Batboard

I have a Unity, Omni Directional ground plane antenna on top of a water tower. The terrain is pretty much the same in all directions. The signal is weaker to one direction, and strong in all three other directions. I use a Minitor IV pager to measure signal, since it is the worst receiver I know and the most noticable since pager activation is important here.

The antenna (Andrew DB201-M) is new, and mounted on the top of the water tower, towards the side that has the weaker signal (circular railing on top, mounted to weaker side). What I need to know is this...
Can this create a directional signal much like the antenna location on a car? Is it significant enough to sound scratchy only 2-4 miles toward the bad direction, yet sound great 20-30 miles in the other direction?
Is this antenna capable of a directional signal?
Is there another unity antenna that could serve better (note: I have a height restriction, only a few feet to work with)?
I am licensed for 110 watts at 110 ERP, does anyone make an antenna that will allow me to meet the requirements and possibly pattern more ERP to the poor direction?

Any thoughts would be appreciated.
Thanks,
Ett1033

Ett1033 wrote:I have a Unity, Omni Directional ground plane antenna on top of a water tower. The terrain is pretty much the same in all directions. The signal is weaker to one direction, and strong in all three other directions. I use a Minitor IV pager to measure signal, since it is the worst receiver I know and the most noticable since pager activation is important here.

The antenna (Andrew DB201-M) is new, and mounted on the top of the water tower, towards the side that has the weaker signal (circular railing on top, mounted to weaker side). What I need to know is this...
Can this create a directional signal much like the antenna location on a car? Is it significant enough to sound scratchy only 2-4 miles toward the bad direction, yet sound great 20-30 miles in the other direction?
Is this antenna capable of a directional signal?
Is there another unity antenna that could serve better (note: I have a height restriction, only a few feet to work with)?
I am licensed for 110 watts at 110 ERP, does anyone make an antenna that will allow me to meet the requirements and possibly pattern more ERP to the poor direction?

Any thoughts would be appreciated.
Thanks,
Ett1033

Is there something above this antenna? You say that you have a hight restriction and that your using an Unity, Omni Directional which would send a lot of power up.

What band is this on?

Band is VHF.

Nothing above this antenna. We are licensed to not exceed the height of the tower, which is infact the lightning rod. The rod is adjacent to the antenna, and is a whole 2" higher. The rod is located in the center. I doubt that is causing the problem.

Have you checked to make some a-hole hasn't used it as a hanger for spare supplies? (sorry, had to after what I found the other day on ours)

Location on the tank could make it directional, same as on a vehicle, closer to the edge the worse it would be..
How close is it to that lightning rod? That could do it too.

How much power is your transmitter putting out, and how much cable loss do you have?
You might do better with a gain antenna and power adjustment, for example, if you've got a 110W radio running full out, 2dB cable loss and unity gain, you're 2dB below ERP because of the cable. Since you can't get more output power to make up the loss, switching to a 3dB gain antenna and dropping the power back 1dB would give you rated ERP. This should help the receive too (unless of course it's a paging transmitter).
A gain antenna might help with the directional problem, hard to say though... any chance of borrowing one to try?

We have approximately 160' run of Heliax super flex 1/2" I think. It is a continuous run. I am told we put out 110 watts at the base and by the time it comes out up top, we are about 98 watts (repeater type system with cavitys). Power is not the most important, since we have great elevation. But, if we are sending most of the signal in the other directions, that could be why the pagers are scratchy towards the one side only a couple of mile away. Pagers are great 20 miles the other directions.

Look at the pattern created by mounting to one side. Generally
the best signal is the one that goes across the ground plane
(the water tank) not away from it. Just like a whip antenna on the
left rear fender of a car will radiate best in a line from the antenna
to a point that has the most metal under it (right front). The fact that an antenna has a small set of radials on it counts for nothing with a much more massive metal structure immediately under it. Find a way to
put the antenna centered and use the tank as the primary ground
pland and you will find your pattern MUCH more symmetrical.
Or, move the antenna to the side of the tower where you need
the least signal and take advantage f the gain and null effects
to place signal where you want it most. Phrawg

You may be into the issue of the LB antenna nbeing mounted too close to all that steel of the water tower. Those low band antennas do like to be out in the open and not have much real close below them. If you have the radials near or almost touching the tank top, that also may be doing in the omni pattern.

Where is the height restriction coming from? I would try raising the antenna up some and see if that doesn't correct the problem your telling us about.

Jim

Ahhh, now that I recall, one of the radials is using the railing to rest on. It just so happens to be the direction that the poor reception is coming from. Can something this minor cause problems that big? If I raise it a couple of inches so it does not rest, do you think that will even out the signal?

IMSA regulated the height since we are in proximity of airport.

The DB 201 folded dipole ofver a FLAT ground plane (four rods) has a radiation angle at above the horizion. Anything metal closer than about 2 to 3 feet can affect the radiation pattern, even some metal at or near any of the groundplane rods can screw up the pattern.
Accually we used some of these, VHF police system, upside down so there was a significant downtilt on a 3000' mountain serving a area at 400' to 800' elevation and less than 10 miles. Sure got rid on the co-channel at 100 miles!!

A antenna simmular to the Comtelco BS150U-B would be a better bet with a much flatter radiation pattern at the horizon where the signal is needed. Stay away from the Antenex and others as they do NOT duplex well. If you need more help I can put you in touch with the engineer at ComTelco.