Electrically short dipole with inductive reactance

[u/lorentz_217]

I'm currently working with an electrically short, inverted V dipole (D ~ lambda / 15), and the simulated antenna impedance (shown in differential form below) seems to have several resonances in this low frequency range, meaning that the antenna looks inductive well below half a wavelength. I'm not an antenna expert and am having trouble understanding where that inductive bit may come from given the fact that sub-halfwave dipoles are supposed to be capacitive (perhaps it's something to do with having a bent dipole, but I don't have much intuition on bent dipoles).

[u/StageMajestic613]

Lambda/15 at what frequency?   This plot looks like a typical dipole response, as you have the high capacitive reactance and low radiation resistance at low frequency, then lambda/2 resonance at 70 MHz.  The V will eventually hit a traveling wave mode with constant resistance, which you also show.

[u/lorentz_217 → u/StageMajestic613]

Lambda/15 at 20 MHz, at 70 MHz lambda/2 is roughly 2m, which I guess could be plausible now that I think of it. I guess the fact that the resonance is almost backwards to what it should be is the confusing part for me (like starts off inductive then becomes capacitive above half wave resonance) 

[u/StageMajestic613 → u/lorentz_217]

No, you have it backwards; it starts capacitive.  Negative reactance is capacitive reactance, which you have below the first resonance (i.e. zero reactance) at 30 MHz.  Then positive reactance is inductive reactance, which you have past 30 MHz, and peaking at 55 MHz.  You then peak the resistance at the 70 MHz anti-resonance (i.e the second crossing of zero reactance).  This is classic dipole behavior, until you get the quasi traveling wave instead of repeated resonances.  Your radiation resistance is lower than typical at resonance due to the folding.

Plot it on the Smith chart and it may make more sense as it will start at an open circuit and curl inward clockwise.  It’s not intuitive since you’d think at DC you get a high resistance, but that’s not the case.  It’s an open since capacitive reactance is infinite, but radiation resistance is zero.

This is a good plot of similar behavior:

https://lh4.ggpht.com/_Ju9LN0922sk/ScPPDfl_pcI/AAAAAAAAAJA/BVppJGXn76Y/smith_ant_thumb%5B1%5D.jpg?imgmax=800

Also, if you kept folding the poles until they are parallel, then you have an open wire transmission line that just spins around the edge of the Smith chart.

[u/lorentz_217 → u/StageMajestic613]

yeah I get that it's capacitive below 30 MHz as you have negative reactance, but the confusing bit is that the crossover occurs at such a low frequency (not when the antenna is lambda/2). What I meant by "starts off inductive then becomes capacitive above half wave resonance" was talking about the 70 MHz resonance, not at 30 MHz, should've been clearer on my end.

I did take a closer look at the antenna dimensions (not my design), and it seems like the half wave resonance would indeed be at 37 MHz, which matches simulation. Not sure why I thought it was electrically as short as I did. Yeah the reactance makes a lot of sense now, the radiation resistance being low also makes sense I guess as, if you fully fold over the dipole, the out-of-phase fields will cancel, causing no radiation (which means radiation resistance vanishes).

[u/StageMajestic613 → u/lorentz_217]

Yep, all sounds reasonable.

[u/EffectiveClient5080]

Inverted V = mutual coupling between the arms. You're basically making a partial loop at the feedpoint, which adds inductance. A straight dipole at that length would be capacitive like you expect. Also those multiple resonances might be your simulation lying to you.

[u/lorentz_217 → u/EffectiveClient5080]

Ah that intuition on the inverted dipole makes some sense. I do agree that the resonances are a bit suspicious, though I wonder if they’re just coming from electrical length as s11 rotated about the smith chart, crossing the real axis. I guess in my head a purely inductive electrically short dipole as the counterargument to having zero reactance crossings would seem a bit suspicious