[TowerTalk] 80 mts four square or yagi?

[Joe Subich, W4TV]

> HFTA's problems with close spacing begin *much* closer than that -
> IIRC, <10 ft.

 From the HFTA Help file:

>> The internal Yagi model in HFTA is a very simple mathematical
>> model. It does not compute interactions between individual Yagis
>> in a stack — HFTA assumes that each antenna is a “point source.”
>> For antennas stacked more than about a half wavelength apart this
>> is not a problem. For example, you should be cautious specifying
>> spacings less than about 20 feet on 20 meters (and proportionately
>> scaled on other bands) because of mutual-coupling effects between
>> real antennas. Spacings less about 20 feet on 20 meters will show a
>> false increased gain in HFTA, even though the real effects of
>> interaction between the beams will actually be to decrease the
>> gain.

On any band HFTA seems to increase the gain from stacking as the
antennas get closer but, as Dean says, that does not factor in the
interaction between antennas.  Dean's "rule of thumb" is 20 feet on
20 meters (0.28 wavelength) but inflated gain begins to appear at
any separation less than about 1/2 wavelength if one compares the
vertical pattern generated by HFTA over flat ground with similar
patterns generated by EZNEC using the high accuracy ground.

73,

    ... Joe, W4TV


On 2015-03-10 9:19 AM, Pete Smith N4ZR wrote:
> The *magic* of 1X/2X spacing is that it tends to cancel the second
> vertical lobe.  Depending on your takeoff angle needs, this might not be
> a good thing, but on the other hand if you can switch to use just the
> lower antenna you'll find that it emphasizes the same angles that the
> stack cancels. You might look at 50/100 ft spacing too.  HFTA's problems
> with close spacing begin *much* closer than that - IIRC, <10 ft.
>
> 73, Pete N4ZR
> Check out the Reverse Beacon Network at
> http://reversebeacon.net,
> blog at reversebeacon.blogspot.com.
> For spots, please go to your favorite
> ARC V6 or VE7CC DX cluster node.
>
> On 3/9/2015 9:11 PM, W0MU Mike Fatchett wrote:
>> What values did you get using HFTA.  HFTA has some issues with very
>> close spaced stacks.  The numbers are wrong.
>>
>> Mike W0MU
>>
>> On 3/9/2015 7:09 PM, Jorge Diez - CX6VM wrote:
>>> Thanks Jim
>>>
>>> Sorry for the delay, just returning from along trip after ARRLDX SSB
>>>
>>> Is curious HFTA. It stopped my project for a two stack for 20 mts yagís.
>>> Since many contesters told me to put it at 69/139 ft, HFTA told me
>>> that this
>>> is not the best distances.
>>>
>>> OK about your document, will read it, but here we don´t have such
>>> facilities
>>> than in USA to know ho´w the ground conductivity, the terrain data,
>>> etc, etc
>>>
>>> 73,
>>> Jorge
>>>
>>>
>>> -----Mensaje original-----
>>> De: TowerTalk [mailto:towertalk-bounces at contesting.com] En nombre de Jim
>>> Brown
>>> Enviado el: domingo, 08 de marzo de 2015 02:44 p.m.
>>> Para: towertalk at contesting.com
>>> Asunto: Re: [TowerTalk] 80 mts four square or yagi?
>>>
>>> On Sun,3/8/2015 10:06 AM, Jorge Diez CX6VM wrote:
>>>> Wish someone have some analysis or field test to compare a 4SQ with
>>>> sixty 1/4 WL radials each vertical and a 3 element wire yagi with boom
>>>> at 115ft
>>> This is not exactly the answer, but it should help you think about it.
>>> http://k9yc.com/VertOrHorizontal-Slides.pdf
>>>
>>> Also, if the land around you is not flat, you also need to study and use
>>> HFTA extensively. It comes on the CD with the ARRL Antenna Book. HFTA
>>> tells
>>> only about horizontally polarized antennas, NOT verticals, because they
>>> interact with the earth very differently. The slides tell a big part
>>> of that
>>> story.
>>>
>>> 73, Jim K9YC
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