Diplexer Help Please

[joey_corleone82]

Hello all. I have a uniden 796d mobile scanner. I would like to hook up 2 antenna's , 1 antenex scanner antenna for VHF/UHF and an antenex PHANTOM for 800mhz digital trunked. I understand I need a diplexer to do this, but I can't seem to find much information on what connectors are on the diplexers or what I need.

The scanner and both antennas have BNC connectors. all the diplexers I've seen say something about N connectors, or PL-259 and I have no idea how I'd connect those interfaces, please help!

Also, if anybody has any input on using a good 800mhz antenna for michigan digital trunk (mpscs) let me know

thanks!

 

[MacombMonitor]

Hello all. I have a uniden 796d mobile scanner. I would like to hook up 2 antenna's , 1 antenex scanner antenna for VHF/UHF and an antenex PHANTOM for 800mhz digital trunked. I understand I need a diplexer to do this, but I can't seem to find much information on what connectors are on the diplexers or what I need.

The scanner and both antennas have BNC connectors. all the diplexers I've seen say something about N connectors, or PL-259 and I have no idea how I'd connect those interfaces, please help!

Also, if anybody has any input on using a good 800mhz antenna for michigan digital trunk (mpscs) let me know

thanks!

The RF Systems SP-3 can be used as an antenna splitter or combiner. It allows two scanners to be connected to one antenna without interaction and with minimal signal loss. Or it may be used for two antennas connected (combined) into one scanner. Operational from 10 to 2500 MHz. This is a passive (not amplified) device.

Click here:

http://www.universal-radio.com/catalog/preamps/0973.html

 

[kb2vxa]

Hi Joey, Macomb and all,

"It allows two scanners to be connected to one antenna without interaction and with minimal signal loss."
3dB insertion loss is hardly minimal when it amounts to cutting the signal level in half. "Without interaction" only applies when you're using it as a splitter and it means one scanner will not interfere with the other. When used as a combiner it plays hob with the signals, just follow me on this.

That's not a diplexer, it's a passive splitter-combiner having no band separation whatsoever. None of you guys seem to be following my thread about Diplexer Trek in which the fearless Captain Picknose is searching the galaxy for a diplexer for that exact purpose. Geordie LaFog finally came up with one custom made to my exacting specifications here in a nut shell.

Nominal bandwidth;
Low port 120-600MHz 1dB down @ 600MHz
High port 800-900MHz 1dB down @ 800MHz
Crossover @ 700MHz
Insertion loss 0.75dB
Available in 50 or 75 ohms impedance
Connectors customer specified

Tin Lee Electronics Ltd. Toronto Canada www.tinlee.com
Link to page http://www.tinlee.com/Diplexers.htm#Table: Standard models LH7 and Order Information

Once you have sussed it out as I have you may enquire of sales;
Mrs. Sneva Lee, E-mail: sneva@tinlee.com

This is still a work in progress and so far the best lead I have. A point worth mentioning is a diplexer affords filtering and band separation quite unlike a combiner. I'm using a combiner now and have a nasty problem with stray signals arriving out of phase from the 800MHz antenna and partly cancelling signals arriving from the VHF/UHF antenna. Naturally such signal loss is something we don't want, that's why we use the best coax and coupling devices available at reasonable price. Enter the diplexer, those stray signals are attenuated by more than 35dB which eliminates them for all intents and purposes! Add to that 0.75dB insertion loss vs. 3dB for a combiner and you can plainly see the advantage, it's almost as good as using each antenna singly and scanning each band separately. Well, we don't want to do that so the diplexer combining signals from two antennas with no interference one to another and negligable insertion loss is the way to go.

 

[MacombMonitor]

Hi Joey, Macomb and all,

"It allows two scanners to be connected to one antenna without interaction and with minimal signal loss."
3dB insertion loss is hardly minimal when it amounts to cutting the signal level in half. "Without interaction" only applies when you're using it as a splitter and it means one scanner will not interfere with the other. When used as a combiner it plays hob with the signals, just follow me on this.

That's not a diplexer, it's a passive splitter-combiner having no band separation whatsoever. None of you guys seem to be following my thread about Diplexer Trek in which the fearless.

I didn't imply my suggestion was a "diplexer". I offered a solution to his needs, at a reasonable price. I understand insertion loss is a consideration, but at what price? I suppose he could buy an active (amplified) version from someone like a Stridsberg's MCA102M, for $148.00.

When combining two antennas, to one scanner, what do you gain with your suggested "band separation", if in fact your combining the output of both separated bands back into one BNC connector, to the radio? What ever separation you may have had is gone at that point! Besides, he doesn't want band separation, he wants all the frequencies to get through equally.

I could understand possible benefits of band separation if you were attaching two scanners to one antenna, but that's not what he's doing.

No question, single band antennas tuned for the band you want to receive are better than multi-band antennas. However if you take two different single band antennas, and combine them, I think even the loss involved with the numerous coax connectors required to accomplish the hook-up, would almost defeat what was gained to begin with.

Bottom line, it probably isn't even necessary. A good multi-band antenna should do the job nicely. I can understand if you're using the 800MHz Phantom, to keep a low profile. But if he's also installing a good VHF/UHF antenna...there goes the stealth advantage of the 800MHz Phantom. You can say what you want, but the inexpensive wide-band Valor PMM3B for $12.00 has thousands of very satisfied customers! Or the Antennex ASCANC would be another good choice.


You didn't say what your device cost?

Thanks!

 

[joey_corleone82]

thanks for the replies guys. Macomb, I believe what vxa is saying is that with a true diplexer, it is essentially just a low pass filter and a high pass filter, so you wouldn't be combining signals and passing a combined signal to the output, what you'd be doing is taking the signal from both antennas into the diplexer , then the diplexer would pass it's output to the scanner from one of the 2 antennas based on the frequency.

For instance lets say port 1 was 0-150 Mhz and port 2 was > 150Mhz (just an example I made up)
so you tune your radio to say 130Mhz or whatever...... port 1 would pass the signal to output, port 2 would not pass whatever signal it happens to be getting to output. Is that right VXA?

- Joe (kd8bxf)

 

[joey_corleone82]

oh btw, I ordered the ASCANC and the 821-896Mhz tuned PHANTOM they should be here today. I'm going to experiment, but I would have to imagine that the PHANTOM tuned for specifically 800mhz band will outperform the ASCANC significantly on that particular band...and monitoring MPSCS and 800mhz digital is alot of what I want to do, so thats the whole reason for this mess

- Joe (kd8bxf)

 

[MacombMonitor]

oh btw, I ordered the ASCANC and the 821-896Mhz tuned PHANTOM they should be here today. I'm going to experiment, but I would have to imagine that the PHANTOM tuned for specifically 800mhz band will outperform the ASCANC significantly on that particular band...and monitoring MPSCS and 800mhz digital is alot of what I want to do, so thats the whole reason for this mess

- Joe (kd8bxf)

Where are you in Michigan? There are so many towers, I can monitor MPSCS on a stock rubber ducky inside my SUV, with the hand-held sitting in a cup holder, down on the floor.

Bill

 

[MacombMonitor]

thanks for the replies guys. Macomb, I believe what vxa is saying is that with a true diplexer, it is essentially just a low pass filter and a high pass filter, so you wouldn't be combining signals and passing a combined signal to the output, what you'd be doing is taking the signal from both antennas into the diplexer , then the diplexer would pass it's output to the scanner from one of the 2 antennas based on the frequency.

For instance lets say port 1 was 0-150 Mhz and port 2 was > 150Mhz (just an example I made up)
so you tune your radio to say 130Mhz or whatever...... port 1 would pass the signal to output, port 2 would not pass whatever signal it happens to be getting to output. Is that right VXA?

- Joe (kd8bxf)

I'm still trying to grasp this theory. How would the diplexer sense what frequency your scanner was tuned to?

 

[joey_corleone82]

hey macombmonitor, I live in Sterling Hts, but I commute down to Dearborn for work everyday, and am interested in monitoring state police when I drive around different places.

 

[MacombMonitor]

hey combinator, I live in Sterling Hts, but I commute down to Dearborn for work everyday, and am interested in monitoring state police when I drive around different places.

You're probably aware of RR's Database, but if not, they have a section that shows all the towers on MPSCS, and their frequencies. The frequencies in RED are the primary control channels, that you should program as a minimum. The frequencies in BLUE are the secondary control frequencies that they fall back on if there is a problem using the primary, but that doesn't happen too often. The frequencies in BLACK are the voice channels, and do not need to be programmed at all.

Here is the link for the tower frequencies:
http://www.radioreference.com/modules.php?name=TRSDB&sid=100

This is another helpful link that shows tower locations.
http://radiowurx.com/mpscs/

If you click on a tower icon, it will show you the related site I.D., frequencies, etc. The blue icons have been confirmed, the red icons have not been confirmed. The map has not been updated in some time, so it may not be complete, and you might want to double check the maps info against RR's Database, which is more current at this time.

On your BC796D, you can only program one tower's primary, and secondary per bank. Program only the necessary towers along your routine path of daily travel.

Also, on the BC796D (and BC296D for those of you reading this), be sure to refer to page 45 of your manual, regarding the setting of the proper PLAN, (1,2,3, or 4), on a per tower (and in this case, per bank) basis! This is very important when attempting to decode in the control channel only mode.


You should not require any exotic, or expensive antennas to receive MPSCS. There have been some issues with Detroit, and Genesee County's simulcast portions of the system, but there not much anyone can do about it at this time. You can find more info in RR's archives about this never ending dilemma.

Keep us posted on your ability to receive Detroit in particular, if you would. We'd appreciate the input!

 

[Al42]

I'm still trying to grasp this theory. How would the diplexer sense what frequency your scanner was tuned to?

It wouldn't. As joey said, it's basically a low-pass filter and a high-pass filter combined. 150 MHz signals impinging on the 850 MHz antenna never reach the receiver, since they have to get through a filter that cuts off at 700 MHz.

As far as the advantage over a combiner, totally aside from the 2.25db less loss per antenna, a (for example) 150 MHz signal would ONLY be arriving at the scanner from the 150 MHz antenna - the out-of-phase signal at the 850 MHz antenna would be attenuated in the diplexer by more than 1,000 times, so it wouldn't be there to interfere with the signal coming from the 150 MHz antenna. Signals coming from more than 1 antenna at a time can produce strange effects, up to and including complete cancellation (which is how phased arrays work). And the cost of a decent diplexer is a lot less than the cost of an amplified multicoupler - Diamond has them for a few different frequency range combinations.

 

[joey_corleone82]

OK I'm still unsure of the connectors though. do they make one of these diplexers that will work with bnc connectors?

 

[MacombMonitor]

Help me to understand...if the scanner is programmed to receive both 850MHz signals, and 155MHz signals, and both signals are present, and active at the same time, how does the diplexer determine which one to give preference, or priority to?

Thanks!

 

[joey_corleone82]

MacombMonitor said: "On your BC796D, you can only program one tower's primary, and secondary per bank"

Right now I have all the unique primary's and secondary's programmed into 1 bank. I understand that the 796d cannot and will not track all of the ones it receives at the same time, but I figure what will happen is that it will lock on and trunk track the first one it picks up...this way I can travel across the whole state and always have the closest tower....if I'm in range of 2 or more towers I can always find out the one that is clearly closer with better signal strength by manually scrolling through them all, and then lock out the weaker one temporarily. This solution will work yes?

 

[joey_corleone82]

Help me to understand...if the scanner is programmed to receive both 850MHz signals, and 155MHz signals, and both signals are present, and active at the same time, how does the diplexer determine which one to give preference, or priority to?

Thanks!

Yes I'm actually curious about this too. I wonder if it has anything to do with what freq the radio is tuned to receive at that particular time?

 

[K5MAR]

Help me to understand...if the scanner is programmed to receive both 850MHz signals, and 155MHz signals, and both signals are present, and active at the same time, how does the diplexer determine which one to give preference, or priority to?

Thanks!

The diplexer combines the signals from two different antennas. That's all. It doesn't (ideally, at any rate) give preference to any band. (This is when using it to combine two different band's antennas to one receiver.) It provides a signal to the receiver that contains all the bands. It's the receiver that selects the signal appropriate to the band. Think of it as a little multi-gate entrance inside the scanner dividing all the VHF-low, VHF-high, UHF, and 800 MHz into their respective RF stages. That's overly simplified, I know.

Somebody also inquired about a diplexer with BNC connectors. I haven't seen any, at least not by the majors like Diamond or Comet. You could use adapters, but you would be better off feeding the diplexer with the proper connectors it needs, PL or N-style, then making an output jumper with the PL or N connector on one end and the BNC on the other.

Mark S.

 

[K5MAR]

MacombMonitor said: "On your BC796D, you can only program one tower's primary, and secondary per bank"

Right now I have all the unique primary's and secondary's programmed into 1 bank. I understand that the 796d cannot and will not track all of the ones it receives at the same time, but I figure what will happen is that it will lock on and trunk track the first one it picks up...this way I can travel across the whole state and always have the closest tower....if I'm in range of 2 or more towers I can always find out the one that is clearly closer with better signal strength by manually scrolling through them all, and then lock out the weaker one temporarily. This solution will work yes?

Works for me here in Oklahoma. It may not be the optimum solution, but it's either that or use one scanner just to scan the state's trunked system. That's something I'm going to when I get my new console configured and installed in my truck. One 780 for conventional radio systems, and another for the state trunked systems.

Where you might run into problems is when you are in an area where two systems are equal in strength but weak. As the signal level fluctuates, your scanner could end up hopping from one to the other, which would cause you to miss parts of conversations. On a statewide system, which has one set of IDs statewide, that's not a big problem. If you were scanning two systems that had the same ID assigned to different users, it might be more annoying. You might go from system A police to system B sanitation.

Mark S.

 

[Al42]

Help me to understand...if the scanner is programmed to receive both 850MHz signals, and 155MHz signals, and both signals are present, and active at the same time, how does the diplexer determine which one to give preference, or priority to?

Thanks!

It doesn't and that's not the problem. The problem is a single signal, say a 150 MHz signal, coming in on both antennas. Unless the signal is at exactly the correct angle, it'll be at different phases on the two antennas. If there's a 90 degree difference you lose half the signal. If there's a 180 degree difference you lose all the signal - which is how a direction-finding antenna works - you tune for zero signal, knowing exactly where, relative to the antenna base, the zero-signal point is. If the phase varies (say one of the paths is going through a tree that's waving in the breeze), the phase changes, so the signal fades in and out.

The diplexer keeps the scanner from getting any 150 MHz signal from the 850 MHz antenna (since that antenna is fed through a 700 MHz high-pass filter), so that particular problem is eliminated. Regardless of what frequency the scanner is tuned to. 850 MHz signals come down only one pipe, 150 MHz signals come down only the other pipe. (700 MHz signals come down both, but the crossover point is where no one is listening [at the moment].)

The scanner doesn't care if there also an 850 MHz signal at its antenna connection at the same time. 1) It's listening to 150 MHz and 2) there's a filter in the scanner between the antenna and the first RF amp (or should be there) that passes only 130-174 MHz (or 108-174, or however the designer cut the spectrum up), but definitely not 150-850 MHz in one chunk. (The filters are switched in and out depending on the frequency the scanner is listening to. Which is one reason a scanner not designed for mil air may be tunable to mil air, but you won't hear much - you're using either a 150 MHz filter or a 450 MHz filter, and neither one passes much 350 MHz signal.)

 

[Al42]

Right now I have all the unique primary's and secondary's programmed into 1 bank. I understand that the 796d cannot and will not track all of the ones it receives at the same time, but I figure what will happen is that it will lock on and trunk track the first one it picks up...this way I can travel across the whole state and always have the closest tower....if I'm in range of 2 or more towers I can always find out the one that is clearly closer with better signal strength by manually scrolling through them all, and then lock out the weaker one temporarily. This solution will work yes?

No - the scanner will lock to the FIRST one it finds, not the strongest one. That could even be the weakest one. If you manually lock it onto one control channel you're not trunking, you're listening to the control channel, which might be interesting if you were a computer running a tracker program, but meaningless to a human being. About the only way you can do it manually is to lock out all but the local tower frequencies. Considering that there may be 4 of them on each site, you'd better carry a list of all the sites you have programmed. At least program all the control frequencies - in the order you have them on your list - so that you can lock out everything, then just go to the first of the four channels and unlock 4 adjacent ones.

 

[kb2vxa]

Hi again,

It seems that some of you are confused even though I tried to keep it as simple as possible. OK, check out some of the posts further explaining band separation, no need to reiterate. Edit; Well, a bit downstream I changed my mind, read on. To answer a few questions I deliberately left hanging (I thought you understood I'm having a diplexer custom made), here we go.

Price varies with what you're having made as with anything just like a custom bike or rod. Nobody mentions how much they paid to have the Teutels build thier custom chopper, they're all different so they cost different, that's what makes them custom. I'll give you a clue, mine will set me back $100 including shipping. No, not the chopper, the diplexer. MIKEY RULES! (;->)

As for the connector question, same as above, it's a custom job. You can have the connectors of your choice. Just submit the electrical and mechanical design specifications and they'll build it.

Now to tidy up a few loose ends.

"However if you take two different single band antennas, and combine them, I think even the loss involved with the numerous coax connectors required to accomplish the hook-up, would almost defeat what was gained to begin with."
No Macomb, the idea is to keep the loss as low as possible and that's exactly what I'm doing. Please can that notion that connectors introduce loss, they don't. If that were the case radiocommunications systems couldn't operate with scores of connectors holding it all together.

"Bottom line, it probably isn't even necessary."
Oh but it IS and that's exactly why I'm doing it that way.

"A good multi-band antenna should do the job nicely."
There's no such animal, that's what makes it necessary. ALL multi-band antennas are a compromise and I refuse to compromise on quality.

You've got it right Mafia Son, while all signals are combined the STRAY signals arriving from the "wrong" antenna aren't.

To clarify Al's point on insertion loss it's 0.75dB for the diplexer vs. 3dB for the combiner. I don't know where he came up with 2.25dB but I'm not splitting hairs.

"And the cost of a decent diplexer is a lot less than the cost of an amplified multicoupler..."
Al, that depends a LOT on the diplexer and the combiner, prices vary WIDELY. More to the point, an amplified "multicoupler" is unidirectional so is totally useless for this application. Somewhere in this forum I came across "unity gain" CATV multicouplers not designed to amplify but just enough to overcome insertion loss. On the other hand CATV diplexers are made specifically with return path in mind, one port provides the return on the 5-40MHz sub band.

OK, I think that about covers all the bases. Macomb, Al pretty much covered it so go back and read it again. (pause) OK, now you're back so think filters. The low port passes signals below 600MHz only and the high port passes signals above 800MHz only. The problem with the combiner (no filtering) is that stray signals below 600MHz coming from the 800MHz antenna are interfering with those coming from the VHF/UHF antenna. By filtering them out and allowing signals from only one antenna to pass there are no strays to interfere. My VHF/UHF antenna is stone deaf at 800MHz so the reverse is not a problem. In other cases stray 800MHz signals may also interfere on that band but in any case a diplexer keeps the antennas separate. Think in the abstract and you'll get it.

Now notice I said my main antenna is deaf on 800MHz. That's an extreme case (ham antennas are rather like that) but 800 is always a problem of one sort or another no matter what antenna you use unless it's specifically for 800MHz. That's the whole idea behind using two antennas and a diplexer. No comments from the strong signal peanut gallery please, you never notice problems when you live under the tower. (;->)

Since a picture is worth a thousand words this little graphic should help you visualize how a diplexer works.