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If you had a choice between stacking either two Delhi VIP 303SR's or two Winegard YA 1713's to obtain Ch. 7 or higher in your area, which would you choose?
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Doubling-up your antennas gets you 3 dB of extra gain, at best. If the setup is not done right, you probably won't get the full 3 dB improvement.
For high-VHF, the VIP-303SR and YA-1713 are very similar in performance (around 9-10 dB), however, the VIP antenna is much bulkier because it also has elements for picking up low-VHF. The YA-1713 will be much lighter and have less of a wind/snow/ice load because of this.
A single VIP-307SR (out of production) gets about 13 dB in high-VHF, so it would actually out-perform a double VIP-303SR setup (9dB + 3dB = 12 dB).
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Also, do you think stacking two 91 XG's would make for a more directional forward gain and reject signals off the sides and back better than a single 91XG?
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As a general rule of thumb, stacking antennas vertically (one on top of the other) will squash the antenna pattern in the vertical direction (i.e., flatten it more like a pancake). It usually won't alter the side-to-side beam width by much.
Stacking antennas horizontally (side by side) will squash the antenna pattern from the sides, creating a narrower side-to-side beam width, but leaves the vertical beam pattern about the same.
Either form of stacking can improve the forward gain by as much as 3 dB.
Vertical stacking will not do much to improve the rejection ratio from the sides. Horizontal stacking WILL improve the rejection ratio from the sides.
Neither stacking method will do much to change the front to back ratio.
If you have a specific signal to the side or back of your antenna that you wish to screen out, there may be other tricks to help eliminate it (e.g., antenna staggering, tuning of phasing lines, physical screens, etc.).
If you are really desperate to get every ounce of signal possible, you should also consider getting a very low noise figure pre-amp (with NF around 1 dB or less). This will get you another dB or two better than the more commonplace amps like the CM 7777 or Winegard AP-8700.