# Tag Info

## New answers tagged antenna-theory

3

And why is an antenna resonant, because the voltage and current are in phase or because the reactances cancel out, or both ? Reactance ($X$), resistance ($R$), and the phase difference between voltage and current ($\theta$) are related by: $$\tan \theta = { X \over R }$$ So if reactance is zero then $X/R$ is zero, so $\theta$ must be zero, meaning voltage ...

0

This is commonly described as the ground plane acting as a reflector, so that there is effectively a second quarter-wave element “below the ground plane”. It IS commonly described that way, but it is wrong. Tom Rauch, W8JI —in a 2010 post on qrz.com— correctly stated: The "image" is not formed by the radials any significant amount, and it is a ...

3

The original question was, "Why do ham radio operators insist that a half wave dipole is resonant when the inductive and capacitive reactances cancel out?" That question seems to be about psychology, and I will answer accordingly. I don't know that my answer will be very useful to others, but I'll proceed anyway. People who are hams come from all ...

0

To add to what Hotpaw stated ... There's the theoretical half-wave dipole, which is 492/f. However, a practical resonant dipole is shorter. When making one, we must account for the end effect, by cutting its length to 468/f. The end effect results from the fact that the antenna is normally operating surrounded by air, and the signal is travelling in a ...

3

What you miss is that a resonant dipole doesn't need to be a half wave length long. You can add series inductors near the center or part way out from the middle, and/or capacitors (hats) near the tip ends, with L's and C's varying in value from small to large, to vary the dipole length from a just tiny bit shorter to significantly smaller (e.g. HT rubber ...

5

Those antennas are held together with aluminum pop rivets which are easily drilled out. In less than an hour you can completely disassemble a TV antenna like that and salvage the main mast, all the dipole elements, and the plastic element mounts- and use these and some fresh pop rivets and an electric drill to build yourself a two meter yagi beam with ...

6

I'm sorry to say that's not a simple log-periodic antenna, it's a hybrid monstrosity to meet some particular need. Guessing from the element lengths: 136 cm each side would work at about 50 MHz, so low-band TV. Two-element log-P with a reflector. OK. 12 cm each side would work at about 550 MHz, so mid UHF TV. Some sort of Yagi 15 cm total would be a ...

2

Log-periodic antennas have a very wide bandwidth. One designed for TV would not work at HF, but it may very well work on 2 meters, 6 meters, and some UHF amateur bands. I suggest that you measure the SWR and see. However, unless its impedance is 50 to 75 ohms, you will need a suitable matching device. Can you post photos of it? While you're at it, include ...

0

For a theoretical answer to the first two questions, the skin effect is caused by magnetic fields generated by the currents repelling each other, so the current flows as far away from current of the same direction as it can get. The overall result spreads the current out over the entire outer surface, with an exponential weakening of the current as you go ...

1

On a closed tube, the current only flows on the outside. On a strip, the current flows on both sides. It might not be equally distributed, but I don't believe that there is enough difference between the current density between the center and the edges to matter.

1

Bandwidth of a yagi is largely controlled by the diameter of the elements. Generally, the larger the diameter, the larger the bandwidth. However, the yagi is also helped by very slightly tapered elements. The impedance matching network may also affect the SWR bandwidth. The boom conductivity does not affect bandwidth, as well explained by Phil.

1

You certainly can feed the dipole with coax without a balun at the feedpoint, you will be able to work stations that would not be good on the dipole alone. The downside is that the noise level of the whole system dipole + coax will be higher. As the coax will radiate and also receive signals you will have a very different "radiation pattern" ...

2

If "ill effects" include common-mode current on the feedline, no. You can of course just tolerate the common-mode feedline current. In some situations it may not matter.

4

Theoretically yes, if you carefully control the lengths of the cables and the placement of the antennas. This is called a phased array. However, you would get a far better improvement in range by using an antenna which is not a rubber ducky, like a 1/4 wave whip or a dipole. Rubber ducky antennas are electrically shortened, making them smaller. The price of ...

-3

The boom is like two end-fed right-angle fan-dipole elements for some completely different frequency added to the driven element. Elements of a fan dipole for a very off frequency typically do not take much of the Rf current away from the on-frequency resonant element of a fan-dipole. My guess is that some RF current will travel down the boom (and back), ...

6

The elements of a Yagi are dipoles. If you look at a dipole at any instant, the voltage at one tip will be some value, and the voltage at the other tip has the same magnitude, but opposite polarity. So let's say at some instant, the left side is at +50V and the right side is at -50V. Consider these relative to the soil at the base of the mast, if you like. ...

1

While someone flagged this question as off-topic for this site, I think as a “technology of radio” question, it still stands. And I would say that it should be amateur radio-related, as I would recommend using ham bands for this application. Simply because a licensed amateur can use more power than is possible on an unlicensed band. With a good yagi-uda ...

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