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I am adding a second antenna to an existing mast that will be pointed in a different direction.

The old antenna is FM/VHF/UHF (Winegard HD7080 I think) and there are four channels I care about on it, two VHF hi and two UHF - it was installed 10 years ago when there was one VHF lo station that converted to UHF when they went digital so the VHF lo / FM capability is unused/unneeded. The furthest is RF channel 7 about 50 miles away, which I get at nearly 100% in good weather, the other three I get at 100% in good weather.

The new antenna will be UHF only (Winegard HD9032) which will pick up two stations I care about 70 miles away that are 120* offline from the old antenna. The old antenna was temporarily aimed in that direction to verify reception and it picked up one channel at 60% and the other at 90% in clear weather, so with the additional UHF gain the HD9032 has compared to the old antenna I think I should be able to get a very good signal on both of those stations.

From reading posts here and links elsewhere, there is a minimum spacing between antennas but this always seem to be dealing with trying to point two antennas in the same direction to increase gain. Is there still a minimum spacing when the antennas are pointing in completely different directions and picking up different stations?

If there is, how do those formulas that use the lowest channel number work? Do I use channel 7 since that's the lowest I care about receiving, channel 2 since that's the lowest the old antenna can pick up? Or maybe channel 14 since that's the lowest channel they both have 'in common'?

Does it matter which antenna goes on top? For wind/ice load the UHF only would be preferable. But that may mean putting the current antenna a bit lower and I care much more about continuing to successfully receive the four stations in all weather with my old antenna. The two stations the new antenna will pick up are 'nice to have' but I'm thinking I should put the old antenna on top to insure its reception is not compromised. I'll already be taking a 3.5db hit from the combiner, and may have to tweak the aim of one or the other if I encounter multipath though hopefully not since there are no large hills or buildings in the vicinity.

I'm also wondering if there is some minimum distance above the top of the tripod that an antenna must be mounted? I currently have a five foot 1 1/4" mast, it will be replaced with a seven foot 1 5/8" mast to make room for the new antenna. I haven't bought that yet since I want to figure out the height between antennas and from the top of the tripod in case I need to go to 8 or 9 feet.

One final question, would it be a bad idea to break off the VHF lo elements on the old antenna to reduce the potential wind/ice load? One already broke off a few years ago after an ice storm. Or maybe even figure out how long elements to receive channel 7 should be and trim them to that length...would that help or would the spacing between them not be right?
 

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Please follow the sticky at the top of this forum about antenna threads. We need your TV Fool link.

3' spacing is usually enough especially when the elements are nearly cross aligned. If you want to know exactly how two antennas will affect each other the entire setup needs to be modeled and that's not a trivial task.

How much gain difference is there between the two antennas on UHF? It's unlikely the UHF only antenna will take a 60% station and turn it into a 100% station, more like 60% to 75%. With your TV Fool report we can know if a preamp is appropriate. If you don't have one now, adding a preamp can make a bigger difference than the antenna.

I would not break off the low VHF elements. It might affect the high VHF performance.

Your comments about the weather are telling me you don't understand what's going on. Rain or clouds do not attenuate TV signals. Rainy weather is the best time to check your reception because temperature inversions which create ducting and extend/enhance reception are not present. Your system needs to receive the weakest stations 100% when it's raining or you'll never have good reception. Clear weather indicates high pressure and that's when temperature inversions are most likely to form. Most people report best reception in the morning and evening and problems in the afternoon. Solar heating in the afternoon on clear days breaks up inversions so distant signals are weaker. LOS signals are normally not affected much by inversions unless the distance is great.
 

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When stacking two dis-similar Antennas I found that Metal-to-Metal separation COULD be as close as 2-ft before there were small degradations to Raw Gain....but to minimize degradation to Front-to-Back and Front-to-Rear Ratio performance required AT LEAST 4-ft and preferably MORE separation. This was with both pointed in the same direction. An EXACT modeling would be needed to verify, but I don't think there is much difference when pointing in different directions, given the similarity of these results for all combinations tested, incl the highly compact CM-4228 UHF Antenna:
http://imageevent.com/holl_ands/stacked

Tripod should also follow the greater than 4-ft from the lower Antenna advice.
UHF is usually on top to take advantage of a few more feet of Antenna Height Gain (but it's isn't critical)...AND more importantly to (usually) reduce weight and wind load at the top of the mast...but it depends on the choice of Antennas as to which is lower weight and wind load.

I did an analysis for a R-S VU-190XR with FOUR Missing or Broken Elements, finding that it was serious degraded and TRIED a Re-Optimization to see if it could be "cut" for better performance...which LOST 2 dB in lower Hi-VHF Band and GAINED several db in upper Hi-VHF Band vs an UNDAMAGED Antenna:
http://imageevent.com/holl_ands/zigzaglpa/vhflpda/vhfrsvu190xrlpdanouhf

If you can provide the Antenna Model number (and Measurements if we don't already have the 4nec2 model) and a description of the damage, I can run a similar Re-Optimization to see if "simply" cutting back the remaining VHF Elements (in their current positions along the Boom) will actually work or not.

However, bear in mind that the old Lo-VHF/Hi-VHF Antennas had Elements that were 3/2-wavelength long in the Hi-VHF Band...which inherently have about 2-3 dB more Raw Gain than the usual 1/2-wavelength long Elements found in Antennas designed for Hi-VHF Band. So some old Ch2-13 UHF Combo Antennas (e.g. Monster W-G HD-8200) typically provide significantly more Hi-VHF Gain than any of the latest Ch7-13+UHF Combos [Bigger IS Better Rule Applies.]

Need your TVFool results URL before we can talk about Preamps.....
 
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