Home Repeater Antenna Setup

[jcemt911]

Hey guys,

I need some help/guidence with setting up a home repeater antenna system. I have searched and searched without being too successful on the answer to a few questions.

I will be running a 30 watt maxtrac home brewed repeater with duplexer. I am thinking I would like to buy a high gain nmo UHF antenna and a NMO to base kit. Would that be a usable antena set up or would it be a very poor choose compared to an actual base omni antenna?

How would I mount the NMO to base kit? It looks like it just screws into the mounting surface. So my plan was to mount the kit onto my wooded eaves by simply screwing it into the wood. Does that sound like an appropriate way to mount it? Would any other steps be needed, or just run the cable into the house and hook
It up to repeater?

This is not a professional repeater and is just a project. It will not be hooked up and powered on 24/7. Just looking for an antena setup that is safe for the repeater, the performance is ok and somewhat portable setup if you will.

Thanks for you help,

Jared

[jcemt911]

If the NMO to base is a poor performing option, I have no problem buying a omni stick. I just don't understand the whole ground plane theory. I would probably mount the omni stick on the wooden eaves or on a flat portion of the roof using a tripod. So would I need a ground plane model omni antenna or standard for either of the eave mounting or the tripod?

For any of these options cable will be 75' or less and on a single story residence. If using a omni I will be using a short stick about 3' or so. Would it be necessary to ground either setup to earth for lightning precautions?

As for cable, I understand from researching it that LMR400 is not the best, but should be suitable for this low budget repeater application?

Thanks

[Bill_G]

Mornin Jared. If the vswr is right, and it's mechanically tight, it will work. How long it will last, and what kind of range you'll get is a different matter.

We have used those nmo base mounts indoors on distributed antenna systems, and they work fine. Cut the antenna and the ground radials according to the chart. LMR400 is okay as long as the connectors are put on correctly. Because your antenna elevation is not significantly higher than your mobile, you won't see a big difference in coverage over simplex. But, it will work. The key is good workmanship. That is what keeps the noise down.

On grounding, that is your call. If lightning is common in your area, then you should put a piece of #6 straight to earth below the antenna.

[Wowbagger]

I just don't understand the whole ground plane theory.

Maybe this will help:

Imagine a stick. Imagine a mirror. The stick is your antenna. The mirror is a ground plane. Place mirror on table. Place stick upright on mirror. See how there is a reflection of the stick in the mirror, making the stick twice as long?

A quarter wave antenna is really only half of an antenna. It really needs another quarter wave, at the opposite polarity, to make a full antenna. The ground plane is a mirror to the RF, making what is called an image antenna that is the reflection of the real antenna, but with the opposite polarity - and now you have your other half of the antenna.

The better the ground plane, the shinier the mirror; the worse the ground plane, the less shiny. A couple of skinny little wires is like trying to use mud as a mirror. Lots of thicker wires is like using a reflecting pool. An actual plate of metal, like a metal roof, is a good mirror.

If the ground plane is smaller than the wavelength you are working at, it's like using a dental mirror - you really can't see the image antenna and it won't work well. If the ground plane is many wavelengths, that's like using a full sized mirror in your bathroom.

[AEC]

I see no logical reason to use any mobile antenna for repeater service, the receive capability will suffer, and then for all the work involved, you might as well use a base antenna on a portable.
To get maximum range, use a good quality antenna, D.C grounded for noise reduction and P.I.M(passive Intermod) as well as water related detuning effects.
Good cable choice means more signal gets into and out of your equipment, with little to none making its way into external equipment or nearby transceivers.
Crank the Maxtrac down to no less than 10 watts to keep heat build up to a minimum, and use a fan to keep airflow moving to take away that heat.
Maxtracs become unstable with output powers being adjusted to below a certain limit, and certain band of frequencies.
If you are not desiring to hear and work others from long distances, keep the TX power low, and make sure the duplexer is properly tuned for each side, TX and RX. ANY leakage will cause big problems for anybody using this. Range will suffer, and reception will be severely degraded if any amount of RF power is getting into the receive side of the duplexer. his will cause desense and limit how far you can hear others. The other problem with transmitter power getting into the receiver, is burnout of the front end of the receiver, and if a preamp is used, you will probably blow out the active device. GaAsFETs do NOT like ANY power applied to their gate leads, and they will burn out faster than you can dekey!
Proper planning and construction/adjustments go a ong way to making a system work well.

Good luck with your endeavor!

[jcemt911]

Thank you for all the advise guys!

You sold me on doing it right and getting an actual base antenna. I was going to go with a commercial Tram 3db, but after some research the Comet amateur omni gain sticks seem to be well liked. I think I'm going to go with one of Comet sticks.


Bill_g, With your advise I will make sure the antenna is as high as I can mount it. I'm thinking I will mount it on a pole with the antanna base maybe 3-6' higher the my roofline for a total of approx 20-25' height. I deffenately want a set up that works about as good as possible with the repeater I'm running. I would be really disappointed if the range was not much greater then simplex.

Wowbagger I don't think it could have been explained any better---I totally understand now. Thanks!

As for the lightning protection, I'm thinking I'll mount the antenna to a metal pipe (that goes all the way to the ground), copper clamp a ground rod to the pipe, and put a polyphaser on the coax right before it enters the house. I live in a area that in not prone to lightning, so I think that setup would be proficient.

What do you guys think of that setup?

Thanks again!

[cleb]

I just bought a Comtelco BS450XL6-B from a member here (Will). Not sure how much the Comet is going to run you, but the Comtelco has great gain (6dBd, 8.15dBi) and construction for the price. I'm going to be putting it in service on a Amateur repeater. Hasn't arrived yet, but I've been told its a good choice. Not sure what your budget is, but might be worth looking at. Should run you a little over $200.

[Bill_G]

Jared - antenna elevation of 20-25ft is a good start *if* it is above the surrounding tree canopy. There is a reason we put repeaters on top of hills and tall towers. 25 feet doesn't push your horizon out very much. If you are below the trees, you probably won't see much increase. Trees gobble up RF.

[Will]

Crank the Maxtrac down to no less than 10 watts to keep heat build up to a minimum, and use a fan to keep airflow moving to take away that heat.
Maxtracs become unstable with output powers being adjusted to below a certain limit, and certain band of frequencies.

The minimum TX power on a 25 watt Maxtrac IS 18 watts before unstable/spurious operation. So 20 watts WITH a fan is a good setting.

The antenna is one of the most important parts of a repeater system.

[motorola_otaku]

You sold me on doing it right and getting an actual base antenna. I was going to go with a commercial Tram 3db, but after some research the Comet amateur omni gain sticks seem to be well liked. I think I'm going to go with one of Comet sticks.

I would advise against using any Japanese-made ham fiberglass antenna (Comet, Diamond, etc.) for repeater use. The elements are not held very securely in the tube (I think one I took apart used blocks of styrofoam??) and over time they will vibrate and weaken the solder joints, causing noise in your transmit signal and eventual high SWR. I know guys who swear by the Tram antennas as being exact copies of "real" Laird/Antenex or PCTel Maxrad commercial antennas, but their Chinese manufacture makes me leery of using them.

I can tell you that any fiberglass antenna made by Comprod, Comtelco, PCTel Maxrad, Laird, Andrew Decibel, or Telewave will work and will last you for years. A small folded dipole array like a Decibel DB 404 will work even better and, speaking from experience, make great backyard repeater antennas.

[Will]

The PCTel, Laird, Maxrad all use a steal wire as the element. Not much different than the Comet, Diamond and Tram.