Antenna for In-Building Coverage (VHF Hi)

[70cutlass442]

I am adding RX site to a local high school for better in building coverage for the local police. Today we did coverage testing with a Larid 3.5DB fiberglass and had mostly favorable results.

My question for you guys is, should I stick with the 3.5DB larid or should I go with a unity gain? Or perhaps a DB222. I ask these as I know I am trying to cover directly below the antenna and the immediate area surrounding. I have heard that gain antennas are not as good for this as they focus more on the horizon. Can someone better explain this? I like the idea of a DB 222 as they are more durable than the fiberglass.

Thanks.

 

[mmckenna]

Depends a lot on where/how it's mounted in relation to what you are trying to cover.
If it's a small area, lower gain can work better.

But, if the results were favorable, why change it? Do you have specific areas that it's not covering?

 

[prcguy]

How big is the building? How many floors? What is the building made of? Are you placing the antenna inside the building, on top or outside?

 

[70cutlass442]

Depends a lot on where/how it's mounted in relation to what you are trying to cover.
If it's a small area, lower gain can work better.

But, if the results were favorable, why change it? Do you have specific areas that it's not covering?

there were some areas that were a little scratchy. Much better than the current setup where the RX is several miles away. I have been adding preselectors and RX preamps which seem to clean this issue up. I mainly want to know if someone has done something similar and had the opportunity to compare several antennas. Time is limited so I just used what had (being the above mentioned stick).

 

[mmckenna]

Not in an application like that.
I do have a couple of UHF repeaters that are high up and directly adjacent to very deep canyons. I solved a number of coverage issues by swapping out 6dB gain antennas to a single bay folded dipole. Coverage improved quite a bit.

Higher gain antennas work by compressing the pattern towards the horizon. That can work against you. Lower gain antennas don't do that, the radiation pattern can be more favorable in some situations.

Might be worth trying a 0dB gain antenna and see if it improves. You could even whip up a quick 1/4 wave ground plane to try it out. I've even seen some shipboard repeaters (UHF) where they had the a 1/4 wave ground plane antenna mounted upside down on an upper deck.

 

[12dbsinad]

How was the testing antenna mounted? Rooftop? Center of building? Etc.

 

[70cutlass442]

How big is the building? How many floors? What is the building made of? Are you placing the antenna inside the building, on top or outside?

It is a large building. Three gyms, large performing arts center, two stories. 520' x 1000' See attached photo and note the orange dot where the Antenna was placed today. The distal corners opposite the antenna are where I could use just a bit better signal.

 

[70cutlass442]

How was the testing antenna mounted? Rooftop? Center of building? Etc.

Rooftop, only a couple feed above the roofline. It is important to know there are many rooflines on this building. See the above photo.

 

[12dbsinad]

We have a plexiglass company near buy with 2 VHF repeaters mounted on top of silo tanks. these tanks are located in the center of several large industrial buildings. One is a unity gain and the other is a 7 db ASP fiberglass stick. The 7 DB covers better inside the buildings directly below vs the unity. On paper this shouldn't be, but it does. Just giving you a little real world results..

 

[70cutlass442]

I am looking for real world results so thanks for sharing. I have a VHF Station Master sitting here, I am half inclined to lug that thing up there and try it now.

 

[prcguy]

If you use an omni I would use low gain and put it on the 1st floor center of building but high near the ceiling and well away from any computer or RFI generating equipment. Otherwise I've had great success using a Yagi mounted well away from the building but pointed at the building of interest. A three element Yagi would nice to hit the building you show broadside from across the street if there is a place to house it.

On UHF I've used a 10dBd gain Yagi on a roof tower of a 10 story building pointed straight down. That saturated the entire building to the basement for a paging system transmitter.