Capacity hat size for AM

I haven't tried it (yet) but I pondered using 6" gas vent pipe. It's fairly inexpensive and the joints can be soldered togather to make a sturdy element..

I don't know how an F.C.C. field person would look at a "Hat" made with the top section cut down the side with "Tangs" sticking out horizontally if the original length was legal..

Would the "Tangs" give more benefit with the loss of length?

Below are some numbers and comments on these topics.

NEC was used to model the three antennas below, and determine their performance. Here are the results for the groundwave field at one mile for ground-mounted antennas with 25 ohm r-f ground loss and the r-f loss of a typical loading coil:

Configuration > NEC Calculated Field for 1 kW Tx Power > Computed Field for 80 mW Tx Output Power

1. 3-meter, no hat > 19.52 mV/m > 17.46 µV/m

2. 3-meter, 4 spoke hat, spokes at 90 degree interval, each spoke 1 m long > 28.61 mV/m > 25.59 µV/m

3. 2-meter, 2-spoke hat (in line), each spoke 1/2-m long > 16.15 mV/m > 14.44 µV/m

Configuration 1 is a classic, ground-mounted 3-meter vertical. The other two are configurations suggested by earlier posters in this thread.

If the spokes lie in the horizontal plane and are arranged symmetrically they will not produce useful radiation. This is because the currents that flow in them always move in opposite directions in each in-line pair, and the radiation from one cancels the radiation from the other. Getting an FCC field inspector that understands this probably is dicey, and the 4-spoke configuration above might be seen as exceeding the 3-meter definition in Part 15. It does give the best performance of the three, though.

Configuration 3 uses a 2 meter vertical section and two 1/2-meter spokes, so the total structure is 3 meters. But it doesn't perform as well as configuration 1 (no spokes), because even though the average current in the vertical section has increased, it is flowing along a shorter radiator.

For a 3-meter vertical with no hat, using a 6" OD pipe versus a 1/2" OD pipe has a negligible affect on the radiation resistance and current distribution of the radiator, so there is no real change in the radiated field. The 6" OD pipe does have lower reactance, and it changes at a slower rate with frequency -- so there is a benefit to it in that its VSWR bandwidth is better, and the station might have better audio bandwidth at receivers. Using the 6" pipe might save a bit of r-f loss in the loading coil needed to resonate it, also.

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Thanks! I guess that unless I put a 1 or 2 meter diameter ball at the top of the antenna, I won't see much if any gain. And then the wind loading will be huge!

I wonder how much 6 inch copper pipe would cost???? I'll have to ask a few of the broadcaster if they have any rigid copper coax that they can't use and would like to donate. Then all I would need to do is remove the center conductor and build it to length.

Hi Guys (and also all the Gals here),

Rich surely provided a scholarly presentation on this topic and I don't have much to add except that I read somewhere where the function of a "top hat" is to alter the current distribution in the radiator such that the current is increased or "raised up" in the body of the radiator relative to a non "top hat" antenna.

Rich's numbers indicate some difference, and the legalities could be discussed forever, but technically it doesn't appear to me that there is much advantage in practical range by employing these hats. Perhaps when the radiator is a significant fraction of a wavelength it is worth while but though I see no harm I see no real advantage.

By the way, has anyone heard of a "Slinky" antenna? Rich, your program would probably like this. I know you have simulated a helix but have you tackled a genuine Slinky, perhaps center fed or link coupled?

Neil

Rich's numbers indicate some difference ...

Hi Neil - It is all a matter of definition, I guess, but the 3-meter vertical radiator system showed about 47 % more field with that 4-spoke top hat than without it. Other things equal, the power applied to the 3-meter vertical system without that top hat would have to be increased by 2.16 X (to 173 mW) in order to produce the same field at a given location that 80 mW did using the top-hat version.

That 2.16 X improvement in radiated power would extend the distance to a given, usable groundwave field strength contour by about 23%. Some might find that to be useful.

While discussing my earlier post here, I should re-state something I wrote in it. I said that the performance of configuration 3 was not as good as # 1 because # 3 was shorter. Looking at the NEC data and thinking about this some more, I realized that the reason for this is that the radiation resistance of # 3 is less than # 1, and therefore a greater percentage of the available power is dissipated in system losses.

By the way, has anyone heard of a "Slinky" antenna?

The basic belief about these types of antennas is that the electrical length of a radiator can be extended by using a spiral conductor over a short(er) linear dimension. Modeling them would be a chore, but my intuition for practical Part 15 use is that the "OD" of the slinky would tend to reduce the reactance compared to a straight, small OD conductor of the same linear aperture dimension, but that its system radiation efficiency would be little different. This would be similar in effect to using a large OD pipe instead of a small OD pipe or wire.
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