About Resonant Dipoles
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- čas přidán 11. 08. 2023
- #199 In this video I set out to correct one of my previous mistakes and look at the finer details of dipole antennas and observe when exactly they are resonant. I have a look a some basic simulation results, and then proceed to build some antennas and try to observe what constructive parameters impact their resonance.
Related content:
Antenna Equivalent Model: • Antenna Equivalent Model
Further reading:
courses.ansys.com/wp-content/...
www.antenna-theory.com/antenn...
Antenna theory and design; Stutzman, Thiele; 1981;
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That was interesting and gave me some new insights. Impressed how you turned a (small) mistake in an earlier video into a very useful new video! Keep up the good work…
re: why the diameter of the material matters, an explanation I came up with which may not be technically correct, but seems to be close enough to build useful antennas and easier to wrap my head around:
when you have many elements fanning out from one point, the radio waves will use the one that's the best 'fit' for the wavelength and ignore the others (best visualization is a HF multiband dipole)
when you have a thick antenna element, you can think of it as a whole bunch of wires fanning out from the feed point to all the points around the far end.
the minimum frequency it will resonate at is the length of the element * velocity factor
the maximum frequency is the 'diagonal' from the feedpoint to the far side of the element at the other end.
At HF frequencies, this is negligible, but I've built 2m antennas with 4" diameter elements where this is significant and in practice (as measured by test equipment), this way of thinking got me within normal antenna trimming range of being correct
just a quick thanks for all your videos. They have helped me greatly in my attempt to learn hobby electronics. Thankyou!
I already signed up for the first video, great content.
Can I just say that this channel is probably the best RF theory resource there is? It helped me understand SO many concepts already, and I haven't watched even through half of the videos yet. Fesz, you are an awesome teacher, and you deserve several times more subscribers.
Thank you, a lot
This is an excellent presentation!
Nice video and great explanation.
Excellent work.
The voltage graph (6:39 into the video) shows a zero crossing at the center. This is a common mistake we also see in text books. If the voltage at the feedpoint was zero at resonance, then the impedance (V/I) would also be zero. Clearly, this can't be right. The actual voltage is not zero at the center (feedpoint). For 1 Watt of driving power it must be sqrt (P x Z) = sqrt (1 W x 72 Ohms) = sqrt (72) Volts = 8.49 V or half of that for each arm, in the context of the red graph trace.
👍 7:00 There is a simpler way of thinking about the effective length. A half wave dipole are two pieces of wires than can be considered a capacitor. AC current can flow to a capacitor despite the fact that it's an open circuit - its plates are accumulating electrons and thus constantly charged and discharged. Thus the effective current on the edged of the dipole is (!) not zero - the current still flows (charges the capacitor) despite the fact that there is nowhere to flow anymore. As a result the effective length is larger that the physical one.
No.
Good point. Yes, the dipole must be a "closed circuit". At the ends of a dipole the conduction current must approach zero but the antenna circuit extends when we include the fields. In this case the electric field.
The capacitor analogy is not entirely wrong. The equivalent circuit of a halfwave dipole is indeed an inductor in series with a capacitor followed by a resistor.
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Very interesting, as always. Thanks.
I was wondering about how to predict and measure and improve the efficiency of an antenna (without an anechoic chamber) ?
I mean that you can have a perfect match using an antenna that doesn't radiate anything (some commercial wideband products event contain a suspicious dummy load).
Perhaps using a field meter far away enough ?
One way is - search for the term "Evolved antenna"
"In radio communications, an evolved antenna is an antenna designed fully or substantially by an automatic computer design program that uses an evolutionary algorithm that mimics Darwinian evolution. This procedure has been used in recent years to design a few antennas for mission-critical applications involving stringent, conflicting, or unusual design requirements, such as unusual radiation patterns, for which none of the many existing antenna types are adequate."
@@pakistaniraveasylum1396made by Chat GPT 😂
What firmware are you running on your nanovna? It looked like your vna was set to 1024 sweep points…. You must have one of the higher end vnas. Great video as always!
I'm using a "LiteVNA"; but I noticed that there is PC software (Nanovna saver) that can be used to sweep any number of points, so if you are interested in more measurement points in a single scan, then that should be the way to go. Another really interesting thing about the mentioned program is that it allows logarithmic frequency axis display, something that the base device cannot really do
Very interesting video. Thank you for sharing. It’s a little distracting how your voice has only 3 inflection tones. Medium for most of the video, with high pitch flicks at end of comma, and finishing low at end of sentence. Which makes seem like it’s an endless run on sentence.
Thanks for an excellent video. If you don't mind, can I ask a quick question because I can't find an answer on the internet and you may be one of the only people on the planet who knows.....? Do you know whether it is possible to add both real and imaginary terms to the inductance of an inductor in LTSpice? I need to simulate both the inductance and loss - the latter being important for thermal noise. Ideally, I would like to have a look up table for the frequency dependancy of both terms, but I guess that is asking too much! At the moment, I run repeat simulations at different frequencies using modified values of L for the real part. I also add a series resistance for the imaginary (loss) part. As these need manually editing as a pair for each frequency, I feel that there must be a batter way...... Have you ever come across anything?
In case the simulation type you want to perform is "AC", then what might help is the FREQ function I discussed a few videos back - with that you could define a resistor with both real and imaginary parameters and also have these varied based on frequency.
FesZ you gonna be taking a look at the new QSpice from Qorvo ?
That's not something I plan at the moment. I will stick to LTspice for now in my various videos
@@FesZElectronics No worries. Thanks.
Is the ballun necessary? The antenna dont care about DC and the current between the Terminals should also always be equal.
The importance of the balun in a balanced load like a dipole antenna is that it forces the current from one branch of the antenna to be equal with the current in the other branch; without the balun, part of the current from the branch connected to the center conductor of a coax will close back to the coax shield rather than the other antenna branch.
Can I put multiple length antennas in parallel to tune to their frequencies?
Like one antenna for 400MHz, one for 940MHz, one fore 1.2 GHz,... and so one all connected in parallel.
To some extent, I think what you are describing is the Log-Periodic Antenna. Its a type of antenna that uses arms of different length to cover a wide frequency range. However, if you just need a few very specific frequencies, and not the interval in-between, maybe its best to use a each antenna with a filter and then combine them - similar to how a diplexer is used to combine a 2m and 70cm antenna; but a more complex filter for more than 2 frequencies...
@@FesZElectronicsThanks for the explanation.
Have never tried it at these higher frequencies but to answer the question, yes. Over the years I have made several fan dipole antennas for the HF Amateur radio band. I have one up now for 10M, 20M, 40M and 80M. Just Google "fan dipole" or "parallel dipole". There are also free online calculators for calculating the element length's. When tuning start with the longest element (lowest frequency) first and work towards the shortest element (highest frequency). Mike KC3OSD
@@mikesradiorepair Will do. thanks.
@@mikesradiorepair A variation on the fan dipole that folds around on itself is a 'cobweb antenna'.
Ok... I don't understand the '50' ohm calibration load you made... at 13.58 in the vid. Looks to me... like you soldered a 49.9 SMD ... in parallel... with a 200K through hole. If so? ... how does that get you closer to 50 ohms?
It doesn't. But since the goal here is to make relative impedance measurements, the absolute error in the measured impedance is not very important.
@@janhamaldvorak560 For resistors... pure real resistance.... no reactance... if you put them in parallel... the total resistance is less than the least. So... yes... its going to be less than 49.9 ohms... always. But another meaning is if Z = 49.9 + Xj ... where reducing the reactance... by adding additional components...gets you closer to a real resistance.
Have you tried qspice ?
Have you tried iodine on that nasty zit?