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Author Topic: Closely stacked beams on tower - consequences?  (Read 1612 times)
EI4HQ
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« on: February 14, 2010, 10:18:08 AM »

Hi,

Space at towertop available for antennas is only about 2 metres (6' or so). This is a design decision due tower limitations and allowances for bad winds at QTH.

I have 3 antennas:
1. TH3 Mk IV (3 ele tribander)
2. A50S (5 ele for 6m) and
3. A27010S (5 ele on 2m & 5 ele on 70cm).

I'm worried about the negative consequences to any/all antennas by close stacking...

In theory I could mount all 3 separated by circa 1m (TH3 at 0m, A50S at 1m and A27010S at 2m), but I expect that would result in 'significant' radiation pattern distortion and/or tuning issues for each/all of the antennas?

I'd appreciate the insights of those who've close stacked antennas, and some idea of how 'bad' the consequences are likely to be...

Thanks in advance
BR & 73
Cormac, EI4HQ
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W8JI
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« Reply #1 on: February 14, 2010, 11:19:41 AM »

I read a lot of stuff about stacking antennas, but the only real way to tell is to model the antennas, mast, and feedlines.

The PATTERN and resonances (even from out of band antennas) is what really controls the interaction of antennas, not the "capture area" or effective aperture. We have a tendency in Ham radio to give very good sounding and concise simple answers, even when they are wrong.

Let me give an example of why it is so unpredictable. I can mount a 144 MHz and a 432 MHz antenna on a common boom without undue interaction if the antennas have minimal induced currents from the other band. At the same time, I might have problems if they are a wavelength apart if one of the antenna happens to have resonances on the operating frequency of the other antenna that allow a significant level of unwanted current to be induced.

You really just never know, and sometimes even feedline and mast lengths affect it all.

Personally if I were in your position and I could not model the antennas, I would mount the TH3 at the very bottom and the A50S at the very top, and squeeze the A270 in the middle. The A270 needs a lot less spacing, so that would be the best shot at success.

73 Tom



 
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EI4HQ
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« Reply #2 on: February 14, 2010, 11:50:20 AM »

Tom,

Words of wisdom - as you say - we tend to look for the 'easy' answer knowing full well there probably isn't one ;-) The triumph of hope over experience...

Your suggestion is a good one - its easy enough for me to give different mounting scenarios a go and see how things shape up - I'll start with that one (TH3 at 0m, A27010S at 1m and A50S at 2m) and see how it looks.

BR & 73
Cormac
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N3OX
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« Reply #3 on: February 14, 2010, 05:02:15 PM »

You can use G4HFQ's Polar Plot and some kind of signal source feeding a horizontally polarized distant antenna or a local ham with a well-choked horizontal beam to take patterns on your antennas after you've installed them.

73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
K2OWR
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Posts: 36




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« Reply #4 on: February 20, 2010, 11:02:56 AM »

Stacking those particular antennas you mentioned is something I have done many times. Although 6' is a little tight, I doubt any of those antennas will have their pattern seriously affected. Tri-bander at the bottom of course, then six, then two, should not be a problem.

Mount the HF beam only an inch or so above the very top of the tower and the two meter one right at the tippy top with six in the middle and you should be good to go.

This is my experience....not based on any actual measurements, but I got out pretty well!
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W2MC
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« Reply #5 on: March 10, 2010, 06:41:44 AM »

I have used a short stack on top of my ez-way tower for several years, at two different QTHs.  The overall length of the mast is 12 feet, and I have about 10 sticking out of the top of the tower.

The stack, currently, is (top to bottom):


Create Log Periodic for 1.2 ghz to 50 Mhz
Cushcraft A3W for 12 and 17 Meters
Mosley TA-33 for 10, 15, and 20 Meters

Underneath it all is an inverted-v wire for 80 meters.


Are they too close?  Yes; the patterns on the bottom two are probably influenced by each other to some degree.  I havent noticed it, however.  The only issue has been the Mosley, where I had to re-tune it slightly to bring the SWR down on 20 meters, accomplished by adjusting the tips of the driven element.  Other than that, they all seem to work fine.

Just do it!

Jon
W2MC
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W4KVW
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« Reply #6 on: March 13, 2010, 07:35:44 PM »

I have a MOSLEY TA-53M at 50 ft,a CUSHCRAFT 13B2 at 53 ft,an M2 6M5X at 55ft,& a CUSHCRAFT 215WB at 61 ft.on the same mast pipe.I have had ZERO issues with this spacing on either of the antennas!

Clayton
W4KVW
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KF6A
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« Reply #7 on: March 18, 2010, 04:07:09 AM »

I was afraid of the same thing so I ended up purchasing EZNEC and modeling my antennas as best I could. The model indicated some interaction, assuming my model was correct, with slightly degraded gain on my 2m stack by something like .5db. I put the antennas up and everything is working great, although my spacing isn't as compressed as yours will be.
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WX7G
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« Reply #8 on: March 18, 2010, 06:31:16 AM »

Some hams say to mount the antennas at right angles. I have not modeled this and so I don't know if this is good or bad advice.
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EI4HQ
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« Reply #9 on: April 24, 2010, 02:33:37 PM »

Hi All,

Thanks for input from all contributors. I thought I'd provide an update on how things actually worked out...

In the end I had to mount the antennas with A270 at the top, A50S in the middle and the TH3 Mk IV at the bottom. I tried the A50S at the top and the A270 in the middle, but the wind loading would not have been acceptable so I had to swap them around. The vertical distance between each antenna is about 4 feet and all booms are parallel.

First, the good bits; the A270S tunes as per spec., the TH3 Mk IV tunes as per spec. on 15 and 10m.

The bad bit is the A50S is resonating 1.3MHz above spec. (approx. 51.4Mhz instead of down around 50.100MHz) and the TH3 Mk IV is resonating 500KHz above spec. on 20m (14.550MHz instead of around 14.050Mhz). The TH3 Mk IV is showing an SWR of just under 2:1 at 14.010MHz and the A50S an SWR of about 1.5 at 50.100MHz. There appears to be interaction between the two antennas; I temporarily removed the A50S and the TH3 Mk IV then behaved as per spec. on 20m.

While not ideal on 20m/6m, and goodness knows what the radiation patterns look like, the setup is very usuable with the greatest compromise on 20m - since installation I've done some test driving on 20m and it's working well enough to satisfy me... As I'll be removing the A50S for each Winter season anyway, the compromise is quite acceptable.

Thanks to all for your insights/contributions.

BR
Cormac, EI4HQ
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W4VR
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« Reply #10 on: April 25, 2010, 08:52:20 AM »

Guess it's too late now since you've already done it, but as one poster said the best way to find out and save yourself a lot of trouble is to get a copy of EZNEC and model everything you've got on the tower to get the spacing with least interraction.  I guess that's why the guys with "real" good antenna systems use a separate tower for each rotary beam.
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WX7G
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« Reply #11 on: April 26, 2010, 10:44:00 AM »

A NEC model will tell the story. Anything else is guesswork.
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