Posted: 6/30/2012 1:37:13 PM EDT
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I just drove 400 miles round trip to pick up a Cushcraft A3S (20/15/10 meter) 3 element beam, Ham IV rotator, a 10' center section & a top section with a HD bearing & rotor mounting plate of Rohn 25. $500 + $80 for gas. And the guy threw in a Cushcraft A50-3S 6 meter 3 element beam. All items are <5 years old.
Did I do good? I plan on mounting all of the above on my 60' (minus current top section) 50', Rohn 25, TV tower. A) How far above the top section of tower should I mount the A3S? B) How far above the A3S should I mount the A50-3S? C) Or am I asking for trouble with the two antennas on one mast? |
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Quoted:
I just drove 400 miles round trip to pick up a Cushcraft A3S (20/15/10 meter) 3 element beam, Ham IV rotator, a 10' center section & a top section with a HD bearing & rotor mounting plate of Rohn 25. $500 + $80 for gas. And the guy threw in a Cushcraft A50-3S 6 meter 3 element beam. All items are <5 years old. Did I do good? You really have to rub it in don't you 
I plan on mounting all of the above on my 60' (minus current top section) 50', Rohn 25, TV tower. A) How far above the top section of tower should I mount the A3S? I would suggest as close as is feasible, to reduce bending moment on the mast and stress on the thrust bearing, rotator etc. B) How far above the A3S should I mount the A50-3S? In theory if you want to minimize/eliminate performance degradation to the 6m antenna you'd want it like 9 1/2 feet above the HF beam. That's not going to be very practical, you can reduce it to something a lot more reasonable, like 5 feet or less, and probably never notice the difference. Less spacing will put a lot less stress on the mechanical components and likely lead to a more satisfactory installation overall. Intuitively I would probably avoid a 1/4 wave spacing which would be like 54 to 60 inches, and as the article I linked to below suggests, more than 3' to avoid excessive coupling of TX energy. Here's a webpage with info about stacking VHF/UHF antennas for reference: http://www.directivesystems.com/STACKING.htm C) Or am I asking for trouble with the two antennas on one mast? Not as long as the components you are using are up to the stresses from windload etc. Make sure your tower is guyed well, and the mast is sufficiently strong, and that your rotator is rated for the total amount of windload. There are plenty of installations out there with more on one mast than what you're proposing. |
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A. Far enough for it to rotate freely with the coax. I'd say 1 '.
B. 1/2 of the H-plane aperture of the higher frequency antenna. Directive Systems had a great article but they seem to be redoing it right now. I'll say 7' for your setup, less is ok too, just more interaction. ETA: Gamma finished his post while I was typing and found the article too! 
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Quoted:
Quoted:
I just drove 400 miles round trip to pick up a Cushcraft A3S (20/15/10 meter) 3 element beam, Ham IV rotator, a 10' center section & a top section with a HD bearing & rotor mounting plate of Rohn 25. $500 + $80 for gas. And the guy threw in a Cushcraft A50-3S 6 meter 3 element beam. All items are <5 years old. Did I do good? You really have to rub it in don't you
I guess that means yes. 
I plan on mounting all of the above on my 60' (minus current top section) 50', Rohn 25, TV tower. A) How far above the top section of tower should I mount the A3S? I would suggest as close as is feasible, to reduce bending moment on the mast and stress on the thrust bearing, rotator etc. B) How far above the A3S should I mount the A50-3S? In theory if you want to minimize/eliminate performance degradation to the 6m antenna you'd want it like 9 1/2 feet above the HF beam. That's not going to be very practical, you can reduce it to something a lot more reasonable, like 5 feet or less, and probably never notice the difference. Less spacing will put a lot less stress on the mechanical components and likely lead to a more satisfactory installation overall. Intuitively I would probably avoid a 1/4 wave spacing which would be like 54 to 60 inches, and as the article I linked to below suggests, more than 3' to avoid excessive coupling of TX energy. Here's a webpage with info about stacking VHF/UHF antennas for reference: http://www.directivesystems.com/STACKING.htm C) Or am I asking for trouble with the two antennas on one mast? Not as long as the components you are using are up to the stresses from windload etc. Make sure your tower is guyed well, and the mast is sufficiently strong, and that your rotator is rated for the total amount of windload. There are plenty of installations out there with more on one mast than what you're proposing. I think all of the components are plenty strong enough for what I have. The HF & 6 meter beams both present less than 7 ft sq wind load total and the Ham IV rotor can handle up to 15 ft sq. My main concern was the two antennas interacting negatively with each other and degrading performance. Right now I don't have a 6 meter radio, but the thought was to get everything set up properly now so there was no need to take the antenna down again later on..... ..... I just ran off and read the link from above (and a few others) and now I'm like 
The HF antenna instructions say to stack antennas as far apart as you can. So no matter how I mount it, (within reason) there is going to be interaction with the tower and any other antennas on the mast because the aperture is so large. The 6 meter antenna instructions say to stack it at least 1/2 the boom length apart which would only be 3 ft. So if I used a heavy wall 10' section of pipe I would have about 8' above the tower top to work with. If I set the HF antenna 1' above the tower I could get about 7' of separation. My brain hurts.  I'll come back to this after I eat.
Here's a picture before it came down. 
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Last year I put two Tennadyne log antennas (HF and 6M) on a 100' tower for a friend. I forget the exact distance but we put them 7-8' apart. As of last week he hasn't complained at all about the antennas adversely affecting each other. They're rock solid and burning up the airwaves.
Before the Antennas 
Hmm, I'll have to see if I have better pics of the final install. 
ETA: The best I found. At 100' it's hard to get a profile shot. 
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I think I'll just put as much separation between the antennas as I can & go from there.
I guess I also need to look at adding guy lines to my tower. It's a standard 60' Rohn 25 TV tower that is bracketed to the peak of the roof (18' +/-), but if I add another 10' or so to it and a larger antenna I might run into trouble during the next good thunder storm or hurricane. I wish I had a wad of extra cash laying around. A guy that lives down the road has a 199' Rohn 25 tower that he is willing to give me, but he wants whoever takes it down to be licensed & insured (it's right next to a house so I don't blame him). It's been up for 20+ years and the couple of guys I talked to want more than I can justify spending right now to take it down. 
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