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Antenna interaction question

We have a system in place in the local area whereby the fire departments monitor fire alarm signals at the fire stations in most of the area towns. It is mandatory that all businesses as well as commercial properties are equipped with fire alarm systems. Two of the most common methods of communicating with these fire departments is either by master box telegraph which sends a coded signal by operating a wind up eccentric interrupter via municipal overhead wire, or by radio transmitters which are connected to these fire alarm systems.

With the radio system, in the event of a fire, three short bursts of data are sent over approximately a one second period. The location, event, and zone information are all sent during this period. The maximum allowable power output is one watt The signal is AM at a frequency of 72MHZ. The terrain is hilly. Most transmitting antennas are omni directional affairs which seem to work adequately however there are some cases where it is necessary to use a directional antenna.

It should also be noted that most fire departments have more than one receiver with antennas located in different locations. Sometimes these antennas are miles apart. Depending on topography, it is sometimes not possible for a transmitter to hit one receiver, however it may hit the other with a good signal. Redundancy is important but most fire departments understand propagation somewhat and are satisfied with a good signal at one receiving location.

We have one transmitter location which is about 5 miles from the fire station. The terrain is somewhat hilly however not pronounced. The fire station has two receivers with both antennas that were installed on the roof about ten feet apart. This makes no sense to me. It seems like they should have been put in two different locations as other towns do it to be able to receive signals coming from other locations which may be blocked to the other antenna.

In this instance, these two receiving antennas and the transmitter location form a straight line. This particular fire department is insisting on reception on both its receivers. The complaint is that the fire department is receiving a good signal on one of its receivers but not the other one.

We have tried moving the antenna to another part of the building, raising it 25 feet, and replacing the RG 58U cable with RG 213. Nothing seems to work. I've also considered using a Yagi however It has been suggested to me that in this case perhaps the second receiving antenna may be blocked by the first, which has created a dead spot to this particular transmitter location.

In my opinion, this two antennas in the same location arrangement also eliminates any advantage the fire department would realize of receiving any signals from a blocked transmitting location. I'm also wondering if two receiving antennas operating on the same frequency should have been installed that close together in the first place.

Does anyone have any suggestions The fire inspector knows little about radio and is a real pain in the ass. Thanks, Lenny.

Reply to
captainvideo462002
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Perhaps a polarization change would help. Sometimes as radio signals get reflected/refracted they can swing 90degree pol.

Perhaps try mounting the Tx or one of the Rx antennas horizontally as a test?

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Reply to
Peter Kolbe

I would put a signal ref on the input of the question receiver. If both are on the same frequency with separate antennas of the same type I assume, there is something wrong!

This also assumes that both receivers are of the same model..

It makes me think that maybe the front end of the question receiver is weak or, passing through a narrow filter which is not tuned correctly.

Have you tried switching the feeds at the receivers to see if the bad receiver becomes good?

--
"I'd rather have a bottle in front of me than a frontal lobotomy"

"Daily Thought:

  SOME PEOPLE ARE LIKE SLINKIES. NOT REALLY GOOD FOR ANYTHING BUT
  THEY BRING A SMILE TO YOUR FACE WHEN PUSHED DOWN THE STAIRS.
http://webpages.charter.net/jamie_5"
Reply to
Jamie

Jamie Inscribed thus:

Switching feeds would be the common sense thing to do first ! One receiver could have a bad front end.

--
Best Regards:
                     Baron.
Reply to
Baron

Receiver voting: it allows the selection of the strongest received signal from multiple sites.

Two antenna used for receiving shouldn't affect each other. Do you have a TV field strength meter available? You should be able to compare the signals from both antennas. A 50 to 74 ohm adapter will give a better match, but the resistive type will introduce a 6 dB loss. One can be made from one of the six hole ferrite beads. Wind the wire through all six holes, and a tap at the fifth turn. The tap is 50 ohms, and the sixth turn is 72 ohms.

Is there a local ham radio club? You might find a member who builds and maintains a repeater system who could help.

--
Greed is the root of all eBay.
Reply to
Michael A. Terrell

"Michael A. Terrell"

** What signals ?

Certainly NOT very brief, very weak data bursts on 72 MHz narrowband AM.

Only if there is a VHF channel 4 TV transmitter in the vicinity, could the meter be of any use.

.... Phil

Reply to
Phil Allison

are

M.

e

FYI since the Digital TV analog shutdown last summer, there are no channel 4s and 8 channel 5s in the country. Low band VHF is no longer coveted real estate.

G=B2

Reply to
stratus46

There are other signals, like FM broadcast that will be picked up by the antennas that can be measured with a TV FSM. All you are looking for is something to compare the two antenna systems.

--
Greed is the root of all eBay.
Reply to
Michael A. Terrell

Actually, Chicago has a digital signal on channel

4.(WOCK-CD). See:

formatting link

David

Reply to
David

When I made that statement I looked through this list

formatting link

I thought this list was part of the FCC. In any event they don't list WOCK which appears to be a comparatively low power station. Suffice it to say they are very few low band VHF stations operating in the US.

G=B2

Reply to
stratus46

I am 36 miles west from their transmitting site and also not in the main lobe. I only noticed they were on the air when I was wondering where some interference on my master antenna was originating. I use a combination band stop - band pass filter to insert an analog channel 4 into my antenna distribution system which is used throughout the house to serve several TV sets with ATSC tuners or digital converters. I use analog channel 4 to send whatever is tuned by a receiver connected to a pay TV service and controlled by RF remotes. This arrangement has worked very well until WOCK increased their power to 3KW. For digital on that channel this is really not low power. Before the transition, the local CBS station was on channel 3 for several years with 4.4KW ERP. Retuning the filters more accurately eliminated the problem and I really was not interested in Korean programming anyhow. I suppose I could have moved the analog channel somewhere else, but that would be a lot of work.

David

Reply to
David

Diversity receiver. It's much the same as with the common wireless router that has two antennas. One antenna is considered the main antenna, the other the auxillary antenna. The receiver sits on the main antenna until it hears something. If the error rate is high on what it's hearing, it switches to the aux antenna.

The idea is to eliminate frequency selective fading. That's where the direct path and a reflected path both converge on an antenna, but due to the differences in path length, may cancel or distort the received signal. It's only necessary to move 1/2 wavelength in order to eliminate this effect, which is what the aux antenna does.

There are pleny of other methods of accomplishing the same thing. For example, redundant receivers, both listening to the same channel, and comparing results are possible. Same with two receivers on different RF frequencies, in order to accomidate congestion.

See receiver voting systems. These are common in municipal communications systems, where it is difficult to hear a handheld radio at all points of the city. Mutliple receivers are used as you describe. The hard part is deciding which receiver should be heard by the dispatcher, which is what the voting system does.

Are you sure that there are two receivers and not one receiver and an antenna switch? Some makers and model numbers would be useful. At

72Mhz, two closely spaced antennas should hear roughly the same thing. If one doesn't, it's broken somewhere. What's not clear is whether the signal is weak or just not there.

A 72Mhz Yagi is rather large and expensive.

It's possible, but not likely. How far apart are the fire departments two antennas spaced? Are they perhaps on opposite sides of a tower or pole?

With careful construction, it's possible to create a cardioid pattern with two omni antennas and a phaseing harness, but the beast is so critical in construction, that it's unlikely to have been constructed by accident. Similarly, if one antenna were acting as a reflector for the other, the maximum gain or loss would be about 3dB, which should not be noticable.

I doubt it but it's possible. If the antennas are side mounted on a tower, the tower will do some blocking. Spacing of the antennas away from the tower also have a big effect. There's a section in the ARRL antenna handbook and other publications showing the effects to an omnidirectional pattern caused by various antenna to tower spacing.

A photograph of the fire dept antenna structure would be helpful.

It's not a problem.

Replace the sick receiver with a cheap scanner receiver and see if it hears your transmissions. If it does, fix the receiver.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann

I'm told that the other receiver does pick up signals from other transmitters using simple omni directional antennas in the same general area as ours. The fire dept will not let us tamper with their equipment so confirming a weak front end is not possible. Its an almost impossible situation. While I might be able to place a scanner down in the FD office and try to hear the signal, however it won't be off the suspect receiving antenna. And even if I can hear a signal if it doesn't come in on their receiver, it still is not going to make them happy. We may have to get the receiver contractor involved. But who's going to pay for that? How do I get these jobs anyway? Lenny

Reply to
captainvideo462002

How does this prevent you from taking a photograph of the antenna structure in order to answer my questions? Yes, the fire department will not let you touch the hardware. However, you can identify the probable cause by substitution (i.e. scanner and 3rd antenna) which will surely attract the attention of their radio service contractor or department.

That's fine. You can then say. It works with my equipment. It doesn't work with your equipment. Your equipment is broken.

This is a technical newsgroup, not a political. We can identify and solve technical problems. However, if you have a credibility problem with the fire department, I suggest you obtain the assistance of either a technically qualified communications engineer or tech. If that is insufficient to make the fire department "happy", perhaps a local politician might inspire cooperation.

Hint: The more people involved in YOUR problem, the more complicated it will become. Find one person in your area that is familiar with the equipment and let them do the necessary testing. If you call out the troops, there will be plenty of activity and noise, but little useful in the way of a solution.

You are. It's *YOUR* problem. As far as the fire department is concerned, at this time, they're equipment works just fine. They can also demonstrate it with tests from other users.

Karma failure. Good deeds often help.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann

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