Biquad antenna measurment confusion

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AronDouglas

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After reading on how effective biquad antennas where for wifi, I'm experimenting with an 855.00mhz antenna designed for a truncked system 40+ miles north of my house (I'll build a yagi some day and see which one preforms best). I used a biquad calculator for some exact measurements and I am coming up with conflicting information. Each element should be 1/4 wave length long, correct? The elements should be 3 5/16" (84.1375mm) each. But the calculator I used states that the elements should be 3 21/23" (85.1mm). The calculator also states that the over-all measurement of the antenna should be 9 3/8" (238.2mm) and my finished antenna is around 10 3/8".

I used the following calculator, Online Calculator .:. Bi-Quad Antenna

I think I'll spend my night tonight re constructing a new antenna with true 1/4 wave length elements. I just wanted someone elses opinion on the subject because I'm only a 3 day old biquad enthusiast :)
 

AronDouglas

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That's what I was thinking, but the overall dimensions is what got me concerned since I was a whole inch too big. But thanks for the reassurance, it saves me the time of making a new one.
 

ko6jw_2

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In any antenna design there is the theory and then there is the real world. The bottom line is that any calculated dimension of an antenna will be slightly different in a real antenna. For example, a half wave dipole wire antenna will end up shorter than the theoretical calculation due to end effect. In the present case, you might assume that the person who wrote the design software had some practical experience in antenna design and, therefore, took these considerations into account. Or, you can assume that the calculator produces only theoretical values. The significance of this is not worth worrying about where the difference between the theory and practice is only a few mm. The only way to know for sure is if you have the necessary test equipment to measure the performance of the antenna. Also, you will use the antenna for receiving and not transmitting. Your receiver will not care if the impedance of the antenna is not exactly 50 ohms.

A friend once held a competition for new hams to design a 1.2Ghz antenna. The winning design was a rhombic. It beat out yagis etc.

My feeling is that if you need to receive distant 855Mhz signals something with higher gain than a biquad would be a better way to go. A multi-element yagi would be a better choice. Unfortunately, the price of copper has made it economically unfeasible to build many types of antennas. This makes commercial antennas more attractive.
 

WA4HHG

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Factors Limiting Range

40 Miles + on 855 mHz is indeed a haul. Some sort of gain antenna plays an important role but the antenna path itself is problematic. There may be too much distance involved for the height(s) that are available:

The distance, D1, to the radio horizon for the transmitter is 1.415 times the square root of h1 (feet). The theoretical maximum line-of-sight distance between two elevated points, presumably the transmitter (h1) and the receiver (h2), is the sum of the two distances to the radio horizon (D1 + D2).

(Credit: DXFM.com)

In the sceneario above, if there were 100' antennas at both ends of the path, the maximum theoretical transmission range (sum of D1 and D2) would be something on the order of ~ 29 miles.

Before expending much more effort, consider how distance and available antenna height(s) factor into the viability of your project/installation.

-73-
 

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jackj

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Chuck,
Those figures assume that the earth is a perfect sphere, perfect weather conditions and no obstructions between you and the transmitter. In reality you will be lucky to get much more than half of that distance. Edge effect doesn't affect 800 MHz much at all so the signal won't bend over the horizon like VHF or 450 MHz UHF does. Also, 800 MHz doesn't penetrate well at all so a stand of trees on your horizon will kill your signal.
 

WA4HHG

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Distance to Horizon calcs

Your comments below are well noted. The post simply demonstrates the relationship between antenna height and the pure distance to the radio horizon. Trees, buildings, hills, etc means YMMV hence the use of the (~) symbol on the mileage outcome. The visual horizon uses slightly a different constant in the formula (if memory serves, the USCG teaches that to be something on the order of 1.213). The formula posted earlier also factors radio waves extending slightly beyond the visual horizon though how far is frequency dependent. Yes, the earth is actually pear shaped but those calculations would include location based operators, factoring path azimuth and calculated using a much more involved formula; the results would minimially affect the outcome, especially on a purely latitudinal transmission path.

Heck, I'm working on trying to hear 800 mHz transmission from VA Beach and Norfolk from my home in Chesapeake, VA. Been following this topic and want to build one of the antennas, put it on my tower at about the 90' level and feed it with CATV hardline. I can see the lights from the Norfolk skyline at night but darned if I can hear 'em using a discone at 35' unless I drive about 6 miles toward the city.

-73-

Chuck,
Those figures assume that the earth is a perfect sphere, perfect weather conditions and no obstructions between you and the transmitter. In reality you will be lucky to get much more than half of that distance. Edge effect doesn't affect 800 MHz much at all so the signal won't bend over the horizon like VHF or 450 MHz UHF does. Also, 800 MHz doesn't penetrate well at all so a stand of trees on your horizon will kill your signal.
 

AronDouglas

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The reason I started working on a bi-quad is because I was researching antenna designs. I came across that specific design and saw how it was used for higher frequency ranges and it offered a bit more gain than a dipole. Plus I like researching and building antennas.

My line of thinking with a bi-quad was that since my Austin Spectra could (sometimes in special situations) pick up random trunked transmissions. So I thought any directional gain antenna mounted higher than my truck (the Spectra is on my truck) would allow me to pick something up. But hey, if it doesn't work I'll build a yagi :) I have a 40' tower (with a 16' extension pole added on later) so I'm hoping that will be high enough. I have the antenna element built, I'm just dragging my feet on getting the reflector made since I recently ran out of plate copper and brass.
 
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