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Vertical Antenna Stacking: In-Depth
Date Modified: 2009-04-21

Question:

Question 1

I am trying to stack vertically two YA-1713 antennas for VHF Channel 9 WNBW. I have them 61 inches apart. The feed from each is 5 feet long to a Winegard CC-7870 combiner. The bottom antenna is 10 ft above a metal roof. Do I have them at the right distance apart? I am not seeing much improvement over a single YA-1713.

Question 2

I have a pair of YA-1713s and a CC-7870 to combine them.

First is that the best thing to combine them? I thought about direct phasing lines, but there is also a Ch7 and a Ch11 I watch sometimes but could sacrifice if using a phasing harness would be better than the CC-7870.

First I had one up 20ft. Then I added the second one above (new extened pole) 61 inches up. I did seem to get few drops, but no better reading on my TV sets meter (maybe because it's quality (SNR) and not signal level?).

Winegard Technical Service recommended I reduce it to 42 inches. Unfortunately that was a bad day to test, and ended up leaving at 50 inches for the last couple weeks. It seems worse, but there are so many variables I can't really tell.

I had read that closer together will cause bigger grating lobes but increases the main forward gain. Right now letting it run 2 weeks at 50 inches, I am pretty sure that it's not as good through varying conditions as it was at 61 inches.

Also from reading pushing them out to a wavelength reduces the grating lobes, makes the main lobe broader and reduces gain.

So two questions in a nutshell...

1) is the CC-7870 the best way to combine them.

2) Is it still worth experimenting between that and say 70 inches. Do you have any in house data on what happens at different stacking distances?



Tech Tip:

Answer 1

The vertical spacing for your antennas should be at 42” which is 2/3 of a wave length for channel 9. A full wave length would be 61’’ but we stack at 2/3 wave length to obtain 3dB gain. It is possible that the bottom antenna may be seeing a reflection off the metal roof. Try raising the stack 3-4 feet higher.

Answer 2

The best way to combine two antennas in a stacked configuration is through using the Winegard CC-7870.  Phasing lines can be employed within stacked antenna arrays but only under the best of conditions do they produce a measurable improvement in DTV signal reception.  Each antenna within the array must be in "full illumination" of the RF field or generally greater than 20 wavelengths above the ground.  This is possible in a CATV system using a tall antenna tower. 

In practice however, one antenna is in a different field strength as compared to the other within the array.  Thus, the desired signal voltages are different and combine out-of-phase.  Under some out-of-phase conditions, one antenna may become an electrical load to the other greatly reducing signal levels.  Using a transformer-type antenna coupler as the CC-7870 provides isolation between antennas limiting the combining loss and possible antenna loading to no greater than -3.5 dB to the output signal level.   
There are good references to antenna stacking - horizontal and vertical.  Working from any antenna polar pattern, one can predict the resulting antenna array pattern for various stacking distances.  The calculations can be done on a simple hand calculator.  The information can be found in:
TV Antennas and Signal Distribution Systems
by M. J. Salvati
Howard W. Sams
Copyright (1979)
The method is very good for placing antenna array null or nulls where you desire maximum signal rejection and improving received DTV signal levels.  Another reference was distributed by Winegard at technical seminars in late 1970s:
Stacking TV Antennas
by James E. Kluge
(Reprint from June 1975 issue of)
Electronic Technician/Dealer
This is a good reference for stacking information and gives further details about the CC-7870.



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