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AM Broadcast Antenna Questions

T

trusty

Star Participant
Maybe this should go under "Engineering" - but the questions are more historical than technical, so here it is under "Classic" - which you have to go through "Formats" ??

Anyway, here's the question:

Almost all the pics I've seen of AM antennas back in the '20s show them to be a wire (or three wires) strung between two towers with a feed line from the transmitter building. (I even saw them years ago at the old VOA complex north of Cincinnati for SW broadcast.) I think these were called "dipole" antennas.

These evolved in the 30s into vertical sticks - either stand-alone or guyed.

The question is: why the change? (I'm assuming the "dipoles" didn't have a ground system, or the vertical sticks just worked better (?).

Also, because of the nature of their construction, did the dipoles have a directional signal?

And finally, what was the first station to change to a vertical tower/antenna?

(Please help before my grandson asks another question I can't fully answer.) Thanx.
 
trusty said:
Maybe this should go under "Engineering" - but the questions are more historical than technical, so here it is under "Classic" - which you have to go through "Formats" ??

Anyway, here's the question:

Almost all the pics I've seen of AM antennas back in the '20s show them to be a wire (or three wires) strung between two towers with a feed line from the transmitter building. (I even saw them years ago at the old VOA complex north of Cincinnati for SW broadcast.) I think these were called "dipole" antennas.

These evolved in the 30s into vertical sticks - either stand-alone or guyed.

The question is: why the change? (I'm assuming the "dipoles" didn't have a ground system, or the vertical sticks just worked better (?).

Also, because of the nature of their construction, did the dipoles have a directional signal?

And finally, what was the first station to change to a vertical tower/antenna?

(Please help before my grandson asks another question I can't fully answer.) Thanx.
Click to expand...

I'll give some general responses and hope that posters Fry and Cat give the deeper technical aspects.

In any "T" (often called a flat-top") or "Inverted L" antenna radiates principally from its vertical component. A flat-top has two non-radiating support structures (poles or towers or even two buildings) and a center, vertical feed line. The horizontal parts act like top loading on a more modern antenna.

Early medium wave (the US AM band) stations found it easier to hang some kind of a vertically fed horizontal antenna, as the support structures did not necessarily have to be insulated and the required insulators were just in the wire radiator assembly.

Most of those antennas were not dipoles. They did not have two separate elements, but just one with a horizontal and a vertical part to them. But they were directional because of the support structures and the directionality of the horizontal element.

Most of those antennas had a ground system, although many simply had a batch of ground rods or some less extensive net or web at the feed point.

Short wave is different... it is intended to radiate a skywave, and the wave lengths are shorter so dipoles and adaptations such as certain types of curtain antenna systems have "both sides" hung in the air.

When I built my first AM station (570, non-directional) in Quito, of the 31 stations in the market only 3 had towers. The rest used inverted L antennas hung between two masts, usually the trunks of two eucalyptus trees. They often buried old car radiators and other metal as a ground. Most of them did not get out of the immediate city area; I built a tower as trying to use a longwire on a low frequency was not a good idea and ended up covering about a 100 mile radius in the daytime.
 
The modern FCC imposes a minimum efficiency on AM station antennas in section 189 of the FCC's regulations. The least expensive antenna that meets those requirements is a vertical tower.

In modern decades I've personally only heard of horizontal antennas being used with special temporary authority (e.g. in the event of a tower collapse while the regular tower is being refurbished).
 
PTBoardOp94 said:
The modern FCC imposes a minimum efficiency on AM station antennas in section 189 of the FCC's regulations. The least expensive antenna that meets those requirements is a vertical tower.

In modern decades I've personally only heard of horizontal antennas being used with special temporary authority (e.g. in the event of a tower collapse while the regular tower is being refurbished).
Click to expand...

However, if you make a horizontal antenna high enough, and have a good ground system, the vertical component is enough to meet minimum efficiency. But it is going to be somewhat directional, and requires two towers to hold it up. Today, electrical "heightening" of a tower can be done by top loading so we don't see that kind of antenna much any more. In the past, it was the best way to achieve a goal at a reasonable cost before we understood the characteristics of different kinds of antennas... meaning before the 1930's.

This was also the time period when stations often located the antenna on the roof of a building or hotel... two towers and a "T" antenna system. They used a counterpoise ground. Some later built on buildings with single towers and a counterpoise and there are still a few of those left... even thous a larger site at ground level is to be preferred.

A flat-top was used by LA's 1230 AM up until just a few years ago. I believe there are pics at Sr. Fybush's site!
 
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