We would like our transmitter, which lies on the lower end of the AM band to cover the small college campus (3/4 mile radius from transmitter). The exciter and power amplifier is finished, and we can get up to 60 Watts of output from the final RF stage.
I am experienced with constructing antennas with similar wavelength to the signal (Ex. 1/4 wave ground plane , Dipole, J-pole, slim jim etc.), but I have never worked the lower bands, and would like some suggestions for electrically short AM antennas.
I realize the radiation efficiency is very low for electrically short radiators, and that is why we have a large power amplifier. I would like to keep the total length under 15 feet due to physical restrictions at the site. What is the best practice here as far as loop antennas, or verticals with loading coils and capacitance hat etc?
The characteristic impedance obviously has to be 50Ω, so any ideas better than a dummy load for short radiators?
Get the antenna as high as you can within your means. Strap on a capacitance hat on the top of the radiator or a 1 to 3 wire horizontal section tied at the middle. The ground radials are equally important. Use as much wire as you can near the base of the antenna and spread it out like bicycle spokes along under the dirt, or connect it to large metal objects that go into the ground like pipes.
There is a reason why the old schematic symbol for an antenna looks like an upside down triangle with a line from the top to bottom, it's because that is how the marconi T antennas looked while viewing from the narrow side. You'd stretch three wires across trees all connected electrically at ends, then in the center connect them again all electrically to a tieoff that goes down with one wire to the transmitter shack. Then it's just a matter of tuning up the T antenna.
Inverted L antennas work well on the medium wave AM broadcast band too and lots of ham radio operators use them on the 1800-2000kHz band. It's basically the same as the T antenna but with some horizontal takeoff which can help at night when horizontal skip gets out well.
Now given that this is short there is one thing to keep in mind.. the larger the top hat / capacitive hat, the more it will radiate signal out to the horizon. Electrically it helps drag the RF up to the top of the antenna where you want it and allows you to use a far shorter antenna than a 1/2 wave dipole vertically which none of us pirates have the ability to use, or a 1/4 wave marconi. The trick is to use a tuning network to make it radiate. The tuner will act as part of a dummy load because there will always be losses with short antennas, but the goal is to get as much out into the wire as you can. If you push 60 watts into the wire and you only get .6 watts out with something like a 30' T antenna, that is still far better than a dummy load! In fact this is exactly how longwave beacon stations operate. Often with far more power which is wasted from losses, but as long as the radiated power gets out then the loss is negligible if you have the light bill to blow.
In my experience it seems like a few 10s of watts into a T or L antenna can yield close to a single watt out of radiated power. It greatly depends on your ground soil conductivity, number of radials installed or other metal objects you can wire up to at foot level, and how high you can get the wire up in the sky and how long you can make the capacitance hat.
My goto setup would be a 50' pole or wire up in the air with a horizontal T section across two trees as long as I can get it, or if no trees I would use a lightweight "hat" that is wide like an oversize bicycle wheel and spokes on the top of the pole stretching out, and work hard on the ground part of the shortened 1/4 wave antenna. Stretch radials out as far as you can get practically and as many as you can. Tuning network can be a simple LC circuit. Lots of examples lingering around here showing how I did it and others. Get the most RF current up you can into that wire and the more you get out.
Getting on the AM broadcast band is not easy at all. It is so simple to get on FM BCB from 88-107MHz with a small antenna that it makes AM BCB broadcasting look like some kind of hidden black art, and it really is. At such low frequencies you have to measure RF current, unlike with VHF and FM where you measure RF wattage. Goofy things like that make the 530-1700kHz band a recipe for disaster if you don't have some understanding of low frequency work. Not trying to discourage, it's easy to get on shortwave and even the AM BCB if you study antenna theory for low frequencies. Problem is at those frequencies it's ALL IN THE ANTENNA. With FM and VHF you can get away with using a god damn metal measuring stick as an antenna and probably get listeners. I wouldn't doubt if you sat the transmitter into a toilet if it would radiate to a block of listeners, but with low frequencies it's THE OPPOSITE, it's the antenna and the antenna and the antenna.
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