Probably good advice Chip. I tend to only mention the degree when it seems relevant; I do not list it on my QRZ home page and generally do not advertise it. I think you did a nice job restating my impedance explanation and I hope it makes sense to others. Two (equivalent) ways to look at it are adding high impedances in parallel with a low impedance yields a combined impedance roughly equivalent to the low impedance. Perspective 2: A very, very small current (very near zero) will flow in the high impedance branches; no current, no radiation. We can all learn. As a matter of fact, this thread forced me to review the reflectivity of earth to RF. I thought the earth was more of an absorber than a reflector. Now that I see to the contrary, it makes sense why higher antennas have a lower take of angle when you think about reflection angles and reinforcing interference patterns. I am glad I learned something.
Please model it. Not at all what I said. I am very aware of the law of conservation of energy. The "parasitic elements" re-radiate the same energy they absorbed to create an interface pattern in space that creates the gain. I never said it was limited to 3dB; not sure where you got that. The director elements take back and sideline power and focus the beam. A very large portion of the "volume" of the beam is redirected. Gains can be much higher that 3dB. You are thinking very 2 dimensionally. Look at a parabolic dish for example. The parasitic elements direct the energy forward, not upward. Parasitic elements generally (overs simplification) change the pattern in the plane where they live. Dipoles are great, I have one.... but they are not that effective if they were why does the military, NASA, tv service, etc invest in other antenna types
I was under the impression that an endfire type beam antenna, IN FREE SPACE, focuses the energy from ALL directions toward the main lobe. Am I wrong? A parabolic dish is a completely different animal.
Generally when the term end-fire array is used they are referred to arrays with multiple driven elements. In general, this terminology is not used with beams (yagis). With that said, the parasitic elements refocus energy from all directions in the plane of the antenna. It had a very minimal effect in the orthoganal plane. To maybe be more clear, it will take the energy from the back and sidelobes and redirect it into the forward lobe. This larger forward lobe will still be 3 dimensional with some of that energy going downward (not directly downward mind you). Just like with a dipole, that downward energy is what is rereflected and adds to the forward gain. Parabolic dishes are really not different animals. The still focus energy forward and still have small side lobes and back lobes. I think they have a better capability in focusing the power in both the vertical and horizontal dimension, but there is still a 3D front lobe and there will still be some re-radiation (reflection) from the earth
There are broadside arrays, collinear arrays, and end fire arrays. Yagis and other antennas with parisitically excited elements still radiate like an end fire array. You keep making my point for me, there IS downward energy from an end fire array, JUST NOT AS MUCH AS FROM A DIPOLE, therefore the amount of gain increase from the earth reflection is not as great for a horizontally polarized end fire beam as it is for a dipole. Parabolic dishes are the same as other antennas in that they radiate RF, but in the context of this discussion of practical ham antennas for HF, they are never used at HF frequencies and they are never used at less than 1 wavelength above earth. They're a different animal in the context of this discussion.
The downward energy from a dipole is going to be very similar to a dipole. You do know dipoles are in general set up with horizontal polarization? Dishes are impractical at hf frequencies obviously but the directivity analogy still holds. Do you really believe dipoles are as good as beams?
This should have said: The downward energy from a beam is going to be very similar to a dipole. You do know dipoles are in general set up with horizontal polarization? I should not try and type responses on my phone...
You might not be aware of cocoaNEC which should work for you: http://www.w7ay.net/site/Applications/cocoaNEC/index.html
I just used cocoaNEC to model a 20m hex beam 5/8 lambda above the earth. It has a gain of 11.06dBi. I plan to add all the bands to the same model to show they do not interact. This is a fun exercise. I will be adding antenna modeling to my resume
For those interested, the file below is a 5 band hexbeam excited at 20m (14.2MHz) only 30 ft above the ground. It was modeled with cocoaNEC. FYI, if you try and model a hex beam, you have to use a lot of segments to simulate the patter correctly The model gives you a 9.91dBi gain even when below half a wavelength high. A dipole at the same height gives 7.2dBi gain.
