3 meter Marconi "Tee" antenna optimization?

[JasonW]

Hello All,

Has anyone here ever experimented, either with an actual antenna or high-fidelity computer modeling, with optimizing the performance of a Marconi "Tee" antenna within the 3 meter Part 15 length limit?

The least efficient one would obviously be two 1.5 meter horizontal wires or rods on top of the transmitter's antenna terminal and pointing in opposite directions (no vertical section at all), while the other extreme is a 3 meter vertical wire or rod with no horizontal top loading. Somewhere between these two extremes is an optimum (for each given frequency) ratio of height to top load span. I'm assuming a 3 meter total length for the top load span (not half-span) + vertical section, and no ground lead length (transmitter or ATU sitting directly atop the ground (metal mast, billboard, ground rod, etc.) -- Jason

 

[rfry]

> I'm assuming a 3 meter total length for the top load span (not
> half-span) + vertical section, and no ground lead length
> (transmitter or ATU sitting directly atop the ground (metal
> mast, billboard, ground rod, etc.) -- Jason
_____________

True r-f ground conditions don't exist at the top of a metal mast, "ground wire" or billboard. Until those conductors reach the mass of physical earth (their connection point at/with radials, stakes etc), they are part of the radiating structure of the antenna system. This is not a debateable point or a personal opinion, it is an engineering reality.

The tee is useful when it is easier/cheaper to erect horizontal conductors than vertical ones, such as in LF beacons and LW broadcasting. But in those cases the horizontal part is always much longer than the vertical part is high.

The relative performance of a MW tee antenna where the total length of the horizontal and vertical wires (including the length of the conductor to physical earth) cannot exceed 3 meters could be much worse than a 3-meter vertical without the horizontal part.

//

 

[xoanon]

From my experiments (with an antenna of 32 ft overall length) adding a horizontal section (loading hat, in this case, 3ft dia, 6 spokes) will improve performance greatly. In this instance it doubled the range and increased radiation resistance noticeably (at least the apparent load resistance went up several ohms and I don't see how that could have been a change in ground R!) Now, using the 3m limits the tradeoff is more critical, of course. Using a hat of several spokes (not just a tee top) maybe a foot in radius will add a lot more capacitance and should raise the RR quite a bit. I believe using the radius figure of the hat rather than the diameter to subtract from the 3m length is fair, but that's just me. Some don't believe loading hats count at all toward the 3m limit. Anyway the more spokes you put on there (plus a circumference loop around the outside) the better. In my case the main loading coil is at the top of the mast, under the hat, making the mast under the coil carry the high current part of the wave.

> Hello All,
>
> Has anyone here ever experimented, either with an actual
> antenna or high-fidelity computer modeling, with optimizing
> the performance of a Marconi "Tee" antenna within the 3
> meter Part 15 length limit?
>
> The least efficient one would obviously be two 1.5 meter
> horizontal wires or rods on top of the transmitter's antenna
> terminal and pointing in opposite directions (no vertical
> section at all), while the other extreme is a 3 meter
> vertical wire or rod with no horizontal top loading.
> Somewhere between these two extremes is an optimum (for each
> given frequency) ratio of height to top load span. I'm
> assuming a 3 meter total length for the top load span (not
> half-span) + vertical section, and no ground lead length
> (transmitter or ATU sitting directly atop the ground (metal
> mast, billboard, ground rod, etc.) -- Jason
>

 

[JasonW]

I use the hat diameter rather than the radius just to be conservative and err on the side of caution.

That's a remarkable improvement you got with such a comparatively small hat, but the multi-spoke hats on LF beacon antennas are quite small too. I may try a solid metal square or disc as a hat, since the dimensions within the 3 m limit or so small. -- JasonW

> From my experiments (with an antenna of 32 ft overall
> length) adding a horizontal section (loading hat, in this
> case, 3ft dia, 6 spokes) will improve performance greatly.
> In this instance it doubled the range and increased
> radiation resistance noticeably (at least the apparent load
> resistance went up several ohms and I don't see how that
> could have been a change in ground R!) Now, using the 3m
> limits the tradeoff is more critical, of course. Using a hat
> of several spokes (not just a tee top) maybe a foot in
> radius will add a lot more capacitance and should raise the
> RR quite a bit. I believe using the radius figure of the hat
> rather than the diameter to subtract from the 3m length is
> fair, but that's just me. Some don't believe loading hats
> count at all toward the 3m limit. Anyway the more spokes you
> put on there (plus a circumference loop around the outside)
> the better. In my case the main loading coil is at the top
> of the mast, under the hat, making the mast under the coil
> carry the high current part of the wave.