RG6QS vs LMR400 for scanner

Status
Not open for further replies.

topgun1986

Member
Premium Subscriber
Joined
Sep 25, 2012
Messages
138
Location
Southeastern Missouri
RG6QS @ 400 MHz has an attenuation of 3.73 dB, at 100'.
LMR400 @ 450MHz has an attenuation of 2.71 dB, at 100'.

It is 80' from my shack radio scanners to their corresponding antenna. I went el cheapo and ran RG6QS between all and the results have been dismal to say the least.

Question #1) Is 3.73dB (RG6QS) down to 2.71dB (LMR400) that noticeable for an 80' run of coax just for a scanner?

Question #2) Could my problem be in the added dB for the various connectors I am having to use?
Example: antenna #1 (centerfire discone) has a SO239 connector and I installed a PL259 (50 Ohm) to F connector (75 Ohm) connector so I could run RG6 to the scanner. Then at the scanner, I installed a BNC connector (Radio Shack 278-0126) on the RG6QS.
 

n5ims

Member
Joined
Jul 25, 2004
Messages
3,843
1 dB isn't very noticeable in most situations. Basically it'll make a really noisy signal only very noisy (or vice-versa). Also just a single adapter (assuming you have good quality ones installed correctly) won't make a noticeable difference in signal levels.

One thing that may help is changing from a very wide bandwith, but no gain discone to an antenna designed to cover only the band or bands you actually monitor. For example, if everything you monitor are in the 700/800/900 MHz band, you could increase the signal level by quite a bit simply by changing out the no-gain (aka 2.15 dBi gain) discone to a single band 800 MHz antenna with 8 or 9 dBd of gain.

Doing the simple math, you start out now with 0 gain and subtract 3.73 dB in cable loss so you have 3.73 dB less signal at the scanner than when you started at the antenna. With a 9 dBd gain antenna you'll start out with 9 dB and subtract the same 3.73 in cable loss, ending up with a net gain of 5.27 dB of signal gain (not quite twice the signal strength).

You can do similar upgrades by changing to single band VHF-Hi or UHF ham antennas or even dual VHF-Hi/UHF ham antennas. Trying to cover the full range of VHF-Low, VHF-Hi, UHF, and 700/800/900 MHz bands with a single antenna will give you an antenna that works everywhere, but works well nowhere. Somewhat like buying a semi with a 55 foot box trailer as a daily driver since it will haul everything that a small pickup, medium pickup, large pickup, box truck, or a semi will even though you rarely haul anything at all (and when you do it will fit into the bed of a small or medium sized pickup).
 

teufler

Member
Premium Subscriber
Joined
Dec 19, 2002
Messages
2,365
Location
ST PETERS, MISSOURI
you could have installed a bad connector. You would have to go and climb the tower, unscrewing from the radio, and from the discone. Check a continuity check with an ohn meter, positive to negative, If you have a needle swing, you have the coax shorted. Now it looks like you had coax that had a connector pl259 then you got a convertor so this sounds fine. You said you installed a bnc, bnc's are tricky so there might be the place to look, though you do have to have the other end disconnected.
 

mmckenna

I ♥ Ø
Joined
Jul 27, 2005
Messages
14,684
Location
SNCZCA01DS0
Not all RG-6 is created equal. Are you using a name brand cable?
And, I agree, check for issues with your connectors. It's really easy with some F connectors to get a stray piece of braid touching the center conductor. If they have been up there for awhile, check for corrosion/water ingress.

The advice about dedicated band antennas is good. Discones are great for covering wide swaths of spectrum, but their lack of gain can be a real hindrance.
 

topgun1986

Member
Premium Subscriber
Joined
Sep 25, 2012
Messages
138
Location
Southeastern Missouri
The RG6 QS was purchased from a local big box store. Tappan I believe was the manufacturer.

The coax is also connected to a basic grounding block outside. I'm sure that adds to my dB loss as well. I did not mention that earlier, my apologies.

I pick up a few frequencies having the discone connected to my Pro 106, so would that rule out a short somewhere, like was mentioned earlier?

I am scanning from 29 MHz (HAM) up to 800 MHz (local utility company) with this discone and nothing.
The only channels I am getting are 150MHz Public Safety.
This run has a Radio Shack BNC connector at the scanner...
Then 50' of RG6QS to my 12"x12" outdoor box...
Then connected to a basic F connector style grounding block/terminal in my 12"x12" box...
Then 30' of RG6QS out and up to the discone where I installed the F to PL259 adapter.

Would just putting some type of amplifier in line with the cable before it goes into the Pro 106 get me the same result as replacing the RG6 with LMR400?
 

mmckenna

I ♥ Ø
Joined
Jul 27, 2005
Messages
14,684
Location
SNCZCA01DS0
Would just putting some type of amplifier in line with the cable before it goes into the Pro 106 get me the same result as replacing the RG6 with LMR400?
It might help, but amplifiers can amplify noise and unwanted signals, so keep that in mind if it starts performing worse. Put the amp as close to the antenna as you can.

Also,
How high is your antenna mounted?
Antenna height is key, especially the higher you go in frequency.
 

topgun1986

Member
Premium Subscriber
Joined
Sep 25, 2012
Messages
138
Location
Southeastern Missouri
The antenna amplifiers I have seen all require 120V power so putting them at the antenna would be a no go, unless there is some other type of preamp I am unaware of that did not require power. I am currently not versed in antenna theory enough to understand what the difference would be (power vs no power pre amp).

As far as height, my antenna tower "farm" consist of 3-10' sections of Rohn 25G tower sections mounted roughly 6 apart in my backyard. Each 10' 25G tower section then has a 10' piece of galvanized pipe (fence rail) mounted vertically to each leg, which would give me the ability to have 3 antennae per tower section for a total of 9 different antennae. This way I can manually loosen any 10' pipe section and slide down as needed to work on whichever antenna or connector I need to. So they are minimum 20' above the ground, with some of my homemade j poles up to 28' above the ground.

I found out how to make my own j poles & slim Jim's from a few different online sites and I went a little wild with how many different ones I built. When my neighbors ask about my farm in the backyard, I explain to them it is only temporary. That should buy me a year or so.
 

n5ims

Member
Joined
Jul 25, 2004
Messages
3,843
The antenna amplifiers I have seen all require 120V power so putting them at the antenna would be a no go, unless there is some other type of preamp I am unaware of that did not require power. I am currently not versed in antenna theory enough to understand what the difference would be (power vs no power pre amp).
They make a "mast mounted" preamp where the preamp itself mounts outside as close to the antenna as practical. Inside you have the power supply attached to the coax, which delivers power to the preamp. The device where the power is inserted generally is designed to push the power on the antenna side of the cable, but not down to the TV (or scanner in your case) side of the cable. Instructions (quoted below) are here Mast Mounted Amplifiers with pictures of the various parts here http://dennysantennaservice.com/1136577.html. Note: Although this is for a TV preamp, a similarly designed scanner preamp will work the same.

A preamp consists of two units: a preamp and a power supply. The preamp itself is mounted on the TV antenna boom or on the mast as close to the TV antenna as possible. The power supply unit is mounted indoors. Power is supplied to the preamp unit through the transmission line. The preamp is located as close as possible to the TV antenna feedpoint because the weak received signal must be amplified before it is attenuated by the transmission line, and also before it can be subjected to interference from sources between the antenna and the receiver. Preamps mounted farther from the TV antenna usually amplify (magnify) the interference along with the signal. Preamp units come with U-bolts for easy mounting on the mast or the antenna boom.
Also note: since the frequency ranges for most scanners and TV signals are pretty much the same, a TV antenna preamp can generally be used for scanners with good results - beware that TV antenna preamps are not the same as Cable TV preamps, antenna preamps are designed for very low signal levels while CATV preamps are designed for fairly high signal levels. TV signals range from about 54 MHz to around 900 MHz (This range is for channels 2 through 83. If your selected preamp doesn't cover the full range, e.g. a UHF only channel 14 - 69 - 470 - 806 MHz, your range will be different.)
 
Last edited:

doublescan

Member
Joined
Aug 23, 2011
Messages
119
Location
Blount Co, AL
cabling antennas

Topgun I have been running RG6 on my scanners for 2yrs or so, with good results on VHF, and the 118-136mhz airband. The tv-type amps do help my airband reception quite a bit, but I haven't gotten my antennas up more than 25' or so yet, and I expect once I do I will find a much better signal strength on everything. Google earth says I'm 26miles from the Birmingham airport, didn't think it was that far, but I have just a little elevation to help me with that. For what it's worth, I ordered a 50' RG8X cable off amazon, and plugged that up to my home-built airband ground plane today, and I could tell there was an improvement. For the experiment I tuned to the airport weather recording, with the RG6 I could hear speech in the noise, but couldn't understand many of the words. Switched out the cable and could immediately understand every word they were saying. Still noisy, but understandable.
I don't listen to any UHF frequencies, but I suspect you may find the 50ohm cable will help.
Height of your antenna is key, of course, too!
 
Last edited:

topgun1986

Member
Premium Subscriber
Joined
Sep 25, 2012
Messages
138
Location
Southeastern Missouri
Thanks everyone for the tips.

The RG6QS was made from Southwire, not Tappan. It feels a lot heavier/ better built than RG6QS I have used in the past. Or maybe it's just my imagination.

I did a continuity test on all my cable runs with no issues. But I did go back and redo a few of the Radio Shack BNC right angle crimp connectors I installed on the RG6QS. The reception on my Pro 82 that I use for my civil air scanner (fed from my home made air band J pole) did seem to improve. Not sure if trying to add some solder on that fitting would help any. Aluminum foil on the coax is no fun to mess with.

n5ims, I appreciate the info regarding the pre amps. I have never paid that close attention between tv amps and cable amps. Now I have a better idea what specs to look for.

I have a second Pro 82 (programmed with rail road frequencies) connected to another home made j pole (designed for 160MHz) with RG6QS that is working great. I was very impressed as soon as I turned it on.
There are some rail yards 20 to 40 miles from my location and I was picking up some of them very well, or at least I believe it was one of the yards.

My first Pro 82 (programmed with the civil air frequencies) is the one I would really like to improve. Looks like I will be shopping for a pre amp this week.
 
Status
Not open for further replies.
Top