More cage monopole

Cage Monopole is right now my favorite type of both AM and FM antenna, and I have ongoing experiments in both departments.

A three element cage presents a solid signal but a four element adds a very slight but measurable increase in the signal power.

I have not tried more than four elements.

What astounds me (FM, probably also AM) is that the 4-element cage monopole totally stops multi-pathing caused by persons walking near the antenna or within the signal field.

Hi Ken... I thought of another aspect that is interesting to think about.

A four-element cage monopole has been described as placing four 10-foot antennas in parallel, a practice which lowers the resulting impedance.

Compare it to four 16-ohm loudspeakers in parallel, which would give you a final load of 4-ohms.

Because of that the tuning gets adjusted accordingly. With an SStran3000, for example, it might even be cause for making the modification for lowered impedance operation described in the manual.

"A four-element cage monopole has been described as placing four 10-foot antennas in parallel, a practice which lowers the resulting impedance."

I would hope so ... isn't that what it's supposed to do?

"Because of that the tuning gets adjusted accordingly. With an SStran3000, for example, it might even be cause for making the modification for lowered impedance operation described in the manual."

Yep, it's going to need a loading coil. The questions become:

1) How do we attach the wires of the cage to it?

2) How to measure for a match?

3) Will matching with the coil and whatever else (air cap?) to get it to resonance kill too much of our miniscule power?

Indicators are such that I think it could work to advantage, but figuring the physical dimensions, wire gauge and number of turns, inductance Q and impedance, etc. hasn't been so easy, at least for me.

Your questions are out of the ballpark for me, except that I can talk about it anyway.

The math is on my list of "must learn" stuff. But I will guess that the lowered impedance of 4 wires in parallel might be close to that of a very large solo copper pipe.

I haven't yet built the coil to go with my AM cage, but when I do I will use that home remedy of trying different taps until I find a peak in power.

Just tuning it up with a stock SStran3000 (unmodified) at 1680kHz lowers my S5 setting from 82uH, the normal setting for a single wire, down one notch to 138uH.

I wondering if the any of the characteristics of the cage monopole will be much different at AM BCB freqs, or if the cage will appear as a single element to the large wave lengths in the AM band.

I'd be inclined to treat it as a single, large diameter vertical element, as my first try...

1) How do we attach the wires of the cage to it? Have a top and bottom ring that shorts all the verticals together and attach to the bottom ring?

2) How to measure for a match? Same as with any ANT - check the4 current going to the ANT - the better the match, the more current flows

3) Will matching with the coil and whatever else (air cap?) to get it to resonance kill too much of our minuscule power? Should need much less coil, should be less resistance than a thinner radiator, air cap would be the same.

The 4-individual wires of the 4-element should not be electrically connected to each other at the top, in my opinion, otherwise you'd be loading the signal back into each wire from each other wire and there could be some phase cancelation. That is entirely an unscientific attitude, like avoiding ground loops being another example where electrical connections should not be made at more than one point.

The tuning, matching, I think, is done as you would to a single wire, element or pipe.

The bottom should be fed from a single point outward like a spider of equal-length wires to the 4-elements, which should be secured to be equidistant from each other all the way from top to bottom.

There are no estimates yet as to how close or far apart the elements need to be from each other.

I wouldn't recommend a metal electronically attached ring at the bottom, unless the ring was a solid piece of metal, because the RF wire from the transmitter needs to feed the 4-elements from a center point, so perhaps a non conductive pair of rings for securing the top and bottom of the 4-elements would be desirable.

For my FM cage I used securing devices (top and bottom) constructed from toothpicks and corks which I will photograph and post rather than attempt to verbally describe. The FM cage is held in the air from a three-leg tower built from bamboo poles.

In the picture you see the 3-legs of the bamboo tower, and hanging within you see a little construct made from corks and toothpicks. Each cork has a hole drilled through the center through which the wire can pass, so that the four-sided construct holds the wires in the correct relation to each other.

Up at the top of the tower is an identical cork/toothpick construct which holds the 4-wires from the top. To make sure that the wires don't slip free from the corks each hole at the top of each cork is plugged with tips from toothpicks to lock the wires in place.

At the bottom the four wires projecting from 4-corks come together in a single centered connection, are soldered together and attached to the Wholehouse FM 2.0 antenna output with an alligator clip.

The black hook comes from a pack of socks, and holds the transmitter under the antenna by a triangular bent paperclip that slips through the transmitter's belt clip.

Since the four-element antenna is dangling from above, it is only good indoors where there is no wind to blow it into motion.

Looks like some sort of gizmo Tarzan might have in his treehouse. Jungle radio!

BTW, why would it make any difference if the wires are connected together at the top if they are already connected at the bottom? AFAIK, the worth of a cage monopole is that is 'sides' are connected, i.e., making it all one large dia. antenna, i.e., it gives it a dramatic increase of skin effect ... idea-wise, similar to what a coil does to antenna length.

There are three possible outcomes by connecting the 4-antenna wires at the top

1.) it will be somehow detrimental;

2.) it will not matter;

3.) it will improve something.

Although I have sided with Number One, I really don't have a clue. The good thing is that part15.us has many experienced engineers who can help us understand the situation.

Also we could run tests and experiments (connecting the wires at the top) which I have not done.

The easiest and most efficient way to tune a caged monopole is to first connect all the "fold" conductors both top and bottom. A return or ground side conductor connects at the common point at the top and runs down to the ground side of the feedline at the bottom (usually through the center of the antenna).

Then, a shorting ring is constructed so that it encircles the entire antenna horizontally connecting to all fold conductors at the same elevation. The point of the connection is determined by the feed impedance of the transmitter. This can closely impedance match the antenna to the transmitter.

The antenna constructed this way most often exhibits inductive reactance which can be tuned to resonance by a simple variable capacitor in series with the hot side of the feedline to the antenna feed connection (usually a feed ring similar to the shorting ring). This may eliminate the need for any coil in the feed to the antenna.

In broadcasting, this antenna design is called a folded monopole and can be used at any frequency. This antenna design is used to raise the antenna efficiency for systems that are very short radiators. This design would be a good choice for Part 15 AM BCB antennae. I have used a very similar design (with success) for the AM BCB, as well as, vhf and uhf antennae.

Examples and drawings of the construction and tuning of this antenna system is common place by doing a Google search on the web. This antenna can be made to exhibit a wide bandwidth and nearly any impedance required by the transmitter.

The picture shown previously on this thread is a simple example of a series feed system found on many AM station broadcast towers across the country. Just think of each vertical wire as a separate leg of the tower.

Marshall, your very informative information makes the cage-monopole an even more interesting antenna than I first believed.

My view is (was) simplistic, seeing 4-common 10-foot antennas combined at the bottom. But with your input, it becomes obvious there can be more to it than that.

I'm going to seek links with the details you speak of, so the part 15 community can be equipped with road maps for getting the most out of cage monopoles.

If anyone else finds links, please post them here for our benefit.

The Part 15 AM antenna, that is...

I'm just going to try different
configurations.

It's so funny about this Part 15 stuff.

Things are never quite as they seem.

Best Wishes,
Bruce, MICRO1690/1700

Bruce MICRO1700 sparked a "not as it seems" part 15 idea.

Imagine a large transmitter room, filled with giant cases from a huge 50kW transmitter, except the part 15 hobbyist would totally gut the insides and build in a new AMT5000 from sstran.com, hooked to one of those giant "ON" switches with huge cooling blowers that would sweep to life. Many lights would flash and meters would swing back and forth.

100mW to the final, 3-meter total antenna.

Part 15 on a large scale

I had a similar idea for my Part 15
station - except it was thought to
be a big piece of gear - and it would
have a bunch of glowing vacuum
tubes on the top.

This would go with the Gates board,
the reel to reel deck, the turntable,
and the couple
of antique radios that I have sitting
near by.

It would be THE LOOK, just like your
big huge transmitter that would be
Part 15.

Bruce, MICRO1690/1700