Your assertion that
“The one lobe per half wave in height is accurate unless one is on the side of
a cliff or very substantial downslope”
is far too simplistic and not accurate.
I have neither situation and yet the sharp varying elevation changes up and
down of 10’ – 30’ within 100’ of my tower of my towers, have large and
unexpected effects on the patterns in specific directions according the highly
accurate K6STI software that I have used for almost 30 years.
I have no interest in arguing with you – I am pointing this out so that Warren
and others will be aware that yes, it DOES matter, even for undulations,
ravines, ridgelines, etc, especially when within close proximity to your
towers, and when one does the actual modeling, they can see it for themselves.
The time spent to model the antenna at a given qth is to learn what your
pattern looks like at your qth – not to discern whether or not 63’ vs. 67’
matters. (It really doesn’t although you may see small differences – which may
be useful for other reasons). Personally, I like to LEARN things and how to
use available resources to learn more – and I know that there are a lot of
other smart people on this reflector that like to do that as well. I would
never presume that someone doesn’t want to learn when given the opportunity to
do so.
And Modeling is a great way to learn – far better than relying on
generalizations in a handbook. The patterns will also vary depending on the
specific antenna being modeled, and who is to say that that other antennas
won’t be erected at some point? Having the modeling software and knowing how
to use it would allow someone to try different scenarios with other antennas
that might be worthwhile to build, especially in close proximity to the yagi on
the tower.
Warren will decide what he wants to do – I was giving him reasons why he might
want to consider learning how to model his antennas over his terrain rather
than making the assumption as you did that he has flat terrain and no interest
in learning.- neither of which I saw in his post.
Bob KQ2M
From: Joe Subich, W4TV
Sent: Thursday, January 17, 2019 6:29 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Half Wave?
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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|