[NLRS] [BC'ers] Need advice on yagi stacking

Mon Jun 29 19:00:25 EDT 2015

Friends,

Thanks so much for the antenna stacking advice, both to the whole group and
the private replies.  It's excellent news that the inverted tree is best,
since it puts the 902 and 1296 at the bottom where it's easily to tinker.
I'm probably going to have to start with the little "cheap yagis" from
WB5VJB --which I already have built and working well -- and then upgrade as
I can.

I'm going to try one more time to get this home station properly decked out
with at least 50 - 1296 MHz.  Maybe by September?  Wish me luck.

73,

David  W9HQ

On Sat, Jun 20, 2015 at 7:49 PM, Dr. Gerald N. Johnson <geraldj at netins.net>
wrote:

> 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
>> ______________________________________________________________
>>
>>
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