Question about dipole antenna theory

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wbswetnam

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With any simple dipole antenna, one "side" of the antenna is connected to the center terminal of the coax which runs to the transmitter (let's call this side A), while the other "side" is connected to the shielding of the coax (let's call this side B). Maybe I am oversimplifying this, but... is the side A of the dipole "hot" while the corresponding side B simply passive and not an active radiator? Or are both sides equally radiating the signal, therefore both "hot"? I hope my question makes sense.
 

nd5y

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A dipole is a balanced antenna. Both sides are hot. Both sides of a dipole are supposed to have equal but opposite currents and radiate equally but in the real word that only occurs when a dipole is fed with a balanced transmission line (twin lead) or with a balun at the feedpoint and there are no other conductive objects nearby.
 

jackj

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Tom has over simplified things a little, you won't get any current flow if you don't have a return path. So your dipole that is fed with coax will be noisy compared to the same antenna fed with a balanced feedline. But both sides of the dipole DO radiate regardless of how you feed it, try taking your side "B" out of the system and see what happens to your match. Tom is right in that the antenna will be more efficient, the radiation pattern is closer to the ideal and the antenna system won't pick up as much noise if it is fed with a balanced source.
 

wbswetnam

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OK I am setting up a fan dipole with four sets of elements: 40m, 20m, 17m and 10m (this way the 40m elements will also work well on 15m and the 17m elements will also work on 6m, thus, a sixbander). I have a 1:1 current balun for the central feedpoint. Now, next question: if using twin lead feedline is so great then why do almost all ham transceivers have 50 ohm line-in antenna sockets and not 300 or 450 ohm for the antenna input?

BTW my rig is a Kenwood TS480SAT with the 50 ohm "pigtail" antenna inputs... no way to attach a 450 ohm twin lead directly to that... or is there?
 

n5ims

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OK I am setting up a fan dipole with four sets of elements: 40m, 20m, 17m and 10m (this way the 40m elements will also work well on 15m and the 17m elements will also work on 6m, thus, a sixbander). I have a 1:1 current balun for the central feedpoint. Now, next question: if using twin lead feedline is so great then why do almost all ham transceivers have 50 ohm line-in antenna sockets and not 300 or 450 ohm for the antenna input?

BTW my rig is a Kenwood TS480SAT with the 50 ohm "pigtail" antenna inputs... no way to attach a 450 ohm twin lead directly to that... or is there?
The issue with twin lead is that anything nearby will affect the impedance and signal transfer. This will cause transfer of the 60 Hz hum from any nearby AC lines. This will cause the 20 Hz ring signal from phone lines to be picked up by the twin lead. This will cause noise of any kind to transfer to the twin lead from any nearby nail, screw, gutter, etc. It also tends to act as part of the antenna and cause mixing of signals and that associated interference.

Coax doesn't have this problem since the center conductor is surrounded (and by the correct distance) by the shield so it is generally shielded from the extra noise and keep the impedance at or near the rated values.

This is the main reason that TV antenna installations switched from 300 ohm twin lead to 75 ohm coax. This was in spite of the additional cost and the fact that the antennas and TVs were designed to use 300 ohm feedlines and to use coax, they also needed to add transformers on each end to change the impedance from 300 ohm to 75 ohm and back. This eliminated nearly all of the ghosting and much of the interference from passing planes.
 

LtDoc

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Contrary to popular belief, parallel feed line/ladder-line is no more susceptible to interference than coax if it's installed correctly.
Also contrary to popular belief, if using ladder-line, the impedance mismatch at the antenna end matters very little.
Using a balun with a dipole and ladder-line is silly, it's not needed at all, they are both balanced.
If that dipole is only for single band use then matching impedances isn't a bad idea, but a balun is not suited for that application, neither is an unun.
If that dipole is to be used on more than a single band then there's no single balan impedance ratio that will come close to being right. You might use a 9:1 transformation ratio on one band, but that ratio is ONLY good for one band. That antenna's input impedance won't be close to 50 ohms other than on it's primary band of resonance.
Think about it...
- 'Doc
 

zz0468

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Forget the feedline issue for a minute. It's balanced, both sides are "hot".

Feeding it with an unbalanced line (coax) disturbs the balance, but both sides are still "hot". But with the feedline unbalanced, it becomes part of the antenna. Just how, is subject to an infinite variety of variables.
 

LtDoc

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As to the original question. RF is AC, it swaps polarity half way through every cycle. If you think of it in terms of current flow then for half of one cycle 'A' is positive and 'B' is negative. Both sides are producing an electromagnetic field (no matter the current's polarity) which is what's being 'radiated' not the current or voltage. The combination of those two 'EMF' fields, one '+' and one '-', results in a total 'composite' 'EMF' field of some particular 'shape' or radiation pattern. (Remember, we're thinking in terms of current flow which accounts for that '+/-' polarity thingy.)
Confused yet :) ? It's just another step 'deeper' in understanding what's happening, and what that radio signal consists of. It doesn't consist of voltage or current, but voltage and current produces the electromagnetic field that's being radiated, so the radiated signal can be characterized or described by using them.
- 'Doc
 

wbswetnam

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As to the original question. RF is AC, it swaps polarity half way through every cycle. If you think of it in terms of current flow then for half of one cycle 'A' is positive and 'B' is negative. Both sides are producing an electromagnetic field (no matter the current's polarity) which is what's being 'radiated' not the current or voltage. The combination of those two 'EMF' fields, one '+' and one '-', results in a total 'composite' 'EMF' field of some particular 'shape' or radiation pattern. (Remember, we're thinking in terms of current flow which accounts for that '+/-' polarity thingy.)
Confused yet :) ? It's just another step 'deeper' in understanding what's happening, and what that radio signal consists of. It doesn't consist of voltage or current, but voltage and current produces the electromagnetic field that's being radiated, so the radiated signal can be characterized or described by using them.
- 'Doc
Thanks yes that's pretty clear. Also I went back and reviewed the chapter on antennas and radio wave propagation in my General Class License Manual... I should have remembered all of this to begin with.
 

K7MEM

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OK I am setting up a fan dipole with four sets of elements: 40m, 20m, 17m and 10m (this way the 40m elements will also work well on 15m and the 17m elements will also work on 6m, thus, a sixbander).
I wouldn't count on the 40 meter section working very well on 15 meters. It all depends on which part of 15 meters you want to use. If your trying to use the high end of 15 meters, you should be OK. But if your interest is the low end of 15 meters, you may not be able to get a good SWR without a tuner.

The usual method to bring the resonance down is to add a capacitive hat at the 21.1 MHz - 1/4 wavelength position, on the 40 meter section. That's about 1/3 of the way from the center, along the 40 Meter wire. This works well in some situations. It's usually explained in the ARRL Handbook or any of the ARRL's Antenna Handbooks.
 

wbswetnam

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I wouldn't count on the 40 meter section working very well on 15 meters. It all depends on which part of 15 meters you want to use. If your trying to use the high end of 15 meters, you should be OK. But if your interest is the low end of 15 meters, you may not be able to get a good SWR without a tuner.

The usual method to bring the resonance down is to add a capacitive hat at the 21.1 MHz - 1/4 wavelength position, on the 40 meter section. That's about 1/3 of the way from the center, along the 40 Meter wire. This works well in some situations. It's usually explained in the ARRL Handbook or any of the ARRL's Antenna Handbooks.
Thanks for the advice. My Kenwood TS-480SAT has an internal automatic tuner, which can adjust for SWR levels as high as 3:1. Right now I have a smaller multiband dipole antenna with elements cut for 20m, 15m and 10m. For the 20m and 15m meter bands I trimmed the elements dead-on, but my 10m elements are resonant at 29.200. If I want to use SSB on the far lower end of the band I have to tune the antenna. It takes the autotuner about 2 - 4 seconds and oilla! I have a matched antenna. It'll even manage to tune the antenna for the 6m band after about 6 seconds, strangely enough.
 

LtDoc

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Sounds like your 10 meter antenna is slightly too short, adding an inch or two woul put it further down in frequency. If it's working to your satisfaction now, I think I'd leave it alone.
- 'Doc
 
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