = 0 deg.
Bearing,
Degrees V Fld H Fld Tot Fld
0 16.324 3.51E-14 16.324
5 16.324 3.50E-14 16.324
10 16.324 3.46E-14 16.324
15 16.324 3.39E-14 16.324
20 16.324 3.30E-14 16.324
25 16.324 3.19E-14 16.324
30 16.324 3.04E-14 16.324
35 16.324 2.88E-14 16.324
40 16.324 2.69E-14 16.324
45 16.324 2.49E-14 16.324
50 16.324 2.26E-14 16.324
55 16.324 2.02E-14 16.324
60 16.324 1.76E-14 16.324
65 16.324 1.49E-14 16.324
70 16.324 1.20E-14 16.324
75 16.324 9.10E-15 16.324
80 16.324 6.10E-15 16.324
85 16.324 3.06E-15 16.324
90 16.324 0 16.324
95 16.324 3.06E-15 16.324
100 16.324 6.10E-15 16.324
105 16.324 9.10E-15 16.324
110 16.324 1.20E-14 16.324
115 16.324 1.49E-14 16.324
120 16.324 1.76E-14 16.324
125 16.324 2.02E-14 16.324
130 16.324 2.26E-14 16.324
135 16.324 2.49E-14 16.324
140 16.324 2.69E-14 16.324
145 16.324 2.88E-14 16.324
150 16.324 3.04E-14 16.324
155 16.324 3.19E-14 16.324
160 16.324 3.30E-14 16.324
165 16.324 3.39E-14 16.324
170 16.324 3.46E-14 16.324
175 16.324 3.50E-14 16.324
180 16.324 3.51E-14 16.324
185 16.324 3.50E-14 16.324
190 16.324 3.46E-14 16.324
195 16.324 3.39E-14 16.324
200 16.324 3.30E-14 16.324
205 16.324 3.19E-14 16.324
210 16.324 3.04E-14 16.324
215 16.324 2.88E-14 16.324
220 16.324 2.69E-14 16.324
225 16.324 2.49E-14 16.324
230 16.324 2.26E-14 16.324
235 16.324 2.02E-14 16.324
240 16.324 1.76E-14 16.324
245 16.324 1.49E-14 16.324
250 16.324 1.20E-14 16.324
255 16.324 9.10E-15 16.324
260 16.324 6.10E-15 16.324
265 16.324 3.06E-15 16.324
270 16.324 3.07E-21 16.324
275 16.324 3.06E-15 16.324
280 16.324 6.10E-15 16.324
285 16.324 9.10E-15 16.324
290 16.324 1.20E-14 16.324
295 16.324 1.49E-14 16.324
300 16.324 1.76E-14 16.324
305 16.324 2.02E-14 16.324
310 16.324 2.26E-14 16.324
315 16.324 2.49E-14 16.324
320 16.324 2.69E-14 16.324
325 16.324 2.88E-14 16.324
330 16.324 3.04E-14 16.324
335 16.324 3.19E-14 16.324
340 16.324 3.30E-14 16.324
345 16.324 3.39E-14 16.324
350 16.324 3.46E-14 16.324
355 16.324 3.50E-14 16.324
360 16.324 3.51E-14 16.324
IF there is no horizontal polarization energy, then why are all antennas in any table top AM radio positioned horizontally? Even small pocket recievers have the ferrite bar laying down flat. (Assuming standing upright on its designed base, or lying down on its back for that matter..) Could it be to the nature of ambient noise having vertical polarization and trying to elude it?
I understand why automotive antennas are vertical due to the polarization of most FM broadcast stations. (Possibly circular)
I don't see a "legal" Part-15 antenna being no more than an isotropic radiator spewing radio waves out in an disorganized pattern. There's not enough antenna to build polarity, IMO..
Another good question: IF there is no horizontal polarization energy, then why are all antennas in any table top AM radio positioned horizontally?
The loopstick antennas inside portable and many desktop AM receivers respond to the magnetic field of the radiated wave -- which is rotated 90 degrees from the electric field. So the loopstick must lie in the horizontal plane to receive maximum field from a vertically-polarized wave.
The physical orientation of the electric field defines the polarization of the radiated wave.
I don't see a "legal" Part-15 antenna being no more than an isotropic radiator spewing radio waves out in an disorganized pattern. There's not enough antenna to build polarity, IMO.
Strange as it may seem, physics shows that a linear radiator even as electrically short as 1/1000 of a wavelength still radiates a linearly polarized field roughly in the shape of a "donut," with essentially zero field off the ends of the radiator.
In the case of a vertical linear radiator with its base on/near the earth, the pattern shape is that of the upper half of a donut sliced in two across its widest dimension.
A horizontal medium wave AM antenna placed at the appropriate altitude based on the operating frequency would restore the lower half of the donut-shaped pattern, and would perform as a directional array, with lobes on either side and nulls off each end.
A horizontal medium wave AM antenna placed at the appropriate altitude based on the operating frequency would restore the lower half of the donut-shaped pattern, etc
Actually a vertical radiator with its base at/near the earth doesn't lose its radiation toward the lower hemisphere. That radiation still exists, but is reflected by the earth. As the reflection is in phase with the energy radiated directly into the upper hemisphere by the antenna, the two fields reinforce each other.
For a perfect, infinite ground plane the "half donut" shape produced by a vertical with its base at/near the ground plane has twice the peak directivity that the full donut has in free space.
This is still true for the electrically short vertical radiators used in Part 15 AM setups.
Hope this isn't too much detail.
WOW.. Full donut.. Directional.. Possibly more efficient.. More horizon..
Think we could talk people into bendin' their car antennas and get the neighbors to sit their radios on end? LOL..
This Part-15 Am thing with all of its limitations just intrigues me to no end. FM is too easy! Even if one could figure an efficient antenna that's legal by discription, they'll zap ya with the field strength thing. Ya just can't win. It's proven that 100 mW can exceed the radiation limits..
Let's go horizontal! They'll be checkin' us vertically and we'll look like a bunch of wimps! We have a select listening pool so we could just let them in on our secret..
Just try sleeping in a vertical position. Miserable.
Sleeping only works horizontally.
But there actually are professional radio engineers who use horizontal AM antennas. Somewhere in the depths of this website is a description how one of the members, actually the design engineer behind a very popular part 15 transmitter, uses a horizontal antenna in his basement because he needs to have his equipment up on the bench and there would be no room for a vertical. He reported very satisfactory results all the way upstairs and beyond.