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What would be the difference between a two-bay antenna designed like this: enter image description here

and a four-bay antenna designed like this:

enter image description here

Assuming that everything about these two antennas are identical aside from the number of bays, what exactly does "adding more bays" do for a bowtie antenna?

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It increases the directionality and gain of the antenna. This helps it reject noise coming from directions other than towards the transmitter. The higher signal to noise ratio leads to better reception.

For this antenna as drawn, the directions of maximum gain are two: one coming out of the page, and one in the opposite direction. Sometimes these antennas will have a reflector behind them to reduce that to just one direction.

For a wave arriving from the maximum gain direction, the wavefront hits all the bowties at the same time. The individual signals received from each bowtie then add constructively.

It's not immediately obvious why there's the crossover in the antenna. That is, why not are all the blue halves on one side, and the reds on the other? The reason is the two outer bowties are farther away from the feedpoint than the inner bowties. If the incoming wave hits all the bowties at the same time, but then the bowties are at different lengths from the feedpoint, by the time the signals have traveled from the bowtie to the feedpoint they will no longer be in phase, and no longer add constructively.

The crossover compensates for this delay. The wave is oscillating with some period; if we wait half this period the polarity at that instant will be reversed. The crossover accomplishes the same thing, not by waiting but by crossing the wires. Meanwhile, the extra distance that must be travelled from the farther bowties introduces another half-period of delay. That half-period, plus the additional half-period of the crossover, means that we are effectively delaying the signal by a full period, which is the same as not delaying it at all.

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  • $\begingroup$ It has crossover for the same reason the original log-periodic antenna does. In the end it turned out not to matter much so most of what we call log-periodics no longer bother. $\endgroup$ – SDsolar Apr 26 '17 at 17:06
  • $\begingroup$ @SDsolar What? Show me an LPDA which doesn't cross over between each element. $\endgroup$ – Phil Frost - W8II Apr 27 '17 at 14:42
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I find I learn a lot by reading catalogs, and this was no exception.

Going from THIS AD FOR BOWTIE ANTENNAS they claim longer range with more bays.

That would make sense because there would be more surface area with which to capture signals.

Obviously, an antenna like your diagram would be bidirectional like any dipole, but the one listed in that ad has a screen reflector on one side to make it unidirectional.

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    $\begingroup$ Just making the antenna bigger doesn't necessarily give it a bigger aperture. $\endgroup$ – Phil Frost - W8II Apr 26 '17 at 5:00
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    $\begingroup$ In the case of a reflector-concentrated (dish) type antenna it does. I worked with HIPAS, the precursor to HAARP, where we had crossed dipoles standing above a steel mesh, and the total aperture was measured in fractions of an acre. This antenna is similar. $\endgroup$ – SDsolar Apr 26 '17 at 5:55
  • $\begingroup$ A dish antenna is a kind of aperture antenna: the dish is the aperture, less an efficiency factor to account for diffraction, shadowing, resistive losses, etc. The dish is just a mirror. This antenna isn't at all like that: it's a dipole array, and making it bigger will increase the gain only if additional dipoles are added in the right position such that they add constructively. $\endgroup$ – Phil Frost - W8II Apr 26 '17 at 13:36
  • $\begingroup$ OK, I changed my wording. I didn't mean to spark a debate about whether a screen reflector has an aperture. My whole point was that the way the sell them is to enhance signal reception in a single direction, and not in two directions like his drawing would do without the reflector. I like your wording that they add constructively. But in all my years I have never seen someone erect a free-standing Bowtie like for HF. In the ad, they are more like driven elements of a 2-element beam, with the reflector being a mesh. That's all I was trying to say, even though my words were clumsy. $\endgroup$ – SDsolar Apr 26 '17 at 17:01
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    $\begingroup$ The aperture is just another way to express gain: $(G \lambda^2)/(4\pi) = A$. While it's generally true that a higher gain requires a larger antenna, you can't just arbitrarily make an antenna bigger and say it has a larger aperture. Nor does the cross-sectional area seen from the transmitter need to relate to the aperture. For example, an endfire array does not have "more surface area with which to capture signals", and yet its gain, and thus its aperture, is much larger than an individual element. $\endgroup$ – Phil Frost - W8II Apr 26 '17 at 20:17

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