Coax Loss

[ab5r]

I am NOT trying to create a controversy or anything like that, but I am confused by what a constantly read here about antenna coax losses. It is understanding that losses published are usually for 100 feet lengths on the particular coax.

Question (1): Are these "losses" for reception or transmission? These are NOT simply resistive loss of the center conductor.

Question (2): Are the "losses" given even decernable to the human ear? Would you even notice a difference? (A friend of mine once said that you could hand a wet noodle out the window and still receive on it.) Nope, I haven't tried that.

I agree that certain coax and connectors are best for higher frequencies. When you get into the "space" frequencies, it is instrumental to have proper feed line and fittings! When it comes to the basic scanner listener (especially for cost savings), is it always necessary to obtain the alleged best?

I will close with a controversial statement: "In my opinion, there ain't no BEST antenna or BEST coax for receiving only." There are infinitesimal factors to one's location and setup, but "blanket" statements can misleading and expensive for the OP.

Please ,no flaming!

Regards,
Jerry

 

[radioman2001]

Loss factors for coax go both ways, but more power means more loss. As far as decernable to the ear depends on the quality of the signal being received. If you have a signal on the ragged edge of reception then it makes a big difference, also the frequencies used has effect on what coax should be used. If you a re listening to a statewide system with towers all over the place yea a wet noddle will work, but if the system you are listening to is a simplex (no repeaters) it makes a big difference.
My opinion is like anything else that's a hobby, spend as much as you can afford.

 

[jcop225]

They are insertion loss values in dB, the amount that an RF signal is attenuated passing through. They effect both'transmitted' and 'received' levels the same. So a transmitted signal may be +40dBm at the base of an antenna line and a received signal may be at -83dBm at the antenna port but they will both be decreased by the same amount at the other end.

One thing that is important is that the reported values for transmission line loss are based on the devices at both ends having the exact same characteristic impedance as the cable. The impedance mismatches present in real world scenarios is what leads to the possibility of differing performance on transmit and receive for the same antenna/line system.

Based on their design receivers have a static sensitivity, an amount of signal they need to reach a standard performance level (usually 12dB Sinad for analog and 5% BER for digital). The 'noticability' of attenuated signal in my opinion depends on where you received signal level falls in relation to that receiver sensitivity. With most digital vocoders once you reach that 5% BER threshold there is no way to discern if the signal is stronger or weaker from the audio. Keep in mind that the reported static sensitivity is a laboratory measurement and will be degraded by ambient RF levels.

 

[trentbob]

I'm going to answer in very simple terms but I'm sure others will get more complicated. I see now that others are responding as I am writing this.

I remember as a kid for CB we always used, RG58 on the rooftop antennas my dad would put up... the first radio in my first car for police was a slide rule dial VHF low-band high band receiver in the early 70s, RG58 worked great.

When we went to UHF and T band frequencies I remember installing a Regency scanner in my car and I used RG58, what did I know? We only read magazines we didn't have RR.

Reception was terrible but I figured that's the way it is, I was using a good antenna but...

Ended up getting the house, listened to UHF and T band with the back of the set antenna but decided to get a rooftop antenna. What did I use? About a hundred feet of RG58. I could hear basically nothing that I could hear with my back of the set antenna just fine. That's when I found out about coax loss of signal. It happens both transmitting and receiving.

The lower the frequency like CB 27 megahertz the lower the signal loss, when you get to UHF 400 megahertz or t band 500 megahertz or 800 megahertz you lose signal through the coax, it leaks out so to say.

In those days I replace the RG58 with RG8 and it was like night and day.

Today I have three rooftop antennas and I use lmr400. I'm sure others here will give you the laundry list of all the great low loss coax. You consider price, length of coax and frequencies you're listening to. As far as connectors you have signal loss through them too.

 

[iMONITOR]

For the typical hobbyist scanner listeners I think concerns regarding loss are exhagaerated. Just about any cable, connectors, and antenna will work to some degree. But there are many of us that want maximum performance out of radios and investment. There are legitimated facts regarding specifications/quality and what works best. I'd rather spend say $350 on an excellent antenna system and be satisfied vs. spending $150 on a system that is disappointing. There are numerous factors involved in determining what works well and what will get you by. It's up to the individual and what they are trying to achieve. Often you won't even know what your missing until you find what was there all along!

 

[prcguy]

Coax loss doesn't really matter when signals are strong and coming in "full quieting" on an FM receiver. Its when you are trying to pick up weak noisy signals when you will really notice loss in coax or other things in your transmission line. If you are receiving a signal that is noisy but otherwise 100% copy, reducing your feedline loss by 3dB will be noticeable and you will have less noise. Now go for a signal that is very difficult to copy, add 3dB loss and now they are too far in the noise.

For amateur (hamster) use and weak signal reception you can notice very small improvements since you might be listening to signals that are barely able to copy in SSB or CW mode, that's a little different than FM mode with a police scanner.

For scanner use I would not spend a lot to reduce loss by say .5dB, but if your going to buy new coax anyway why not put in something better than will guarantee the lowest loss within reason for the life of your antenna system? You might only listen to VHF or UHF now but in the future you may want good performance at 800MHz or higher, so put in the best coax you can now and sleep better.

 

[ko6jw_2]

My view is that low loss coax is very important, but there are other factors. Mainly the type of connectors used and the expertise of the operator in attaching them. Cheap PL259's improperly soldered will negate the benefit of expensive coax. Use N connectors when at all possible. If you buy already made up lengths of coax, check them carefully. I bought a hundred foot length of RG-213 from a national ham radio distributor and found it was performing very poorly. I found out the the center conductor on one end had been cut too short and was not actually soldered to the pin. I now use crimp on connectors where the shield is crimped and the center pin is soldered. I never liked PL259's (I've used them for 50 years) and hate soldering the braid to the connector body.

One other point, many people on these threads like to use RG6 because it has a lower loss than RG8X. This is true except for the impedance mismatch from 75 to 50 ohms. For receiving only this may not be much of an issue. However, it is worth noting that not all RG6 is created equal. The home center stuff is probably not up to spec. If you want to use RG6, buy it from someplace like DX Engineering where you will get a high quality product. Never use F connectors.

 

[buddrousa]

Simple electronics.
3DB loss = Loss of 1/2 of your signal
so say starting at 100% signal
3Db = 50% of 100
3DB = 25% of 100
3DB = 12.5% of 100
3DB = 6.25% or 100
So saying 12DB of loss you only have 6.25 percent of your original 100.
As stated here the higher the frequency the more the loss.

 

[dlwtrunked]

Simple electronics.
3DB loss = Loss of 1/2 of your signal
so say starting at 100% signal
3Db = 50% of 100
3DB = 25% of 100
3DB = 12.5% of 100
3DB = 6.25% or 100
So saying 12DB of loss you only have 6.25 percent of your original 100.
As stated here the higher the frequency the more the loss.

In a picky mood today, the correct abbreviation is dB but your point might be a little clearer if wrote (fixing things a bit) - I cannot help being a mathematician (really):
-3 dB (3 dB loss) results in 50% of the original [note more accurately it is -3.0103 dB]
-6 dB (3 dB more loss) results in 25% of the original [note more accurately it is -6.0206 dB]
-9 dB (3 dB more loss) results in 12.5% of the original [note more accurately it is -9.0309 dB]
-12 dB (3 dB more loss) results in 6.25% of the original [note more accurately it is -12.0412 dB]
and of course, -10 dB results in 10% of the original

In general, a loss of L dB gives (where ^ indicates exponent)
10^(L/10) of the original (multiply the result by 100% to get percentage of the original)
-7 dB (a 7 dB loss) gives 10^(-7/10)=10^(-.7) using a calculator allowing exponents = 0.1995 or multiplying by 100%, we get 19.95% or about 20% the original.

 

[ab5r]

Thank you, gentlemen. ( Is that OK, or should I say gentlepersons?)

So, may we agree that in an "average" scanner installation: (1) Outside antenna is better than inside; (2) On an outside antenna, the higher the better; (3) Choose coax in mind of NOW and Later, if not satisfied; and (4) BUY WHAT YOU CAN AFFORD.

ALL that depends upon a LOT of variables for each location: Distance to agencies monitored, topography at your location, frequency range(s) needed, and on and on.

I appreciate each input (even the math nerd -smile)! Hopefully some newcomers have benefited.

73,
Jerry

 

[Firekite]

So, may we agree that in an "average" scanner installation: (1) Outside antenna is better than inside; (2) On an outside antenna, the higher the better; (3) Choose coax in mind of NOW and Later, if not satisfied; and (4) BUY WHAT YOU CAN AFFORD.

Yep. Don’t skimp on the infrastructure. Then no matter what scanner or transceiver you attach to it, you should have the best results.

 

[a417]

(4) BUY WHAT YOU CAN AFFORD.

...at the same time understand that spending money on a quality item, and then doing a piss poor installation or implementation of it will negate the value of the purchase in the first place.

buy what you can afford, and take the time to learn how to do it right.

 

[MisterLongwire]

Buy the best you can given the information provided. Don't rely on too much technical info as it may throw you off. But from what I read here everyone is correct

 

[mmckenna]

So, may we agree that in an "average" scanner installation: (1) Outside antenna is better than inside; (2) On an outside antenna, the higher the better; (3) Choose coax in mind of NOW and Later, if not satisfied; and (4) BUY WHAT YOU CAN AFFORD.

For hobby use, sure, buy what you can afford. Usually the first question I ask when someone wants to know what the "best" coax is would be what their budget allows. No point in recommending high cost coax if it's a low budget job.

But, hobbyists often overlook the cost of installation. I know, hobbyists don't get paid, but they do put themselves at risk climbing up on the roof, hanging over the side, crawling around in the attic, maybe paying someone else to do that, etc. That should be considered as a cost.
And then there's the sloppy installs with no weatherproofing, figure being up on the roof next spiring. That's a cost.

So, cheap coax, sloppy job, figure you'll be replacing it sooner rather than later. So, figure that in.
Or,
Use quality coax and connectors, do a proper job weatherproofing them. Cable install will last much longer. Less long term costs.

For LMR stuff, skimping on the coax is kind of stupid. You can kill a system doing that. Someones life on the line isn't worth cutting corners. I've fixed installs where someone used RG-58 for a repeater install. Did it work? Yes. Did it work well? No.

ALL that depends upon a LOT of variables for each location: Distance to agencies monitored, topography at your location, frequency range(s) needed, and on and on.

Yep, and why I dislike it when people ask "What's the BEST coax?" Answer:
1. You probably cannot afford the best.
2. Depends.
3. Depends.
4. Still depends.

 

[trentbob]

Buy the best you can given the information provided. Don't rely on too much technical info as it may throw you off. But from what I read here everyone is correct

Yes that's why I was keeping my reply rather simple... If I might add something important. Go for the good stuff as once the thing is up, it's up LOL.

Also, invest in something I've been using for a long time. It's called coax- seal.

 

[trentbob]

For hobby use, sure, buy what you can afford. Usually the first question I ask when someone wants to know what the "best" coax is would be what their budget allows. No point in recommending high cost coax if it's a low budget job.

But, hobbyists often overlook the cost of installation. I know, hobbyists don't get paid, but they do put themselves at risk climbing up on the roof, hanging over the side, crawling around in the attic, maybe paying someone else to do that, etc. That should be considered as a cost.
And then there's the sloppy installs with no weatherproofing, figure being up on the roof next spiring. That's a cost.

So, cheap coax, sloppy job, figure you'll be replacing it sooner rather than later. So, figure that in.
Or,
Use quality coax and connectors, do a proper job weatherproofing them. Cable install will last much longer. Less long term costs.

For LMR stuff, skimping on the coax is kind of stupid. You can kill a system doing that. Someones life on the line isn't worth cutting corners. I've fixed installs where someone used RG-58 for a repeater install. Did it work? Yes. Did it work well? No.



Yep, and why I dislike it when people ask "What's the BEST coax?" Answer:
1. You probably cannot afford the best.
2. Depends.
3. Depends.
4. Still depends.

I didn't see this post before I posted when I said the good stuff I don't mean the really really good stuff because that is very expensive. LOL

 

[questnz]

Coax Calculator

 

[prcguy]

Nothing that special about RG-6 from DX Engineering. For some of the best RG-6 go to a satellite TV warehouse company, you can get excellent RG-6 sweep tested to 3GHz for about half what DX Engineering charges. I was and sort of still am in the Satellite business and have purchased and installed literally miles and miles of RG-6. I've also had to test miles of RG-6 after installation for loss and VSWR.

Nothing wrong with modern type F connectors, they have excellent VSWR specs past 2.5GHz and are very weather resistant. I prefer Thomas & Betts compression types but there are many other good mfrs of F connectors.

In fact M2 antennas uses RG-6 in their balun cables for VHF and UHF Yagi's with what looks like T&B F connectors and the antennas are rated at 2.5kW through the balun. That should put some trust in using F connectors or an F connector to a PL-259 or type N adapter.

If you want to use RG6, buy it from someplace like DX Engineering where you will get a high quality product. Never use F connectors.

 

[radioman2001]

If you are looking to listen to 700/800 mhz, I doubt you are going to spend upwards of $10.00 a foot for 7/8 coax $50.00 per connector which is the prefered size for under 100ft of cable. Antenna could cost you upwards of $2,000.00. So again you need to evaulate what you want to listen to, where is its located compared to your position, do you have to deal with simulcast issues, the list goes on and on.
For VHF or UHF for simplex 1/2 hard line or 9913 (don't use for duplex that is TX/RX on the same line) a good monitor antenna, and you might spend $200.00. Take your worst case reception and buy what you can afford for that.

As far as RG-6 that's 70 ohm cable there will be some loss due to the difference in impedance. When use for satellite reception you are not looking to receive at 3ghz, you are transporting signals that have been down converted. YMMV

 

[WB9YBM]

I am NOT trying to create a controversy or anything like that, but I am confused by what a constantly read here about antenna coax losses. It is understanding that losses published are usually for 100 feet lengths on the particular coax.

Question (1): Are these "losses" for reception or transmission? These are NOT simply resistive loss of the center conductor.

Question (2): Are the "losses" given even decernable to the human ear? Would you even notice a difference? (A friend of mine once said that you could hand a wet noodle out the window and still receive on it.) Nope, I haven't tried that.

I agree that certain coax and connectors are best for higher frequencies. When you get into the "space" frequencies, it is instrumental to have proper feed line and fittings! When it comes to the basic scanner listener (especially for cost savings), is it always necessary to obtain the alleged best?

I will close with a controversial statement: "In my opinion, there ain't no BEST antenna or BEST coax for receiving only." There are infinitesimal factors to one's location and setup, but "blanket" statements can misleading and expensive for the OP.

Please ,no flaming!

Regards,
Jerry

yes, it goes both ways. As far as being able to hear a difference, a person with average hearing will hear a signal that changes one decible.