Omnidirectional antenna theory

[HeavyThumper]

Got a question - not sure where to ask it. Hopefully someone here can at least point me in the right direction.

I use a radio monitoring system that operates on a licensed UHF frequency (465Mhz). The manufacturer of the radios provides some simple cheap antennas - I've been exploring some quality aftermarket options and gotten very surprising performance. But I have an usual situation I'm trying to remedy.

Normally, these radios are each themselves repeaters for my network. So the more I add the better my overall network performs. But I've got a new location that's very RF unfriendly. The primary sending unit is located on the 1st floor inside an electrical room - that's where the monitored equipment is. Signal reception in & out of this location is poor. So...

I went up to the roof on the 4th floor, there's a supply closet, where I've installed a second radio to act as a relay. This relay sees the rest of the city, and the 1st floor is able to communicate with the relay. So...communication link established. And this communication is approximately 50' vertically from the 1st floor to the roof, and about 100' horizontally, and of course is through the floors/walls. The communication to the closest remote links in the city is a few miles.

But it's not a very strong link in either direction. Let me explain more. The standard antennas used for these radios are simple 8" "rubber duck" type. I know just enough theory to get into a lot of trouble...I know the top of the radio's metal cabinet normally provides the ground plane for the antenna. So that is indeed the case for the 1st floor location. Now, the 4th floor location - the radio is inside the closet, and the antenna is mounted on the outside. That external antenna is another simple rubber duck - but it current has no ground plane.

The radio locations have to remain as is - for reasons of power & physical security.

Now that I've provided the background...some questions:

1. I'm theorizing that adding a small ground plane, maybe a 6" metal plate, to the exterior antenna will have a benefit on its communicating with the city. But...will that affect it's communication to the 1st floor?

2. I tried a small 3' antenna for the external - it didn't talk to the 1st floor. My assumption is that the radiation pattern of the 3' antenna is too flat compared to the 8". Probably correct?

3. The antennas are all vertical...hence I assume my network is "vertical polarity". What would be the likely change if I were to angle the 1st floor antenna - so it was closer to perpendicular to the roof location?

I'm still looking for a good application reference for the novice.

 

[mmckenna]

Now that I've provided the background...some questions:

1. I'm theorizing that adding a small ground plane, maybe a 6" metal plate, to the exterior antenna will have a benefit on its communicating with the city. But...will that affect it's communication to the 1st floor?

It will probably help the overall performance, but may also reduce the ability to communicate with the first floor. Sounds to me like the manufacturer is just supplying a "good enough" antenna. You are on the right track, no reason why you shouldn't be able to improve performance by playing with antenna design.

There is a function of many vertical antenna designs called "Down tilt". This down tilt can either be mechanical or electronic. It's used heavily in the cellular industry to adjust system coverage.
Mechanical down tilt is where the antenna or individual antenna elements are moved to adjust the coverage close in around the antenna base. I've swapped out antennas on an 800MHz trunked radio system to ones with more down tilt to improve coverage close in around the system.
Electronic down tilt is where the antenna is specifically designed to drop the radiation pattern down to below the horizon. Same effect as above, improve coverage.

A compromise antenna like a rubber/flexible antenna is going to be a poor performer on many levels.

2. I tried a small 3' antenna for the external - it didn't talk to the 1st floor. My assumption is that the radiation pattern of the 3' antenna is too flat compared to the 8". Probably correct?

Probably. The 3' antenna probably had much higher gain, but that gain is achieved by narrowing the radiation pattern down to focus it more at the horizon. This is a case where a lower gain antenna might work better than a higher gain antenna.

3. The antennas are all vertical...hence I assume my network is "vertical polarity". What would be the likely change if I were to angle the 1st floor antenna - so it was closer to perpendicular to the roof location?

It might. Vertical antennas are poor performers when directly under or directly over the area you are trying to cover. They tend to have a null area in these planes.

Some other ideas:

Can you remote the sensor? In other words, could you use twisted pair cabling from the 5th floor down to the first floor to bring the sensor data up to the radio? This would likely solve the issue.

Try turning the antenna on the roof upside down. This might improve the coverage downstairs while maintaining enough signal to reach out to the rest of the city. I've seen this done on ships to improve coverage below decks.

Directional antenna on the first floor might get enough signal out to connect to the rest of the system, or might be able to push enough signal upstairs.

Using some high quality coax, can you extend the antenna from the first floor up to a higher floor? Even getting the antenna outside might help a bit.

I'm still looking for a good application reference for the novice.

ARRL antenna handbook is a good resource, but I'm not always impressed with it. It's probably one of the better resources out there.
My grandfather always told me that antenna design was a lot like voodoo. You can measure the heck out of everything down to the fraction of an inch, plot out patterns, etc. but in the end there is still some "spit ball" adjustments that need to be made. So many things come into play that it's difficult to narrow it down completely to a science. I've got a friend that works in EMI engineering. He does a lot of work at antenna test ranges. Even with all of todays modern computers, it still comes down to setting up an antenna at a range and measuring by hand. Minor tweaks to get things to work right, etc.

 

[krokus]

2. I tried a small 3' antenna for the external - it didn't talk to the 1st floor. My assumption is that the radiation pattern of the 3' antenna is too flat compared to the 8". Probably correct?

Is the 3' antenna meant for 465MHz use? The length is not the only factor, and the longer is not necessarily better.

Sent via Tapatalk

 

[radioman2001]

You can figure an antenna's signal to be like a doughnut over the antenna, higher gain antenna's squash the signal flatter and compromise antenna's like the 3' one really distort it, and a 5/8 antenna (roughly 12" with a good ground plane of 6") might work a lot better than a -20db helical antenna. Is this for the old Lanier alarm reporting system?

 

[HeavyThumper]

Loads of info. Thank you.

There is a function of many vertical antenna designs called "Down tilt". This down tilt can either be mechanical or electronic. It's used heavily in the cellular industry to adjust system coverage.
Mechanical down tilt is where the antenna or individual antenna elements are moved to adjust the coverage close in around the antenna base. I've swapped out antennas on an 800MHz trunked radio system to ones with more down tilt to improve coverage close in around the system.
Electronic down tilt is where the antenna is specifically designed to drop the radiation pattern down to below the horizon. Same effect as above, improve coverage.

Is the same as the vertical coverage pattern - just focused on the lower section? Or does this mean something else?

Can you remote the sensor? In other words, could you use twisted pair cabling from the 5th floor down to the first floor to bring the sensor data up to the radio? This would likely solve the issue.

Theoretically possible - physically an extreme challenge as well as expensive. Right now not a practical option.

Try turning the antenna on the roof upside down. This might improve the coverage downstairs while maintaining enough signal to reach out to the rest of the city. I've seen this done on ships to improve coverage below decks.

I will certainly try this! Hadn't thought of that one!

Using some high quality coax, can you extend the antenna from the first floor up to a higher floor? Even getting the antenna outside might help a bit.

The problem is the site doesn't want anything exposed because they've had vandalism problems. So I need to keep equipment, cable, and antennas in secure rooms. The rooftop location is questionable...but rubber ducks are cheap...

 

[HeavyThumper]

Yes, the antenna is selected for the frequency.

 

[HeavyThumper]

You can figure an antenna's signal to be like a doughnut over the antenna, higher gain antenna's squash the signal flatter and compromise antenna's like the 3' one really distort it, and a 5/8 antenna (roughly 12" with a good ground plane of 6") might work a lot better than a -20db helical antenna. Is this for the old Lanier alarm reporting system?

Not Lanier - AES Intellinet.

 

[HeavyThumper]

Ground plane definitely improved things, and communication to 1st floor is still good. I'm probably not going to get better than this...though I'm still considering the upside-down trick...