Electronics > RF, Microwave, Ham Radio

Yagi-Uda antenna driven element impedance question

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Yansi:
Hello,

just needed to order a stash of different aluminium parts from a warehouse and thought adding some stuff for a diy yagi antenna construction, to make the shipping price less % of the whole order :)

I thought it would be kind of handy for me to make a 70cm band antenna (432MHz). So I have looked on different yagi calculators google could find withing a couple of searches. What I am quite surprised is that most of the calculators just throw lengths of the elements and their positions on the boom, but I think none of them did show any info about resulting impedance of the driven element.  ???

Even the popular VK5DJ Yagi Calculator does not show any impedance info, rather just spits dimensions for a folded dipole with a 4:1 balun.

I find that strange for one part the impedance of the folded dipole (guessing) can't be 300 ohm anymore and second even if the dipole would somehow had 300 ohms, 4:1 balun would not cut it, not at least from a 50 ohm coax feedline (resulting in just 200 ohm at the dipole terminals). Or is this what is needed?

I am quite sure that the proximity of the parasitic elements inherently means the impedance of the driven element must be different from a "standard" free air impedance.

As a side note, due to the folded dipole being difficult to make precisely, wouldn't it be possible to use just a half-wave dipole? That one should have normally 73 ohm free air impedance, not sure how could one make a coaxial (or high power capable) balun from unbalanced 50 to balanced ~73 ohms.

In terms of antennas, I am quite a n00b, any hints really appreciated.

Thank you,
Y.

tautech:
After much deliberation and not needing a directional antenna I settled on a J-Pole (after member recommendations) for 315 MHz and 432 MHz will be just as easy to make and tune. Yagi just seemed a lot more work given I'm an RF noob too.
Looking back now just a SA and reflection bridge need be used to get a quite reasonable tune.

My battles and success are written up here:
https://www.eevblog.com/forum/rf-microwave/antenna-project-log/

Other antenna investigations:
https://www.eevblog.com/forum/rf-microwave/what-really-is-this-antenna/

Howardlong:
This, from an expert in the field.

https://www.wa5vjb.com/yagi-pdf/cheapyagi.pdf

Key here is that while they might not squeeze the last fraction of a dB against a carefully tuned and machined antenna, they are very reproducible.

aurelZ:
In fact it is easy
If your driven element is closed into loop then impedance is 300
if is open dipol type then impedance is 75 ohm and then you don't need asimetric baloon tranformer 4:1.

For 400mhz you can build double biquad antenna and u must use baloon.

Yansi:
No, it is not easy.

The dipole more than likely when near parasitic elements in a yagi configuration will have different impedance, than free air one.

Half-wave ("Open") dipole does have free space impedance of 73ohm (close to 75, sure), but still you can't connect that to a unbalanced coaxial 75 ohm cable, not to mention, that 75ohm is not the choice for transmitting, 50 is.

For open dipole with a 50ohm feed line, you would still need a 1.5 : 1 impedance balun (baloon is a different thing). 75 ohm unbalanced coaxial  feed line would just get away with a 1:1 balun.

I know, there are many hacks and things that kind of work and kind of don't. I am not interested in those.  I am not targeting "the simplest or weirdest antenna build", I just want to make a good quality yagi antenna. Not a slapped-together one, I have lot of those already.

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