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[TowerTalk] Antenna measurements

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Subject: [TowerTalk] Antenna measurements
From: (Larry L Moore)
Date: Sat, 19 Jun 1999 08:59:08 -0700
I appreciate the numerious contributions that have been posted on this
subject. I am learning things and hope they lead to better
testing/understanding. I have several questions and comments.
1 Guy - are you sure the ground path does not skew or compromise the
measurements in some manner antenna to antenna? We "know" that the shape
of the gain lobes of simple dipoles and mulit-element/long boom yagi's
differ. I think measurements in free space [ ie at higer freq's ]
establish this - and computer programs predict this. Is it not almost
certain that the additional materials of tri-banders existing in the
beam forming area [ ie traps, other band AL , linear loading etc ]
distort the patterns even futher [ and in unknown/ unpredictical ways
]??  What specifically in the root assumptions explain why ground would
have no influence when the patterns must vary widely antenna to
2 Guy -I would like to understand why in elimininating reflection/far
field effects, and seeing no change in antenna impedance "proves"
comparisons are valid. Isn't the the antenna impedance determined  by
design factors and by coupling to materials in the near field almost
exclusively ??  The impedance of dipoles, as they are raised above
ground, increases to and above [ and then below ] 75 ohms and then
cycles to this value as it approaches free space. I think Yagi's do the
same but only to different values.
Can you help me understand the thought/ theory behind your "proof"
comment ??
3 I accept the primis that perfect ground and surfaces do not exist in
the real world and that the null's lose their sharp "attenuation". If
the tested antenna's have different patterns [ due to design and
materials hung in its near field ] doesn't this fact introduce, rather
than eliminate, error unit to unit?
4 Mosley, Hi-Gain, Cushcraft and others are guilty of using coils/traps
shunted by metal pipe - they and others are also guilty of using
lossey/distorting linear loading.
These trade-offs certainly comprise performance and all are guilty of
mis-representing their impact - with out exception.
5 But questions still exist as to why some trapped tri-banders look
"good" and others look "bad" if  the cause is exclusively traps? [ check
the data on TH-11, TH-6/7, KT-34, X-9, PRO-57's and others.]  Will the
increased gain of long booms off-set the gain lose of traps so as to
provide a net improvement [ ie C-3 vs X-9 ]?
6 I agree that Mosley's response is unproffesional [ I'm ok you are not
ok]. Unfortunately all of the other designers tend to take the same
approach in presenting their products. I agree SHAME SHAME on all of
them - but thats a different discussion.
7 If a measuring method such as K7LXC used can be defended and validated
the suppliers comments will be moot. I want to believe the data but to
much information indicates one can't.

Guy - can you help us futher on these points???  I for one really
appreciate your contributions.  LARRY

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