Loop Transmit Antennas for Part 15 AM

[rfry]

Posted on another website was this link http://www.techlib.com/electronics/amxmit.htm#Unfair Radio Transmitter . Here is a short clip from it:

Those who have experimented with electric field transmitters like those above have probably discovered that they only work well when the antenna is free and clear and there is little solid matter between the transmitter and receiver. The electric field is easily "shorted out" by even slightly conductive materials. In contrast, this loop transmitter generates a magnetic field that can cut right through the thickest walls.

Just to note that electromagnetic radiation (a radio wave) ALWAYS includes both a magnetic and an electric field. Neither one can exist without the other. In the far field, they always have the same relationship to each other -- no matter what geometric form of antenna (vertical monopole, loop etc) produced that radiation.

Most consumer-grade, indoor AM broadcast band receivers use a horizontally-oriented loopstick antenna, which responds only to the magnetic field of the radio wave. Those receivers have no problem receiving the radiation from the vertical monopole radiators of AM radio stations (licensed or unlicensed), because the magnetic field is oriented +/-90 physical degrees from the electric field, AND they are using only the magnetic field for reception.

 

[Tom Wells]

I've thought about trying this for years, and next would come the question,
If ALL the rf current were directed into the loop ( let's assume it's tuned ideally ), then does any rf flow on a ground lead?
And if not, through what coupling mode/angle/interface does the magnetic field produce the e field?

I can visualize a figure radiation pattern off the coil, but at which reference to zenith would most effective
coupling exist? Laid flat we have a figure 8, but up and down we shoot signal into the ground.

In both cases I vizualize the returns of the coil's of lines of force as a torus, but not sure which
condition would couple the most into a groud wave, circular type pattern, or if a directianal signal
becomes necessary with a loop transmit antenna.

 

[Carmine5]

Has anyone tried a TAK-tenna for part 15 AM? Interesting design.

http://www.taktenna.com/

 

[rfry]

Below is a link to the 1.65 MHz performance of a loop having a circumference of 3 meters. Its radiation efficiency is 0.059%, as compared to 0.17% for a 3-m base-loaded monopole (base at the earth) with a 15 ohm coil and 50-ohm r-f ground connection.

The 3 dB r-f bandwidth of the loop is 3 kHz, which would restrict the audio bandwidth heard by an AM receiver. The 3 dB r-f BW of the 3-m monopole system described above is about 36 kHz.

The radiation pattern of the loop is donut shaped, and the loop would have to be oriented in the horizontal plane to produce an omnidirectional E-field radiation pattern for the ground wave. That E-field will be horizontally polarized, which is not desired for MW groundwave propagation. Mounting the loop vertically produces vertical polarization of the E-field, but the radiation pattern is directional in the horizontal plane.

Matching the impedance of the loop to the impedance expected by the transmitter would be difficult.

http://i62.photobucket.com/albums/h85/rfry-100/Loop_Params.jpg

 

[rfry]

Has anyone tried a TAK-tenna for part 15 AM?

From inspection and comments on various antenna sites it appears to be a short dipole using the spiral conductors at each end of the dipole arms to resonate the feedpoint. Performance measurements on the 40-m ham band (~7.2 MHz) show rather high dependence on radiation from the coax feedline (sound familiar)?

 

[druidhillsradio]

The TAK is another miracle antenna. A friend once told me that you can't pour a mechanic out of a can. The same goes for antennas.