multicoupler using RG-6 (75 ohm coax)?

[Progline]

The threads here regarding running multiple scanners from one antenna have been interesting. In my case, I now live in a rural area, mostly VHF-Hi and some UHF activity. I have a run of RG-6 available here in my home office that used to go to a DISH antenna. I successfully use this coax on an older RS VHF Hi-band quarter wave ground-plane vertical (the old "sputnik" antenna they used to sell) for one of my scanners. I would like to be able to use this antenna on both my RS-PRO-2005 (yeah, I know, it's old but an excellent scanner and we're still mostly analog here) and a GRE PSR-600. I can use a BNC T to split the feeds, but I wondered if maybe a multi-coupler would be a better approach for isolation? At some point, I might want to add a 3rd scanner to the mix (RS Pro-2055).

I'm kinda confused as to which way to go...Stridsberg MCA204M or some kind of CATV drop amp? The Stridsberg looks good but I'm not sure if it will play well with the 75-ohm RG-6? If you're familiar with the GRE's...they will roll over and die with any kind of pre-amp/gain so I'd need something as passive as possible.

Any thoughts?

 

[737mech]

Start out cheap and try. EDA 8 port has 3.5db gain and port isolation. Also put a FM trap in front of it. Strids aren’t doing so well on customer service lately according to some posts I’ve read here rr forums. Those are not considered cheap multicouplers. The catv amps are not specifically designed for scanners but I have used them for years and they do work.

 

[cavmedic]

If you are already running RG6, and didn't hack off the F connecter, you could get away with using a simple cable two way splitter. I am running a unity gain , 8 port drop amp being fed from a scantenna, but running three scanners , weather radio and have extra ports for add on later if I want. All RG6 feeds as well,

 

[Progline]

All the F-connectors are still on the RG-6. When I checked electroline's website, it seems they are out of the DropAmp market. I'm not sure which model would be best for a no-gain situation? I'd never need more than 4 ports.

 

[737mech]

EDA 2402 4 port 0db gain. Be careful on 4 port models they tend to be 8db gain (too hot!). 8 port models are usually 3-4 db gain per port.

 

[gmclam]

...I successfully use this coax on an older RS VHF Hi-band quarter wave ground-plane vertical (the old "sputnik" antenna they used to sell) for one of my scanners.

LOL. I have to ask what you mean by Sputnik antenna. Is it the dish? Which way is it pointing? If that's what you're using, I wouldn't expect it to be effective. Nothing wrong with older model radios.

I'm kinda confused as to which way to go...Stridsberg MCA204M or some kind of CATV drop amp? The Stridsberg looks good but I'm not sure if it will play well with the 75-ohm RG-6? If you're familiar with the GRE's...they will roll over and die with any kind of pre-amp/gain so I'd need something as passive as possible.

Any thoughts?

I wouldn't worry about RG-6 75 ohm vs something 50 ohm for receiving. If you are only driving 2 receivers, it's probably not worth the effort to amplify. If you need to amplify, be mindful of any strong signals. I specifically filter out AM & FM broadcast. I'd like to filter out a couple of cell or paging signals, but that's a lot more challenging.

 

[Progline]

"sputnik" was the slang term used on a YT video for this VHF-hi/800Mc groundplane. it's the simple one RS sold for years. Product ID: 20-176
https://www.eham.net/reviews/view-product/8026

 

[Progline]

EDA 2402 appears to be the no-gain one, vs the 2400 with gain. Unfortunately, the EDA-2400 and EDA-UG2402 seem to be hard to find these days, given Electroline's exit from the market. Plus no documentation available online. I think I have some CATV 4 port splitters here (with their attendant loss) I can try. As this is a rural area, RF congestion is not an issue...heck, it's hard to find an FM broadcast station!

 

[737mech]

There are other companies that make the same product. PCT for example. I have 2 8 port PCT amps. Model # PCT-VB-8P. +4db per port. 5-42 and 54-1000 MHz.

 

[cavmedic]

There are other companies that make the same product. PCT for example. I have 2 8 port PCT amps. Model # PCT-VB-8P. +4db per port. 5-42 and 54-1000 MHz.

Keep in mind that that 5-42 is amplifying those signals going outbound . Designed for cable modem and set top box return signals back to the Head end.

 

[737mech]

Keep in mind that that 5-42 is amplifying those signals going outbound . Designed for cable modem and set top box return signals back to the Head end.

Thanks I never paid attention much to that number as I do not monitor anything that low band. I pretty much stay with the normal scanner bands from aircraft and up to business 900.

 

[prcguy]

Keep in mind that that 5-42 is amplifying those signals going outbound . Designed for cable modem and set top box return signals back to the Head end.

I think these things are passive in the 5-42MHz reverse mode, no amplifier on that side.

 

[Progline]

I ended up using a CATV 3-way splitter I had on hand and it actually works pretty well! I'll follow up on this when it's time to upgrade the antenna further and use something other than the leftover RG-6 that was in place.

 

[cavmedic]

I think these things are passive in the 5-42MHz reverse mode, no amplifier on that side.

Some do , some don’t. Towards my exit from that career field, we stopped getting the drop amps with the active return because clowns weren’t cleaning up the noise generated in the houses, and never failed to be a dispatch at 2 am for the noise covering up the entire node pocket return path knocking hundreds of modems offline.

Plus we were moving towards E streamers instead of traditional set top boxes, so you only needed RF to the modem, the streamers either worked via WiFi or could use MOCA to backfeed the house.

 

[prcguy]

Some do , some don’t. Towards my exit from that career field, we stopped getting the drop amps with the active return because clowns weren’t cleaning up the noise generated in the houses, and never failed to be a dispatch at 2 am for the noise covering up the entire node pocket return path knocking hundreds of modems offline.

Plus we were moving towards E streamers instead of traditional set top boxes, so you only needed RF to the modem, the streamers either worked via WiFi or could use MOCA to backfeed the house.

I was DTH satellite, we were one of the first to use MoCA but I have a lot of experience designing big building coax distribution for satellite with three different satellite services in the same buildings, traditional amplifier/coupler/divider plus multiswitch, SWIM, MoCA, you name it . I would still prefer a 50 ohm LNA feeding a divider in a 50 ohm coax system for feeding multiple police scanners. It just works, no impedance problems, little to no noise from CATV amplifiers never intended to be used on an antenna and so on. There is always a right way to do things and compromised ways. Why compromise?

 

[cavmedic]

No doubt.

I played with them because I had them on hand.

 

[Ubbe]

Most scanners don't have a 50 ohm impedance over its frequency band. For a BC780 it looks as if 75 ohm are closer to the average impedance than 50 ohm. There are 3 filters in series for the 300-500MHz bandpass filter in a 780 so you will see 3 waves where the filters interact with the signal impedance.

It's a 0,2dB loss at each point of connection that have a 50 to 75 ohm mismatch. At 75 ohm you have a higher voltage in the coax compared to 50 ohm, that's why it it used for small signal systems that use levels in micro volt to keep as a high voltage as possible to distance it from the noise floor of the noise figure. For transmit you use power and 50 ohm are more suitable there but even lower are better but are a compromise if the coax are to be used with both TX and RX signals.

There are cheaper and better quality coax for 75 ohm as there are a huge amount of it being used and the competition keeps the price down compared to less use of good 50 ohm coax suitable for home use that makes it more expensive for the quality you get.

For a 1/2 wave dipol antenna its impedance will be 75 ohm and for most antennas that are designed for 50 ohm they only have that at the exact frequency they are cut for and outside of that frequency they usually begin to rise its impedance the further away you go in frequency. For the old Antennacraft ST2 antenna they used a 300 to 75 ohm transformer to make use of that higher impedance outside of its cut frequency but made it a bad mismatch to the cut frequency but as the final result made it more even in reception over a wide frequency range.

/Ubbe

 

[prcguy]

Most scanners don't have a 50 ohm impedance over its frequency band. For a BC780 it looks as if 75 ohm are closer to the average impedance than 50 ohm. There are 3 filters in series for the 300-500MHz bandpass filter in a 780 so you will see 3 waves where the filters interact with the signal impedance.
View attachment 181369
It's a 0,2dB loss at each point of connection that have a 50 to 75 ohm mismatch. At 75 ohm you have a higher voltage in the coax compared to 50 ohm, that's why it it used for small signal systems that use levels in micro volt to keep as a high voltage as possible to distance it from the noise floor of the noise figure. For transmit you use power and 50 ohm are more suitable there but even lower are better but are a compromise if the coax are to be used with both TX and RX signals.

There are cheaper and better quality coax for 75 ohm as there are a huge amount of it being used and the competition keeps the price down compared to less use of good 50 ohm coax suitable for home use that makes it more expensive for the quality you get.

For a 1/2 wave dipol antenna its impedance will be 75 ohm and for most antennas that are designed for 50 ohm they only have that at the exact frequency they are cut for and outside of that frequency they usually begin to rise its impedance the further away you go in frequency. For the old Antennacraft ST2 antenna they used a 300 to 75 ohm transformer to make use of that higher impedance outside of its cut frequency but made it a bad mismatch to the cut frequency but as the final result made it more even in reception over a wide frequency range.

/Ubbe

It may only be a 0,2dB loss in a resistive circuit but mixing 50 ohm components and 75 ohm components with mismatched coax will introduce more loss depending on length of cable, etc.

 

[Ubbe]

It may only be a 0,2dB loss in a resistive circuit but mixing 50 ohm components and 75 ohm components with mismatched coax will introduce more loss depending on length of cable, etc.

Maybe, but if 75 ohm are actually closer to the impedance of the scanner and the antenna, then a 50 ohm coax are more mismatched than a 75 ohm one.

/Ubbe

 

[vagrant]

I urge everyone to experiment and view the difference using the same length of 75 and 50 Ohm coax, but use the same antenna. Even if you need to use adapters, test the difference. Test a length you would actually use, versus say 500’. Use an SDR to get a visual of the dB results. Even if you use LMR-400 for the test and regular old RG6. Remember, this test is for receiving.

Perhaps review the following for testing, but not a signal so strong it’s blasting:
Local ATIS frequency (more than one if you can)
Local NOAA weather broadcasts
Local PD, Fire or whatnot repeaters around 450 MHz.
Go higher for 700, 800 and 900 MHz

Now look at the signal strength difference when using a cable TV splitter and without. Do not forget to take into account any advertised gain, or not, of the splitter.

Once you have your numbers, consider WHAT you will actually monitor and compare. Experiment people. You will be better for it.