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Author Topic: Antenna Height Above Average Ground  (Read 14442 times)
AF6D
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« on: January 24, 2014, 11:46:30 PM »

I am a repeater owner with commercial experience dating back to the 80's but new to ham repeaters beginning in 2009. I have built repeaters from HT's and added a Hamtronics COR-3 and done just fine. I've pulled duplexers from old IMTS radios and it worked out fine. But I still find that with my mentor my opinions matter little compared to his experience that he touts. While I respect him I often disagree with him.

Such is the case with a newly launched site 2 intended to be voted and simulcast. Using software to endlessly analyize parameters and telephone calls to antenna manufacturers I still hear "...but I've been doing this for years. Just throw the antenna up and you get what you get." Well, if I am paying good money to put up a machine and even though I am the rookie I think I am an educated rookie.

Consider the new site that sits on a 4500 foot mountain surrounded by nothing lower than 2500 feet. Yet the experienced ham insists that mounting a DB224e 20 feet above the bottom of the tower would be just fine. I disagree. When looking from the door of the repeater vault I could not even see my antenna on the 200 foot tall tower with ample space. It is my opinion based on many years of using commercial antennas and mobile radios to listen to forest fires 100+ miles away that antenna type and elevation make a huge difference. It is 100 miles across the desert floor and such low mounting suggests that any elevation throughout the valley will cause a weak spot to appear.

So how should I express my opinion without pissing off the experienced operator who wants me to just accept coverage based on being 20 feet off the ground? Technical talk like ground coupling mean nothing. It seems to me that we should mount the antenna as high as possible using no more than the 100 feet of hard line I already have. We could go higher but I'd have to invest in 7/8" or larger. Heck, we could get near the top if I wanted to invest in the hard line!

Or, should I just expect that tripling the elevation won't make much difference? Am I misguided?
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NJ1K
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« Reply #1 on: January 26, 2014, 06:14:09 AM »

There comes a time when the rookie understands certain things better than the elmer.  Sometimes you just have to break out on your own and do what you think is best.

That said, there is software on the web that will generate your own path loss map based on your GPS coordinates, antenna HAMSL, ERP, and other criteria.  Sorry, but I don't have a link to the one I used several years ago.  I used it to determine whether a UHF link could be established between two certain VHF repeaters.  It worked well. 

Good luck with your project.
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K1CJS
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« Reply #2 on: January 26, 2014, 08:48:37 AM »

If the heights you give are accurate, that mountain gives you plenty of elevation--2000 feet worth.  Another hundred feet or two isn't going to make much of a difference.  That is probably what your elmer is thinking--and he's right!  What you're probably concerned with is the antenna being side mounted, and if the antenna serves the community that it's supposed to, that means little.
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KB4QAA
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« Reply #3 on: January 26, 2014, 01:14:44 PM »

You already have 2,000 feet of altitude.  80 feet won't make much difference, and is a very small percentage of what you already have.   Mother Nature has blessed you with 'free elevation".  Don't look a gift horse in the mouth.

Side mounts are standard and perform excellently.  If you question the performance then you need to pull out the specifications, do a pattern analysis, and a terrain analysis, and show whether this installation meets your signal requirements.

A properly installed VHF vertical antenna up 20 feet should have very low losses from ground coupling.

p.s.  Saying this in the kindliest way:  Having watched your questions on other topics recently, you seem to have very strong opinions and tend to argue with good advice from experienced people which is different from what you have already formed.   Not good or bad. Just an observation.  Try to be open to advice.

DB224 Spec sheet
http://www.repeater-builder.com/db/pdfs/db-224-instruction-sheet.pdf
« Last Edit: January 26, 2014, 01:33:18 PM by KB4QAA » Logged
AF6D
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« Reply #4 on: January 26, 2014, 04:37:16 PM »

I have used the software you mentioned to model the coverage. I have used software for the DB224 that shows its patterns based on mounting. They all tell me that the antenna is too low. The element not mentioned is the mountain behind me blocking me to a community that doesn't need to be given some altitude on the antenna. Please note that I said nothing less than 2500 feet lower. The area is mountainous terrain with a string behind the tower mounted 20 feet off the ground leaves no chance of clearing the mountain. I didn't mention this the first time and that is my error. There are also natural formations throughout the valley that are significantly shadowed. What I was trying to convey, albeit poorly, was the attitude of putting it up and accepting what you can get rather than putting it up higher and mitigating the shadowing. I suck as a writer!!!

Bill, thank you for your comment intended in the kindest of ways but I would have preferred a private message. The first reply of this thread points me in one direction while two others point me in another. With the utmost respect, what one person calls arguing with experienced users is an inexperienced user asking questions trying to get a full understanding of conflicting comments. The student is always encouraged to ask the teacher more questions when the student doesn't understand.

I tend to use facts from software or books that are contradicted by experience and I should know better than trust the books. Just like the books that often contradict one another when a new user is subjected to conflicting "experience" and he doesn't question the contradiction how is he to understand? I really just want to learn. A follow-up question to clarify a point isn't intended to be arguing for the sake of argument. It means that I don't understand. I will try to do better. I really will. I am more receptive to advice than apparently it comes across. Maybe its in the style of writing. I don't know. I come here for the experience even if the book in front of me tells me differently.

I already have the PDF and it doesn't mention mounting the elements inches from the galvanized tower. I telephoned Commscope and their engineer told me 5 feet clearance. This isn't intended as arguing but passing on information from the engineer in the hope that it may be helpful to someone else down the road.

The real issue is that I want to use the mountain at 6,450 that sees down into the entire area rather than side-mounting an antenna mere inches from the tower in omni configuration rather than all elements pointed out. There are pros and cons both ways by covering it from the side has already resulted in user complaints. From the alternate location I was able to carry on a mobile-to-mobile conversation over 120 miles away that would not be possible from site 2. Again this isn't arguing with my elmer but having a difference of opinion based on field tests. I hope you understand. but I will try hard to write more clearly and in a way that doesn't appear to be argumentative. My mentor has taught me an awful lot and I wouldn't want to disrespect his judgment at all. I hope that I've said this correctly.
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AF6D
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« Reply #5 on: January 26, 2014, 11:33:50 PM »

As a note, I spoke with my mentor this evening at length and after being presented with the results of field tests done while he has been away he came to the conclusion on his own that the site that looks down into the deep valley is better than the site that looks across the valley. "Heck, we tried." We both agreed that even if we used an available Station Master at the top of the 198 foot tower it wouldn't make a difference.

A note on my conversation with the Engineer at Commscope.

If one wishes to mount the DB224e in an omni-directional configuration it should be mounted at least 5 feet out from the tower. A similar conversation with an Engineer at Telewave recommended that their antennas are designed to be mounted with the elements facing all in one direction. He also suggested that in a high lightning environment, which we are at two of three sites we're building the DB224e style antenna is a better choice than a Station Master type.

Your mileage may vary and your experience may indicate something different. But in this case I am only the messenger trying to share what I learned by talking to experts. I hope and pray that it is w2ell received.
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KA7YAZ
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« Reply #6 on: January 27, 2014, 09:49:00 AM »

After reading his post several times it dawned on me that none of the replies addressed his question:

"So how should I express my opinion without pissing off the experienced operator who wants me to just accept coverage based on being 20 feet off the ground?"

A common occurrence on message boards. Some were for his position while others were not. He even tried to a apologize and do better only to be slammed by old-timer that makes having an elmer difficult I read what he wrote and found no arrogance or argument in it at all. What I found was a self-professed new guy looking for away to talk to those of you that are behind a keyboard and so easily dismiss and slap down the new guy, He asked how to talk to some of you and offered what I read as a heartfelt apology. Give the kid a break and move on.

He was not incorrect when he said nothing lower than 2500 feet. His issue is with formations that increase from 2500 feet to 3500 feet or more leaving holes. That sounds like a fair discussion to have,
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WB2WIK
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« Reply #7 on: January 27, 2014, 03:07:08 PM »

An antenna on a hilltop is never "on the edge of a cliff" in more than one direction.  Most mountains here in southern CA aren't such major prominences because they're almost all in mountainous areas with lots of other mountains around.  Unlike Mt. Rainier or Mt. Hood which stand up thousands of feet above everything around them for many miles, I can't think of anything like that around here.

As such, a repeater antenna only 20' above ground might work well in one direction but it's unlikely to work well in all directions, as the mountain it's sitting on will shadow it to the valleys below.  The only places where an antenna that low to the ground will usually work just fine is to other mountaintops -- while "usually" the intended coverage isn't those mountaintops, it's to the valleys between them.

The Mt. Wilson "radio tower row" sits at >5700' above sea level with a great view "downtown," but not such a great view along the ridge or to the north where it's not "above average terrain" at all.  Note the VHF and UHF systems up there are all on tall towers.  I'm pretty sure the reason wasn't because everyone wanted to spend a lot of extra money installing tall towers. Wink
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K1CJS
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« Reply #8 on: January 28, 2014, 07:08:13 AM »

Re:  Reply #6, KA7YAZ

Granted he wasn't answered properly, but his question was unclear to begin with--WAY unclear.  For example, he stated the site was at a 4500 ft. elevation with nothing surrounding it LOWER than 2500 ft.  Most looking at that would interpret it the wrong way--as I did--and think the site had a great advantage compared to the surrounding area.  It turns out that it does not.

Heck, everyone expresses themselves poorly sometimes.  As you said, give the poster a break and move on--and while you're at it, give the responders a break as well.
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KA7YAZ
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« Reply #9 on: January 28, 2014, 07:45:40 AM »

His question was:

"So how should I express my opinion without pissing off the experienced operator who wants me to just accept coverage based on being 20 feet off the ground? "

What part of that is unclear? You and several others went on to try and address technical issues that were not part of the question. Only one person in here answered the question. The rest of you attacked the technical issues and him needlessly. But you ask that I give you a break? You didn't give him a break and answered the wrong question. I see a young ham trying to figure out how to talk to his elmer that seems difficult to talk to.

If you must attack him to make yourself correct rethink it. He said that the site elevation was 4,500 feet with nothing lower than 2,500 feet. This means that the site sits 2,000 feet and less above the average terrain. Using basic trigonometry one can easily see that if there were no increases in elevation within the field he would be correct in his statement that at some point his signal would hit buildings broadside. This was echoed by WB2WIK. You want so desperately to save face that you now ask me to let you off the hook.

No sir. His question was crystal clear.

"So how should I express my opinion without pissing off the experienced operator who wants me to just accept coverage based on being 20 feet off the ground? "

Just as clear as his assumptions that because of varied terrain within a 2,000 foot window was obscured. "Consider the new site that sits on a 4500 foot mountain surrounded by nothing lower than 2500 feet." He's looking from the site sown and realizes that the average terrain is 2,500 feet or higher and some of you chose to make assumptions that were incorrect. He told you from the beginning that he was dealing with a window with some things higher than 2,500 feet. You chose not to understand something that wasn't even the question. One of you then chose to openly attack him based on answering the wrong question.

You wanna break? Go back and answer the actual question and help him understand how he might approach an elmer that is set in his ways. THAT was his question. While you're at it consider an apology that I wouldn't hold my breath for. After all you've asked me to give you a break and I find that rather selfish.

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K1CJS
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« Reply #10 on: January 28, 2014, 03:11:27 PM »

"YAZ, The extra info he provided was unclear.  BTW, just how was my answer an attack?  It wasn't--and in addition, you seem to be nothing more than a pot stirrer, you're attacking others and causing trouble.  Welcome to the ignore list.
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KA7YAZ
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« Reply #11 on: January 28, 2014, 06:43:19 PM »

"YAZ, The extra info he provided was unclear.  BTW, just how was my answer an attack?  It wasn't--and in addition, you seem to be nothing more than a pot stirrer, you're attacking others and causing trouble.  Welcome to the ignore list.

Not at all. But its guys like you that use the ignore list as a weapon. I think you just don't like being called out for what you are. You asked for a break but would not give the OP a break. All I did was point out that out of the few replies only one actually answered his question. As it turns out those that did reply were incorrect in their assumptions. Only one poster stepped forward and stated the obvious -- that a narrowing +/- 2,000 foot window in one direction was insufficient.
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AF6D
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« Reply #12 on: January 28, 2014, 06:49:32 PM »

Gentlemen,

Please stop this bickering. I asked a question and some of you misunderstood it. Please let it go. Please?
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KA7YAZ
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« Reply #13 on: January 28, 2014, 06:57:37 PM »

AF6D,

Your question was simple and the facts you provided were not unclear. You asked how to talk to an elmer that seems set in his ways? Stand your ground. Here and with him. If you stand for nothing you'll fall for anything. There are too many people in the world that hide behind things like the Ignore List (scarey!) and it keeps new hams from joining.
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AF6D
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« Reply #14 on: January 28, 2014, 07:10:38 PM »

Just please stop. I like it here and the guys are great with answers. Just let it go and let's get back to radio. I don't need this and I didn't ask for your help.
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