[TowerTalk] Re: Yagi's
art unwin
aunwin@fgi.net
Mon, 05 Feb 2001 12:31:18 -0600
At 10:27 PM 2/4/01 -0500, you wrote:
>Hi Art,
>
>You made the claim yagis can't have more than 20-25 dB F/B or
>F/R, and that is not factual. My 40 meter antenna is an example,
>and I'm sure there are countless others.
I did ?
I used the word " can't" ?
I wish you had specifically mentioned that sooner.
Ofcourse, that is completely incorrect as a single statement on it,s own
I am bemused that no one took me to task other than you
>As a matter of fact with two driven elements in a phased array, I
>can get a null that for all practical purposes goes to zero.
True Tom but for amateur antennas you would be foolish to purchase
an antenna based on one statistic, let's keep apples to apples
and keep to the theme of the thread or it becomes just a bunch
random words or statistics.
>stations do that all the time, to protect other stations.
Oh I,m sure you are correct.I've heard of zero reception when
in 'spitting' distance of the antenna. But that is with verticals!
I don,t think you can extrapolate those experiences and relate
them to Yagi's. But since you have brought it forward I am
interested in hearing more about that particular yagi,
it sure sounds unique and raises interesting questions
>If you doubt my antenna or other three element yagis can have that
>null depth, I can e-mail you an Eznec file showing they do.
Oh theres no doubt that any antenna can get a spot rejection
of high levels but if I remember correctly I stated a particular
rejectionb in association with it's accompanying high
REAR REJECTION figures.
Surely you recocognise the difference. You focussed on the
spot rejection null and choose to ignore the other salient information
when you brought forward your massive 3 element beam and moderate
rejection point figures.
This choice of manipulating supplied statistics
and ignoring others distorts the debate, and that was why another poster
made mention of it. Ofcourse every body knows that the the antenna
you brought forward has very poor REAR rejection rates across the band,
even more so when you look at the spot frequency you stated.
ie F/B plus corresponding F/Rear (they should go together)
>I made no extraordinary gain, null depth, or bandwidth claims. My
>antennas work like most other antennas in the world.
Regarding the example that you provided I agree that you did not imply
extraordinary features but to focus on one statistic to make a point
while ignoring others is good for gottchas but has no place in a
serious discussion
>
> > I would like a yagi to have 'good'flat front to rear across the band.
>> The same goes for flat gain and a flat realistic drive impedance The
>> antenna I posed with 3 reflectors meets that target or goes along way
>> towards that end. This antenna came up as a response to a post
>> regarding extra reflecters and their physical length and we are
>> certainly along way from that theme.
>
>As I understand this, you are saying you have an antenna (with
>three reflectors) that meets the goal of being very broadbanded by
>virtue of having three reflectors.
Yes, that is a correct interpretation except I would not go the
extra step by adding " by virtue".
>It appears you are saying it has a deep wide rear null (more than
>50 dB?) at useful angles, a flat and modest driving impedance
>across "the band" (whatever band that is), and as I recall it has
>comparable or more gain than other antennas the same physical
>size (I believe you mentioned that in a prior post).
Yes. In it's basic form that is what I stated tho it came up initially
as a response to a poster who focussed on multi reflectors of long physical
length My response being that my antenna has three reflectors each
with a physical length less than the driven element, but yes
I stand by the statement .What are you leading up to?.
>Many people would love to have an antenna like that!
>
>I can't deliver one, it is far beyond my technical skills and meager
>understanding of antennas.
I don't believe that for one moment Tom!
You may take issue as to the value of the priorities taken
for such a design which would be a debate of it's own.
You have already raised the value of emphasis placed on
any rejection figures because of the occurance of "back scatter"
Some would prefer to compromise in other areas.(no free lunch)
>> Does your 3 element yagi meet all YOUR goals with respect to
>> performance across the band. Are there some areas YOU 'wish' you could
>> improve upon, putting aside heavy emphasis on known resons why it
>> CANNOT be done ?
>
>It does everything I expect it to do, which is nearly all it can do
>according to theory.
Well no one is advocating new break thu's in theory, just a reexamination
on how we approach things and a reevaluation of prior sacred cows.
A past post talked of selsyns and prop pitch rotators a 50 year old
association
made before cheap real time cameras were available.
The first part of this thread suggested that reflectors were always longer
than the driven element and this is not ALWAYS so.
Let me get back to the antennas where you imply that the extrordinary
skills required
are beyond you. I think you are being modest here.
In this thread I suggested an antenna that could be termed a combination
or interlacing of three antennas. One would be designed for the lower portion
of the band, another for the center and another for the upper portion .
When these are interlaced for a single antenna with a single driver on a
single
boom you have a high density element array with three reflectors.
Not really that hard to do, go with the flow Tom and hold off with the"
butts."
Since you mentioned computor programs it suggests you somewhat embrace
the idea and accuracy of simulations.(when done correctly ofcourse)
The following computor stats (just the stats maam, just the stats)
shows what one should expect from such an aproach
Yagi, < 1 WL boom,13 elements 3 of which are reflectors at 65 feet.
FREQ GAIN F/B F/B are generally also the 'Worst Case'
28.0 15.56 25.6 lobe rejection figures except for
28.1 15.67 31.69 portions of the band where F/B really
28.2 15.75 37.31 escalates
28.3 15.82 42.54 This antenna provides a natural 50 ohm
28.4 15.87 47.07 impedance and allows good SWR across
28.5 15.92 48.18 the band.
28.6 15.93 35.28 Feed line is placed in a metal tube
28.7 16.0 44.34 parallel to the boom to prevent near field
28.8 16.03 39.46 distortions
28.9 16.07 35.81
If you compare the above with the Hygain antenna improvement/modification
article in the Handbook 17 th edition for a 1 WL boom I think you will agree
that my statements are basically correct (no nitpicking !)
The antenna pattern is clean with 54 degrees half power with
with no side lobes at the front.
So that's where I am coming from Tom, It is a bit long but should put the
whole thread in context plus figures(meat) to substantiate what I am saying.
Best Regards
Art Unwin KB9MZ
>73, Tom W8JI
>w8ji@contesting.com
>
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