I have got stuck on the theory around the matter of tweaking the electrical length of an antenna by adding L or C somewhere along the transmission line.

From transmission theory, the phase velocity in a conducting medium is 1 over the square root of the L and C values per meter, as in the equation here.

This indicates that if you increase either the inductance (series) or the capacitance (shunt) of the line, the electrical length G will increase. That all seems fine, but I have read various sources saying that if an antenna is electrically 'long', due to too much series inductance, then you can electrically ‘shorten’ it by adding capacitance to counteract the inductive reactance and return the electrical length to the optimum value for the particular frequency and conductive medium.

I am unable to unite these two statements since, according to the equation above, adding capacitance will increase G and not reduce it.

What am I missing here in the logic? Is it to do with the series and parallel configuration of the L and C?

There are some excellent articles on AAC about this general area, but there is a lot to read through, and I haven't resolved the issue yet.

Thank you.

 

https://archive.org/details/in.ernet.dli.2015.6364/page/n89/mode/2up

Get reading.

 

https://archive.org/details/in.ernet.dli.2015.6364/page/n89/mode/2up

Get reading.

I expect every question ever posed on this site could be answered by getting the questioner to read enough material. . . . . .

 

I expect every question ever posed on this site could be answered by getting the questioner to read enough material. . . . . .

Yes, that's how you learn. If you were just a beginner then a more detailed answer would be needed. Your question text shows (assuming you didn't just ask some LLM) a level of knowledge that can easily understand the linked text book as a way to derive an answer to your questions.

There is a lot to read through, no shortcuts from me.

 

Yes, that's how you learn. If you were just a beginner then a more detailed answer would be needed. Your question text shows (assuming you didn't just ask some LLM) a level of knowledge that can easily understand the linked text book as a way to derive an answer to your questions.

There is a lot to read through, no shortcuts from me.

My point is that I have read quite deeply into this query but so far not found a suitable and believable answer. The prospect of doing the same with ‘100’ pages of a well used book does not inspire me for a positive outcome.

I am new to RF issues but it is my Physics training that gives the impression otherwise.

 

I don’t see that this contribution provides any insight into my query. Yes I get it that people have to adjust their antennas for all sorts of reasons, such as coaxial line type, bandwidth and impedance matching, but that does not explain why when the basic equation, indicating that increasing either L or C increases your electrical length, it is often stated that increasing C ‘shortens’ an antenna.

My approach to learning about this stuff is to gather information from a range of sources (including LLM) and then write a summary in my own words. So far I have been unable to do so with this bit about adjusting G.

 

I expect every question ever posed on this site could be answered by getting the questioner to read enough material. . . . . .

Most likely. But isn't it nice to have someone point you to specific material that they think will help you along? After reviewing it, you are then in a better position to have a discussion based on a common reference point.

 

I have read various sources saying that if an antenna is electrically 'long', due to too much series inductance, then you can electrically ‘shorten’ it by adding capacitance to counteract the inductive reactance...

You add series Cs to an antenna that is too long (inductive). The transmission line C is shunt capacity. These two, not being in parallel, do not add up.

 

I don’t see that this contribution provides any insight into my query. Yes I get it that people have to adjust their antennas for all sorts of reasons, such as coaxial line type, bandwidth and impedance matching, but that does not explain why when the basic equation, indicating that increasing either L or C increases your electrical length, it is often stated that increasing C ‘shortens’ an antenna.

My approach to learning about this stuff is to gather information from a range of sources (including LLM) and then write a summary in my own words. So far I have been unable to do so with this bit about adjusting G.

So do that. Provide your best shot at summarizing it so that we can discuss what parts of your summary are problematic. You might include relevant snippets from the material you are drawing information from and the conclusions you are drawing from them, as well, so that we can get a better idea of your thought processes as you approach things.

For instance, I just asked Copilot (though, as a rule, I don't use LLMs for much) and simply asked, "how can adding capacitance to a transmission line shorten the electrical length of an antenna?" This is what it returned:

Great question—this dives right into the heart of RF and antenna theory!

Adding **capacitance** to a transmission line or antenna system can **electrically shorten** an antenna by **counteracting inductive reactance**. Here's how it works:

The Basics
An antenna that is **physically longer than its resonant length** (say, longer than ½ wavelength for a dipole) behaves **inductively**—it has excess inductive reactance. To bring it back to resonance, you can **add a capacitive element**, which introduces **capacitive reactance** that cancels out the excess inductance.

This doesn’t physically shrink the antenna, of course—but it **alters the phase relationship** of voltage and current along the structure, making it behave **as if it were shorter** at the operating frequency.

Transmission Line Analogy
In transmission line terms, the **electrical length** is determined by the **phase velocity**, which depends on the **inductance (L)** and **capacitance (C)** per unit length. The formula is:

\(
v_p = \frac{1}{\sqrt{LC}}
\)

So increasing **shunt capacitance** (or series inductance) slows the wave down, increasing the **electrical length**. But when you're **tuning an antenna**, you're not just changing wave speed—you’re **adjusting the impedance profile** to make the antenna resonate at a lower frequency, effectively **shortening its electrical behavior**.

TL;DR
Adding capacitance to an inductively long antenna **brings it back to resonance**, making it behave like a shorter antenna at the desired frequency. It’s a clever trick used in everything from **car antennas** to **compact HF designs**.

Want to explore how this plays out in real-world antenna designs? I’ve got examples!

Now, as a rule, I take anything produced by an LLM with a grain of salt, but notice what it says about the difference between tuning an antenna and determining the electrical length of a transmission line.

I then asked the great Google "how to adjust the electrical length of an antenna" and one of the top results was the following:

https://electriciantraining.tpub.com/14182/css/Basic-Antennas-184.htm

Which included the following:

The electrical length of any antenna wire can be increased or decreased by loading. If the antenna is too short for the wavelength being used, it is resonant at a higher frequency than that at which it is being excited. Therefore, it offers a capacitive reactance at the excitation frequency. This capacitive reactance can be compensated for by introducing a lumped-inductive reactance, as shown in view A. Similarly, if the antenna is too long for the transmitting frequency, it offers an inductive reactance. Inductive reactance can be compensated for by introducing a lumped-capacitive reactance, as shown in view B.

Just this much information from these two basic searches hints that the notion of "electrical length" for a transmission line might be somewhat different than that for an antenna. So perhaps that is something worth exploring a bit.

Another thing to explore is whether these capacitances and inductances are being added in the same way. How is a capacitor being added to the transmission line to lengthen it? How is a capacitor being added to an antenna to shorten it?

 

I have consulted a range of sources, but it is also very true that the way one phrases a question determines the quality of the answer. As the truism from days gone by, "Rubbish in, rubbish out".

Yes, I had already seen and digested the responses you quoted, but they still don't quite answer my query. Yes, it seems perfectly sensible that adding capacitance will counteract too much inductance and so adjust and correct the resonant length, as might show on a Smith chart.

But I think you are onto something there, that the issue of increasing or reducing electrical length is a little bit different from fine-tuning an antenna, when I had assumed they were almost synonymous (even though they are surely linked). So I will fine-tune my questions to minimise the amount of "rubbish out".

Thanks

 

While all you have stated may be true, it does not address my question at all. I fully understand why and how adding capacitive reactance will 'offset' inductive reactance and so bring an antenna back into resonance. That is not what my question is about and all you are doing it trying to make your knowledge work and fit my query; which it doesn't. Thanks for your efforts but I will seek answers elsewhere.

 

Hello,

Here is a page about the top load antenna:
https://www.robkalmeijer.nl/technie...hniek/hambladen/qst/1990/03/page26/index.html

Bertus

 

Will give it a whirl, thanks

 

Did you miss this?

You said:

"This indicates that if you increase either the inductance (series) or the capacitance (shunt) of the line, the electrical length G will increase. That all seems fine, but I have read various sources saying that if an antenna is electrically 'long', due to too much series inductance, then you can electrically ‘shorten’ it by adding capacitance to counteract the inductive reactance and return the electrical length to the optimum value for the particular frequency and conductive medium."

You add series Cs to an antenna that is too long (inductive). The transmission line C is shunt capacity. These two, not being in parallel, do not add up.

Shunt vs. series capacitance. Capacitve loading is in a series with the antenna, just like the "load coils" used to electrically lengthen an antenna, a series capacitance is used to shorten it (see: capacitive hat).

 

So the original equation I stated refers to series capacitance, even though transmission line theory equates a propagation path to a set of inductors in series and capacitors in parallel?

The capacitance in my original equation is shunt not series and yet I often see it written that to shorten the electrical length you add shunt capacitance. That would appear to be wrong since, as I show in this table, at 100MHz increasing either increases G.

 

The capacitance in my original equation is shunt not series and yet I often see it written that to shorten the electrical length you add shunt capacitance.

You can also use a shunt C instead of series C to shorten a transmission line (or antenna) that is too long. Choice is yours.
The problem is that you take the sum of both C's, the concentrated element C used to shorten the line and the distributed C per unit length of the transmission line, to calculate "G" as if these are somehow in parallel. That is not the case.

 

Transmission lines and antennas are totally different, which may have been mentioned already. And while it may be nice to totally be able to derive the equations to prove things, that is needed in actual "Rocket Science", but it is not quite so vital in the rest of the world.
To lengthen an antenna electrically we add a series inductance , to shorten that antenna that has been lengthened by adding a series inductance, we cancel some of that added inductance by shunting it with a capacitance. It should be clearer now that the lack of understanding the "where" is what promoted all of the confusion.

 

Sounds about right

 

Adding capacitance at the ends of an antenna segment is similar to adding parallel capacitance to a parallel L-C tuned circuit in that it effects a lowering of the resonant frequency. The effect is similar to lengthening the antenna element. The detailed mathematics are rather tedious, in my opinion.

 

That seems opposite of what other sources say which is that adding shunt capacitance to an antenna has the effect of ‘shortening’ it (and adding inductance lengthens it). You can see the reason for my query.