multicoupler using RG-6 (75 ohm coax)?

[Ubbe]

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.

It's both costly and time consuming doing double installations and the differences will be so small that the weather and time of day will probably have a bigger impact on the result. You will have to test most of your frequencies and then quickly switch coax and any other devices in line before external conditions changes.

But if having dual coaxes you probably also get another antenna of a different type or model and as antennas are frequency dependent you shouldn't replace the old antenna, keep it and install the new one next to it. It will be one antenna that works best on some frequencies and the other will hopefully work best on other frequencies to complement each other.

Use one coax with one scanner and the other coax with another scanner, or have a coax switch or perhaps a diplexer to one single scanner.

/Ubbe

 

[G7RUX]

So this is quite a complex area to consider.

Firstly, very few scanner receivers have a 50 Ohm input impedance across anything like an appreciable portion of their operating range.

Secondly, 75 Ohm coax is actually lower loss than 50 Ohm, all other things being equal.

Thirdly, and perhaps most importantly, you already have the 75 Ohm feeder in place so there would have to be a significant advantage to support swapping it out. Personally, if I was swapping it then I would use the absolute best, lowest-loss cable I could get my hands on.

Finally, even if the receiver input were 50 Ohms the mismatch losses from 75-50 Ohm are less than half a dB so you would be really hard pressed to notice that, even with decent test kit to measure it.

Using a low-ish gain multicoupler/splitter fed with 75 Ohm feeder will be absolutely fine and won't introduce any noticeable effects; most LNAs that work over a reasonably wide band similarly don't have terribly flat input impedance across that range. Just make sure to keep the gain low, just to overcome the splitter losses (3 dB for a 2-way split, 6 dB for a 4-way, and so on) so you don't introduce other issues. There are decent quality, cheap TV/receiver splitters and distribution amplifiers which should do the job nicely.

 

[vagrant]

@Ubbe - Ohhh…that’s right about the time of day. Yes, testing in the morning, afternoon and night makes a difference on the results. I should have noted that. It was a slight pain, but I tested and then swapped the coax several times one day and logged the numbers. For another test I even compared LDF4-50A against LMR-400, LMR-240UF and RG6 over the course of several days, but noted the times to keep things fair-ish.

In summary, looking at the results there were no surprises, but if one wanted to keep their costs low RG6 definitely works. No need to chase gain when one monitors local repeaters/transmitters already blasting in. Alternatively, I enjoy listening to military aircraft chatter, so LMR-400 works for me. LDF4-50A has a premium cost and its weight along with low flexibility makes it a solution I would use if I was ”transmitting“ on 33cm and up. Receive only allows one to use 75 Ohm RG6 and have plenty of fun.

* The easier and better solution is using the right antenna, or multiple appropriate antennas for the tasks. A diplexer, switch and or multiple receivers may the prudent solution depending on one’s needs. Some inline filters will also keep things friendly regardless of antenna or coaxial cable used.

 

[N8YX]

Weighing into this as an EDA/multiple scanner user:

I'm located a ways away from the transmitter sites of major metro areas, and the antenna I've got installed at the moment is only up around 25 feet. Plus it feeds a bunch of different receivers. In this situation an EDA 2400 isn't enough amplification. What I did was to cascade EDA2800s off each of the 2400's ports and drive the receivers from them.

Upstairs listening post (in my main floor office) is a different set of challenges. There are a pair of PRO-2096s in use scanning the Ohio MARCS-IP 800MHz systems and I cannot get enough signal with cascaded 2400-2800s to get them to decode Summit and Portage County agencies. (They worked fine when I lived in Summit.) So...a 2200 will replace the 2800 as an experiment.

Does anyone in thread know whether or not the EDAs are active-return or passive over 5-42MHz?

Another thing I'm looking at eventually is the installation of some type of antenna for 40-55MHz coverage. A couple NRD-525s with converters and at least one PCR-1000/R-7000 pair will be used to scan and monitor that range.

 

[Ubbe]

In this situation an EDA 2400 isn't enough amplification.

Does anyone in thread know whether or not the EDAs are active-return or passive over 5-42MHz?

I would put a 15dB amplifier at the antenna and then use passive splitters, standard 1-6 CATV or whatever you need. Then you put an EDA at one of it output ports if you need more signal outputs than 6.

As I have seen, I think it was a schematic, the VHF-low return are just a passive lowpass filter that will direct signals in both directions without any gain. The amplifier section then also use a highpass filter to not get feedback from output to input. So there will be no gain, maybe 3dB loss below 40MHz and up to 50MHz there's a gap with maybe 6-10dB attenuation and then above that it starts to have a 7dB gain for the 2400 series.

/Ubbe

 

[N8YX]

I would put a 15dB amplifier at the antenna and then use passive splitters, standard 1-6 CATV or whatever you need. Then you put an EDA at one of it output ports if you need more signal outputs than 6.

This was tried initially. Problem is, if the EDA units sense anything on the line besides themselves they won't amplify.

One other gotcha comes from DC feed on the coax (used to power an inline device): It's not uncommon to see connector failure due to electrolysis if these are utilized in areas with high average rainfall amounts. AZ and NM can likely get away with them without issue.

 

[Ubbe]

This was tried initially. Problem is, if the EDA units sense anything on the line besides themselves they won't amplify.

They are designed to be splitting an amplified signal from the antenna to several outputs without much gain from input to output. Maybe you used a power supply or/and a t-bias that could not give enough current to both EDA and antenna amplifier.

/Ubbe

 

[N8YX]

They are designed to be splitting an amplified signal from the antenna to several outputs without much gain from input to output. Maybe you used a power supply or/and a t-bias that could not give enough current to both EDA and antenna amplifier.

/Ubbe

Separate power supplies were used for each. Also tried cascading an EDA 2100 and a 2200 (each with its own power supply) off an EDA 2800 and a Channel Master equivalent of a 2400 - same results.

I'm thinking that a DC block is needed between them to be run in cascade. Theory is if there's any DC present on the RF In port that amplifier will reduce its gain. So...a handful of them are inbound.

 

[Ubbe]

I'm thinking that a DC block is needed between them to be run in cascade. Theory is if there's any DC present on the RF In port that amplifier will reduce its gain. So...a handful of them are inbound.

What kind of bizarre bias-T are you using that gives DC voltage out both to the antenna coax and to the receiver coax? The input of amplifiers are usually protected by parallel diodes that conduct at signal levels higher than 1,4V rms and if there are no DC blocking capacitor at its input it will shortcircuit any DC voltage and the amplifier that needed that DC voltage will die, and as soon as the protection diodes conduct from that DC voltage it will also shortcircuit any RF signals, that's probably what happens to your setup.

The EDA2100 have a 15dB gain and if you cascade amplifiers you are sure to get into serious overload problems. Only use one amplifier at the antenna and then if it has enough gain it will overcome the loss in a splitter. Usually you'll need to attenuate the signal even more to not get overload issues.

If the coax attenuates a lot and/or you split to a lot of outputs you can add a drop amplifier that has no or little gain to overcome a splitter loss and the loss of any long coax lengths that are connected to the splitter outputs.

You never cascade amplifiers that have a reasonable gain. The amplifier at the antenna are the important one that needs to use a very low noise figure. As soon as the signal are amplified you can use higher noise figure amplifiers to drive splitters. EDA usually has a high 3dB noise figure not suitable as an antenna amplifier for weak signal monitoring.

/Ubbe

 

[N8YX]

I'm starting to think I have a few bad EDA units.

Did one more experiment this afternoon: A 10dB RadShack inline unit (5-905MHz) into an EDA2800 and a bank of scanners hooked to that. Now we have signals.

What I'm planning in the main listening post as a result is to feed the antenna into a passive 4 port (Mini Circuits) splitter. I may or may not put a low gain wideband amp ahead of it. Off each port will be connected a separate inline amp. 10dB/5-905MHz, 10dB 50/2200MHz and 20dB/430-1450MHz are my choices. These will feed individual EDA2800 drop amps and the 4th port may be left unamplified for use with the various TVs and FM receivers in the house. Different scanners are tasked to different monitoring ranges so I can connect them to the appropriate drop amps.

This approach will lend itself to multiple antenna feeds as well. I'm planning on a dedicated low band ground plane, an array for VHF Hi and an 800MHz Yagi for our MARCS-IP system in addition to the discone I currently use.

 

[Ubbe]

What I'm planning in the main listening post as a result is to feed the antenna into a passive 4 port (Mini Circuits) splitter. I may or may not put a low gain wideband amp ahead of it.

The antenna coax and the splitter will give a lot of attenuation and your noise figure will rise dramatically, not suitable for weak signal monitoring. If the signal disappears along the way you cannot bring it back up by amplifiers too far down in the chain. You will have to amplify directly at your antennas and then attenuate along the way in the coax and splitters and use drop amplifiers with low gain and good strong signal handling to feed any splitters if the signal becomes too much attenuated at your scanners/receivers.

If you plan to use several antennas and each for a dedicated frequency band then use a triplexer that has virtually no loss and combines several antennas to one coax and then use a good amplifier to feed a single coax. If will give enough signal to use a 1-4 or 1-6 splitter and if you need more outputs you can use a drop amplifier at their outputs.

You absolutely need to get a variable attenuator to check out what the optimum signal level are at different places in the chain, as you probably will need additional attenuation to a scanner/receiver depending of its receive capability. One place to get one are ScannerMaster.

/Ubbe