[NLRS] Need advice on yagi stacking

Sat Jun 20 20:49:58 EDT 2015

Christmas tree stacking is based on wind loads antenna bending the mast 
(and rotor). The wind load is proportional to the wind area of the boom 
and elements. Its nearly always the 6m beam that has the greatest wind 
area, so in Christmas tree stacking its given the shortest lever arm.

Every vhf antenna works better the higher it is wavelengths above the 
ground plane. Otherwise the lowest elevation lobe is well above the 
horizon, the effect is greatest on 6m (or HF). This argues for turning 
the Christmas tree upside down most of the time. Actually the best 
elevation for 6m depends on the mode. For tropo and F2, the higher the 
antenna the better, for sporadic E often a lower antenna with a higher 
elevation lobe works better. W0WOI, long ago the VHF editor for QST has 
a couple 6m yagis at different heights and switching so he can use 
either one or both and has repeatedly found the lower yagi gives 
noticeably better signals on sporadic E. He quit mentioning that on the 
air because so many thought he was drunk. But I agree that his 
observation should be the case because the refraction point on sporadic 
E can be at a high elevation compared to the elevation angle for F2 or 
tropo distributed refraction where the lowest elevation angle gets the 
signal propagated further.

Occasionally a magazine VHF column editor has opined that we'd all be 
better off with inverted Christmas tree stacking on VHF, so long as the 
wind didn't bend the mast.

Sporadic E does happen occasionally on 2m and 222 but not nearly as 
often as on 6m. So higher elevation radiation angle is less critical on 
those bands.

HF DX contest stations like N0NI stack four yagis and work DX hours 
before the signal yagi stations because of the lower elevation angle 
(and antenna gain).

I have used a cross boom for higher bands, I made it U shaped so there 
wasn't a boom in the plane of the yagi elements. I used thin wall 
conduit bent in a couple right angles. Many have used an H frame to hold 
4 unrelated yagis.

As for spacing, the classic theory requires half a boom length spacing. 
Same as for optimum gain when stacking for the same band. A few years 
ago I spent months modeling stacked harmonically related yagies (mostly 
2m and 70cm). What I found was that the low band antenna was not 
affected at any spacing, but the high band antenna gain and elevation 
pattern was modified at spacings under 1 wave of the high band. At a 
half wave spacing the penalty was about 1 dB gain whether the high band 
antenna had 10 or 18 dBd gain. The gain was lost by increasing the up 
and down lobes.

There are many details in my paper from 2011 CSVHF on line at:
www.geraldj.networkiowa.com/papers/CSVHF2011/HowCloseB.pdf

The last two graphs show the conclusion most clearly that the gain loss 
vs boom spacing in boom lengths has little correlation but the gain loss 
vs boom spacing in high band wavelengths bunches fairly tightly. I also 
set up an antenna range and to the best of my experimental (not 
something I'd done before) precision, there is good correlation between 
the computed gain loss and the measured gain loss.

I didn't do any calculations or measurements of horizontally polarized 
yagis side by side, because the coupling off the ends of the elements I 
expect to be very small because there's a null in the radiation pattern 
off the end of a dipole. So I think a quarter or half wave spacing from 
element ends to element ends will be plenty to keep the antennas from 
interaction costing gain.

One thing to consider is that the signal coupled across bands may be 
enough to fry receiver front ends even if it doesn't affect the radiated 
signal significantly, so its critical to put all radios into transmit 
mode or be disconnected from the antenna by switching the TR relay when 
any band is transmitting.

One more reference. 1960s vintage ARRL VHF handbook showed a chart of 
height gain where an over the horizon signal was always better the 
higher the antenna.

My plans generated by that research are for a pair of FO-9 on 2m stacked 
about 12 feet apart, with a 5 or 6 element 6m yagi halfway between them 
and with three FO-9 for 432 in that space between the 2m yagis. For 902 
and 1296 I'm thinking of a 2.4 network skeleton dish with dual band feed 
mixed close to one of the 2m yagis. The 432 yagis will be slightly 
closer spaced than optimum for gain. The 2m beams will be spaced further 
than optimum to make room for the other antennas between them. Compared 
to a single long yagi for each of those bands the FO-9 has about twice 
the azimuth beamwidth making for easier aiming. These aren't as clean as 
those optimized for the highest G/T but I think some side and back lobes 
help find signals out here near the Boondocks were signals are not 
plentiful.

73, Jerry, K0CQ

On 6/20/2015 3:32 PM, David Palm wrote:
>
>
> Friends,
>
> The Palm family got on the air as much as we could in this last contest and
> we enjoyed handing out some points.  Things are little by little freeing up
> here, so I'm starting work on revamping the antenna stack, in anticipation
> of the September ARRL contest.
>
> The antennas I currently have are a Cushcraft A50-5S 5-element yagi on 50
> MHz, a Cushcraft A148-10S 10-element yagi on 144 MHz (not optimal, I know,
> but it's what I have), a home-brewed N6NB 8-element quagi on 222 MHz, I'm
> about to buy a 22-element 432 yagi on a 14 foot boom.I also want 900 and
> 1296 MHz in the same stack.
>
> My question to the group is how best to stack these?  Can the 900 and 1296
> MHz antennas go under the larger antennas, since they need less clearance
> without "seeing" the other antennas, or should I just go ahead and do the
> classic "Christmas tree", going up from 50, 144, 222, 432, 900, and 1296?
>
> Thanks and 73,
>
> David  W9HQ
> ______________________________________________________________
>