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I'm considering building some stacked 70cm quad antennas (home brew / DIY build) -- and... I'm having a devil of a time trying to figure out how/where to feed these antennas. For a single home-brew antenna, there are instructions out there for how to feed/tune them -- but for stacked yagi/quad home-brew antennas... I'm not coming up with much as far as 'here's how you do it'.

For all my googling, I'm coming up short with how to feed a stacked antenna system from a single feed line (either coax or 'ladder line') -- am I missing something/overcomplicating things?

Know any good methods/instructions out there?

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  • $\begingroup$ Serious question though - why go through all the pain of stacking them when you can just make them longer? Doubling the boom length of a quad or yagi is as good as stacking two, and much less complicated, less to go wrong. Stacking is done for moonbounce etc when they're already 5 or 10 metres long and it's mechanically more realistic to add more in parallel. But if you have to ask, you're probably not in that league yet. Also, 2 element quads beat 2 element yagis, in gain and smaller turning radius, but longer than that Yagis are better. You'll find tons of long boom 70 cm designs on the net. $\endgroup$
    – tomnexus
    Jun 28 at 21:42
  • $\begingroup$ You're right, I'm definitely not in the EME league yet -- still learning. I don't have an electrical engineering background, and haven't done antenna modeling before either, but have built 1 70cm 'hiking stick' quad that never got finished before I left for college, because I couldn't figure out how to feed the thing. I'll see if I can brush up on my 'google-fu' to pick up on designs that other folks have made in the UHF/VHF segment. $\endgroup$
    – Alan T
    Jun 30 at 16:56
  • $\begingroup$ That's great! Yagis are tricky but there are good designs out there, if you follow the dimensions and other details exactly then they should work. For 70cm I've made a few with just a wooden dowel / broomstick, some coathanger wire and hot glue. This works fine up to about 2 m long which gets you over 12 dBi. Antennas in practice has a table of simple designs that work. $\endgroup$
    – tomnexus
    Jun 30 at 18:27

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Assumption: your antennas are placed such that they don't directly couple, i.e., in a way that they are practically in a null of the other antennas' patterns. Otherwise, you're not designing a stacked antenna system, but one giant 3D antenna, and the only way to do that is honestly simulation and loads of experience. Rule-of-thumbs quickly break down for complex 3D antennas, and a lot of the "results" you find online are very questionable. This assumption is, however, well justified, as Yagis can well be placed parallely with > 1/2 wavelength distance.

For a yagi, you have no choice, essentially: you feed the driver element of each individual antenna. You'll hence need a power divider that takes your one feedline (70cm: probably coax) and divides it for the multiple antennas, such that the impedance still matches. Same for quad, the feedpoint is inherent to the antenna design.

I've seen such antennas being stacked for reasons of increasing gain / narrowing beamwidth (that's essentially the same thing). The question that arises is

In which direction to you want to decrease the beamwidth?

You'd need to stack in that same direction. Want your beam to be narrower vertically? Stack vertically. Want it to be narrower horizontally? Stack horizontally.

From that, what remains to be answered is

  1. How do you divide the power between your $N$ antennas, and
  2. How do you want them to be phased?

Let's address 2. first: Imagine you put two Yagis on top of each other, such that they don't directly couple (not too hard, as a yagi has directivity "up front", not to the top). Now, if you supply them both with exactly the same signal (i.e., "in phase"), the waves emanating from them add up constructively exactly in points that are exactly equally distant from both antennas' phase origin, because they have the same phase there. In other words: in the ray in the middle between these two antennas main axes.

Now, add, say, 1/8 of a wavelength in additional cable leading to the upper one. Now, the points where the two wavefronts emanating from both antennas add up constructively is still where the waves are in phase. Only that the wave from the bottom antenna has 1/8 wavelength of an advantage – therefore, the points where the waves add up constructively end up on curve where each point is 1/8 wavelength further from the lower antenna's phase center than from the top one's.

Congratulations! You just implemented beamforming through means of phased array.

So, your antennas' relative phase (this extends to arbitrarily many antennas) defines the antenna pattern you get. You define that phase through cable lengths, or other means of delaying the signal.

So, in conclusion:

trying to figure out how/where to feed these antennas

  1. how: using a matched cabling, and a power divider, with the desired length of cabling. If you "just" want to increase gain in the direction they already have, stack them equidistantly, and feed all with exactly the same length
  2. where: like you'd feed any individual antenna.

The question of "oh, I get a lot of freedom when I feed antennas, how to use that optimally" is answered by designing the antenna array to have the so-called array-factor you want. The array factor is like an antenna pattern, only that you multiply it with the antenna pattern of your individual antenna, which in this context is called element factor.

I really can't (not good enough, and also, too much for an answer) introduce you to antenna and multi-antenna design, but it happens so that if you put your antennas in a regular distance on a straight line in space, that antenna factor can be calculated through the DFT of a vector of the individual antenna's excitation factors, i.e., a complex value having the magnitude representing the amount of amplitude of the signal they're getting and a phase representing the signal's phase.

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