[TowerTalk] Re: 20M Yagi Interaction with nearby objects

[Jim Lux]

At 02:05 PM 10/20/2003 -0400, tongaloa wrote:
>corneliuspaul at gmx.net wrote:
>
>>VE6WZ  wrote:
>>
>>
>>
>>>Put both elements back pointing the same direction.
>>>Shorten all the elements on the 20 the same amount to bring it back to
>>>resonance.
>>>
>>>The fact that the 20m Yagi has been "loaded down" because of the 40m
>>>does not mean that there will be a problem with the pattern AFTER it is
>>>re-tuned.  There IS interaction....but only to the extent that the 20m
>>>elements now resonate lower in frequency.
>>>
>>
>>
>>
>>I recently have been asked a similar question by a friend who is trying 
>>to put up a 3ele 20m yagi 6m (18ft) above a sheet metal roof.
>>
>>The resonance (point of lowest SWR he measured with his SWR bridge) moves 
>>down to 13.8 MHz.
>>The interaction seems to be much less pronounced when he moves the 
>>antenna to only 2m (6ft) above the metal roof.
>>
>>I was hesitating a bit to give the advise of shortening first the driven
>>element,
>>and afterwards shortening also the parasitic elements by the same percentage.
>>
>>Do you really think this will work?
>>OK, I understand your point, the roof is "capacitively loading" all the
>>elements
>>and therefore all the elements have to be shortened accordingly. True?
>>I am still afraid that even with the shortened elements, the antenna pattern
>>will not be restored.
>>
>>I have tried to model the situation with NEC2 but had no succes in making
>>a workable model (too many segments)
>>
>>Any more comments?
>>Thank you


I don't know that capacitively loading is actually the entire story of 
what's going on with a antenna above a metal roof.  There's not only 
capacitance from elements to the ground plane (several pF/meter), but also 
the currents induced in the roof. The pattern will almost certainly change, 
but whether for good or bad is hard to tell.

You're also going to form an "image" antenna in the roof, much as an 
antenna over good conducting ground forms an image, which can change the 
pattern.  A lot depends on the size and conductivity of the roof.

It's no surprise that the interactions are very noticeable at 6m above the 
roof.  That's getting close to a quarter wavelength, and one would expect 
significant impedance variations. (think of the "reflected" wave from the 
roof coming back up to the antenna and being almost exactly out of phase 
and, so, causing a cancellation of part of the element current...)  Up 
close, though, the coupling is very strong, but, also, close to being in 
phase, so things like resistive loss might dominate, but field cancellation 
less so.

Modeling in NEC, you might get a feel for it by modeling the antenna over 
perfect ground, 2m above the ground.  You could even attempt to adjust the 
conductivity of the ground to try and match the roof, just to see what happens.

What you want to watch out for is that you don't want to use a reflection 
coefficient approximation, which is more suited to figuring out the far field.

You might also try specifying the ground radials on the GN card, although, 
the radials are at 0,0,0.  I don't recall off hand how to appropriately 
model the "cliff" problem.