Back off! The one lobe per half wave in height is accurate unless
one is on the side of a cliff or very substantial downslope and
that was not part of the question as presented. I also indicated
that terrain irregularities were likely to be more significant than
the few feet of difference.
*HOWEVER* there is no need to spend hundreds or even dozens of hours
to answer the question as it was presented. While I have spent many
weeks modelling antenna systems over many locations, for most amateurs
there is little to be gained when the question is simply whether there
will be a significant difference in a few feet of height. Most amateurs
are limited to a very small area for a tower. Those who are installing
big stacks or are located on challenging terrain (as I was 25 years ago
in West Virginia) are well served to model the terrain but there is no
way a four foot difference on generally flat ground justifies the time,
effort, and potential cost certainly - particularly for someone with
no prior experience or software.
73,
... Joe, W4TV
On 2019-01-17 6:03 PM, Bob Shohet, KQ2M wrote:
That’s a REALLY presumptuous response on your part Joe. I have spent MONTHS of ACTUAL
TIME time modeling antennas of all types over all types of terrain at multiple qth’s
over the past 28 years with at least five different modeling programs and I guaranty that I
have run far more antenna/terrain models of antennas than you have and far more than most of
the people on this reflector.
I don’t live on flat ground and most of the people that I know don’t either; but even
if they did have a flat qth, their patterns would still be affected by the topography around them as
well as soil conductivity, which is why you should always model your ACTUAL antennas over ACTUAL
terrain or else your model won’t be reasonably accurate and not likely to be particularly
useful.
It’s not about how many lobes you have, it is also about how broad or narrow
the lobes are and at what wave angles they are located and how useful those wave
angles are relative to the wave angles that you receive from the high volume DX areas
at your qth. I suggest that you spend some meaningful time doing modeling over ACTUAL
real-world terrain to see what I am talking about rather than relying on generalized
concepts that may be misleading and highly inaccurate at a specific qth.
Bob KQ2M
From: Joe Subich, W4TV
Sent: Thursday, January 17, 2019 5:43 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Half Wave?
On 2019-01-17 5:24 PM, Bob Shohet, KQ2M wrote:
Simply put, antennas at a given height have MULTIPLE lobes at
different wave angles of varying gain over flat ground over average
soil.
Not quite accurate ... dig out an ARRL Antenna Book or modeling program
and you will find that a horizontal antenna does not develop a second
lobe until it is more than 1/2 wave above flat ground, a third lobe
until it is more than 1 wave above flat ground, a fourth lobe until it
is more than 1.5 wave above flat ground, etc.
Since the first lobe is rather broad and the original question was about
the difference between 63 an 67 feet for a 40 meter yagi, the answer is
"not enough to worry about." The difference between 63 and 67 feet will
be more than swamped by any irregularities in the terrain. Now, if the
question had been about the difference between 40 and 65 feet, the
answer would have been significantly different.
73,
... Joe, W4TV
On 2019-01-17 5:24 PM, Bob Shohet, KQ2M wrote:
Simply put, antennas at a given height have MULTIPLE lobes at different wave angles of
varying gain over flat ground over average soil. If you have anything other than flat
ground and especially if you have complex terrain or are on the side of a hill or
adjacent to water, your antenna at 1/4 or 3/8 wave height above ground might have
useful high angle lobes and some unexpected low angle lobes despite the low height.
This is why it is important to model your antenna over actual terrain – and the
ground conductivity with fertile ground vs. sandy soil or ledge also makes a
difference and can be quite helpful.
I live in Western CT and on 160 I use a wire Inverted L with a top height of 85’ with the rest of the wire horizontal and 3 elevated 1/4 wave radials no greater
than 10’ above ground. It is a proverbial “cloud warmer” and yet I have worked 200+ countries and 33 zones with it along with a handful of
JA’s on 160. And I haven’t tried very hard. And the antenna is on ledge and deep forest “soil” on the top of a hill with awful ground
conductivity. The hill helps but it is primarily a high angle antenna – no matter – it works fairly well most of the time. It would work better if it was
all vertical but that is not possible at my qth. If I listened to the “doom and gloom” of some opinions about height above ground I wouldn’t have
bothered to put it up or even tune the band.
Bottom line:
MODEL several proposed antennas over your specific terrain at your qth. You
may discover advantages at a given height that are counter-intuitive, and then
put it up and get on the air. Often your antenna will be far more useful than
you could imagine despite what others think.
BTW, most antennas are louder over some path at different times depending on the height (or lack of
it). My 3 L wire beam @ 50’ on 40 is VERY loud to EU for about 2 hours early in the opening;
as loud as a yagi would be at the same time at a high height. The difference is that after those two
hours, the Yagi would be 10 db – 2 s-units louder from the combination of antenna gain PLUS
being at a higher and more optimum height for the given arrival wave angles from EU. The wire beam
has its usefulness even though a quality Yagi at a high height would be better. What would be best
is to have both – each for its advantages at different times.
Actual results trump theory. If it works then use it. If it works well then
enjoy it regardless of what anyone else thinks about it!
63’ vs. 67’ on 40 is not even worth considering if it looks close enough after
modeling it over your terrain. Go with whatever is easiest for you.
73
Bob KQ2M
From: Peter Voelpel
Sent: Thursday, January 17, 2019 3:37 PM
To: 'Tower Talk'
Subject: Re: [TowerTalk] Half Wave?
A 1/4 wave length height IS exactly a cloud warmer.
At least 3/8 wave length height is necessary for a lower take off angle.
73
Peter
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Keith
Dutson
Sent: Donnerstag, 17. Januar 2019 21:27
To: Tower Talk
Subject: Re: [TowerTalk] Half Wave?
My experience is that 1/4 wavelength is okay to avoid a cloud warmer.
However, higher is always better for DX, aka lower take-off angle.
73, Keith NM5G
-----Original Message-----
From: TowerTalk <towertalk-bounces@contesting.com> On Behalf Of Warren Wolff
via TowerTalk
Sent: Thursday, January 17, 2019 11:47 AM
To: towertalk@contesting.com
Subject: [TowerTalk] Half Wave?
Greetings: My understanding is that a yagi should be at least 1/2wave
above ground to avoid warming the clouds. So, considering that thought,
how much does an antennasuffer if such antenna is 63 feet up instead of 67
feetfor 40 meters over VERY rocky "soil"? Thanks, Warren; W7WY
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