Duplexer Question

[ThomasMcKean]

Hi guys.

I am trying to figure a way to hook up a Channel Master 5094-A. I am trying to figure a way to wire it into two radios. A splitter is out for obvious reasons (duh), but a duplexer might work in my situation if I could find one?

What I am looking to do is put one end into a general scanner and the other end into a CB base. Now I know y'all are likely rolling your eyes that I am interested in hearing CB (and I do that myself about it sometimes), but yew have to understand I live close to two highways and I thought it would be fun to hear the truckers who drive by. Also note that the base (A Robyn SB-520D) has the microphone UN-plugged and stored away.

So what I need is something that separates around 30 or so. There are many out there and I have found them, but the situation is made more difficult because here in my area, the public safety is on EDACS (for now, anyway) and they are around 850 or thereabouts. And the duplexers I have found only go up to like 512 at the highest.

Is there a way to accomplish this or do I need to give it up?

 

[rescue161]

Duplexers are typically frequency bound, meaning that they are tuned to two different frequencies so that only those two frequencies are either blocked or will pass. They have a receive side and a transmit side.

You're not likely going to find anything close to what you are looking for.

Best bet is to just program the 40 CB channels into the scanner and hook it up, leaving the CB out of the loop. If you really need the CB and the scanner, then two antennas are going to be the cheapest and easiest solution.

 

[ThomasMcKean]

Duplexers are typically frequency bound, meaning that they are tuned to two different frequencies so that only those two frequencies are either blocked or will pass. They have a receive side and a transmit side.

You're not likely going to find anything close to what you are looking for.

Best bet is to just program the 40 CB channels into the scanner and hook it up, leaving the CB out of the loop. If you really need the CB and the scanner, then two antennas are going to be the cheapest and easiest solution.

I have CB channels programmed into a PRO-2006. That works well because banks are 40 channels. The PRO-2053, which I want to hook up the antenna to, only does 30 channels per bank but has pre programmed searches, one of which is for CB channels. So I may have to go that route.

 

[zz0468]

Mini-Circuits has some resistive splitters that cover the frequency range of interest. Isolation isn't that high, but in your application, that won't matter.

 

[prcguy]

An amateur diplexer like the Diamond MX610 goes way past 470MHz on the hi pass side but the loss will be a little worse. Using one of these would allow you to transmit on the CB while receiving on the scanner at the same time if the common antenna has a reasonable match on CB. I could sweep one of these diplexers and post the loss up to 800MHz if interested.
prcguy

 

[902]

Personally, I would go for a quick and easy solution, as you don't intend to transmit. I would go to your favorite store that carries television antenna supplies, get a 2 port 75 ohm splitter, then hook one port into the CB, the other into the scanner. Then hook up the antenna. You're good to go. Yes, there is a mismatch, but I don't think you'd notice it if you were using it for receive only.

 

[ThomasMcKean]

Personally, I would go for a quick and easy solution, as you don't intend to transmit. I would go to your favorite store that carries television antenna supplies, get a 2 port 75 ohm splitter, then hook one port into the CB, the other into the scanner. Then hook up the antenna. You're good to go. Yes, there is a mismatch, but I don't think you'd notice it if you were using it for receive only.

This is what I have decided to do. As yew know, splitting the signal with one of those creates a loss of 3.25dB, which is unacceptable to any true scannerphile. So what I did was get an inline 12dB amplifier that connects at the antenna. That way I can split the signal and still come out ahead. I think?

Would this solve the problem? I hope?

 

[Ed_Seedhouse]

Would this solve the problem? I hope?

It may well. The two problems are that the amplifier my be too strong and overload the front end of your radios, and this can cause all sorts of problems. Also TV amps tend to roll of below around 50 mhz and may actually attenuate your signal on the C.B. frequencies, depending on the design. If you have a local very strong signal the attenuation given by the splitter may actually help.

 

[902]

This is what I have decided to do. As yew know, splitting the signal with one of those creates a loss of 3.25dB, which is unacceptable to any true scannerphile. So what I did was get an inline 12dB amplifier that connects at the antenna. That way I can split the signal and still come out ahead. I think?

Would this solve the problem? I hope?

Be careful. You might solve one problem and create another, particularly with overload. So, let's say you lose that 3.25 dB in the split. With a 12 dB preamp, you'll have net gain of 8.75 dB. The preamp may become a place where mixing could occur. Additionally, without filtering in front of it, you may have some interesting issues from strong signals that are not even in the frequency range you intend to hear. You will be using a very broad device. Try it, but your results may depend on what kind of environment you are in.

What I would recommend is that you try it with just the antenna into the splitter. Then, if you can hear everything you need to hear, you don't need a preamp. Believe me, a 3 dB difference is negligible in the grand scheme of things. Either way, post your results. I'd love to see how it works for you.

 

[DisasterGuy]

I really would not worry about a 3.5dB loss. Think about it this way, what is the ERP of a 100 watt radio when the antenna has a 7.5dB gain, feed line has a 2.5dB loss and a cavity filter has a 3dB loss? The point is that the loss is minute in the equation...


Sent from my iPhone using Tapatalk

 

[zz0468]

This is what I have decided to do. As yew know, splitting the signal with one of those creates a loss of 3.25dB, which is unacceptable to any true scannerphile. So what I did was get an inline 12dB amplifier that connects at the antenna. That way I can split the signal and still come out ahead. I think?

Would this solve the problem? I hope?

It may not really help, and in fact will probably hurt. Look up the threads that talk about preamplifiers, particularly noise figure and third order intercept (IP3).

Then take a look at this chart:

http://wa8lmf.net/miscinfo/dBm-to-Microvolts.pdf

The extra loss only makes a difference in signals that are not saturating (full quieting), and when you get down to that level of signal, the 3db loss going through the splitter is like the difference between 0.3 uv and 0.2 uv.

Play with a receiver and a signal generator, and you will quickly realize that it's hard to distinguish the difference between two weak signals only 3 db apart. But adding a poor quality preamp can harm the overall noise figure and IP3 of the receiver.

Those "true scannerphiles" can relax about their 3 db splitter loss. They'll never miss it.

 

[ThomasMcKean]

It may not really help, and in fact will probably hurt. Look up the threads that talk about preamplifiers, particularly noise figure and third order intercept (IP3).

Yew guys are all giving me info contrary to what I have read and heard elsewhere. Certainly this requires some testing and experimentation. I'll def have to get back to yew on this with my findings!

 

[902]

This is from Hamtronics:

WHEN IS A PREAMP BENEFICIAL?

It is tempting to hope that a preamp can make any receiver more sensitive, in any situation. But, it is important to understand what happens when you add a preamp before a receiver.

A preamp can help overcome a deficiency in receiver sensitivity only if the noise figure is poor, either due to the design or because a lossy filter or cable adds to the noise figure. For instance, a preamp up at the antenna can overcome the effects of coax cable loss.

However, adding gain in the front end raises all signal levels; so, in effect, every dB of gain added overrides one dB of i*f selectivity or dynamic range. Therefore, adding a preamp can result in intermod or desense. The only way to know is to try it.

===

All that said, I'm all for experimenting. That's how you learn. Good luck!

 

[ThomasMcKean]

However, adding gain in the front end raises all signal levels; so, in effect, every dB of gain added overrides one dB of i*f selectivity or dynamic range. Therefore, adding a preamp can result in intermod or desense. The only way to know is to try it.

===

All that said, I'm all for experimenting. That's how you learn. Good luck!

It would be nice if these inline preamps had an on/off switch at the base. But then again maybe they do? Maybe I can just plug it in and unplug it? Awkward, but it may work. If it does, I may have to rig up some kinda less awkward switch for it.

 

[zz0468]

Yew guys are all giving me info contrary to what I have read and heard elsewhere.

Considering how much bad information on preamps is out there, this shouldn't be too surprising. You've been given some solid, sound advice in this thread. Adding a preamp to a receiver and having it actually help more than it hurts is a science unto itself, and there's no black magic or "art" to it.

The noise figure MUST be lower than the receiver that follows it, or it's not helping anything. The IP3 spec MUST be better than the receiver that follows it. You MUST not add too much gain, or the following stages WILL overload and cause problems. A preamp MUST only have just enough gain to overcome the noise figure of the following stages.

Adding a preamp just to compensate for the 3 db loss of a single splitter is probably a waste of money and effort, unless the receivers after the splitter could benefit from a better noise figure to start with.

Certainly this requires some testing and experimentation. I'll def have to get back to yew on this with my findings!

"you" or "yew"?

A "yew" is a species of evergreen plant.

 

[ThomasMcKean]

Considering how much bad information on preamps is out there, this shouldn't be too surprising. You've been given some solid, sound advice in this thread. Adding a preamp to a receiver and having it actually help more than it hurts is a science unto itself, and there's no black magic or "art" to it.

it is the same in the autism field I work in. There is SO MUCH MISINFORMATION out there that new parents have no idea what to do, where to go, or what to think.

I guess, as others have noted earlier, that the only way to find out personally whether or not it will work for ME is to experiment. That I will do. Here is some good news. Since I last posted, I have heard from the electric company that I have "no conflicts" regarding putting the antenna up and I can safely pound the mast into the ground without fear of hitting any wires.

 

[902]

"you" or "yew"?

A "yew" is a species of evergreen plant.

At least we're not ewes or eews.

 

[prcguy]

If you use a Diplexer there will be minimal loss, maybe less than .3dB to 470MHz and even if the diplexer is not rated to 800MHz the loss may only be a dB or so up there.
prcguy

Yew guys are all giving me info contrary to what I have read and heard elsewhere. Certainly this requires some testing and experimentation. I'll def have to get back to yew on this with my findings!

 

[ThomasMcKean]

If you use a Diplexer there will be minimal loss, maybe less than .3dB to 470MHz and even if the diplexer is not rated to 800MHz the loss may only be a dB or so up there.
prcguy

??? Are yew saying if a duplexer only goes up to 470 or 512 it will still pass signals that are around 800? I am still trying to figure all this out....

 

[902]

??? Are yew saying if a duplexer only goes up to 470 or 512 it will still pass signals that are around 800? I am still trying to figure all this out....

There is some confusion in the industry about the term "duplexer" especially when the line crosses from commercial/public safety to ham radio. To the commercial/public safety guy, a duplexer is a series of cavities or ferro-resonant devices that allow you to transmit and receive from one antenna. The things that ham radio manufacturers make would pass as "cross band couplers" in the commercial/public safety world. They are usually a low pass and high pass filter. Not much more than one or a few capacitors and inductors (coils). The guy at the ham radio store might understand duplexer to be either that thousand dollar set of filters, or the $75 coupler. The commercial/public safety guys will just tilt their heads at a 45 degree angle.

Sorry, I had to get that out (I've also got a hang up about "public safety" vs. "public service").

Would that coupler work up to 800? Short answer is "maybe." The test is to put a spectrum analyzer on one output port, terminate the other with a 50 ohm load, then put the sweep generator into the input. Then you look at the trace to see how wide the response is, where the cutoffs are, and what the insertion loss is. Depending on how it's put together, "maybe."

The CATV 3 dB splitter has no frequency-dependent components. It just takes whatever goes in and splits it to two ports.

Can you take a preamp and vary the gain? Maybe. You might be able to vary the bias (the supply voltage). I've seen that work, but these devices are intended to operate within a given range and strange things might happen, like they might break into oscillation, and that would create noise. Or, you could pad down the input with 50 ohm pads. These are resistor networks that attenuate a given amount of signal. That probably would not be worth your while, but go ahead and try it for yourself. Make notes and if you can, take measurements. See your results.

I think it's great that you want to do this stuff! I did the same a long time ago!

BTW, I also found some of your articles. I appreciate you writing in the first person and sharing your perspectives.