View Full Version : Antenna spacing on a mast
10-30-2007, 02:12 AM
Ok guys ,
# #I am really enjoying 2 ssb and would love a 17b2 or something like it..... I currently have on my tower starting at the bottom up # #hf tri band right on top of the tower, then 5' above that 6m horz 5 ele beam, 5 ' above that the cushcraft 20t the horz vert combo beam, and 5 ' above that a tri band vert 6-2-440. That is a total of 15' of mast out of the top of the tower all turned by a tail twister and it WORKS GREAT !!!!!!!
I was talking to my antenna guy about a 17b2 and he was like oh no problem it can fit between the hf and the 6m beam. he is saying that putting it 2.5 ' between these antennas ( the hf and 6m) will NOT affect any of the 3, I guess I question that.
So any thoughts on this?
10-30-2007, 04:59 AM
That is about the spacing that I have between my 6 meter, 2 meter, and 1.25 meter antennas. They work fine.
The VHF antennas are in the 2nd photo.
10-30-2007, 12:32 PM
Thanks for your reply, I was worried about the 6 meter and the HF beams mainly being affected.................
Looking to read other comments on this
10-30-2007, 12:45 PM
This site may help:
Stacking Yagi Antennas
10-30-2007, 01:31 PM
WOW that is good info !!!
10-30-2007, 04:29 PM
Per G3SEK, and I agree with him as his advice follows everything I've ever read on the subject, you would not want to stack any other beams (for other bands) any closer than 6-1/2 feet from a 17B2.
2-1/2 feet violates this by quite a bit.
Frankly, even a "small" 5 element six meter beam like a Cushcraft A505S (only 12' long) has a stacking distance of twelve feet, and half that is six feet; so stacking anything within six feet of that antenna wouldn't really be recommended, either.
I have 3 antennas on my VHF tower. 30ft yagis for 6m and 2m, and a 26ft yagi for 432. All are less than 5ft apart and work fine. Lot's of DX on 6m Es, and very successful with tropo, Es, meteors, EME with the 2m and 432 antennas.
Some pics on my website
10-30-2007, 08:39 PM
Quote[/b] (AB3BK @ Oct. 30 2007,09:38)]I have 3 antennas on my VHF tower. 30ft yagis for 6m and 2m, and a 26ft yagi for 432. #All are less than 5ft apart and work fine.
"Working fine" is a relative term; unless you've tried different stacking distances and measured results, it's impossible to know if the proximity of the other antennas is negatively impacting performance or not.
My guess is: Yes, it is negatively impacting performance.
I can place VHF yagis so close together the elements are essentially interlaced on a single common boom (with all separate feedlines) and they still "work." Problem is, they don't work well. The reason nobody actually does this is because it's not a reasonable compromise; if it worked well, everybody would be using VHF-UHF antennas of interlaced yagis on a common boom -- think of the space and cost savings!
Now, separating Yagis by a couple of wavelengths makes them quite independent, and this is what most strive to do. Where is the reasonable compromise between infinite stacking distance and having interlaced elements?
From both empirical trials run by VHF antenna manufacturers in the 1950s (many of which were performed by Telrex using the excellent antenna range at the U.S. Army Signal Corps R&D labs at Ft. Monmouth, NJ) and also modern-day computer modeling, it appears the reasonable compromise occurs at just about exactly where Ian G3SEK says it does: When you space booms half the distance of the antenna's vertical aperture, which is defined also as the optimum stacking distance for two identical antennas to achieve maximum increase in forward gain.
Typically, this is about a boom length for shorter Yagis (in terms of wavelengths) and reduces to about half a boom length for very long Yagis (also in terms of wavelengths) and is highly calculable.
Some good discussion on the subject, albeit regarding HF antennas, is here:
Since I always use telescoping towers and masts I can reach from a work platform or roof, I experiment with this stuff quite a lot. My current HF/6m/2m beams are spaced 7' between HF and 6m and 5' between 6m and 2m, and it's actually "too close." I can see the difference in weak, distant beacon signals continuously arriving via tropo forward scatter. When I increase the distances to 13' and 9' respectively, all distant tropo beacon signals improve -- all of them, from distances of 60 to 300 miles, in all directions. Putting the same beams closer together reduces signals again, consistently, by as much as 3-4 dB.
However, I settled on a reasonable compromise for me; balancing wind loading and mass stresses against performance. If I had a 24' climbable chromalloy mast I'd consider going back to the wider spacing, because it really does make a difference.
10-31-2007, 01:01 AM
sounds like I should remove the 20t and put it up there ....
11-01-2007, 12:45 AM
I am going to ask this here vs starting a new thread.
I have a Diamond V2000 at the top of the mast and the v2000 has three thin radials so if I had a 17b2 or a 6m yagi just below that would the yagi see the v2000 and have tuning issues or would it see it more as a mast, and of course how would the v2000 be affected? My thinking is if I removed the 20t that sits 5' below the v2000 and shove the 6m beam up just under the v2000 that would give me 12'-13' to place the 17b2 in leaving 6-6.5' top and bottom spacing for the 17b2.
11-01-2007, 05:47 PM
Quote[/b] (n1kon @ Oct. 31 2007,17:45)]
>I am going to ask this here vs starting a new thread.
I have a Diamond V2000 at the top of the mast and the v2000 has three thin radials so if I had a 17b2 or a 6m yagi just below that would the yagi see the v2000 and have tuning issues or would it see it more as a mast, and of course how would the v2000 be affected?<
::The horizontally polarized VHF beam directly below the V2000 will see those radials as undesired parasitic elements and that's not a good thing. The horizontally polarized VHF beam directly below the V2000 will not see the vertical radiating element at all, and that is a good thing. But you want to space the beam about 40" below the V2000 to minimize the interaction with the horizontal radials of the V2000. I've been through exactly this same thing. If the beam below the V2000 were an HF beam, there would be virtually no interaction at all, and you could place them very closely.
>My thinking is if I removed the 20t that sits 5' below the v2000 and shove the 6m beam up just under the v2000 that would give me 12'-13' to place the 17b2 in leaving 6-6.5' top and bottom spacing for the 17b2.<
::Sure. The 20T probably didn't work very well "vertically" anyway, because such an antenna should never be installed on a conductive mast. The 20T is intended for use on a non-conductive mast. I think your new approach is better than what you have.
11-01-2007, 06:41 PM
If you havent bought the 17B2 yet...let me know...I have one that needs a home email@example.com ..73's
11-01-2007, 11:40 PM
YOU HAVE EMAIL!!!
Your 40" in what cushcraft just emailed me it should be vs the 6-7' spacing I was reading about in this thread. I knew the 20T would not do well vert since it is in the middle of the mast. I am really not happy with my 20t over all, it does help both horz and very but I have local hams who has a 16 ele KLM at 30' and sits ALOT lower than me sea level wise and still can hear stations no problem while all I can tell is there is someone there, my 20t is up 60'. In regaurds to the 20t being mounted on a non-conductive mast, I do not see in the manual anything about that can you give me more info on that?
11-02-2007, 02:28 AM
40" is 1/2 wavelength at 144 MHz, and that's why I recommended that distance. That's also why Cushcraft recommended it...
Any vertically polarized beam that is not end-mounted behind the reflector needs to be mounted on a non-conductive mast. The mast in the middle of the boom becomes an undesired, untuned parasitic element and degrades performance; then, so does the coaxial feedline if it is routed down the boom and then down the mast! You shouldn't do that, either.