Antenna Help Needed Big Time Please

[Jonathan KC8RYW]

I am asking the age-old question: what type of antenna setup should I have?

I am debating between three different 900 mhz antennas made by Antenex.

Here are their specs:

1) 0 dbd ... 75 vertical beamwidth
2) 3 dbd ... 33 vertical beamwidth
3) 6 dbd ... 17 vertical beamwidth

This antenna will be mounted on top of a 2-story house, so the height couldn't be any more then around 40 feet above the ground.

I am just wondering which antenna to use.

This is for a future 900 MHz ham repeater.

Yes, I know: get more height. I would love to, but the homeowner's association wouldn't let that happen.

I was thinking the 0dbd antenna would be best [low vertical beamwidth for the suburban/urban area]... but factor in coax loss [~ 3db] then duplexer loss [~ 1db] and I'm already way in the hole. I have pre-amps, so I might be able to save the Rx.. but, as for the Tx.. I have no massive power amps.. it's just 10 watts on the stock PA. I was kind of hoping for an ERP of 125 watts, but given the situation, that isn't going to happen [unless I acquire a bigger PA.]

I've read what broadcast engineer's have said about antennas. One source I read said that going with low gain, and high TPO is the way to go in an urban/suburban enviroment... to reduce the amount of multi-path and also direct the signal down low, to where the users will be [portables and in cars.]

But, broadcasters need to only worry about the Tx power, and they don't have to worry about picking up weak signals from mobile users. And broadcasters antennas are mounted on towers sometimes over 500 feet tall. This antenna will only be 40 feet of the ground, at best.

In sum, help.

[Hartley]

Hi Jonathan,

With your situation, the highest gain antenna makes the most sense, IMHO. If you think of it as sitting at the antenna and looking at the users, all of your users (except you at home!) are going to be pretty much within a few degrees of the horizon. Yes, you're going to have multipath, but it will be in the horizontal plane, not the vertical plane. In your case, the power sent anywhere but toward the horizon is pretty much wasted.

The wide vertical beamwidth would be best for the top of a tall building in a metro area, or perhaps a tall mountain/tower, where the users are sometimes well below the horizon. Also if you wanted to talk to the Space Station!

73 DE Hartley

[Jonathan KC8RYW]

Thank you, Hartley!

You gave some very good points, and I found it quite interesting.

I have also been eyeing the Comet FP-19. It supposedly has 16 dbi of gain [1/2 wave x 19.] I'm a little skeptical of that gain figure. How does that sound for my application?

Does anyone in the 2-way field have any favorite 900 MHz antennas for repeater use?

Thank you!

[n8obu]

if your going to run more power than 75 watt don't use an comet they dont last ! .. running 250 watt on 927.525 pl131.8 n8obu dansville/ lansing mi

[Jonathan KC8RYW]

if your going to run more power than 75 watt don't use an comet they dont last ! .. running 250 watt on 927.525 pl131.8 n8obu dansville/ lansing mi

What antenna do you run with?

Do you have any photos of the repeater?

Just wonderin'

[John G]

I have the Comet. They have revised the gain figure to 9 Db. I like it OK for home use but have a heavier commercial antenna in case I ever get to put up a repeater. The commercial antenna only has 3 Db gain but is very sturdy and covers 806 to 960 MHz. I think I might have an extra if you are interested. Unless you have very flat terrain, don't count on much range at 900 from 40 ft. up. That is about where mine is and I can get more range with a pair of VHF handies.

[n8obu]

i get about 35 miles out of my 900mhz repeater ant at 180 ft 8 db gain 1.1/4 feed line 250 watt in to the cans havent checked what im getting out of cans . Jonathan when you go home give me a call ill show ya the repeater mike n8obu

http://n8obu.com

[n5ztl]

Most ham antennas specify gain over 1/4 wave dipole, most commercial antennas specify gain over 1/2 wave dipole. Difference of 3db comparing 1/2 wave to 1/4 wave. Therefore ham 9 dbd (1/4wave) = commercial 6 dbd (1/2 wave). I would recommend grounded (top) antenna configuration if it is much taller than other lightning targets nearby. (at least where I live) If wind loading is consideration need to go with fiberglass monopole antenna. Commercial antennas are usually rated for 90 to 150 mph wind on a tower 200+ feet above ground. Most ham antennas won't last long if you mount them very high (probably over 100 feet). Good luck with your repeater.
'73 de n5ztl

[Jonathan KC8RYW]

Well, let's hear some makes and models of antennas, and if they are any good.

n8obu, sorry I didn't catch this until now... maybe I can stop by next time up?