Dual 3 Element Yagi Beams

[SpugEddy]

Does anybody have any experience with phasing 2 (or more)
3 element Yagi antennas? I'm looking for a quarantine project
and think this is a good choice with some good challenges.

The main questions are:
Configuration. Both facing the same direction as a directional
or 4 antenna array pointing in all 4 directions (N-E-S-W) for
omnidirectional.

Phasing. Looking for the correct phasing. ie. Space between antennas
length of coax for the phasing harnesses

Use. Going to be used as a GMRS repeater antenna. ULTIMATELY
what should be my resonant frequency? I usually go to 464.5 since
that is the midway point

I've found information online, but none of it seems to directly answer
my very specific questions.

Any help is very much appreciated

 

[chief21]

As to the direction(s) of the antennas, which configuration best supports your coverage requirements? Be sure to consider the beamwidth of the antennas or you might find some significant holes in the coverage.

As to the "center frequency", I would think that the overall bandwidth of the antennas would figure into the equation. Will they support your desired frequency spread while still retaining the desired gain? In most cases, repeaters are usually dedicated to operation on a single frequency.

As to the phasing, it is my understanding that coax phasing harnesses need to be very precise and can be difficult to configure by hand. Proper weatherproofing can also be difficult to achieve.

That's about all I have to offer. Good luck with your project.

 

[Ubbe]

Phasing isn't that critical if it's an inch plus or minus. But cut two coaxes to the same lenght so that the antennas signal are in phase where the coaxes meet. If you manage to get a 7 inch difference in lenght of the coaxes they will totally cancel out each others signal at 800MHz.

If you point antennas in different directions, and you have those 3 element antennas that are almost omni, they will receive the same signal source with different phases, if you don't place all antennas above each other so that their dipole elements are directly in a vertical line.

If only one antenna receives a signal then the other antennas works as dummy loads and the signal will split up and goes both to the other antennas and to the coax going to your receiver, attenuated by the amount of antennas you have. Best solution are always to have one antenna as big as possible. If you want omni coverage then use a vertical antenna with gain.

A stacking coax harness are impedance matched so that several 50 ohm antennas can be connected to a single 50 ohm coax.

If you are only experimenting for fun then google up stacking harness.

/Ubbe

 

[prcguy]

You can calculate the length of 75 ohm coax with velocity factor needed to split a single 50 ohm feed to two 50 ohm antennas. You really need a antenna analyzer to get it right and if you target the center frequency of a repeater pair it works fine because the BW of the coax phasing harness will be wide enough. Or you can buy a 50 ohm power divider and use exact length 50 ohm cables to feed the Yagi's. This is much easier and forgiving.

Every time you double the number of Yagi's pointed in the same direction with the proper spacing you will get up to 3dB more gain and a narrower beam width. Four stacked Yagi's can give up to 6dB gain over one Yagi if everything is done right. However, when you point the antennas in different directions you will loose 3dB of gain between two antennas because you are dividing the power between them. So if the Yagi is 6dBd gain, you would have 3dBd overall gain for two 6dB Yagi's pointing different directions.

Four Yagi's pointed at four equal compass headings is hard to determine the gain because there will be a little spill over where two will combine some in phase and provide gain but there will be nulls between them. Dividing to four Yagi's will supply -6dB power to each antenna so if each Yagi is 6dBd gain you might get a little gain in some directions and less in others but less than a single Yagi without dividing.

Here is a cheap 2-way UHF power that will handle 100w. UHF 300-500MHz 2 Way 50Ω/N Radio Repeater Antenna Power Splitter N Female 100W | eBay

Here is a 4-way and you can sometimes find surplus US made dividers from Antenna Specialists and others for cheap. 4 Way Power Splitter Cavity RF Power Splitter For DAS UHF 380-470MHZ | eBay

 

[SpugEddy]

Thank you for the well wishes Chief21.
Thank you for the input Ubbe. The vertical is not completely out of the question.
However, I am looking for a sanity project.
PRCGUY, I did read about the halving of dB's but I wasn't sure I interpreted it
correctly. So, thank you for the clarification.
With that, my choice will be to put the 2 antennas on the same side pointing
in the same direction. As for using the power splitter, am I understanding you
correctly that I don't have to cut the coax to specific lengths just so long as they
are both the same exact length (give or take a cm or so)?
And this leaves the other question as to how far the antennas have to be spaced
vertically from each other. The antennas will most likely have a 20" boom. This is
another question that I'm not finding a direct answer for Yagi antennas on UHF.

 

[Ubbe]

Distance between antennas are the same as for other single antennas distance to other metal objects. Wen you stack two antennas their useful gain will have a narrower beam width when using bigger distances between antennas. You would like to have the antennas as close as possible without them disturbing each other, or it will become very directional.

If you use power dividers with two antennas you loose 3dB right at the bat. You can compensate for that by doubling the output power but you cannot compensate for the loss at receive. That's why they are called power divider. The whole setup will have the same gain as one single antenna. The same happens if you use 4 antennas and a power divider, it equals one antennas gain. That's why you see stacking harnesses being used, even in professional antennas.

/Ubbe

 

[SpugEddy]

If you use power dividers with two antennas you loose 3dB right at the bat. You can compensate for that by doubling the output power but you cannot compensate for the loss at receive. That's why they are called power divider. The whole setup will have the same gain as one single antenna. The same happens if you use 4 antennas and a power divider, it equals one antennas gain. That's why you see stacking harnesses being used, even in professional antennas.

/Ubbe

Very interesting. Glad you pointed that out because it
never entered my mind. While it certainly would have
made things easier, it would have been moot to add
the 2nd antenna to begin with.