Inside a Gimmick Antenna

I fell for marketing and advertising, again. I bought one of those stupid antennas with the little dial knob on it. What's really stupid is that I already had one similar to it, and it sucked. (It turned out to not be so bad, but still...) My past experience was that these "tuning knob" antennas suck, and that regular dipole and loop antennas - the simplest ones - tend to be the best. Anyway, the antenna I got was OK. It wasn't great, but worked. I'm sure it'll eventually get a parabolic mesh behind it to boost the gain :)

So, I took the old crappy one, and had a look at its guts. What a trip. Here's a photo.

The wheel on the front of the antenna is offset from the encoding wheel by 1/2 a position, so starting from the 11:30 position, which I'll call "0", the positions are wired like this:

clock position 
0 - 
1 - 
2 - vhf out right 
3 -
4 - on the inner ring, uhf out left, dipole right, spiral right. this is always connected.
5 - spiral left, uhf ant right, uhf out right
9 - vhf out left, uhf loop left, zigzag left
10 - dipole left, spiral left

The encoding wheel makes or breaks connections according to this pattern:

0 0 0 0 1 0 0 0 0 0 0 0 - the inner wheel is always connected to position 4
1 1 0 0 0 0 1 1 0 0 1 0  - the outer wheel, at 12 o clock.  this spins.
0 0 1 0 0 0 0 0 0 1 1 0 - the pins, starting at 12 o clock

What turning the wheel does is connect the antennas connected up at position 4 to the antennas at position 5, 9, and 10, and to output wires via positions 2, 5, and 9.

I'm guessing that an antenna is activated when both the left and right antenna terminals are connected.

There are two types of antennas, and since I'm largely ignorant about them, please tell me about my mistakes. First are the "rabbit ears" half dipole. Second is the loop "uhf". Then, on that little circuit board are a zigzag thing, and a spiral thing. They look like antennas, but they might be some kind of inductor that's used to interface to the combo uhf/vhf coaxial output. Maybe it's a balun. Here's a diagram of the traces - it's approximate:

In the above chart I call the left side "zigzag" and the right side "spiral", but I don't really know what they are. The green layer is the upper layer (the side you see), and the red layer is the backside.It appears that the upper area is the ground; the red thing in back is mostly ground, except the P-shaped thing in the middle, which connects to the 3rd pin from the left.

I'd assume that when there are two flat areas on top of each other, that's a capacitor. The spiraly and zigzag things are probably inductors or coils. Digging back into my memory of physics, I think this is an LC circuit. So, maybe it's a filter? Maybe connecting different parts of it will cause it to pass different frequencies.

If that's the case, though, it doesn't seem to offer too many frequencies. It also seems like there are three pins that cause a signal to go into the coaxial cable. Going left to right: pin 1, pin 2, and pin 3. If pin 1 is connected, there's a coil and then a capacitor in series. If pin 2 is connected, it's a capacitor alone. If pin 3 is connected, it's almost like a wire, but with a coil in there (I don't think the capacitor before the coil affects the center pin of the coax.) Here's a diagram that, I hope, is based on the illustration above. Like I said, this isn't my area of expertise, or even a level of amateur knowledge. There's a legend here that maps the terms "spiral left" to a pin number.

So, you spin the wheel, and cause the circuit to be re-wired.

Whoever invented this antenna was a mad genius, except for the fact that the antenna doesn't seem to really improve reception. It definitely changes it.

So the different circuits are encoded by the wheel. I'm guessing that the UHF antenna loop isn't engaged if the loop isn't completed. Likewise the different components on that circuit board. The dipoles probably work if it's connected at all - and one always is.

One way to think about this is as an AND function, where the outer wheel and the pins touch, making sets of connections active. In the 12-o-clock position, one pin is connected to the outer wheel at position 10, which then connects that pin to pin 4. Assuming the wheel is the left number, the pattern is this:

110000110010 AND 001000000110 = 000000000010

For the 1-o-clock position, rotate the left number to the right by one.

011000011001 AND 001000000110 = 001000000000

But what does this mean? To figure out what's connected, refer to the chart. Add all the things that are connected:

000000000010 = dipole left, spiral left
Add in the stuff on pin 4, which is always connected with uhf out left, dipole right, spiral right.
The total set is: dipole left, spiral left, uhf out left, dipole right, spiral right.

So both dipoles are connected, one uhf out is (meaning maybe it's not really connected), and both "spirals" are, meaning that capacitor between 3 and 4 is shorted out.

001000000000 = vhf out right
Total set: vhf out right, uhf out left, dipole right, spiral right.
Hmm... seems useless, but I'm ignoring the fact that some parts of the circuit are always connected (at the pins that aren't being connected to pin 4), and the uhf loop is always connected.

Here's the 2-o-clock position:
001100001100 AND
001000000110 =
001000000100 : vhf out right, vhf out left, uhf loop left, zigzag left
Total set: vhf out right, vhf out left, uhf loop left, zigzag left, uhf out left, dipole right, spiral right.

It's getting late, so I'll decode the rest another time. I'm sure I'm missing a lot here.

I have to admit, though, I'm getting the weird feeling that this "tunable" antenna is just a bogus product that degrades the signal in most of these positions. Of course, as I said, I don't know much about these things, so maybe this is just paranoia.

A normal antenna seems to just be some wire, maybe a balun to make it work with coax, and maybe a little amp to boost the signal. Presumably, they aren't tunable - because an antenna's already tuned to be great at one frequency; the reason why antennas have weird shapes or can be lengthened is to allow them to get a wider range of frequencies. Besides, the tuner's supposed to filter in the correct frequency, and filter out the rest.

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