3 antennas into one scanner/RTL-SDR

[JaydenW]

I'm looking to connect 3 different antenna's into my Uniden BCD536HP or my RTL-SDR(I'm upgrading to an RSPduo soon.)

There are three different things I'm trying to listen to;

VHF Ham Band and the VHF Public safety band: 136-174 Mhz.
UHF Business Band: 460-470 Mhz.
700/800 Mhz Public safety: 769-860 Mhz.


I have a Diamond D130NJ Discone antenna that I will use for the VHF portion,
Either a Comet CA-F72GF or a Comet CA-712EFC for the UHF Business Band portion,
And a L-COM HGV-706U for the Public Safety portion.

Questions:
First off, is this even possible?

Does a triplexer exist that can do this and is it even called a triplexer?

If it does not exist, is there a Diplexer(duplexer?) that exists that can join together the UHF business band and 700-800 public safety band?


I'm only looking to receive signals, not transmit.


Please help

thanks

 

[jtwalker]

Yea such a thing exists. It will need band pass filters for all three inputs (connectors to antennas). I have yet to find one that fits into my budget but someone here may be able to point you appropriately.

 

[JaydenW]

Yea such a thing exists. It will need band pass filters for all three inputs (connectors to antennas). I have yet found one that fits into my budget but someone here may be able to point you appropriately.

Forgive my lack of knowledge and ignorance, but what exactly is a band pass filter?

 

[dave3825]

The diamond is a wideband 25 to 1300 so not sure how that would work on a triplexer with 400 and 7/800 antennas

The point of a triplexer is to combine signals from 3 different ranges.

 

[JaydenW]

The diamond is a wideband 25 to 1300 so not sure how that would work on a triplexer with 400 and 7/800 antennas

I can get a vhf antenna instead of using the discone. I was hoping the discone would be enough but its not good enough at 440+ and 850+

 

[jtwalker]

Forgive my lack of knowledge and ignorance, but what exactly is a band pass filter?

It will be built into one of these devices. They apply a high and low frequency limit to what RF signals are allowed in from the antenna port.

Search for “Microlab BK-24-N” as an example.

 

[JaydenW]

It will be built into one of these devices. They apply a high and low frequency limit to what RF signals are allowed in from the antenna port.

Search for “Microlab BK-24-N” as an example.

Whew, that things costs a pretty penny. Again please forgive my ignorance but is there any chance this thing would work?

Comet Antennas CFX-431A Comet Triplexers | DX Engineering

Free Shipping - Comet Triplexers with qualifying orders of $99. Shop Duplexers and Triplexers at DX Engineering.

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Or this?

Comet Antennas CFX-4310C Comet Triplexers | DX Engineering

Free Shipping - Comet Triplexers with qualifying orders of $99. Shop Duplexers and Triplexers at DX Engineering.

www.dxengineering.com

x2

Comet Antennas CFX-431B Comet Triplexers | DX Engineering

Free Shipping - Comet Triplexers with qualifying orders of $99. Shop Duplexers and Triplexers at DX Engineering.

www.dxengineering.com

 

[jtwalker]

I’ll let someone else provide you with better guidance than I can but I notice all the cheaper options don’t allow the frequencies through that I need. For example, all that you list show the high pass lower limit at 840.

 

[wtp]

how old is the coax ?

 

[dave3825]

How far away are you from the signals you’re looking to receive? Sometimes a cheap LNA on a discone works pretty good.

 

[mmckenna]

A triplexer is what you need, but I wouldn't use those ham grade ones. Those have their bands set up around ham frequencies and that won't help you for public safety type listening as well as one specifically designed for those sections of the band. That'll add to the cost.

You don't want to just "Tee" all the antennas together, as that can make performance worse. The triplexer will have the bandpass filters that allow the specific frequency range for each antenna to pass and be combined into the common feed.

No, they aren't cheap. The cheap solution is to run three separate runs of coaxial cable and either swap them out depending on what you want to listen to, or use a 3 position switch.

Discones are not great antennas. The only thing they have going for them is their wide bandwidth. Most of the hobby ones start to suck pretty bad as you go up in frequency. Usually around 800MHz, the radiation pattern starts to point up above the horizon and won't work as well as the lower bands.

While you are looking at all this, don't overlook the other basics:
Good coaxial cable. That's important. The triplexer will let you run one good run of cable to the radio, which can save you some money as compared to running three separate runs.
Make sure that if you are going to do all this, you properly waterproof all your outdoor connections. A wrap of electrical tape is not appropriate waterproofing.

 

[Ubbe]

Whew, that things costs a pretty penny. Again please forgive my ignorance but is there any chance this thing would work?

The CFX-4310 seems to be the best but will still have some dB of attenuation, at 700MHz its 8dB and at the lowest frequency of 769MHz in the band it's something like 3dB-4dB but that might be acceptable to you if the signal isn't so weak that it barely opens squelch when the antenna are connected directly to the scanner without any triplexer in line.

https://natcommgroup.com/wp-content/uploads/2023/09/CFX-4310C-manual.pdf

/Ubbe

 

[merlin]

Your discone should work well for all 3 bands. If you use 3 antennas for their gain, a combiner/triplexer will work at the expense of a few Db loss.

 

[merlin]

I can get a vhf antenna instead of using the discone. I was hoping the discone would be enough but its not good enough at 440+ and 850+

You could use a preamp befor the scanner/SDR but after a BC band notch filter. Even quality preamps can be found cheaply.
I use a cable TV drop amp ($10 on ebay)Look for very low noise and good 3rd order intercept. 8 Db gain should make up for coax loss.
One I use has 15 Db gain, but I have a 10 Db attenuator in line, or it overloads my scanner, even with that attenuator on.
Thing with CATV preams is they have a slope gain, increasing with frequency. the result is a flatter signal from low band through 900 MHz.