Folded Dipole Array Question

[SpugEddy]

I'm looking at getting a nice folded dipole array antenna.
I am looking at 2 different antennas. I noticed that the 2
antennas are somewhat different in how they are mounted.

Pictures are the 2 antennas.
My main question is this: How does the signal leave the antenna?
From the front of the loop or from the side of the loop?
Looking at the 2, is there a benefit to either one?
On Antenna 2, will the mast interfere with the signal?

I am interested in 2 repeaters which are, just about, 180 degrees
from each other. Should I leave the antennas in the shown config.
or should I set them up as full omni?

 

[SpugEddy]

To clarify on my first question:
In picture 1, does the signal radiate toward me and away from me
or does the signal radiate to the right?

 

[prcguy]

The question is complicated due to the way different mfrs make their antennas. I believe your picture 1 is a Telewave and I'll comment on picture 2 first.

Most stacked dipole arrays used dipoles with fixed distances from the mast, somewhere between 1/8 wavelength and 1/4 wavelength. Doing this makes each individual dipole a two element Yagi and the signal leaves each dipole in a line from the mast through the dipole and away from the mast. In this case the dipoles can be arranged like your picture 2 giving a pattern in one direction and somewhere between 180 and 120 degrees of beamwidth at 3dB down depending on dipole spacing from the mast. Closer spacing gives a little wider beam width and closer to 1/4 wave spacing gives the most gain. The dipole to mast spacing will also change the match of each individual dipole. You can also arrange each dipole to cover a 45 degree area and with each dipole pointing to a single compass heading of N, S, E and W it will give nearly omni-directional coverage.

Picture 1 appears to be a Telewave brand dipole array and they allow you to change the dipole spacing from the mast to further diddle the pattern. For instance, you can move the dipoles 1/2 wavelength away from the mast with all dipoles on one side to give a bi-directional figure 8 pattern, or you can go 1/4 wave spacing from the mast for max gain for each dipole, or 3/8 wave spacing for something else, etc.

There are also dipole arrays where each dipole has second matching dipole at the same height location but on the other side of the mast. In this case placing all dipoles in line with each other will give a bi-directional figure 8 pattern and placing every other set at a right angle will give nearly omni coverage. Going from a single direction to omni usually looses 3dB of gain as in a 4-bay array pointing all in the same direction will be rated around 9dBd gain and when set for omni you get about 6dBd gain.

To clarify on my first question:
In picture 1, does the signal radiate toward me and away from me
or does the signal radiate to the right?

 

[nd5y]

Comprod, Telewave and Sinclair have radiation patterns on the data sheets for each product.

 

[SpugEddy]

Comprod, Telewave and Sinclair have radiation patterns on the data sheets for each product.

True, but I'm not very smart when it comes to reading
those patterns. I sort of get it, but I don't know, in this
case, how the signal comes off the antenna. Sideways,
straight out, etc.

 

[Ubbe]

The distance between the antenna and the mast pole will have an influence on the directivity.

When you have two antennas receiving the same signal it will at best give a 3dB boost and having four antennas gives 6dB.
If you have two antennas pointing out at different angles from the mast pole, lets say one forward and the other facing the opposite direction with the mast pole between them, it will give most gain from the side where the two antennas receive the exact same signal in phase.

If you move from that side location to a different angle so that one antenna receives your signal later than the other, then the signals will be out of phase and depending of the angle, the frequency, and the distance between antennas, they could receive the signal in opposite phases and completely cancelling each other out. It would give unpredictable coverage patterns if several stacked antennas are not lined up exactly in line with each other to receive its signals with the exact same phase.

/Ubbe

 

[ko6jw_2]

About a year ago we installed a two bay folded dipole array from Telewave at one of our repeater sites. When ordered we specified a slight down angle for the array which is set by Telewave as part of the wiring harness. As indicated above, the directional pattern was determined by the spacing from the pole. We opted for a cardioid pattern to gives the best coverage for the service area. Everything was assembled on the ground and then hoisted up on the tower. The array is on a 20ft pole on top of a 40 ft tower at about 3100ft. Working very well. By the way, Telewave offers some discounts to ham groups.

 

[spanky15805]

SpugEddy... When you spend that $1000 for a four bay folded dipole, please make sure your drain holes are in the proper location.

 

[SpugEddy]

I know nothing about drain holes. I'm intrigued.

 

[mmckenna]

I know nothing about drain holes. I'm intrigued.

One end of the loop will have a hole in the bottom to allow any condensed moisture to drain out. That has to be pointed down to work. Usually the "top" of the loop on the Telewave antennas has a white band around it. White band goes up, drain hole goes down.

 

[SpugEddy]

Perfect. Thank you for the heads up and for the explanation.

 

[spanky15805]

Tried finding a picture of a drain hole on a folded dipole, really google... Anyway, the bottom of the "U" will be a small hole for condensation to escape. I have seen them installed wrong, hole at the top for a that rain to enter. And the Lord knows, you get rain in your "neck of the woods". The folded dipole is a very solid design as long as it installed correctly. It should easily handle 90 mph winds with 1/4 inch of ice.

From your general location, with a 4 bay, you will probably hear more 2 meter traffic then you care. For giggles, lets say that you have a sixty foot tower and are mounting the the support pole from 40' to 60', so the top 1/3 of the tower is above the tree line, you should have a 75 mile radius. Almost all of your commercial FM broadcasters use some type of folded dipole configuration, 2 bay, 3 bay, 4 bay, 5 bay up to 12 bay for a reason.

 

[prcguy]

I believe probably 90% of FM broadcast stations use a circular polarized antenna and not a simple stacked dipole array. They stack and phase them in a similar way but the elements don't resemble a dipole.

Tried finding a picture of a drain hole on a folded dipole, really google... Anyway, the bottom of the "U" will be a small hole for condensation to escape. I have seen them installed wrong, hole at the top for a that rain to enter. And the Lord knows, you get rain in your "neck of the woods". The folded dipole is a very solid design as long as it installed correctly. It should easily handle 90 mph winds with 1/4 inch of ice.

From your general location, with a 4 bay, you will probably hear more 2 meter traffic then you care. For giggles, lets say that you have a sixty foot tower and are mounting the the support pole from 40' to 60', so the top 1/3 of the tower is above the tree line, you should have a 75 mile radius. Almost all of your commercial FM broadcasters use some type of folded dipole configuration, 2 bay, 3 bay, 4 bay, 5 bay up to 12 bay for a reason.

 

[wb6uqa]

I'm looking at getting a nice folded dipole array antenna.
I am looking at 2 different antennas. I noticed that the 2
antennas are somewhat different in how they are mounted.

Pictures are the 2 antennas.
My main question is this: How does the signal leave the antenna?
From the front of the loop or from the side of the loop?
Looking at the 2, is there a benefit to either one?
On Antenna 2, will the mast interfere with the signal?

I am interested in 2 repeaters which are, just about, 180 degrees
from each other. Should I leave the antennas in the shown config.
or should I set them up as full omni?

It looks almost like a jpole. Radiates away from the mast. They can be phased and rotated for direction on the mast.

 

[W5lz]

From what I can see, both antennas use folded dipoles in a phased array. They both radiate to the sides of the anenna not much up/down to it, but some. How do you tell? You either measure the output, or probably and more likely take the manufacturer's word for it. You could always run the thing through an antenna analyzing program. Which is better? Beats me...