Base Antenna Question

[smills1]

I am setting up a base station on GMRS and am looking for the best antenna style to get coverage from the base outward. I have read of different type of antennas with higher gain over a basic 1/4 wave, but my understanding of this is I stand the chance of losing close in reception with a high gain antenna in favor of far out reception, is this a correct understanding?

I am thinking I will just build a 1/4 wave antenna as it seems pretty straight forward- other than stability will gauge of wire used for the elements effect the reception of the antenna?

Thanks,

Steve

[Will]

I am thinking I will just build a 1/4 wave antenna as it seems pretty straight forward- other than stability will gauge of wire used for the elements effect the reception of the antenna?

Thanks,

Steve

Yes the gauge or diameter of the elements DOES affect the bandwidth or frequency range of the antenna.

So at UHF a good diameter element for a ground plane would be 1/4 inch. It can be something like brass tubing, available at the local Home Depot.

[Wowbagger]

http://harriscountyares.org/training/KNW/KNW-110.pdf

Grounded at DC, so lightning protection is easier.

Higher gain than a quarter wave.

Good bandwidth.

Sturdy.

You'll need to recalculate the dimensions for your band, but that's a simple scaling problem.

[FMROB]

So I am going to give my uneducated, non-engineer, real world answer to this question. Please take the information as you see fit.

The answer to your question (technically speaking) is yes. The higher the gain antenna 6,8,10, 12 db etc, the signal will tend to look towards the horizion and less towards directly below the antennas base. Thought being in a high height, high gain antenna system, coverage will suffer directly below and for a few miles out of the center point of the antenna vs. using a lower gain antenna.

Now, here comes my controversial opionion.

I have installed many repeater and antenna systems, and own my own communicatons systems. I will take a 10 db gain station master over any other antenna for general wide area coverage. The benefit it affords to the the talk in to the system are tremendous, however it does affect the output power of your station. You may have to lower TX power to stay within license parameters, wink wink. For an even hotter, kick butt firecracker system (site noise floor considered) a good filter preamp asembly on the rx side with a good duplexer (RFS 526) will get you cooking real good.

We have noticed no degredation of signal at antenna centerpoint utilizing high gain antennas. As a note, the highest antenna that we have in this area is about a 500' structure, so I can not honestly answer this for the 500' + antenna systems. My thought is that from antenna center point to lets say 10 mi radius out almost all antennas are unity to some degree in any fashion.

I appologize if this causes indegestion for any other savy radio folk. - Rob

[Bill_G]

When I worked Pontiac Motors, I had to lower the giant 10db Station Master to the first floor roof to give good coverage in the plant. It was originally mounted on the highest building and could talk well into Detroit, but had almost zero coverage in the intended surrounding plant buildings. Once I put the plant in the beam path, things worked just fine.

Over the years, I have slowly taught our rentals dept to put the event repeater high and off to the side rather than mounting it in the highest central location centered which just shoots over their head. You want the signal where the people are.

Like FMRob, putting a colinear with lots of gain on a hilltop miles and miles away from the service area works great. Huge footprint. Depending on the beam width, eventually it will hit dirt and be useful to the users. That is why I prefer a folded dipole for close in city service like building security and hospital facilities. Lots of natural down tilt, some increased surface area to help the receiver, and some gain to help those that drive away from campus. Very little cone of silence under them.

The j pole is a classic design and a good one for someone to attempt on their first antenna construction. Otherwise, you will need to weigh the benefits of manufactured antennas for your intended service. You need to consider your terrain as well. Lots of gain shooting into a mountain is no gain at all.

Many things to consider with apparent gain.

[emtprt]

Bill your comments are so true. I have 2 40 watt gr1225 repeaters at the same site and the antennas are mounted at the same height on the structure. If you drive 1/2 to 3/4s of a mile down hill guess which repeater has portable coverage, The 3.8 db gain DB-404 (27 degree). The other repeater with a 9db gain (14 degree) antenna is dead to portable world . 12 miles away the situation is reversed the 9 db gain antenna is great and the the 3.8 gain antenna is hit or miss. I have swapped antennas, had the repeaters serviced and the results are the same. I still have some bad spots with the 3.8db gain antenna and I am currnelty working a solutions ( a new repeater with increased power, potential voting receivers, and maybe a lower gain once the power can increased).

An area hospital moved their antennas from the roof of the 10 story building to the 5 story parking garage across the street because the roof top antennas were over shooting over the lower levels. At one point the had a yagi antenna on the roof pointed at the ground for in building pager coverage. This worked well on UHF with limitations in the Xray department, due to shielding.

There are dipole antennas that give you the option to space and downward tilt ( 1-6 degrees) to customize your signal. Discussion with Motorola dealers and members of this forum have given mixed reviews about this practice. A local repeater with a down hill coverage issue recently installed this type of setup with improvements on VHF. It's not perfect but the coverage has improved.

[W8UU]

What an interesting thread. Any experience with the DB 201 antennas over other low band antennas, like the co-linear (DB 205?) or a simple ground plane?

[emtprt]

Our police dept. had a DB-201 at their old station. It was used for local communications on an iter-city network and the street crime unit. The had a 30 watt desk top repeater and were able to communicate with an obtstructed, non-line of sight regional system antenna 15 miles away. The building was in a valley and the signal carried well with in the valley and the surrounding hills. Like most UHF systems the coverage was spotty on the back side of the 400-600 foot hills.

I had a ringo 2.5 gain antenna on my roof and (small cheap and not very obvious). With a GR1225 40watt repeater unobstructed I have over 20 miles of mobile coverage and 10-12 portable. I can also penetrate a hospital 1.5 miles away on the lower floors. With a partial obstruction I only have about 4 miles of mobile coverage and 1.5miles with a large hill directly behind me.

[SlimBob]

This is the difference between putting the beam where you want it or simply ignoring the range you want to cover. People forget that more gain == less vertical beamwidth. That donut of RF gets turned into a pancake, and you can't get that pancake if you aren't in it.