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50 to 75 Ohm Connection - Requesting Opinions

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K9ERV

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#1
Hello all. I am running into a minor issue and I am just looking for outside opinions. I have 2 scanners in my house, one Bearcat BCT7 (non digital) in the garage, and a BCD996XT (digital) in the office. My previous lower quality tower (before Mother Nature decided it was time to replace it) had an older Radio Shack scanner antenna with an "F" type connector. To keep everything simple, due to the "F" connector on the antenna, I ran RG6 down the tower, into the attic, used a splitter, and ran a length of RG6 to each scanner, going across the attic and down the walls to a low voltage wall box near each scanner. I just used an "F" to BNC connector to hook to the scanners. That set up seemed to work marginally well. Now that I have replaced my tower, I am using a Diamond D130J discone antenna. This antenna has an SO239 connector. I have a 500' spool of Times Microwave LMR-400, so due to the SO239 connector, I plan to use the LMR400 to come down the tower to a polyphaser and then into the attic.... Here is where my issue begins. LMR-400 is a 50 ohm coax and RG6 is a 75 ohm coax. Anyone who has ran wires across cramped attics and down walls can attest to how much of a pain this task can be. I am asking for opinions on this..... If I connect the LMR-400 to the splitter using an adapter, and leave the RG6 from the splitter to the scanners, does anyone think the ohm mismatch will cause issues? I understand if this was a transmitting radio, there would be issues for sure, but due to this being a receive only scanner, are the same issues there? I'd really like to avoid unnecessary work in the attic by running all new LMR-400 to replace the RG6, but I just don't know what type of issues I will run in to if I don't.

For reference, the LMR-400 run to get down the tower (antenna mounted at the 45' mark), to the polyphaser, and then into the attic to the splitter would be around 75'. After the splitter, the RG6 to the garage non digital scanner is about 40' and the RG6 to the office digital scanner is about 20'.

I also have a healthy sized spool of RG6 which I could run from the antenna, but I figure the loss to be less by using the LMR-400.

Sorry this was long. Just wanted to get all of the details out so I could receive good opinions.
 
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#2
Simple answer would be to run the LMR-400 down to your polyphaser. On the other end of it, use an adapter like this (F-Type Female to UHF Male Adapter | ShowMeCables.com) and attach the existing RG-6 to that adapter. Don't forget to properly waterproof all outdoor connections, even if they're somewhat protected (like under the eves).

Since you're receive-only with a scanner you can simply ignore the 50 vs. 75 ohm issue, it won't matter enough to worry about. If you ever want to transmit over the connection, you'll need to extend the LMR-400 inside to your transceiver and eliminate the splitter. Alternately, you could run a second run of the LMR-400 to a second antenna (at a safe distance from the scanner antenna) for your transceiver to use.
 

K9ERV

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#3
Thank you for the reply. I probably should have known that answer being an amateur radio operator as well, but I just couldn't find it online. I didn't think I'd have too much to worry about , but again, the 2nd opinion makes me feel that much better about it. I already have 3 runs of LMR-400 on the tower (UHF repeater, 6m/2m/70cm play radio, and HF dipole antenna, and I was well aware of the mismatch issues with TX, just wasn't quite sure on the RX only. Thanks again!
 
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#4
You seem to be under a common misunderstanding about impedance mismatch. The only difference between an antenna system used for transmitting and one used for receiving is the direction of signal flow. If the mismatch will effect transmitting then it will also effect receiving.

Your real question is will the effect will be noticeable. The difference in line loss might mask any difference in signal strength caused my the impedance mismatch. RG-6 has about 4.3 dB of loss at 400 MHz while your LMR-400 has about 2.7 dB at 450 MHz per 100 feet. My suggestion is to do as you propose and see if the antenna's performance is acceptable.

You might also consider installing an antenna preamp of around 15-20 dB. You should install it ahead of the splitter but you don't have to install it at the antenna. RG-6 as well as LMR-400 cables are well shielded and won't pickup much, if any, noise so the location of the preamp isn't really going to effect the signal/noise ratio.
 

dlwtrunked

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#5
No disagreement with the advice given but a couple more points:
1. Do NOT consider a 50 ohm to 75 ohm transformer. Generally they lose more signal than the mismatch and they are often expensive.
2. It is wise to add an FM broadcast band block ahead of the pre-amp. Even people who are not that close to a station and think they do not have a problem often do. The station can de-sense the receiver and weaken reception without providing any audio cluing the listener to that being a problem.
3. A pre-amp should be high gain and low noise. The issue is not signal level but signal-to-noise ratio. That being said, it you are close to say a NOAA weather station, you may need to notch that. Nearby VHF pagers etc. can also be a problem and they may be too close to what you are listening to to notch out. Adding a pre-amp is different at different locations, may require notch for nearby strong stations or not even be a good idea at all if they are too strong--in other words, if you are close to strong stations, determining an optimal configuration is an experiment often in trial and error. Some say always use a pre-amp, others will say pre-amps do not work, but they are ignoring the truth above. So the worth of a pre-amp may depend on 1. Your location (nearby strong stations) 2. The pre-amp, 3. What you are listening to 4. You ability to buy filters if needed and 5. Your willingness to experiment.
 
Joined
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New Orleans region
#6
The cable impedance differences is not the issue. The line loss and the splitter is where you need to focus. Using a 2 way splitter is a loss of 3 db to each of the ports. You only get half the signal level at each output port.

The solution is to use a line amp that over comes the losses before the splitter. You can either use an in line amp and the splitter, or use a line amp that has several outputs.

Use of the RG-6 can be used. Just pay attention to the line loss and the use of an amp. The use of adapters at the scanner radio is about an I don't care. Sure they introduce a slight loss, but if your concerned about it and the signals are down, you have a more serious problem some place else.

You can use a higher gain antenna to over come some of the line loss.

Just remember that the use of an in line amp may cause you other problems. The big concern is strong signals from transmitters that may be close to you. These strong signals might swamp the line amp and give you grief trying to pull in weak signals. The strong signals can also cause intermod and bogus signals to show up on frequencies that they don't exist on. These strong signals can mix with other signals and cause a third signal to be created that will be possibly a combination of audio of the 2 signals on the third frequency.
 
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