well with dx coming in i was think of raising my current 5/8 antenna
currently its at 30 to tip. now my question is if i got a antenna
like a astroplane i could eaily get the top of it to 30ft. i know
the 5/8 should have more gain but i cant have a antenna higher
than 30ft. to tip.could a 1/4 wave antenna mounted high
out talk a 5/8 antenna mounted lower.i know ideally the best
thing would be to raise my current 5/8 but im at my restricted
height of 30ft.
hotrod, in my experience it is hard to compare antennas and remain fair to all the different testing ideas in the process. Personally I test antennas side by side and in my experience the longer antenna almost always shows better signals among the locals stations I compare---when I place the feed points at the same height.
When I place the antenna tips to the same height, almost always the shorter antenna shows the best signal by similar comparisons.
So in your example I would go with the AstroPlane antenna with the tip at 30' feet and the feed point at 26' feet.
If you are comparing an Imax in this case, its base would be about 7' feet above the ground and just about everything that is 8' feet tall would interfere with the Imax in some way. The Imax will work, but if you raise it up above most structures around the general area of the antenna, you will see a nice improvement---and it might not be just the height that affects the most change unless you go really high.
In my Signal reports, in my album here on WWRF, I show some reports that support this idea, but in most cases I don't start out with a 30' foot height limit. 30' feet is very low considering all the ground clutter we have around us in the real world and the long antennas we use in 11 meters.
Avanti discusses this affect on antennas in the AstroPlane Patent, where the antenna setup is very low at its current maximums. Avanti refers to this problem as the "Shadow" affect which can be caused by any structure that may tend to block signals even non conductive structures. It is part of their design idea for the AstroPlane.
In my comparison work to test this affect I found that even my all wood home effectively blocked signals when I got too close to the building. However, I could still detect this by operating my radio when I moved the antenna away from the structure, out into the open. The signals were improved some, but I could still see the affects of this shadow affect and I knew it was my home that was 18' feet tall at the peak. Once I got the full antenna above the peak, the antenna really begain to respond nicely and I could no longer sense the Shadow affect.
Since Avanti raised the idea in their Patent, I thought maybe the AP would overcome the problem, but I did not see that happen. I saw this happen with all the vertical antennas I have, so IMO you will likely have to deal with this problem having a 30' tip height limit, and you may be better off with an AstroPlane type antenna that is structurally shorter where you can get the feed point above the typical building in your area. If you are out in the open countryside with open fields around you, then put up the longests antenna you can within that limit.
