Phasing 2 antennas on 2 towers

[VK4HJ]

At present I have 2 x 14m towers,a 4 section Hills ,divided in 2 spaced 20m apart.
One tower carries a 30 yr old 5 el 6m yagi,The other a 6m7jhv.
I would like to stack a pair of 6m7jkv's on one tower but this is mechanically out
of the question.
What opinions are out there regarding placing a 6m7jhv on each tower and running them inphase?

Tnx in advance-Chris

[ZL1RS]

Chris, 20 meters apart is too far to horizontally bay two 6m Yagis ... the result would be a very narrow main lobe and MANY side-lobes of similar intensity. In addition, you would have to progressively change the phase on the feed lines as the two Yagis were rotated to maintain an in-phase condition as one would lag the other as soon as they were no longer parallel to each other.

I would like to stack a pair of 6m7jkv's on one tower but this is mechanically out
of the question.

Why? You would achieve meaningful gain from a pair of 6M7JKVs with as little as 5.56m of vertical stacking distance. See http://www.bigskyspaces.com/w7gj/50%20MHz.txt for a table of optimum and suggested minimum baying and stacking distances for a large number of 6m Yagis.

I have two 6 element LFA Yagis stacked at 5.5m on scaffold tubing with no problem. http://www.qsl.net/zl1rs/6mstn.html They have been up "temporarily" for over a year! I've seen 10m Yagis of similar boom length stacked 10m apart vertically on a high tensile steel mast projecting from the top of a lattice tower with no extra guying on the rotating mast.

73, Bob ZL1RS

[VK3AUU]

I would go along with all that Bob has said. 5.5 metres vertical stacking distance should be quite adequate. My next antenna at a new QTH will be a pair of 9 element YU7EF antennas with probably 6 metres spacing.

The phasing system may be of interest. I will be using heavy 300 ohm ribbon in odd multiples of a quarter wavelength from the folded dipole driven element to the common feedpoint with the 4 to 1 balun connected to the 50 ohm coax. The first director of each yagi is shifted slightly to raise the folded dipole impedance to 225 ohm which if you do the sums, gives the correct match at the balun.

David

[ZL1RS]

... by the way, the gain with less than optimum vertical stacking may not seem much (2.3dB as indicated in the VE7BQH table) but there are other perceived advantages that come with vertical stacking. Several stack users report less QSB than a single Yagi at the same average height (probably due to the space diversity effect of the two Yagis?) This translates to better over-all performance than just the raw stacking gain increase.

73, Bob ZL1RS

[VK2XSO]

It looks like an excuse for playing with a space diversity system on 6m.

http://en.wikipedia.org/wiki/Antenna_diversity

[VK4TS]

Have a good look around the array solutions website regarding stacking - a wealth of info - worth reading the notes on stacking dis-similar antenna which will outline the sort of problems you will have with horizontal stacking - basically the phasing will get out and lobes will go awol .. to counteract that, we have investigated using two six way switches to switch in different phasing lines - we have been setting this up for 20M but the principles are still the same...

The advantages of vertical stacking is way beyond 3db as the angle of arrival of dx signals can vary from 0 to as high as 45 degrees - by having multiple arrays vertically you can capture more of this and more consistently - this is the main reason behind the QSB (fading) improved performance noticed on vertical stacked arrays -on HF and 6.....

Consistent long DX performance is always extremely low angle and this is why stations with high big yagis VK3ZAZ etc have been able to work such long haul DX - but the reality is almost any yagi 5 elements or more at a similar height will have similar performance - refer anyone one with a mountain top location or very good drop offs to DX locations eg VK4MA or VK4ZJB...effective height of antennae is best shown in the ARRL software HFTA although even a simple modelling with MMANA and increasing height will show this up...

http://www.arraysolutions.com/appnotes.htm

[VK4WTN]

Interesting information on 6m stacking.Just a quick enquiry Bob talking to VK6OX he has stacked his 2 x 6el yagis at 0.6
wavelength (about 12ft).This is a little easier stacking distance to survive the strong winds around here.Would there still be a reasonable benifit in stacking 2 x 6m yagis at 12ft separation
regards Wayne VK4WTN

[ZL1RS]

Hi Wayne,

I don't have the 6 ele LFA file in this computer to model the stack so close. Mutual coupling might be an issue, but seems Andy has had no problems.

From a practical results point of view ... since being here at E51EME I've been heard by 3 VK6s that I know of and worked one of them ... VK6OX

73, Bob ZL1RS/E51EME

[ZL3TY]

Stacking 6el yagis at 0.6 wavelengths works well, I have used both 5el and 6 el yagis at 0.625 wavelengths with good results. There are good articles in the UKSMG archives , UKSMG.org, "Zen and the art of stacking" and "Stacking for 50MHz"

When I chose 2x6el stacked I wanted a compact antenna to minimise wind forces on tower and rotator. I also wanted to retain the wider horizontal beamwidth of the smaller yagi while getting more gain and lower vertical angle.

Bob ZL3TY

[VK6OX]

Apologies Chris VK4HJ for this off-topic post...

Interesting information on 6m stacking.Just a quick enquiry Bob talking to VK6OX he has stacked his 2 x 6el yagis at 0.6
wavelength (about 12ft).This is a little easier stacking distance to survive the strong winds around here.Would there still be a reasonable benifit in stacking 2 x 6m yagis at 12ft separation
regards Wayne VK4WTN

Wayne,

If you want the modelling results of my stack, let me know.

Regards

[VK3AUU]

Andy, what are you running and I will have a go at modelling it for you. I have EZNEC v4.
With shorter yagis there seems to be a compromise between maximising gain, minimizing side lobes and maximizing survivability.

David

[VK4WTN]

hi Andy, I would love to have a look at the modelling results. 0.6 wavelength spacing is much more manageable than the larger spacing and your antenna system is working well. How much difference have you found from your previous antenna.( and what was your old antenna)
regards Wayne VK4WTN

[VK4HJ]

Many Thanks for all the comments.I have been given "much food for thought".
Bob, ZL1RS, At present the single 7JHV is mounted 3.5 m above the tower on a 50mm,3mm walled Al tube.
I'm located on the coastal windward side at 250 m ASL 25 km from the coast,with a coastal exposure(SE).That 31 Ft boom scribes
figure 8's,end view. Lifting it another 2m makes me shudder,cyclonic rain depressions havent hit yet.I have not yet been game to crank the tower another 6 m to its full height.
Your comment on the the distance between the sources struck a chord. The interference pattern will broaden for coherent(phased) sources.
So there goes the gain and therefore pattern.I had thought that switching in various lenths of coax could be used to steer and manipulate the pattern
if combined with beam orientation.It probably can: BUT not while working Africa on 6m.

Trent,VK4TI,Thanks for the link---Fantastic reads.They really helped clarify some ideas.

And thanks to all those who showed that reduced stacking distance is a practial solution
to a significant problem.The 7JHV is recommended at 7.62m.

Soon I will crank it up another 6m,see if it stays up for a month as is,then look for some really STRONG 50mm tube.

73,Chris.

[VK5PJ]

Chris,
you will get a better result with a single yagi high up than you could ever get with multiple yagi's at half that height, so in my humble opinion, put your efforts into securley mounting a single yagi and crank that tower up!

Soon I will crank it up another 6m,see if it stays up for a month as is,then look for some really STRONG 50mm tube.
73,Chris.

I have fallen into this trap at an earlier QTH and had several different long boom yagi's mounted at 25' for a long time and then when I finally cranked up the tower to 40' it was like I had been listening on an old mushy donut (lots of antenna modeling on my then new 80268 powered PC showed me it should be better).
Antenna radiation pattern sharpened and ground noise lowered, I was astounded by the change it gave me and have ever since then had the mantra higher is better.

Regards,