Through-the boom element correction---why positive?
John Occolowitz
through insulated and non-insulated booms vs. boom diameter measured
in fractions of a wavelength. For both cases the element must be
longer than its free-space length. For the insulated case the
correction is less than for the non-insulated case.
Would somebody please explain the physical principle involved in
deriving the graphs?
John.
pmarkham
<383da805...@news.indy.net>:
This extract from Brian's AO docs may provide a clue why you are not bowled
over with responses. Ian white lurks around a number of radio
related newsgroups.
Pete/wa4hei
" Copyright 1995 by Brian Beezley, K6STI
All Rights Reserved
Element-Mounting Effects
The method used to mount elements to a boom can affect
antenna performance, particularly for parasitic arrays at VHF/
UHF. Conductive mounting plates or through-the-boom mounting
increase element effective diameter and raise resonant
frequency.
To model conductive mounting plates, use the YO Yagi
Optimizer program or refer to "Physical Design of Yagi Antennas"
by Dr. David B. Leeson, W6QHS, published by the ARRL.
To account for through-the-boom mounting of Yagi
elements, use the formula below to shorten measured element
lengths before modeling. Conversely, when constructing a Yagi,
use the formula to increase calculated element lengths.
For noninsulated through-the-boom mounting, measurements
by Guenter Hoch, DL6WU, have been curve-fitted by Ian White,
G3SEK, to yield the following boom-correction formula for Yagis:
C = 25.195B - 229B^2
C is the correction factor expressed as a fraction of
boom diameter B in wavelengths. B^2 means B-squared. For
example, a .01-wavelength diameter boom requires an element-
length correction of 23% of the boom diameter. The experimental
data underlying this formula were derived from booms with
diameters less than .055 wavelength. G3SEK indicates that
correction factors for insulated, through-the-boom mounting are
close to 50% of C."
Ian White, G3SEK
>joh...@indy.net (John Occolowitz) wrote in
><383da805...@news.indy.net>:
>
>>The ARRL Handbook graphs the correction for yagi elements passing
>>through insulated and non-insulated booms vs. boom diameter measured
>>in fractions of a wavelength. For both cases the element must be
>>longer than its free-space length. For the insulated case the
>>correction is less than for the non-insulated case.
>>Would somebody please explain the physical principle involved in
>>deriving the graphs?
>>
>>John.
>>
>>
>
>This extract from Brian's AO docs may provide a clue why you are not bowled
>over with responses. Ian white lurks around a number of radio
>related newsgroups.
>
<de-cloaking>
Sorry, John's original message hasn't arrived here yet (or maybe fell
foul of one of my newsgroup filters).
The basic principle is that the boom, mounting plate etc. partially
"short-circuits" the centre portion of the boom, and reduces its
effective electrical length. To restore it to the intended electrical
length, you have to make the element physically longer.
The corrections are determined by a combination of theory and
measurement, and are often given as a fraction of the boom diameter, or
length of the mounting plate. However, it's really much more complex
than that. Coming up in QEX for Jan/Feb 2000 is an article by Guy
Fletcher, VK2KU, which pushes the whole subject up to a new level.
The one good thing is that it's only a correction, so any errors in the
correction itself are of much less importance than leaving it out
completely.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
John Occolowitz
<G3...@ifwtech.demon.co.uk> wrotz:
It seems that the insulated and non-insulated cases are quite
different. For the non-insulated case I can see that an effective
increase in element diameter as it passes through the boom would raise
its resonant frequency. For the insulted case the boom represents a
distributed capacitance across the center part of the element and
might lower the resonant frequency, however inductive coupling may
have the opposite effect. Anyway, when I recently built my 432 MHz
yagi I used the published curves for element corrections. Guess I'll
have to wait for VK2KU's article to sort it all out.
Thanks. John KB9MIE (hi Peter)
Ian White, G3SEK
>
>It seems that the insulated and non-insulated cases are quite
>different. For the non-insulated case I can see that an effective
>increase in element diameter as it passes through the boom would raise
>its resonant frequency.
The increase in diameter reduces the inductance per unit length of the
center section, which is what raises the resonant frequency of the whole
element, and makes it need to be lengthened.
>For the insulted case the boom represents a
>distributed capacitance across the center part of the element and
>might lower the resonant frequency,
Not much, because the center of the element is insensitive to capacitive
effects.
>however inductive coupling may
>have the opposite effect.
That's it - the center of the insulated element is inductively coupled
to a loop (the outside surface of the boom, and sometimes the inside
surface too) which again reduces the effective inductance per unit
length.
>Anyway, when I recently built my 432 MHz
>yagi I used the published curves for element corrections. Guess I'll
>have to wait for VK2KU's article to sort it all out.
>
Actually that article does not deal with insulated elements, but it
casts new might on the general problem.
Reg Edwards
What difference in performance, if any, would be detected if element
length corrections were ignored ?
--
=================================
Cheers, Reg, G4FGQ
For free technical radio software go to:-
http://www.btinternet.com/~g4fgq.regp
=================================
Ian White, G3SEK <G3...@ifwtech.demon.co.uk> wrote in message
news:6E3hUEAl...@ifwtech.demon.co.uk...
Richard Harrison
"What difference in performance, if any, would be detected if element
Not much if you consistently applied the correction factor to all
elements. Otherwise, performance could be noticeably degraded by
upsetting the phase relationships between the elements.
Best regards, Richard Harrison, KB5WZI
Reg Edwards
Reg.
===============================
Richard wrote ...
Ian White, G3SEK
>What sort of percentage of element lengths are these corrections ?
>
The corrections are best expressed as a percentage of boom diameter in
wavelengths, so the effect on the element length depends how fat the
boom is.
For a 14MHz yagi with a 2in boom diameter, the boom itself is probably
too slim to have any significant effect. However, HF yagis tend to use
element mounting plates which are considerably longer than the boom
diameter, and these also have electrical shortening effects. Brian
Beezley's manual (quoted in a previous message) gives guidance, and his
YO software does some of the calculations, but the main source of
reference on these corrections is Lawson, W2PV's book on 'Yagi Design'
(ARRL).
At the other extreme, a 1.3GHz long yagi with a very slim 10mm boom
would have a boom diameter of 0.04wl, and elements running through the
boom would require a correction of about 55% of the boom diameter, i.e.
+5.5mm (or about half of that if the elements run through the boom but
are insulated). Since the element length is about 100mm, we're talking
about corrections of up to 5%.
>What difference in performance, if any, would be detected if element
>length corrections were ignored ?
A dramatic difference in front/back/side ratios, and often a significant
difference in gain.
HF yagis tend to be inherently narrowband, so the optimum pattern may be
shifted from the CW section into the phone section, or even right out of
the band. Gain tends to be less severely affected, but in a bad case you
could lose 1-2dB. Usually, the more serious problem for HF yagis is
failure to duplicate the tubing lengths and diameters in telescoping
elements, which again shifts the resonance.
VHF/UHF long yagis vary considerably. Older designs can be very
narrowband, and failure to duplicate published construction methods can
mean they "crash" completely - the pattern can even reverse! Inherent
narrow-band performance and failure to understand the need for boom
corrections gave early long yagis a bad reputation.
More modern long yagi designs have a lot more tolerance to errors in
element lengths, but you still need to make boom corrections in order to
obtain optimum performance at the frequency you want. For example, the
1.3GHz example above, an 5% error will shift the frequency of optimum
performance way out of the band.
In order to change from one element mounting method to another, you need
to remove the old boom corrections and then make the new ones. Several
years ago I wrote a program called ELE.BAS to do this. It's downloadable
from www.marsport.demon.co.uk
Reg Edwards
at VHF. But I often think the delicate adjustments/corrections
recommended for a variety of reasons are of a smaller order than the
effects of shifting frequency from mid-band to one of the edges.
Would my impressions be true, for example, of the 2m amareur band ?
Would a 3 percent mis-adjustment of element length have such a
'dramatic' effect on back-to-front ratio as shifting frequency from
one end of the band to the other ? And how would one ever know ?
Agreed, if one is aware before erection of a particular small
correction it is just as easy to apply it as not.
Reg.
================================
Ian White wrote ..
Roy Lewallen
> . . .
> For a 14MHz yagi with a 2in boom diameter, the boom itself is probably
> too slim to have any significant effect. However, HF yagis tend to use
> element mounting plates which are considerably longer than the boom
> diameter, and these also have electrical shortening effects. Brian
> Beezley's manual (quoted in a previous message) gives guidance, and his
> YO software does some of the calculations, but the main source of
> reference on these corrections is Lawson, W2PV's book on 'Yagi Design'
> (ARRL).
I believe Lawson's book is out of print. There's a lot of good specific
information about several common clamp types in _Physical Design of Yagi
Antennas_ by Dave Leeson, W6QHS, available from the ARRL.
Roy Lewallen, W7EL
Ian White, G3SEK
>Thanks for your comments Ian. I have no practical amateur experiemce
>at VHF. But I often think the delicate adjustments/corrections
>recommended for a variety of reasons are of a smaller order than the
>effects of shifting frequency from mid-band to one of the edges.
>
>Would my impressions be true, for example, of the 2m amareur band ?
>Would a 3 percent mis-adjustment of element length have such a
>'dramatic' effect on back-to-front ratio as shifting frequency from
>one end of the band to the other ?
Yes - B/F ratio and the depth of valuable QRM-reducing nulls in the
pattern are very critically dependent on frequency. The effect on
forward gain is less serious. Try running a few examples through EZNEC.
Fortunately, it isn't necessary to cover the VHF/UHF bands from end to
end with a single yagi, because of the way the bands are divided up
between various operating modes and polarizations. This means that yagis
tend to be used in segments of a few hundred kHz at most.
>And how would one ever know ?
>
For strong-signal operation, you'd never notice, of course. However, if
you're at all interested in DX-chasing, a poor yagi will let you know.
You find you're struggling with marginal contacts that your neighbours
on the air can make successfully; and you're suffering QRM that seems to
trouble your neighbours less. It's only a few dB in gain, and maybe 10dB
in sidelobe levels, but gradually the message gets through that
something isn't quite right.
>Agreed, if one is aware before erection of a particular small
>correction it is just as easy to apply it as not.
Just so!