Newby here, I currently have a 75-ohm coax running as a feedline to my Uniden SDS200. A friend of mine suggested switching out the coax to an LMR-400 cable. Thoughts?
Benefit to using LMR-400?
- Thread starter gorio1961
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ko6jw_2
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You don't mention what kind of antenna you have or how long the feed is. Also what bands you want to receive. Important to know. Your radio is nominally 50 ohms. If you were transmitting this would cause a loss using 75 ohm cable. Not so important for receiving only.
Ubbe
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If it is long length of coax it will give a higher signal level with those weak signals you have now.
Your scanners impedance are all over the place, the same with your antenna. So the impedance doesn't matter.
/Ubbe
Your scanners impedance are all over the place, the same with your antenna. So the impedance doesn't matter.
/Ubbe
Thanks for asking. I should have provided those details. current feed is 33 feet. It's a Diamond Discone Dj-130N with an LMR400 25-foot cable connecting to the feed line (so the total length of the run is 83 feet from the bottom of the antenna. MY SDS200 is also a wideband scanner.
Thank you, I added the additional details to ko6jw_2
One other thing...would it be beneficial to use a wideband pre-amp?
mmckenna
I ♥ Ø
Shortening that cable run will have a lot of benefits. Do you really need 83 feet of cable to get from the antenna to the radio?
And it sounds like you have a mix of LMR-400 and RG-6?
There's a lot of variations in cable. RG-6 is a general specification, and the exact vendor/brand/type will have different spec's. It would be hard to do a comparison with a mix of cable installed like you have, but very generally speaking, LMR-400 will have about half the loss of cheap RG-6.
If I was in your shoes:
- Find a way to shorten the cable length. Move the antenna closer to the radio if you can.
- Coax cable signal losses go up with length and go up with frequency.
- Discone's have 0dB gain at best, so an antenna with no gain + lossy coax will result in you losing a lot of signal.
- Switch to one continuous run of cable from the antenna to your lightning protector, and one continuous run from the lightning protector to your radio location.
- Don't connect LMR-400 directly to the scanner. It's a stiff/heavy cable and will eventually damage the antenna jack on the radio
I really appreciate the advice. You are correct, this is currently a mix of RG-6 / LMR-400. I can't really shorten the distance of the runs due to the floorplan of my house. The cable currently enters my house AFTER passing thru a lightning arrestor via a "window pass-thru" cable. If I convert the coax to LMR-400 Ultraflex, it will run along my baseboard over to my scanner. What are your thoughts on a 0.1 - 4Ghz wideband pre-amp?
What kind of 75ohm? Double or quad shielded?
If it's quad the difference against LMR-400 might not be significant.
Also make sure your grounding setup is good.
You'd be surprised how much ground loops or the lack of proper grounding can throw things off.
If it's quad the difference against LMR-400 might not be significant.
Also make sure your grounding setup is good.
You'd be surprised how much ground loops or the lack of proper grounding can throw things off.
mmckenna
I ♥ Ø
Each connection point you have adds a tiny bit of loss. Getting rid of unnecessary connections would be beneficial. It removes that bit of loss plus removes possible failure points. Keeping continuous runs of cable would be a good idea, whichever cable you choose. Also, don't overlook proper weatherproofing of your outdoor connections.
A preamp can be a useful tool if done right. You do have to be cautious as a wideband preamp will boost everything in it's passband, including signals you do not want, like strong FM broadcast stations, cellular, etc. A preamp can overcome coax cable losses and reduce the need for higher cost cables, so that may be a good direction to go, but you may need to add filters to block FM broadcast and strong nearby cell signals.
So, yeah, if you are going to use this antenna for RX only, then a continuous run of RG-6 and a preamp may be a good solution, but figure in the costs of the filters you may need.
If you plan on doing any transmitting down the road (amateur, GMRS, MURS, etc), then you either need a preamp that will automatically switch to a bypass mode when transmitting, or just use LMR-400 or larger cable.
BF8609
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Look at db loss per frequency @ 100ft on the Cable manufactures site(s) to see which one you can afford.... I just installed 100 feet of 9913 and a Dis-cone last week.
Ubbe
Member
That's probably not the one you'll want, as those specs usually means a bad performing chinese amp.
Look for an amp that uses a PGA-103+ device. Those are great performers even in difficult enviroments and shouldn't cost much, $25-$40.
Then you do not need to worry about your coax type and losses in connectors and so on. You actually will have to use an attenuator to limit the signal level at your receivers or they will overload and create a worse reception than without the amp. You obviously would want to install the amp as close to the antenna as possible. If needed you can usually power it thru the coax by using a Bias-T power inserter.
Remember that most discone antennas have problem at 700MHz and higher as the directional beam points up in the sky that will reduce the signal by 10-20dB at the horizon, while the 100MHz broadcast band receives with no attenuation. That could overload an amp and certainly a scanner receiver, making it necessary to use a FM broadcast trap filter.
If you only monitor 400MHz and higher then FM broadcast filter might not be needed but you should then cut down your discone so that all elements are only half length to improve above 700Mhz reception and will reduce signal levels below 200MHz.
/Ubbe
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Very helpful, thank you.
Ubbe
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Going from RG6 to LMR400 will improve signal levels if the length of the coax are long. Shorter runs doesn't matter.
When measuring impedance of the antenna jack from different scanners they all have impedances from something like 20 ohm to 200 ohm over their frequency range. Antennas are even worse, and not even a discone have a constant 50 ohm over its frequency range.
Those stacking harnesses for antennas when using two in parallel use different impedance coax to match them to a 50 ohm coax, but are frequency dependent as they are designed for a specific wavelength. The same type of impedance mismatch happens to scanners and scanner antennas not being 50 ohm when connected to a 50 ohm or 75 ohm coax. When using a preamp at the antenna it will give an almost constant impedance to the coax and using a CATV splitter or similar device at the scanner will isolate the scanners impedance from the coax and will load the coax with an almost constant impedance. Both preamp and splitter will improve performance due to the constant impedance load and not only from the lower noise figure of an amp and its gain to compensate for any kind of loss of lower performing coax or number of connectors and their quality. So there are different positive aspects to using antenna amplifiers.
/Ubbe
While the feed line is important, the height of the antenna is what's going to determine the 'range/distance' of received signals, especially in the VHF/UHF regions. A signal amplifier isn't going to increase an antenna's 'range' so much as it will "neutralize" the losses of the feed line. Having one available would certainly be nice, but additional height would be 'nicer'. How high should you go? That gets into the 'practical' area of it. Personally, I stop at something like a 1/4 mile high... ... I can dream too, you know?
Ubbe
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That's one aspect but also the internal noise figure of the amp will usually be lower than the scanners that will be the same as increasing the range of the antenna. A scanner has something like a 4dB-6dB noise figure and a good preamp 1dB. That 5dB difference are equal to increasing the antenna gain by 5dB.
/Ubbe
For receiving, it doesn't matter unless you have runs of 100 foot.
RG6 works just fine. LMR-400 is expensive, if you can afford that, 1/2 inch heliax is better.
Adding a preamp may do more harm than good. Too much signal to the scanner can overload the front end causing distortion.
Band pass filtering would be a greater asset for the band you monitor.
IE: My setup is a discone just above the roof. That is fed with 65 foot LMR-600 (just because I have hudreds of feet of it)
In the shack, first is an FM BC trap. that feeds my 'preselector', a combination of band pass filters, a 15 Db preamp, and digital attenuators (-40 Db in 5 Db steps).
That splits to an SDR and to my scanner.
I find with no gain at all and some bit of attenuation, my scanner works best, even weak signals.
The SDR works best with zero to 5 Db gain.
Hope you get the gist of this.
73s
RG6 works just fine. LMR-400 is expensive, if you can afford that, 1/2 inch heliax is better.
Adding a preamp may do more harm than good. Too much signal to the scanner can overload the front end causing distortion.
Band pass filtering would be a greater asset for the band you monitor.
IE: My setup is a discone just above the roof. That is fed with 65 foot LMR-600 (just because I have hudreds of feet of it)
In the shack, first is an FM BC trap. that feeds my 'preselector', a combination of band pass filters, a 15 Db preamp, and digital attenuators (-40 Db in 5 Db steps).
That splits to an SDR and to my scanner.
I find with no gain at all and some bit of attenuation, my scanner works best, even weak signals.
The SDR works best with zero to 5 Db gain.
Hope you get the gist of this.
73s
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Thank you. I get it.
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