Dipole Antenna Length

[A.Z.]

Well, at 0.6 Lambda height it's more than 7 dBi, now maybe you'll need to search for dBi

A dipole is not 7 dBi gain.  It's about 2.15 dBi.

https://en.wikipedia.org/wiki/Dipole_antenna

SIGH

another "internet expert"

[A.Z.]

Well, if you are only interested in SHF signals up to the GHz, that one may be a start, but then don't expect it to give you some decent reception down at UHF/VHF or HF, they just won't perform; if you're going to install an antenna up the roof, my suggestion is to start with a discone, most of them will offer acceptable performance from 25MHz up to about 2GHz

Willing to get down to HF/VHF with decent performance, here's my suggestion

Pick this small 9:1 BalUn https://www.amazon.com/Balun-One-Nine-v2-Barebones/dp/B07XJRTF94 and put it inside a plastic w[e]at[h]erproof junction box, then use some 16AWG insulated wire and lay it out over the roof (or inside attic if you have a wooden roof) forming an horizontal loop and connect the ends of the wire to the balun, cut the loop wire wire at the opposite side from the Balun and insert a resistor with a value around 500...600 Ohm, the latter will flatten the impedance curve allowing to obtain a good match over a wide bandwidth, such a terminated  loop will also offer the advantage of being less "noisy" than other types of antennas, yet it will be able to set the SNR and allow you to pick up signals from MW up to around 50MHz at least

I'm not wanting to spend more money beyond what I have.  I've now purchased these pizza discs, and plan on putting two of them together as shown.  This is a limitation I have placed on myself.  I currently use the SDR to listen to ATC and pick up ADSB signals.

that "pan" antenna will possibly work ok for ADS-B which is in the GHz range, but NOT for ATC which is down to the 100s MHz range, again, consider a discone if you want to get some acceptable performance over the frequency range you are interested in

[Fried Chicken]

Well, if you are only interested in SHF signals up to the GHz, that one may be a start, but then don't expect it to give you some decent reception down at UHF/VHF or HF, they just won't perform; if you're going to install an antenna up the roof, my suggestion is to start with a discone, most of them will offer acceptable performance from 25MHz up to about 2GHz

Willing to get down to HF/VHF with decent performance, here's my suggestion

Pick this small 9:1 BalUn https://www.amazon.com/Balun-One-Nine-v2-Barebones/dp/B07XJRTF94 and put it inside a plastic w[e]at[h]erproof junction box, then use some 16AWG insulated wire and lay it out over the roof (or inside attic if you have a wooden roof) forming an horizontal loop and connect the ends of the wire to the balun, cut the loop wire wire at the opposite side from the Balun and insert a resistor with a value around 500...600 Ohm, the latter will flatten the impedance curve allowing to obtain a good match over a wide bandwidth, such a terminated  loop will also offer the advantage of being less "noisy" than other types of antennas, yet it will be able to set the SNR and allow you to pick up signals from MW up to around 50MHz at least

I'm not wanting to spend more money beyond what I have.  I've now purchased these pizza discs, and plan on putting two of them together as shown.  This is a limitation I have placed on myself.  I currently use the SDR to listen to ATC and pick up ADSB signals.

that "pan" antenna will possibly work ok for ADS-B which is in the GHz range, but NOT for ATC which is down to the 100s MHz range, again, consider a discone if you want to get some acceptable performance over the frequency range you are interested in

Wait really?  Why are all these RTLSDR lunatics recommending this if the rabbit ears work better?  Well what else am I going to do with the pizza pans I have now?

Does a discone antenna need to be oriented vertically?

[Fried Chicken]

No no no no no no no.

This is all a distraction.  I still want to know what's going on with the dipole antenna length.  If I have a dipole antenna at full wavelength, wouldn't it pick up exactly nothing in a theoretical perfect world?  Would it cancel out?  If that guy's video is to be believed this is exactly the case....

But is a radio measuring across the two sides of the dipole, or is it measuring across the two sides of the dipole and then the ground?  Maybe I should ask this in a physics forum.  I really want an ideal theoretical answer, not a practical one.  I believe it's a simple question and there should thus be a simple answer no?

[A.Z.]

a 1/2 Lambda dipole will offer good matching at 50/75 Ohm and a bidirectional pattern with the lobes perpendicular to the dipole axis, making the dipole longer you'll loose the match (but it can be recovered) and the lobes will split into multiple smaller ones, and when you approach or go over a full Lambda the lobes will point more and more toward the ends of the arms

note: a 1/2 wave dipole is one whose arms are 1/4 Lambda each, so the total lenght from end to end is 1/2 Lambda (1/2 wave)

[radiolistener]

On the dinky included dipole antenna, they pulled the shielding and put it on one dipole as the negative, and then took the inside and merely connected it to the other dipole antenna.

There's a resistance here?  I mean it must certainly be negligible?

This is a bad cheap antenna, because such connection leads to use outer braid of coax cable as a part of antenna. So, technically this is not dipole, because it uses coax cable and your computer shell as a part of antenna. And such antenna will receive a lot of noise from your computer and from all electronic equipment placed across coax cable path.

In your case one arm of antenna is a half length of such dipole, but the issue here is that second arm is a half length of such dipole + the length of your coax cable + length of your computer shell + length of mains wire + etc. And the second arm is not a part of dipole, but complicated geometry wire.

Proper connection of coax with dipole antenna needs to use balun and RF choke in order to match symmetric antenna with non-symmetric coax cable and block unwanted currents between antenna and outer braid of coax cable.

It prevents to sucking of noise from equipment around coax cable path and eliminates exposure of nearby equipment with high strength RF field from coax cable during transmission.


Half wavelength dipole in a free space has about 73 Ω impedance. So it well match with 75 Ω coax cable, but there is needs balun in order to match symmetric dipole antenna with non symmetric coax cable.

Since both have the same impedance, you can simply use 1:1 transformer for that. Since primary and secondary coils are isolated it prevents current flow from antenna to outer braid surface of coax cable, in that way coax cable braid is excluded from antenna.

[A.Z.]

a simple W2DU choke could solve that issue, yet, that antenna sucks for a number of reasons

[radiolistener]

a simple W2DU choke could solve that issue, yet, that antenna sucks for a number of reasons

Simple RF choke cannot eliminate parasite currents to enough level, because they are very high. It can reduce these currents, but can't eliminate it at all. Using balun to match symmetric antenna with non-symmetric transmission line is more easy way to eliminate these currents.

So, the best way is to use balun. And then, for better results, you can add some additional RF chokes to block currents due to capacitive/inductive coupling of coax cable braid with antenna within near field zone.

[A.Z.]

you know what a W2DU choke (and BalUn) is, don't you ?

[radiolistener]

When I said "RF choke", I mean specific case - ferrite bead on coax cable braid or winding the coax cable in the form coiled coax choke or both in combination.

And when I said "balun", I mean specific case - the component which is especially designed to match balanced and unbalanced lines, it may include impedance match and it needs to break coax cable and solder/connect balun component in the middle.

What I wanted to say exactly in this case is that using 1:1 transformer balun is better way to match coax cable with dipole than using ferrite beads on coax cable or coiled coax choke.

The reason is that it needs to use a lot of ferrite beads or use a lot of coax windings to get good enough performance and it will be very inconvenient to use. While adding 1-3 ferrite beads or coax windings will be not enough to get good performance in comparison with transformer balun.

But ferrite beads or coiled coax choke can be added in addition to transformer balun in order to reduce effect of capacitive/inductive coupling of coax cable outer braid with antenna within near field region. For better result it should be placed at 1-2 meters away from antenna. While transformer balun needs to be placed at feeding point of antenna. Such combination allows to achieve the best results.

[A.Z.]

ok, you don't know what a W2DU (aka Maxwell) choke/balun is, and how, when and why it was born; fine, no problem

[radiolistener]

ok, you don't know what a W2DU (aka Maxwell) choke/balun is, and how, when and why it was born; fine, no problem

I know about W2DU balun which he talking on his book "Reflections III - Transmission Lines and Antennas":



As you can see and as I said above, this is specific case which I meant exactly when I said "RF Choke".

Do you notice that Walter Maxwell wrote in his book "many baluns embody some form of coupling transformer"?
This is what I meant when I said "balun".

From my point of view it is pretty clean. But I don't understand what you're not agreed with, could you please explain?

[MartinL]

Fried Chicken: unfortunately, this thread has diverged from your quite reasonable beginner's questions about antenna theory, into an argument between experts seeking to demonstrate who among them is the bigger expert. Perhaps said experts could take their dispute elsewhere?

Let's come back to your questions.

I still want to know what's going on with the dipole antenna length.  If I have a dipole antenna at full wavelength, wouldn't it pick up exactly nothing in a theoretical perfect world?  Would it cancel out?  If that guy's video is to be believed this is exactly the case....

That video is, quite simply, wrong.

The argument it presents is that in a full wavelength dipole, you would need to have currents travelling in opposite directions within each element, and that those opposing currents would have to cancel each other out, resulting in nothing being radiated from or received in the antenna.

But the whole idea of those opposite currents cancelling out is false in the first place. The narrator seems to think that either you can't have two currents flowing in opposite directions within the same piece of wire, or that if you do, their effect will somehow cancel out such that there is no radiation from the antenna.

But when talking about AC, it is perfectly normal to have different currents flowing at different points along a wire. In fact it's impossible not to. Whenever you have an AC current flowing in a wire, then the current in that wire will vary, all the way from one direction to the other and back again, at intervals of one wavelength all the way along the length of the wire.

Those "opposite" currents at different points do not cancel out, because they are not in the same places. In fact, they are all part of the same AC current. An AC current does not just vary over time: it also varies over space.

And their conclusion, that a full-wave dipole could not radiate, is also false. In reality a full wave dipole will radiate or receive just fine.

The reasons that half-wave dipoles are preferred are practical, rather than fundamental:

- A resonant antenna acts as a filter that preferentially receives or transmits a particular frequency.
- A half-wave dipole is the shortest resonant antenna for a given frequency, and therefore the smallest and cheapest to build.
- The impedance at the centre of a half-wave dipole is one that is easy to work with.

These properties are of great concern when e.g. building a transmitting station at long wavelengths, but in your case, none of them are critical:

- Your receiver is very capable of selecting what frequency to listen to and filtering out others; a resonant antenna can help, but isn't essential.
- In the case of your rabbit-ears antenna, you're free to vary its length to whatever works best within its range; there's no cost to extend it fully.
- Your receiver has more than enough variable gain to compensate for loss due to the antenna impedance not matching that of the radio.

As such, you're quite likely to find that a different configuration - e.g. extending the rabbit ears fully to get a larger antenna which can collect more of the available signal - will get you better results than trying to achieve a half-wave dipole configuration.

But is a radio measuring across the two sides of the dipole, or is it measuring across the two sides of the dipole and then the ground?  Maybe I should ask this in a physics forum.  I really want an ideal theoretical answer, not a practical one.  I believe it's a simple question and there should thus be a simple answer no?

Simple answer: it is measuring across the two sides of the dipole. There doesn't need to be any "ground" involved. A dipole antenna works just fine floating in space. Having a planet nearby is a complication, rather than a requirement, for this type of antenna.

More correct answer: it is not really measuring across the two "sides", but rather measuring at the feed point. By the feed point, I mean the point in the middle where the antenna has been split. Usually for a half-wave dipole this is in the middle, but you can place the feed point anywhere. The position of the feed point will determine the impedance seen by a radio connected at that point.

[iMo]

As the OP is interesting in reception only (that is my current understanding) the special/exact tuning of his "dipole" antenna is not necessary..
Moreover, you would need a broadband balun in the feedpoint of that dipole (Edit: on that picture there is a "balun/common mode choke" on the coax but it should be closer to the feedpoint, imho), otherwise with unbalanced feeder (the coax cable) one leg of the dipole gets grounded.
But again - with that antenna on your picture above and your SDR receiver it does not matter whether the antenna will be cut exactly for a resonance (and a dipole or "doublet" is a single band antenna anyhow).

So simply calculate the lamda/4 for a frequency of choice and set the length of each leg to that number. That it is.

In case you want focus your reception on special bands (especially vhf/uhf) I would recommend an yagi antenna as that would help you with the gain and directivity much more.

PS: the "dipole antenna" when made properly is lambda/2 of length (left leg + right leg), where a single leg's length is lambda/4.
In reality it will be a little bit shorter as it has been said above.

The coax length plays no role in the calculation (it is not a part of the dipole) when with proper impedance matching, it means when the impedance of coax used is the same as the impedance of the dipole's feedpoint and the input of your SDR receiver.

The impedance of the dipole itself is the impedance in its feedpoint (the point between the legs, in your case at that plastic part), and it depends on the angle between right and left leg.

Thus with changing the angle between the legs in your kit you may do some SWR improvement (with legs in the same plane the impedance is around 73ohm, with some angle like 30-40degree off the plane it will be around 50ohm).

So basically you may tune your telescopic antenna (as depicted in your kit) with the leg's length (frequency) and leg's angle (impedance).

You will get the best signal when the legs are set to the lambda/4 each and the angle to aprox 30-40degree off the plane, provided your coax is 50ohm and the SDR's input is 50ohm as well (with max signal strength when the transmitter is in the max of the front/back lobe of your dipole's radiation pattern and, of course, the transmitter's antenna with the same polarization as your antenna is - it could be "vertical" or "horizontal"). 

[ahbushnell]

Well, at 0.6 Lambda height it's more than 7 dBi, now maybe you'll need to search for dBi

A dipole is not 7 dBi gain.  It's about 2.15 dBi.

https://en.wikipedia.org/wiki/Dipole_antenna

That's gain in free space. 

The height above ground of a half-wave dipole will cause the gain to increase at some elevation angles, and decease at others.  I don't have the numbers handy, but I believe that the 7dBi value is possible.  Obviously the quality of the ground also is a factor.

Find a link.

[A.Z.]

Well, at 0.6 Lambda height it's more than 7 dBi, now maybe you'll need to search for dBi

A dipole is not 7 dBi gain.  It's about 2.15 dBi.

https://en.wikipedia.org/wiki/Dipole_antenna

That's gain in free space. 

The height above ground of a half-wave dipole will cause the gain to increase at some elevation angles, and decease at others.  I don't have the numbers handy, but I believe that the 7dBi value is possible.  Obviously the quality of the ground also is a factor.

Find a link.

a link ? check the pdf I attached (reply #25), then if you want a link I may post it too

Oh and then, if you desperately want a link, you may try this one

https://www.w8ji.com/antennas.htm

[Fried Chicken]

Fried Chicken: unfortunately, this thread has diverged from your quite reasonable beginner's questions about antenna theory, into an argument between experts seeking to demonstrate who among them is the bigger expert. Perhaps said experts could take their dispute elsewhere?

Let's come back to your questions.

I still want to know what's going on with the dipole antenna length.  If I have a dipole antenna at full wavelength, wouldn't it pick up exactly nothing in a theoretical perfect world?  Would it cancel out?  If that guy's video is to be believed this is exactly the case....

That video is, quite simply, wrong.

The argument it presents is that in a full wavelength dipole, you would need to have currents travelling in opposite directions within each element, and that those opposing currents would have to cancel each other out, resulting in nothing being radiated from or received in the antenna.

But the whole idea of those opposite currents cancelling out is false in the first place. The narrator seems to think that either you can't have two currents flowing in opposite directions within the same piece of wire, or that if you do, their effect will somehow cancel out such that there is no radiation from the antenna.

But when talking about AC, it is perfectly normal to have different currents flowing at different points along a wire. In fact it's impossible not to. Whenever you have an AC current flowing in a wire, then the current in that wire will vary, all the way from one direction to the other and back again, at intervals of one wavelength all the way along the length of the wire.

Those "opposite" currents at different points do not cancel out, because they are not in the same places. In fact, they are all part of the same AC current. An AC current does not just vary over time: it also varies over space.

And their conclusion, that a full-wave dipole could not radiate, is also false. In reality a full wave dipole will radiate or receive just fine.

The reasons that half-wave dipoles are preferred are practical, rather than fundamental:

- A resonant antenna acts as a filter that preferentially receives or transmits a particular frequency.
- A half-wave dipole is the shortest resonant antenna for a given frequency, and therefore the smallest and cheapest to build.
- The impedance at the centre of a half-wave dipole is one that is easy to work with.

These properties are of great concern when e.g. building a transmitting station at long wavelengths, but in your case, none of them are critical:

- Your receiver is very capable of selecting what frequency to listen to and filtering out others; a resonant antenna can help, but isn't essential.
- In the case of your rabbit-ears antenna, you're free to vary its length to whatever works best within its range; there's no cost to extend it fully.
- Your receiver has more than enough variable gain to compensate for loss due to the antenna impedance not matching that of the radio.

As such, you're quite likely to find that a different configuration - e.g. extending the rabbit ears fully to get a larger antenna which can collect more of the available signal - will get you better results than trying to achieve a half-wave dipole configuration.

But is a radio measuring across the two sides of the dipole, or is it measuring across the two sides of the dipole and then the ground?  Maybe I should ask this in a physics forum.  I really want an ideal theoretical answer, not a practical one.  I believe it's a simple question and there should thus be a simple answer no?

Simple answer: it is measuring across the two sides of the dipole. There doesn't need to be any "ground" involved. A dipole antenna works just fine floating in space. Having a planet nearby is a complication, rather than a requirement, for this type of antenna.

More correct answer: it is not really measuring across the two "sides", but rather measuring at the feed point. By the feed point, I mean the point in the middle where the antenna has been split. Usually for a half-wave dipole this is in the middle, but you can place the feed point anywhere. The position of the feed point will determine the impedance seen by a radio connected at that point.

This offers a lot of clarity, and explains how I can pick up frequencies my antenna theoretically shouldn't pick up.  The radio picks up FM stations without any antenna attached at all!   I tried finding Walter Lewin's lecture where he explains and builds a working radio antenna (and trolls Harvard by making an illegal broadcast in their name).

[radiolistener]

This offers a lot of clarity, and explains how I can pick up frequencies my antenna theoretically shouldn't pick up.  The radio picks up FM stations without any antenna attached at all!

The example of antenna in visible optic band is a telescope. You can see Mars on the sky without telescope, and you can see it through piece of water pipe. But if you want to see detailed picture, you're needs to use properly designed telescope.

Exactly the same thing for radio antenna. You can see some signal with using rusty nail, you can even use it for transmission. But if you want to get good sensitivity and listen good quality radio with no noise interference and with no distortions, you're needs to use properly designed antenna.

The main issue of a random wire antenna is that it is not selective for the frequency of interest and it sucks the noise from nearby equipment, so the useful signal is flooded in a high noise interferences.

Using random wire instead of proper antenna is like using random piece of glass instead of proper telescope to see Mars on the sky.