[amsat-bb] Cross yagi question

[Hans BX2ABT]

Thanks Greg, you phrased it much better than I did.

I did recall that picture of two separate antennas creating a circular 
pattern, but my thought was that by combining them on one boom radiation 
patterns of  both antennas would influence each other. For through-boom 
antennas the elements would line up, but for top/side mounted elements 
there would be an extra discrepancy, possibly adding to a distortion of 
the radiation pattern. You are right that the offset would be small, 
especially on 2 meters, but on 70 cm this quickly adds up (I use a 1 1/4 
inch boom, so offset would be 5/8").

On the other hand with offset elements the current null would fall 
outside of the boom, so part of the elements that have current flowing 
in them are mounted over the boom. This could negatively influence the SWR.

Thanks again for the nice comment. Back to the workshop.

Hans

BX2ABT


On 01/16/2019 01:20 AM, Greg D wrote:
> I think Hans is observing that when the elements are mounted in an
> insulated saddle on the sides of the boom, if you sight down the antenna
> from the end, the horizontal and vertical sets of elements don't cross
> at their centers.  Rather, they cross about half the diameter of the
> boom, plus half the diameter of the elements, away from the element centers.
>
> I think the answer is that this is just fine.  Recall that a circular
> pattern can be created by having two completely separate antennas
> stacked side by side, 90 degrees in relative polarization, and fed with
> a phasing line.  As long as the antennas are within some reasonable
> distance apart, the resulting pattern will be circular.  I expect that
> being offset by such a small distance as the boom diameter will be of no
> consequence.
>
> Hans, you have a good eye for details!  Good question.  Keep it up.
>
> Greg  KO6TH
>
>
> Burns Fisher wrote:
>> I'm not an antenna designer, but I can tell you that the M2 LEO Pack
>> crossed yagi elements go through the center of the boom (i.e. across the
>> radius) and are insulated from it.  They don't contact each other because
>> they are offset from one another along the length of the boom. Of course
>> the phasing lines have to be adjusted to deal with the elements being
>> offset.
>>
>> Is that what you mean?
>>
>> 73,
>>
>> Burns WB1FJ
>>
>> On Tue, Jan 15, 2019 at 8:16 AM Hans BX2ABT <hans.bx2abt at msa.hinet.net>
>> wrote:
>>
>>> I'm sorry, but I wasn't complete in my description: I'm talking about
>>> yagis with the elements completely isolated from the boom with saddles
>>> (clamps) mounted on the boom to hold the elements in place. Much like
>>> the original LFA yagis are build. For an image go to my blog at
>>> bx2abt.com. --Hans
>>>
>>>
>>> On 01/15/2019 03:12 PM, Hans BX2ABT wrote:
>>>> Should the elements of a RHCP cross yagi be centered at the boom, or
>>>> should they be off center so that the middle of the H and V elements
>>>> cross each other in the middle? My gut feeling says off center, but
>>>> I'm not sure. I wonder how this influences the SWR. Cheers,
>>>>
>>>> Hans
>>>>
>>>> BX2ABT
>>>>
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>
>