[TowerTalk] JK Mid Tri opinions

[jimlux]

On 12/23/19 12:24 PM, Ignacy Misztal wrote:
>   I am getting JK Mid Tri 40 next month so this discussion is interesting.
> 
> Regarding F/B, one question is how to measure it across the band to see
> whether reality reflect simulation.
> I erected Spiderbeam at 45 ft as a temporary beam.and used a wideband noise
> generator connected to a dipole some 200 ft away, with response measured on
> Flex 6600.
> See posting on Spiderbeam group at groups.io:
> 
> https://groups.io/g/SPIDERBEAM/topic/my_new_hd_spiderbeam_and/67470919?p=,,,20,0,0,0::recentpostdate%2Fsticky,,,20,2,0,67470919
> The Spiderbeam had very narrow F/B characteristics on 20m, with peak > 30db
> but just 10db  120 KHz away. This is despite being 3 el.  I think the
> numbers are a few db below simulations in fee space. However the gain seems
> flat.


This isn't surprising - a Yagi is a form of phased array, and just like 
a 4 square, misphasing doesn't change the forward gain very much, but it 
does affect the depth of the nulls/sidelobes/backlobes.

A signal that is -30dB relative to the front is 0.001 the amplitude. If 
the F/B changes to -20dB, now that back signal is 0.01.  That's only 1% 
of the forward power (0.04dB change in forward gain).

On a Yagi-Uda, the current magnitude and phase in the parasitic elements 
is set by mutual coupling from the driven element, which is quite 
sensitive to the relative lengths of the elements.  As a not very 
accurate, but simple, conceptual model, imagine the parasitic elements 
as all being the same length, but with a series L or C in the circuit 
which changes the phase.

The current in a parasitic element that is "resonant" will lag by 
approximately the distance between the elements (since that's when the 
radiated field from the driven element gets there). By adjusting the L 
and C loading on the two elements, you can adjust the relative phase, 
and, as a side effect the amplitude.

You can imagine that a small change in amplitude (1%) or phase (a few 
degrees) will ruin the "cancellation" in the backward direction, so 
instead of cancelling to -30dB you only get -20dB or -10dB.


This is why "low sidelobe antennas" are so hard to make.  Imagine an 
antenna where the requirement is that the sidelobes be down -60dB.  The 
proverbial gnat's eylash on the antenna will probably screw that up.


It's also why almost all 3 element Yagis have about the same forward 
gain - that's the easy parameter to optimize for.


> Of course 200ft is not far away.
> 
> Will try the same with my Mid Tri when it is installed.
> 
> Ignacy, NO9E
> _______________________________________________
> 
> 
> 
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>