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UHF Repeater Antenna Question

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cobraultra

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I know everyone has there own opinion on the matter and the topic can get way out of control but i was interested in know why someone would be using a Omni Fiberglass over a DB404 or DB408?

I cant tell the exact model but im pretty sure it is a Sinclair 450-470 being used for the fiberglass.
https://www.tessco.com/products/displayProductInfo.do?sku=591824

Just trying to understand the reasoning for picking this over the DB's they have access to.

Thanks!
 

dsalomon

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Perhaps they need more power handling. The Sinclair handles 500 watts and the Commscope only 250. Or, they might be mounting the antenna in a very high wind area. A vertical with nothing sticking out the sides must have a smaller wind load. The wind load isn't given for the Commscope antenna, but is 220 mph for the Sinclair, but only 100 mph for the Commscope. Perhaps they have a need for a stealthier installation, or an installation in a tight spot.

Bottom line - you have to read through all the specs to get an idea of where one antenna is "better" than another. Looking at gain alone (the Commscope does have better gain than the Sinclair) doesn't give the whole picture.

Best - David
 

cobraultra

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Thanks Dave

So i know its running on a gr1225 so max 45 watts. So power isnt it. Wind speeds and mounting location are not a factor. Its on the top of a hill on top of basically a utility pole. Wind speeds in the area cant say ive seen over maybe 80 in a bad storm.

Not sure exactly yet what this repeater was running on before they moved it but it had to be a DB only other antenna's on the location it was at. They moved it to this tower and put up this fiberglass one. Service area got worse and its about 50 - 60 feet higher now.

Thanks again!
 

jim202

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I know everyone has there own opinion on the matter and the topic can get way out of control but i was interested in know why someone would be using a Omni Fiberglass over a DB404 or DB408?

I cant tell the exact model but im pretty sure it is a Sinclair 450-470 being used for the fiberglass.
https://www.tessco.com/products/displayProductInfo.do?sku=591824

Just trying to understand the reasoning for picking this over the DB's they have access to.

Thanks!
The antenna selection can depend on a number of issues. The first is just where the antenna will be mounted. Is this going to be on top of a mountain or down on the flat land? Next comes the question of what kind of ice conditions will the antenna be subjected to? What kind of winds will the antenna be subjected to? How bad is the lightning at the location the antenna will be mounted? What kind of range are you expecting from this antenna? Are birds roosting on the tower and antennas? How well built is the tower ground system. If your on sandy soil, a single, 8 foot ground rod will not cut the mustard. If your on the top of a rock mountain, grounding the tower can be a major problem.

This might seem like a long list of questions, but you need to consider all the information that they bring to the table. Wind is a death sentence to some antennas and the way they are constructed. You have to be concerned about the constant flexing the different components of the antenna make in order to survive the mounting location. The stick type antennas flex a considerable amount and cause metal fatigue at some of their internal component connections. This is why you will find standard and heavy duty versions of some of the pole type antennas.

If your in an ice prone area, the ice forming on the antenna will drastically effect the function and SWR of that antenna. The folded dipole type construction is very prone to the ice build up on the dipoles. Yes some companies have come out with a black Teflon type coating on the dipoles to attempt to combat the ice problem.

Lightning damage to the stick antennas is well documented and there are a number of photos around showing a banana pealing look of the antenna that has taken a direct strike. Fiberglass tubes just don't seem to do well being struck by a lightning bolt.

With all that said, where and how you mount an antenna on a tower will have much to do with the performance and survival on the tower. Never, never mount your antenna at the very top of the tower if you want long term survival. Always make sure there is something metallic that is taller than your antenna.

If your planning on using a fiberglass stick on a tower in a high wind region, it is best to use a top brace on it to keep the flexing down to a minimum. There are support arms with an insulating section on the end that slides down over the pole to keep the flexing to a minimum. Even the 700/800 trunking antennas use this top brace. Just make sure that the support arm will not wear through the fiberglass pole where it connects to the pole.

Have I provided a true choice between the 2 types of antennas? No, because the choice has too many variables that are site dependent. You have to look at the pro's and con's to come up with the correct answer.

A couple of comment about the antennas. The fiberglass sticks tend to loose their coating over time and the fiberglass itself becomes rough and the strands start showing up. Once this starts to happen over time, water will be able to penetrate the tube and cause all sorts of problems. The metal dipole antennas will have water that gets into the phasing harness after a number of years. But the metal dipole antennas hold up better to lightning strikes.

With the sale and resale of the different companies that have made the folded dipole type of antenna, the position now is they no longer will sell you just the harness. If the harness goes bad, your now forced to buy a whole new antenna. This has not set well with the radio users and has caused a major thinking on just which antenna to purchase. Many of the radio shops got a hair across their butt when this change took place.

So now your forced to throw away a perfectly good metal dipole antenna because the vendor refuses to sell a replacement phasing harnesses. Not a good marketing move in many peoples eyes. But it makes more money in the eyes of the company that sells the antenna. But how has this effected the bottom line on how many antennas they sell in the long run? If it was me, I would take my business to a different company.

So now you have some feedback on the different antennas. Good luck on your choice.
 

KE5MC

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Thanks Dave

snip...

Its on the top of a hill on top of basically a utility pole.

snip...

Thanks again!
One specification I noted is the 6 degree down tilt of the Sinclair and 0 degree on the DB I looked at on another site. Hill top location might be a reason to use the Sinclair. I have no idea if 6 degrees helps with coverage, but in theory it should/may.

Mike
 

mmckenna

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One specification I noted is the 6 degree down tilt of the Sinclair and 0 degree on the DB I looked at on another site. Hill top location might be a reason to use the Sinclair. I have no idea if 6 degrees helps with coverage, but in theory it should/may.

Mike
I concur. I swapped out some repeater antenna on an 800MHz system for a pair with more downtilt. It did improve coverage in the intended area.

Cobraultra:
You didn't say what this repeater was for or what it's intended to cover. Repeater coverage is a game of give and take. Choosing highest gain antennas isn't always the best answer. Sometimes lower gain, more downtilt or any number of other parameters become important.

If it's for amateur use, price also seems to take a key role.
 
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Looks like they are going specifically after the downtilt. Not too long ago I put up a VHF (folded dipole) antenna for a customer which took advantage of down-tilt phasing (and actually replaced a DB222) to provide better local coverage.

I've found over time (especially in the dusty climate that makse up a good portion of the "High Plains" in the US) the gel coat on fiberglass antennas gets worn away. Water then is able to seep into the fiberglass. Aside from the SWR issues Jim mentioned, I've found this to also be problematic when the antennas get struck by lightning. The water instantly vaporized and literally blows the antenna apart. There was a time when it wasn't all uncommon for me to notice top mounted fiberglass antennas with 2 to 3 foot of "stump" ending in something that looked like a mushroom/mattress factory explosion.
 

cobraultra

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So the repeater is used for our Borough's public works and fire department. I was given access to the repeater location which is about 100 feet from the top of the "utility pole". First thing i saw was they just reused a low band antenna feed... rg-213... which is also 20+ years old. If i had hair i would have pulled it all out. ("Professional Company" relocated the repeater and antenna) It this point i went home grab 50 feet of LMR-400 and a small Laird 0 gain fiberglass. got to the base of the unity pole and placed my antenna with my L400. Lucked out i was able to get a hold of a public works employee who was in one of valleys that NEVER works and i could hear him clear as day. Needless to say changes are being made today to fish new coax up to the antenna site.

Thanks for everyone's input on the antenna difference and other info.

Still interested to see what happens when they connect the 6db on the top of the pole compared to my 0db at the base.
 

mmckenna

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Wow. Just wow.

Good catch.

Replace that with some heliax and a good antenna and you'll be back on the air.
 

lmrtek

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I know everyone has there own opinion on the matter and the topic can get way out of control but i was interested in know why someone would be using a Omni Fiberglass over a DB404 or DB408?

I cant tell the exact model but im pretty sure it is a Sinclair 450-470 being used for the fiberglass.
https://www.tessco.com/products/displayProductInfo.do?sku=591824

Just trying to understand the reasoning for picking this over the DB's they have access to.

Thanks!
I use fiberglass omni antennas side mounted on a tower only

They are cheaper and get the job done as long as you keep them away from lightning

I ALWAYS have nothing by dipoles on the top

The dipoles have taken direct hits and keep right on working
 
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