Running one scanner with multiple antennas

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tonsoffun

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Hi everybody,

There seems to be alot of interest with people to run one scanner with multiple antennas, so what to do? Maybe all of us can collectively come up with solutions and maybe make this a sticky on the antenna forum to discuss this.

You can do a search on the antenna forum (Top right) about running combiners or dividers. You will come up with many posts made in the past.

Lets get at it and lets see where it will take us!. Also be free to post links and any information you can come up with.
Take care
Ron
 
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zz0468

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The general solution in virtually all of the previous threads has been "don't". There are very few circumstances where this is a good idea, and lots where it's a bad one. If someone is aware of a diplexer that would allow separate antennas for lowband, highband, UHF, and 800 to a single receiver, then we'd be onto something. But this proposed wicky should be clear that you can't just combine a bunch of antennas and expect it to work right.
 

zz0468

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Hello zz0468:

The purpose of the four antennas is to allow a very broad banded bandwidth of the VHF and UHF Bands. Not to increase gain significantly.
The BEST way to accomplish that is to use a broadband antenna to start with. Discones serve that purpose very nicely.


The four antennas will have equal lengths of coax feeding the four port combiner. And will add up signals in phase more than not be in phase. Even when a signal arrives exactly out of phase for two antennas the side antennas will only be 1/4 wavelength out of phase. And the Combiner will have a 20 dB or more isolation factor so if a signal does make it in 180 degree's which is out of phase the combiner isolation of 20 dB or more will only allow less that 1 dB to be subtracted from the original signal level. Most acceptable.
This is where your thinking is flawed. While the combiner isolation will keep the ports seperate, you will still have random phase variations summing in the combiner, depending on how your antennas are oriented with reference to the arriving wave front. Even if all cables are identical in length, unless your antennas are all broadside to the arriving signal, one antenna is going to pick up the signal before the others. At VHF, about 3 feet of antenna separation will allow for 180 degree phase shift, and the signals will cancel in your combiner. Yes, the one with 20 db of port isolation. And you don't think that 90 degree phase shift you mention won't be a detriment? With your configuration, not only will you be dealing with random phased signals adding and subtracting, you have combiner loss to deal with. What will you have gained? You will, in effect, have built a phased directional array with a weird, unknown pattern.


The four antennas feeding a four port combiner, that combiner feeding another four port combiner to feed four radios will work just fine. Just as my two antennas feeding a two port combiner that feed another two port combiner feeds two radios. And one antenna feeding a two port combiner, that feeds two radios.
The splitter part to fed multiple radios will work fine.

This design is used thru out industry to add antennas and radios. Some designs will use pre amps, and band pass and notch filters as well. Which significantly increase the use of the scanners on the VHF and UHF bands. By reducing the amount of "Buzz Saw" Intermod interference that use to kill my VHF use. Others have reported back that they have had custom notch filters made to eliminate near by interference
from data, pager, FM Broadcast, or Big Burts Garage with his old 250 watt VHF Radio, two blocks away.
WHAT industry? I've never, ever seen antennas combined this way in any professionally designed installation. Primarily because it doesn't work. As I previously stated, if you were to diplex an 800 antenna, a UHF antenna, and a VHF antenna to a single coax, then split to multiple receivers, that would work. The flaw in your design is that they're all either broadband or VHF antennas. A diplexer, BTW, would have a port that passes JUST high band, another port that JUST passes 800, another port that JUST passes UHF... and so on. It provides frequency isolation between ports that a broad band combiner doesn't.

I used a Par Electronics P/N VHFSYM152HT notch filter that notches out the 152 MHz band filled with high power data transmitters. Again it made the VHF band useable. This filter is inserted in the single coax line feeding the two radios. Insertion loss is less than 1 dB as the frequency gets off the notch area.
A notch filter is fine for stopping a specific interfering signal, but it won't help solve the problems inherent to in-band antenna combining.

You have some good points there, for design considerations. I hope we can discuss and carry on technical differences with out being disagreeable.

Jay in the Mojave
Well... we don't have to be disagreeable to disagree. =)
 
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JayMojave

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Hello ZZ0648:

II have covered all the math in the last posting, that says a 180 degree signal will only give a almost 1 dB subtraction or less, again this is acceptable.

The aircraft and marine industries use this method, having two or more antenna feed a combiner to have more than one antenna feed one or more radios.

Aircraft use a upper antenna and lower antenna feeding a combiner to allow antenna coverage to allow communications when in high pitch, roll, and yaw positions. They rely on the isolation factor not having the out of phase signals cancel each other, its works very well.

My modified Scantenna and Discone Antennas feed a two port combiner, and a single coax runs down to another two port combiner that feeds two scanner radios, it also works very well. For the garage radios, one needs to keep an ear on the goings on when working ah.

I have some large diameter aluminum tubing that I will use to help broadband the VHF Aero Band, and VHF High Band Ground Plane Antennas. Or maybe even use the small 3 or 4 inch dia coffee cans. But I am still a few months away from putting up the 4 antenna array until the new air condition and heater system is put in.

Jay in the Mojave
 

zz0468

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II have covered all the math in the last posting, that says a 180 degree signal will only give a almost 1 dB subtraction or less, again this is acceptable.
Two identical, equal amplitude signals, 180 degrees apart and summed together will cancel each other. This is a manifestation of the same principle that's used to design multielement phased directional arrays. Phase and space things right, and you'll cancel signal in some directions, and add in others. Make it all random, and you end up with a random pattern. This is exactly what AM broadcasters are doing with their 3,4, and 5 tower directional systems. Only, their phasing is anything but random.

Your "math" by the way, is exhibiting a fundamental gap in your understanding of how that combiner with 20 db isolation works. That only means that a signal into the input of port 1 will be 20 db down at the input of port 2, and visa versa. Since there is minimal isolation between ALL input ports, to the common output port, and insertion loss is identical, that 20 db isolation figure is not applicable. It's a simple fact that 180 degree out of phase signals applied to the inputs will result in no signal at the output. Depending on the direction of the incoming wave front hitting the antennas, the signals from all the antennas will arrive at various phase angles, and randomly add and subtract, creating an antenna pattern that would more resemble a bug splat than an omni-directional pattern.

The aircraft and marine industries use this method, having two or more antenna feed a combiner to have more than one antenna feed one or more radios.
You're making a couple of apples and oranges comparisons here. In an aircraft, the upper and lower antennas are isolated, with substantially different coverage patterns. It's also very rare in general aviation, actually. Being a pilot, and having avionics experience, I've NEVER seen multiple antennas combined on anything from a Cessna 150 to a King Air. I HAVE seen antenna splitters used, though. I've never seen VHF antennas combined on a boat, either.

Are you sure you're not thinking about cases with multiple radios, multiple antennas, where each radio gets it's own? That's the way every installation in boats or aircraft that I've ever seen or worked on is done.

Aircraft use a upper antenna and lower antenna feeding a combiner to allow antenna coverage to allow communications when in high pitch, roll, and yaw positions. They rely on the isolation factor not having the out of phase signals cancel each other, its works very well.
Yes, that COULD work ok on an aircraft. See my comment above. Even then, there could be some pattern weirdness. Again, it's not a common technique on general aviation aircraft. Possibly on fighters that DO experience extreme pitch and roll angles, but I'd bet the antenna patterns and placement on the aircraft are very carefully considered. Not my field of expertise though, so I couldn't really say. But the point is moot... you're not trying this on an aircraft, are you.

My modified Scantenna and Discone Antennas feed a two port combiner, and a single coax runs down to another two port combiner that feeds two scanner radios, it also works very well. For the garage radios, one needs to keep an ear on the goings on when working ah.
Well, if it works for you, that's great. But I bet you would be shocked at how weird the pattern looks if you were to plot it. As long as signals are in saturation, a scanner user has no idea whether he's 20 db up or down because of a null or lobe.

I have some large diameter aluminum tubing that I will use to help broadband the VHF Aero Band, and VHF High Band Ground Plane Antennas. Or maybe even use the small 3 or 4 inch dia coffee cans. But I am still a few months away from putting up the 4 antenna array until the new air condition and heater system is put in.
Seems like an awful lot of work for what amounts to questionable benefits. But if it's fun, that's the point. For systems that are not fun, and seem more like work, I would NEVER attempt such a combining system.

Just for clarification, you keep mentioning feeding more than one radio with this. I hope you understand that my objection is NOT in feeding more than one radio. That's fine. It's combining the multiple antennas with overlapping frequency and pattern coverage that's the problem.

But go ahead. *shrug* Build it. Me? I know enough not to. My discones work just fine, thank you.
 
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trooperdude

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Diplexers or Triplexers are the accepted industry method of combining multiple antennas into one receiver.

The Electroline multicouplers are used to distribute ONE signal to multiple receivers, not multiple antennas to one receiver.

That one signal can come from a single antenna or the output of a diplexer or triplexer. (combining 2 or 3 separate antennas).

I use a custom triplexer fed by band-specific antennas, then that ONE output is fed into a multicoupler, and finally to the multiple receivers.

Thus, I have 3 band-specific antennas feed into the triplexer, the single output from the triplexer feeding into the multicoupler, which feeds 8 separate receivers with just enough amplification to have 0db loss throughout the system.
 

zz0468

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trooperdude, that is exactly the way it's supposed to be done. =)

One further refinement you could try, if needed, would be to put separate low noise preamps between each antenna and the triplexer, again, with just enough gain to over come the splitting and combining losses. Any gain applied AFTER the splitters/combiners suffers in degradation of noise figure on a db to db basis, equaling the total losses in front of the preamp, so amplifying AFTER combining to one coax will lose weak signals that no amount of gain can recover.
 

thewenk

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Diplexers or Triplexers are the accepted industry method of combining multiple antennas into one receiver.

The Electroline multicouplers are used to distribute ONE signal to multiple receivers, not multiple antennas to one receiver.

That one signal can come from a single antenna or the output of a diplexer or triplexer. (combining 2 or 3 separate antennas).

I use a custom triplexer fed by band-specific antennas, then that ONE output is fed into a multicoupler, and finally to the multiple receivers.

Thus, I have 3 band-specific antennas feed into the triplexer, the single output from the triplexer feeding into the multicoupler, which feeds 8 separate receivers with just enough amplification to have 0db loss throughout the system.
That is exactly how I do it, except I only use two antennas (Yagi & all band) and a diplexer to separate my 700 MHz and above frequencies. I then use the multicoupler to feed multiple scanners. I tried combining the antennas without the diplexer and that didn't work at all.

Dave
 

kb2vxa

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"You will come up with many posts made in the past."

Yeah? And your point is?

"Lets get at it and lets see where it will take us!."

Right on it boss, these guys can't resist a TROLL even when you tell them straight out you're baiting them.
 

davidmc36

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In an aircraft, the upper and lower antennas are isolated, with substantially different coverage patterns. It's also very rare in general aviation, actually. Being a pilot, and having avionics experience, I've NEVER seen multiple antennas combined on anything from a Cessna 150 to a King Air. I HAVE seen antenna splitters used, though. I've never seen VHF antennas combined on a boat, either.

Are you sure you're not thinking about cases with multiple radios, multiple antennas, where each radio gets it's own? That's the way every installation in boats or aircraft that I've ever seen or worked on is done.

I have to agree with zz here. Having been in the aviation industry for 22 years, several of them doing Avionics installations, working on everything from 152's to 727's, doing all three trades (Mechanical, Structural, Avionics) and currently a licensed Aircraft Mechanic (they call us AME's north of the border) I have never once, not ever, seen two VHF communication antennas hooked to the same radio. The main reason is about redundancy and being able to have both pilots talking at the same time on different radios. Having one antenna on the top and one on the bottom isolates them so they can be used simultaniously. Possibly more important is the redudancy factor (aircraft have two of all the important components in case one fails). A slightly less important factor is being able to choose to use one or the other depending on whether you are talking to somebody that is above you or somebody that is below you, but it is an added bonus and typically the "Number 1" radio, as they are reffered to, is on top and the "Number 2" radio is on bottom. This is not a hard and fast rule, just typical.
 

whitesox4life

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Question then...

I am purchasing a Uniden BCD996T. I am going to set up a live feed of my area with it. Is it wise to purchase band specific antennas and combine them into a single receiver or just go with a broadband antenna?
 

Scott_PHX_APP

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Not an expert on the aircraft top and bottom antenna thing, but when working at a company that designs and manufactures aircraft Xponders and other avionics, the top and bottom comm antennas CAN be connected to the same radio, BUT. There's that "but" part. :D The radio is designed to switch between the top and bottom based on if it's on the ground or in the air. Top is used for ground comms and bottom during flight. The radios have an input to them from the flight computers that tell them what to do, (top/bottom). Also, at least on some of the units I read about, the radio will receive on the antenna with the strongest signal and xmit on that same antenna. Again, not being an expert on this, it's just what I remember reading and from talking with engineers that designed the xponders and other equipment. I don't know how recent their information was or is, but hey, you get what you pay for... :D
Later...
 

davidmc36

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That seems perfectly plausible that some systems may have something that automatically switches to the preffered antenna vs what I described in my post where the pilots would have to remember which one was on top and which was on bottom to chose the best radio to use. The princple remains the same that one radio would never be "hooked to two antennas". The switching unit would take care of it in a "virtual" sense. The radio would never use two at the same time.
 

breadtrk

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I'll bite.

What is the difference in this http://www.radioreference.com/forums/showthread.php?t=23054 and running three different antennas and using a matching network in the shack? What you have there is 3 different antennas with the matching network at the antenna, why not match them in the shack?

Remember, we are talking scanners, RECEPTION only so all the puffery about airplanes and transmitters do not apply to the gentleman's basic question.
 
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N_Jay

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Everything past the matching network IS the antenna.

So I guess you have to decide if you want the transmission line to be "transmission line", or "Antenna".
 

davidmc36

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I'll bite.

What is the difference in this http://www.radioreference.com/forums/showthread.php?t=23054 and running three different antennas and using a matching network in the shack? What you have there is 3 different antennas with the matching network at the antenna, why not match them in the shack?

Remember, we are talking scanners, RECEPTION only so all the puffery about airplanes and transmitters do not apply to the gentleman's basic question.
What scott was talking about was a recieve situation. I actually talked to an avionics guy at work who has worked on systems that have a "magic box" that will do as scott said, sense which antenna is getting the best signal and use that one for the reception of that transmission. It is important to not recieve on more than one antenna at a time, also confirmed by the avionics engineer. As far as the ground-planes that you pointed out in the other thread, they look very similar to what I have on the top of my tower, and mine works very well. Not sure what the difference would be between that and the matching network but I suspect one difference would be that they are very close together and three separate ground planes would have to be space quite a ways apart.
 

breadtrk

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What scott was talking about was a recieve situation. I actually talked to an avionics guy at work who has worked on systems that have a "magic box" that will do as scott said, sense which antenna is getting the best signal and use that one for the reception of that transmission. It is important to not recieve on more than one antenna at a time, also confirmed by the avionics engineer. As far as the ground-planes that you pointed out in the other thread, they look very similar to what I have on the top of my tower, and mine works very well. Not sure what the difference would be between that and the matching network but I suspect one difference would be that they are very close together and three separate ground planes would have to be space quite a ways apart.


I am able to space them several wavelengths apart but not so dern far apart that any multiband gain will be eaten by coax losses. It just seems to me that band specific antennas, properly matched and phased, spaced several wavelengths apart have to be better than the same antennas all bunched together.
 
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N_Jay

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I am able to space them several wavelengths apart but not so dern far apart that any multiband gain will be eaten by coax losses. It just seems to me that band specific antennas, properly matched and phased, spaced several wavelengths apart have to be better than the same antennas all bunched together.
Yes and no.

They will also become directional, so it all depends on what your goal is.
 

danb474

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Here's my situation: I currently have a RS 20-176 mounted on a 5' mast at the peak / side of my roof. My coverage is quite adequate, with the exception of an 800 MHz system that sits on the other side of a hill. That 800 MHz system is hit and miss. I probably receive about 1/3 of the frequencies ok, the rest either won't break the squelch or are very scratchy. There are two control channels used. One I get constantly, the other will sometimes not be received by my scanner.

My plan was to raise the antenna onto a 10' mast, and also to add an Antenex 800 MHz specific vertical with 6db gain (FG8246 I believe). Is this possible and practical? It sounds like I will want a diplexer to combine the 800 antenna with the general coverage RS antenna. If this is correct, would I want to put the diplexer closer to the antennas, or further down the line next to my scanner (about 50' of coax later)?
 
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N_Jay

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It does not matter, but you will save cable by putting it near the antenna.
 
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