[Hitechluddite]

Looking at many of the 8bay designs they are running a short patch cable from each side to a combiner. As I understand it this can raise some performance issues as far as gain curve over usable frequency and is a point where multipath can be introduced. This got me thinking that I could just as well get two 4 bays and stack them vertically using identical patch cables to the combiner and probably end up with a better result. Am I right?

 

[majortom]

It does not sound like you have a sound understanding of stacking, superposition, etc.
Keep reading...

 

[Hitechluddite]

I have read

Stacking multiple antennas

Superposition has an effect on lobes but again a fixed physical distance of spacing it is a compromise and not optimal for every installation and it seems it would be of more consequence in horizontal stacking than vertical.

and I read

Antennas Direct DB8 Hardware Hacks

If I "bent" a DB8 so that each of it's 4 bays was pointing a slightly different direction (say 30 degree differential) would I be; A. A fool that destroyed a perfectly good $100 8-bay antenna? B. An innovator that created a sweet multi-directional antenna with better gain then a 4 bay? C. To...

www.digitalhome.ca

Which did pretty much answer my question to some extent but doesn’t designs using individual harnesses from each 4 bay require a custom combiner to keep the impedance at 75 ohms? Simply using a specific length of cable between each 4 bay and the combiner to keep the impedance at 75 ohms is another compromise because as the frequency changes the cable length would also need to change. So if I was going to vertically stack two four bays from a specific manufacture I would be best off to use their combiner from their horizontally stacked 8bay right?

 

[ExDilbert]

Stacking vertically and horizontally each have different advantages. A 2x2 antenna should offer some advantages of both. For antenna manufacturers, cost and ease of manufacture also come into play. For a small bow tie antenna, a 2x2 array is easier to make than a 1x4 or 4x1. Ease of installation and durability also come into play. A 2x2 array again comes out on top for a small bow tie antennas.

I suggest some reading be done on combining stacked antennas. Using a slitter/combiner is generally regarded as better than using stacking bars, especially for DIY stacking using multiple manufactured antennas. Stacking bars are cheap and easy for antenna manufacturers when making stacked antennas.

 

[Hitechluddite]

When reading about Superposition it got me thinking about the physical distance between two antennas vertically stacked because the physical distance will give different angles of lobes and nulls. In HF it is generally considered that a vertical 10degrees over the horizon is most effective. I know better than to guess that it's the same for UHF but whatever it is the optimal physical spacing would be determined by the most important frequency and optimal angle.

 

[ExDilbert]

Vertical stacking will create nulls on the vertical axis. That may be advantageous to minimize tropo interference on a difficult to receive station. Another advantage is to reduce fading and loss of signal with distant stations. Tropo can play a part in that as tropo may generate out of phase signals that negate the station's own signal or generate multipath interference. Depending on the time of day or tropo and weather conditions, signal strength and quality can be different at different heights. If the signal fades out at one height it may still be usable at another. Cable companies once used multiple antennas at different heights with automatic switching to the best signal in order to provide better service on distant stations.

 

[Hitechluddite]

As I develop my game plan I'll try to determine optimal vertical angle. The 60 degree horizontal should be better for me because where I am there's not much interference. With my current 8 bay I can change the rotor 10 degrees and go from a solid lock to almost no signal on two of my stations I'm hoping a wider horizontal beamwidth with similar gain figures will be more forgiving because in the hills the optimal path changes some with conditions.

 

[majortom]

for HF your using skywave propagation, thus the vertical beam wants to aim above the horizon, and the smaller the angle relative to the horizon, the longer the path you could achieve on the first 'skip'. While for short to medium skywave Paths you would use near vertical incident skywave (NVIS) type of patterns.

For TV, your not using skywave propagation, so the best vertical aim for DX reception would be to aim at the horizon, and not the sky. In most cases this can be achieved keeping your mast plumb. That is unless you lived on a mountain top. maybe you'd wanna downtilt a bit.

 

[Hitechluddite]

for HF your using skywave propagation, thus the vertical beam wants to aim above the horizon, and the smaller the angle relative to the horizon, the longer the path you could achieve on the first 'skip'. While for short to medium skywave Paths you would use near vertical incident skywave (NVIS) type of patterns.

For TV, your not using skywave propagation, so the best vertical aim for DX reception would be to aim at the horizon, and not the sky. In most cases this can be achieved keeping your mast plumb. That is unless you lived on a mountain top. maybe you'd wanna downtilt a bit.

Yep, that's the way I understand it. Specific designs tend to have different main lobes of radiation, i.e. quads tend to have a lower angle of radiation at the same height vs a yagi.

 

[Hitechluddite]

I was picking the brains of some hams on this subject and one came back with this site. It's not directly applicable but it does have some nice splitting & combining ideas to maintain proper impedance.

Stacking Yagis

 

[ExDilbert]

If there are hills nearby and line of site is not possible, then the optimum angle may be to point the antenna at the top or edge of the nearby hills in order to pick up the refracted signal.

To pick up signals with varying tropo and atmospheric conditions, a balance of vertical and horizontal beam width may be best as reflected and refracted signal could vary in direction vertically as well as horizontally. Too narrow a beam on one axis may eliminate some tropo signals. With a steady, usable weak signal, stacking to eliminate tropo may be desired.

 

[Hitechluddite]

Ex Dilbert, Would you say in the grand scheme of things The angle of the vertical lobes is not that critical? Attached is a picture from near the top of my tower the TV antenna is actually about 15 feet above where the pictures were taken.
First pic looking East 2nd Pic Looking South, West is about the same as east. The third picture is looking North from the ground it's the worst but not pulling TV from north.
One of the guys on the HAM forum posted this.
"
A non-modeling method commonly used for VHF and UHF stacking is to divide 57 by the 3 dB beamwidth of one antenna in the stacking plane, in degrees. The result is the recommended spacing in wavelengths from center to center of the antennas. This technique is commonly called D-opt (optimum distance stacking).

It is a simplification of the formula:

D = λ / (2sin(B/2)) [meters]

If your objective is something other than maximum plane gain then most other techniques are purely experimental. "

 

[ExDilbert]

I'm not up on the math. I would say this is a fine installation that will be difficult to improve upon. I wouldn't change the existing 4 bay except to apply minor tweaks that may be found in the forum. Considering tower height I wouldn't be too gung ho on that for the small gains to be achieved. If adding another 4 bay is the objective, I would stack vertically and combine the two 4 bays with equal lengths of coax and a 75 ohm splitter/combiner. The stacking distance probably isn't too critical. Tuning antenna spacing vertically for signal stability or horizontally to create null signal lobes may be an objective in some situations but it doesn't sound like those are big issues. I don't see any info on the antenna. It may be possible to achieve some gains with a better make or model of antenna.

 

[Hitechluddite]

Well the big Ham antenna below the TV antenna needs tuned and that tower is setup with a tilt base to raise and lower so I'm going to drop it all with the winch and rework everything. At Rabbits recommendation I'm adding the Stellar Labs Deep Fringe VHF High antenna and I have a UVSJ. I'm going to buy pre made matched patch cables. I'll also check the voltage getting to my CM-7777 after going thru my 150' of RG6. Once I pull it back up I'm hoping to never have to mess with it again!