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Re: [TowerTalk] JK Mid Tri opinions

Subject: Re: [TowerTalk] JK Mid Tri opinions
From: jimlux <>
Date: Mon, 23 Dec 2019 16:35:26 -0800
List-post: <>
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,,,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

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

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