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Author Topic: Spacing between antennas on the same mast?  (Read 1862 times)
AE5X
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« on: January 29, 2013, 05:45:34 AM »

Is there a formula or even a rule of thumb regarding how far apart two antennas should be when mounted one above the other on the same mast?

I want to put a rotatable 40m dipole above my triband Yagi and am wondering how far above it it needs to be t oavoid interaction.

Tnx/73,

John AE5X
http://www.ae5x.com/blog
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W5JON
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Posts: 167




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« Reply #1 on: January 29, 2013, 05:52:41 AM »

The "rule of thumb" that I have gone by for 50+ years, spacing between antennas should be atleast half  the boom length of the shorter antenna.  It has always worked for me.  In the case of a "rotatable dipole", for minimal interaction, I line up the rotatable dipole with the boom of the beam, not with the elements of the beam, with half the beam boom length separation.  These are MY "rules of thumb", your results may vary. 

73,

John W5JON
« Last Edit: January 29, 2013, 06:03:26 AM by W5JON » Logged
AA4PB
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« Reply #2 on: January 29, 2013, 06:03:59 AM »

"The "rule of thumb" that I have gone by for 50+ years, spacing should be atleast half the boom length of the shorter antenna.  It has always worked for me."

It's always worked for me too but I think it is more applicable to stacking two Yagis. I think you'll have a lot less interaction between a 40M dipole and a 10/15/20M Yagi. Some tri-band Yagis have a 40M rotatable dipole right on the same boom (i.e. using the same driven element).
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K3VAT
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« Reply #3 on: January 29, 2013, 06:19:12 AM »

Is there a formula or even a rule of thumb regarding how far apart two antennas should be when mounted one above the other on the same mast?

I want to put a rotatable 40m dipole above my triband Yagi and am wondering how far above it it needs to be t oavoid interaction.

Tnx/73,  John AE5X http://www.ae5x.com/blog

In addition to the above posts, there are dozens if not hundreds of others available on this popular topic by searching:
http://www.eham.net/ehamforum/smf/index.php?action=search

You may have to spend a bit of time; some of the postings reference empirical data, but most anecdotal.

GL, 73, Rich, K3VAT
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AE5X
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« Reply #4 on: January 29, 2013, 06:19:30 AM »

Some tri-band Yagis have a 40M rotatable dipole right on the same boom (i.e. using the same driven element).


My Yagi is the Force 12 C3 which did have the option of adding the 40m dipole to the boom - until I chose another option instead - an additional element for 10m. Not sure if I can still add the 40m dipole to the boom now or i fthat possibility has been nullified by the 10m reflector addition...

I'll check with Force 12 but either way, a 40m rotatable dipole will be taking up residence here soon ;-)

John AE5X
http://www.ae5x.com/blog
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AD4U
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« Reply #5 on: January 29, 2013, 06:35:32 AM »

You have been given good advice so far.  To the best of my knowledge there is no mathematical formula for your question.  When you put several antennas in close proximity to each other, you can expect some interaction.  How much and whether it is significant depends on many different factors.

There are still several multi-band yagis made with interlaced elements for different bands on the same boom.  I think there is even one yagi that is supposed to cover 20, 17, 15, 12 and 10 meters.  Is there interaction?  Probably so.  Is it significant?   Depends..........

IMO the best you can do is to separate your tri band yagi and your 40M rotatable dipole vertically as much as possible within the structural limits of your mast and tower. 

Probably the best answer to your question is to mount the rotatable 40M dipole at a 90 degree angle to the yagi (parallel to the yagi's boom).  The problem with this arrangement is the main lobe of the yagi and the 40M dipole will be at right angles to each other.

At my previous QTH I had 5 element homebrew yagis for 20-15-10 meters, stacked on a 100 foot tower.  The yagis were vertically separated by no more than 5 feet between each.  I am sure there was some interaction, but THEY WORKED!  I had no problem breaking through ANY pile up.

Dick  AD4U
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AA4HA
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« Reply #6 on: January 29, 2013, 11:03:25 AM »

Interesting question. The question "depends" on a few things;

You can vertically space, vertically polarized antennas as they will be in the "null" of the other antenna. Some rules of thumb vary from 1/4 to 3 wavelengths of spacing to minimize desense and overload.

For example, at 900 MHz we space vertically polarized antennas at least 6' apart (vertically). Horizontal spacing on vertically polarized antennas is much much further as omni antennas would be in the radiation pattern of the other antenna.

Conversely horizontally polarized antennas should be separated horizontally and the same 1/4 to 3 wavelength rule of thumb applies. Vertically stacking horizontally polarized antennas... the spacing should be much further.

This is for antennas operating in the same band. If you are on 20m on one antenna and 30m on another then the rules can change.

Just remember that placing any mass of metal in close proximity to another antenna is going to change the radiation pattern of both antennas. You may change the take-off angle, gain, directivity, front/back ratios. Imagine if you are 80' up in the air with stacked antennas. Will the top antenna see the earth as the ground plane or will it see the other beam that is 15' below it? What happens to the lower antenna? Will it see the upper antenna as some sort of ground plane or a capacity hat?

The possibilities could give anyone who models antennas in NEC and then decides to model "structures" or unintentional arrays the sweaty shakes at night.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
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