OMNI STICK VS. EXTERNAL FOLDED DIPOLE

[efc]

Hello,

Looking for some input on antenna type to replace existing bad ones.

1. Quantar 1/4 K base,VHF

2.Area to cover,approx. 50 miles by 50 miles with some rolling hills and farm land.

3. 170 feet tall mast.

4. I figure 7/8 heliax given the power of the bases and I would like to keep the loss low.Yes I could use 1/2 heliax, but prefer 7/8.

5. Cannot decide whether I would use a Commscope Omni fiberglass stick,approx 23 feet, or an external 2 or 4 element folded dipole by either Sinclair or Commscope.

What would you folks think? any response would be apprec.

The fiber glass stick is 5.25 gain and the folded dipole is any where from 6-9 db gain.I guess my main concern is weathering,I believe that the glass stick would last longer.They are about the same weight, but the glass stick is 1300.00 and the folded dipole 750.00

Thanks much.

[maxkelley_kc2spy]

I would say the opposite... Those fiberglass sticks aren't always the best... If they crack, and water gets into them, you're done! The folded dipoles are definitely the better choice here, in my opinion. More gain, still durable, and cheaper!

Take a look, as well, at Telewave's folded dipole arrays... The antennas are completely sealed with a coating they call Txylan. We've started using them at work and they really, really work well. They use them in extremely rough mountaintop sites, and from what I've heard, they hold up very well.

[efc]

Thank you i did not know that about the folded dipole with the coating.Great advice.And that is true,fiberglass cracks you are done.Lightning is terrible as we all know,seen it do a lot of weird and bad things,but is the folded dipole a little more effective as far as shunting lightning?

[Jim202]

Thank you i did not know that about the folded dipole with the coating.Great advice.And that is true,fiberglass cracks you are done.Lightning is terrible as we all know,seen it do a lot of weird and bad things,but is the folded dipole a little more effective as far as shunting lightning?

The folded dipole is a direct grounded antenna element. The support mast extends above the top dipole element and so provides some direct strike protection. Be careful when using the antenna gain information. The gain depends on if you mount the antenna dipole elements for omni or directional use as to what the gain will be. Larger coax is always better for low loss. However there is some point on the curve where the cost doesn't provide any better performance. In your case, you didn't provide how long the feedline will be, so no one can make a guess on the cost and performance.

I would be concerned on getting coverage for 50 miles with only a 170 foot high antenna. Unless the antenna is up on a hill someplace, you might be disappointed with the final result.

You also didn't mention if this was for a one way transmission or to be used as a base to communicate with a mobile. If it is for 2 way communications, don't forget you also have to hear the mobile. With a 250 watt transmitter, you have unbalanced the communications path well over 3 db. Generally this means you will outalk your ability to hear a mobile vehicle transmission with that high powered transmitter.

Jim

[Bill_G]

VHF folded dipoles are quieter than colinear fiberglass antennas by a factor of 2db or more. Unless there is a local source of desense to kill your noise floor, the dipole is the way to go for improved rcvr performance. They match better into cavities, and with lower noise, lend themselves to preamps well. I've built systems with -120db effective sensitivity using Telewave folded dipoles. I use them whenever possible.

[efc]

Thanks guys.To answer a couple of things:

1.Talked to a local contact who said all together the height of the antenna would be 195 feet, with the mast on a 3 story building.
I could not get exact height with the Anritsu because the measurement was too bad.I have a rough idea where the antenna was on the dispaly, but not a good one due to loss.

2.This is a base controlled by a Spectra tac voter,so yes i will be talking to mobiles and portables.

3.I agree with you guys about the dipole,I have recently started using them more,Comprod,and have pleased with their performance.Because of the height of the mast above the last element, that would make sense why they would be better protection from our favorite natural fireworks.I am tending to lean toward dipoles.Just not 100 % on make.
Thank you folks.

[efc]

By the by, I noticed that Telewave keeps cropping up.I have never used much of their products,just power splitters.I have used Andrew,Comprod,Sinclair and even Astron . So I take it their antennas are pretty good.