Hi all...
After doing some tests and checking the pattern as good as I can....I plan to stack 2 70cm beams..
I was reading something about this and looking for stacking distance and wondering if the guys used or believe in the formula.....52/ 3dB beamwidth (in deg) = spacing (in wl)
Beam stacking
Re: Beam stacking
Oh boy, another can of worms John? 
When stacking antennas there are trade offs involved, and no formula can address them all ... however, using it may be a good to keep you out of trouble from stacking antennas too close or too far apart. How much attention is paid to the various factors in that trade off depends entirely the individual situation and, to some extent, how pedantic vs. pragmatic one is when achieving your own 'happiness level'.
Trade offs that spring to mind:
Gain - is maximum stacking gain at the expense of other factors (like side lobe level) practically useful in the average Ham situation?
G/T - (more relevant to comparing individual antennas?) this is based on a theoretical set of conditions and, to my mind, doubtful that they mean very much in a practical sense to most Ham situations (but great for those who love to pour over spreadsheets comparing different antenna arrays if that is what satisfies their 'happiness level').
Side lobe levels - to a point, closer stacking distances will reduce pick up of unwanted surrounding noise sources, but at the expense of gain. What level is 'good' is purely subjective depending on the individual location. In a "noisy" city location, closer spacings and resulting lower side lobe levels usually improve signal readability (but may be better to use so called "low temperature" Yagi designs). EME'ers have different considerations.
Mechanical aspects - not usually a consideration with the physical stacking distances used at UHF, but becomes a factor when stacking antennas at HF and low VHF.
Mutual coupling effects - this is for the pedantic. I'm not going to explain it.
These trade offs (and there may be others) will vary for the actual characteristics of the antenna used in the array as well as the actual situation the antenna array is used in. To my thinking, no one formula will give the right result for all situations (and the side lobe levels are not known) so I use an antenna simulation program (4NEC2 according to my budget ... zero) to look at the resulting radiation pattern changes at different stacking distances, and choose something that satisfies my 'happiness level' for the particular situation involved. (Keeping firmly in mind that what ever is seen on the screen will be different when implemented practically due to real life factors not in the simulation ... hardly ever is the stacking frame or supporting mast/tower included, and the accuracy limitations of the free program).
If that is too onerous, the design you have built might be in the following table ... http://www.dxmaps.com/VE7BQH70.html ... and the stacking distances given there are probably a better guide than the formula. For what you intend to do according to your other post (changing polarisation), it might be best to use the "H" stacking distance.
Open season ...
73,
When stacking antennas there are trade offs involved, and no formula can address them all ... however, using it may be a good to keep you out of trouble from stacking antennas too close or too far apart. How much attention is paid to the various factors in that trade off depends entirely the individual situation and, to some extent, how pedantic vs. pragmatic one is when achieving your own 'happiness level'.
Trade offs that spring to mind:
Gain - is maximum stacking gain at the expense of other factors (like side lobe level) practically useful in the average Ham situation?
G/T - (more relevant to comparing individual antennas?) this is based on a theoretical set of conditions and, to my mind, doubtful that they mean very much in a practical sense to most Ham situations (but great for those who love to pour over spreadsheets comparing different antenna arrays if that is what satisfies their 'happiness level').
Side lobe levels - to a point, closer stacking distances will reduce pick up of unwanted surrounding noise sources, but at the expense of gain. What level is 'good' is purely subjective depending on the individual location. In a "noisy" city location, closer spacings and resulting lower side lobe levels usually improve signal readability (but may be better to use so called "low temperature" Yagi designs). EME'ers have different considerations.
Mechanical aspects - not usually a consideration with the physical stacking distances used at UHF, but becomes a factor when stacking antennas at HF and low VHF.
Mutual coupling effects - this is for the pedantic. I'm not going to explain it.
These trade offs (and there may be others) will vary for the actual characteristics of the antenna used in the array as well as the actual situation the antenna array is used in. To my thinking, no one formula will give the right result for all situations (and the side lobe levels are not known) so I use an antenna simulation program (4NEC2 according to my budget ... zero) to look at the resulting radiation pattern changes at different stacking distances, and choose something that satisfies my 'happiness level' for the particular situation involved. (Keeping firmly in mind that what ever is seen on the screen will be different when implemented practically due to real life factors not in the simulation ... hardly ever is the stacking frame or supporting mast/tower included, and the accuracy limitations of the free program).
If that is too onerous, the design you have built might be in the following table ... http://www.dxmaps.com/VE7BQH70.html ... and the stacking distances given there are probably a better guide than the formula. For what you intend to do according to your other post (changing polarisation), it might be best to use the "H" stacking distance.
Open season ...
73,
Bob, ZL1RS in the Bay of Islands at RF64vs
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VK3ZAZ
- Forum Diehard
- Posts: 785
- Joined: Sat Dec 18, 2010 1:46 pm
- Location: Hamilton Victoria Australia
Re: Beam stacking
I have to agree.. results speak for themselves I guess.
All things being equal, which clearly on 6M band they are not.
VK3AMZ had stacked bay (gone)
So does VK3DUT maybe they care to comment.
BA4SI has stacked yagis so does VK5PO
So does W6JKV K6QXY and W6BYA.
Remember the same yagi array in different hands will have totally different outcome.
Just look at the 9M2 in the hands of N6AMG and Peter James VK3AWY.
Check the websites of the 6M leaders this cycle you will find M2 single long yagis in most cases.
The 9M2WL designed in 1988 still holds its own two cycle later in 2014.
And some interesting variations now exist like 8s 7s and 11s.
And hindesite is an amazing tool...
Where would I be if I had not had the money and motivation to bring three M2 yagis into VK in 1989
packed with three planes in a shipping container delivered by truck by a young Mike Stahl, to 2 Genes Aviation San Martin California??
No wheelbarrows please...
All things being equal, which clearly on 6M band they are not.
VK3AMZ had stacked bay (gone)
So does VK3DUT maybe they care to comment.
BA4SI has stacked yagis so does VK5PO
So does W6JKV K6QXY and W6BYA.
Remember the same yagi array in different hands will have totally different outcome.
Just look at the 9M2 in the hands of N6AMG and Peter James VK3AWY.
Check the websites of the 6M leaders this cycle you will find M2 single long yagis in most cases.
The 9M2WL designed in 1988 still holds its own two cycle later in 2014.
And some interesting variations now exist like 8s 7s and 11s.
And hindesite is an amazing tool...
Where would I be if I had not had the money and motivation to bring three M2 yagis into VK in 1989
packed with three planes in a shipping container delivered by truck by a young Mike Stahl, to 2 Genes Aviation San Martin California??
No wheelbarrows please...
Tread your own path 
-
VK2FAK
Re: Beam stacking
Hi all...
Yeah kind of thought it was a one size does not fit all sort of thing......as mentioned ...the frame setup is going to have some effect.....I did not know there was a formula of sorts for the stacking...I was assuming something like 1.2wl to be around the target...the formula was changed I think to bring down the sidelobe to a respectable level...will check the link to see there estimates...I think I have it modeled...so may put in another...and maybe the cross boom...to see what happens....
Bob:
Mutual coupling effects - this is for the pedantic. I'm not going to explain it. .......chicken....hahaha
Yeah kind of thought it was a one size does not fit all sort of thing......as mentioned ...the frame setup is going to have some effect.....I did not know there was a formula of sorts for the stacking...I was assuming something like 1.2wl to be around the target...the formula was changed I think to bring down the sidelobe to a respectable level...will check the link to see there estimates...I think I have it modeled...so may put in another...and maybe the cross boom...to see what happens....
Bob:
Mutual coupling effects - this is for the pedantic. I'm not going to explain it. .......chicken....hahaha
-
VK7DB
Re: Beam stacking
Use MMANA and model one antenna. Create a stack and optimise stack distance.
Re: Beam stacking
A good article on stacking different band yagi's in close proximity to each other is
http://www.kcvhfgridbandits.com/linked/ ... gyagis.pdf
I know not the Question here (2x70cm yagi's) but may come in handy for some readers here
Cheerio
Wayne
http://www.kcvhfgridbandits.com/linked/ ... gyagis.pdf
I know not the Question here (2x70cm yagi's) but may come in handy for some readers here
Cheerio
Wayne
Re: Beam stacking
I don't always agree with Steve, but in this case I do.
For one thing, the stacking distance, even for small yagis on six metres has to be at least 6 metres and for the 10 element YU7EF which I have, the optimum distance is almost 8. Secondly, if you start with a beam which has a nice clean pattern, with sidelobes down 20 db or better, which incidentally you can easily get with a longer yagi, you won't get such a clean pattern when you stack. Side lobes will always pick up noise from unwanted directions.
My next project is a slightly modified SV1BTR design which should have 13.0 dbd gain, first side lobe 26 db down and all other lobes better than 31 db down, all on a 19.4 metre boom with 50 ohm feed.
Stay posted.
For one thing, the stacking distance, even for small yagis on six metres has to be at least 6 metres and for the 10 element YU7EF which I have, the optimum distance is almost 8. Secondly, if you start with a beam which has a nice clean pattern, with sidelobes down 20 db or better, which incidentally you can easily get with a longer yagi, you won't get such a clean pattern when you stack. Side lobes will always pick up noise from unwanted directions.
My next project is a slightly modified SV1BTR design which should have 13.0 dbd gain, first side lobe 26 db down and all other lobes better than 31 db down, all on a 19.4 metre boom with 50 ohm feed.
Stay posted.
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VK2FAK
Re: Beam stacking
Hi all....
That sounds like one big heavy antenna.....as your interested in side lobes and such. what would be an acceptable side lobe level ....I understand it would generally depend a lot of the purpose of the beam...but in general terms...
If I put the 70cm beams close...I lose about 1dB gain....but reduce side lobes down to around -20dB..
That sounds like one big heavy antenna.....as your interested in side lobes and such. what would be an acceptable side lobe level ....I understand it would generally depend a lot of the purpose of the beam...but in general terms...
If I put the 70cm beams close...I lose about 1dB gain....but reduce side lobes down to around -20dB..
Re: Beam stacking
There used to be a general consensus that the optimum trade off between gain and sidelobes happened at -13 db sidelobes. This happens when the stacking gain is around 2.8 db. If you go wider for more gain, the side lobes come up. If you go closer, the first side lobes go down, gain goes down and second side lobes start to become significant.
So, if you live in a noisy location, you are probably better to err on the side reduced side lobes at the expense of a slight reduction in gain. Adding one or two more elements will always recover the lost gain, but you then have to adjust the position of the last director to minimize yhe back to front ratio and also adjust the position of the first director to get the feed impedance right.
David
VK3AUU
So, if you live in a noisy location, you are probably better to err on the side reduced side lobes at the expense of a slight reduction in gain. Adding one or two more elements will always recover the lost gain, but you then have to adjust the position of the last director to minimize yhe back to front ratio and also adjust the position of the first director to get the feed impedance right.
David
VK3AUU