Coax Cable

Fred Villiard · Mar 31, 2011

Bill, Idid not see your post before I posted I started about 11 am but between phone calls and interuptions here at work it took that long to get done. your right, should have said more loss. I was thinking that 213 had an extra dB loss over 400.

For the RG213 people I have 120 foot piece that I put up may 09 that has over 30 dB loss at 3.7MHz. It was on my 80 meter dipole, dam chipmonk bit the jacket and water migrated thru the whole cable and was dripping out the end. On the other hand the LMR-400 that is on my beam is about 12 years old and has had a chunk chewed out of the jacket about 70 feet up the tower for at least 3 years and no increase in loss. I'm to old to go climbing towers any more, next fall intend to replace the tower with a crank up, and rune the coaxs up flex armor, let them chew thru that.

Don S/V ILLusion · Mar 31, 2011

Fred
Hardline is your friend

Fred Villiard · Mar 31, 2011

I know, but hardline is a real pain in the butt to work with. I used it back in the 60's when I was working EME used both 1 5/8 and 3 1/8 pressurized with sulfur hex, now Andrews has some nice corrugated that is much easier to work with, but still a pain.

Forrest15112 · Mar 31, 2011

Bill Roosa said:
See my DB math post above. I agree that a 1db loss (at the 1 db level) is 20%. I'm having issues with the "extra" part. Am I to infer that at the 5 db level adding 1 db more to get to 6 is a 20% increse in power loss (in our example)? Pretty sure that will not pass even the giggle test.
See my DB math post above. I
You cannot add and subtract db and get valid asnwers. db are ratios so you can say "more" or "less" but not quote an actual ammount till you calculate the two. You certianly cannot say mine is 10 and yours is 20 so mine is half of yours type stuff.
Another way of looking at it is this, a 1db change at the 1 db level is about 20% change in absolute value, a 1 db change at the 100 db level in either microscopic or huge debending on which way you are calculating it but is not 20%
so 2.1 db loss is almost a 80% increase in power disipated while going up/down the mast while a 1.2 db loss is around 20% so while you are receiving at the .1 MICRO watt level you either have .08 micro watt (1.2) or a .02 micro wat (2.1) power level at the receiver end. As Fred noted this is a pretty big differance.

The 1 dB added to the 5dB will always be equal to roughly 20%. For example: 44dBm is equal to approx. 25 watts. If we add 1 dB of gain to that, we have 45dBm which is 31.6 watts or a gain of just over 5 watts or about 20%. It's the same at any level but I think what you were saying is that measuring gain in dB is a ratio proportionate to what is being measured. I think someone may have already noted that in general, every 3dB of change is either doubling or halving the power. What adds to the confusion are specific measurements such as dBw and dBm.
In the case of the coax cable, the 0.8dB difference in cable loss could sound significant but isn't in the overall scheme. A person with a receiver would not notice a 1dB change in power output from a distant transmitter. Supposedly it takes a minimum 3dB to effect a noticeable change at the receiving station. This is true if you have a 5 watt radio or a 5kw radio. Increasing power to 6kw from the 5kw station will not produce a noticeable change at a receiving station. According to this theory, it will require more like 10kw.

Bill Roosa · Apr 1, 2011

I totally agree Forrest
However a 20% change (at a given power level) is somewhat misleading. 20% at 1 watt is 0.2 watts change (a small number of watts). A 20% change at 1000 watts is 200 watts or more than your radio puts out. db is a great way to compare things that you can't compare otherwise and make decisions about their RELATIVE relationship. It is not at all good at giving you an idea about the actual values being compared.
It lets you tell which is better but it does not give any information if the thing will even work. A 9 db loss is going to have twice the power delivered that a 12 db loss does. That tells you nothing about wither the power delivered will be sufficient to drive the receiver. However.
The original issue was the poster saying something like 2.1 db is better than 3.1 db by 1 db so it will deliver 20% more power to the antenna/receiver. My point is at 3.1 db you are losing slightly over half your power in the line which is pretty bad. Additionally those numbers where for a 100' length of cable which I'm pretty sure is not that actual length you would be using. If you only use 2' of the stuff it really does not matter what you use as long as it is 50 ohms impedance. With longer runs you have to use the actual run length for both cables to tell anything. For very long runs (>200 ft) I would totally agree with the posters statement but we don't have that here.

I hate db as it always creates confusion

Bob Poff · Apr 1, 2011

The dreaded "3db loss" comes up on here every now and then.
As to what change you'll hear at different levels, I did some tests to satisfy myself.

This test simulates your signal as it would appear at the other end. By adding pads (attenuators) in the line I'm simulating either decreasing transmitter power or increased line loss at the transmitting station (both ways the ERP still drops) in 3db steps. Or you could also look at it as a fixed level transmitted signal with increasing line loss at the receiving end.

First here is a signal that is a bit noisy. Reference Level
Then I put in a 3db pad : 3 DB DOWN Some difference, but quite readable.
Another 3 db pad : 6 DB DOWN Getting bad, readable with much difficulty.
And finally another 3 db pad: 9 DB DOWN Pretty much done for.

This is on the workbench with a marine VHF radio on ch 16, the signal is being generated by an HP8924C service monitor.
The tone is being modulated to 3 KC deviation. Normal voice is modulated to 5 KC.

Another interesting test would be to have the radio connected to both the generator and an antenna through a two input coupler to introduce some external source RF noise. That might give slightly different results. I'm not sure. After all most of these marine radios aren't exactly something Art Collins would've been proud to put his name on. I'll do that test sometime later.

Forrest15112 · Apr 1, 2011

Bob Poff said:
The dreaded "3db loss" comes up on here every now and then.
As to what change you'll hear at different levels, I did some tests to satisfy myself.

This test simulates your signal as it would appear at the other end. By adding pads (attenuators) in the line I'm simulating either decreasing transmitter power or increased line loss at the transmitting station (both ways the ERP still drops) in 3db steps. Or you could also look at it as a fixed level transmitted signal with increasing line loss at the receiving end.

First here is a signal that is a bit noisy. Reference Level
Then I put in a 3db pad : 3 DB DOWN Some difference, but quite readable.
Another 3 db pad : 6 DB DOWN Getting bad, readable with much difficulty.
And finally another 3 db pad: 9 DB DOWN Pretty much done for.

This is on the workbench with a marine VHF radio on ch 16, the signal is being generated by an HP8924C service monitor.
The tone is being modulated to 3 KC deviation. Normal voice is modulated to 5 KC.

Another interesting test would be to have the radio connected to both the generator and an antenna through a two input coupler to introduce some external source RF noise. That might give slightly different results. I'm not sure. After all most of these marine radios aren't exactly something Art Collins would've been proud to put his name on. I'll do that test sometime later.

That's a good example but would be very interesting to hear the difference at 1dB increments. I still say it will be next to impossible to discern except maybe at the very end of the sensitivity of the radio. I've done pretty much the same test on a 2 meter radio but just reduced the signal level at the service monitor in 1dB increments.

In my opinion, the slight loss of .8dB between LMR400 & RG213 over an 80' length doesn't justify the cost and lack of flexibility to use the LMR cable. In a longer run, or higher frequency, it makes sense. I think the OP said he was going with the LMR cable anyway. It's definitely higher quality in very way. Most people are oblivious to the cable.

Rick Sylvester · Apr 2, 2011

Wow! Great thread.

Good stuff here. Thanks.

I've got my rig down for a refit. I appreciate all the good info on LMR.

I just bought 100' of LMR-240 because of space issues. The loss numbers aren't as good as the 400 but I've got small conduits. The Pacer plant is nearby so I got it there for a little over a buck a foot.

http://www.pacergroup.net/Categories.aspx?Category=0efe036b-0555-4243-848a-4c644a338e91

If you can accommodate the 400 they've got it for $1.67 a foot.

kenn · Apr 2, 2011

A friend redid his small boat's antenna cable using RG-8 and PL-259/238s. We were checking his radio, when we observed that I was getting better reception with a $25 handheld. Some poking around revealed that one of the PL-259s (a compression-type) hadn't been connected properly.

On our little boat, the run from radio to masthead is around 30 ft. The previous owner ran RG-58, with BNCs as the mast-deck connection. Works fine.

Moral - mid-grade cable properly installed beats fancy cable poorly installed.

FourPoints · Apr 2, 2011

Maine Sail said:
None of the guys I know are reporting any issues with them provided they are installed with the correct crimp tool. The draw back is that this is yet another tool to own but so is the proper soldering equipment. The pin end is soldered and the braid gets crimped. If you properly cover your connections with tape and adhesive heat shrink the risk of an issue is very, very small.

I used to work with LMR-600 daily for long range high frequency data transmissions (in the 5Ghz range). Our equipment was very sensitive to signal loass, and our connections needed to be top notch, and were installed outdoors on cell towers.

We used crimp only connectors, (like this one http://www.streakwave.com/Itemdesc.asp?ic=C600-NF&eq=&Tp=), and use a ratcheting hexagonal crimper like this one (http://www.google.com/products/cata...a=X&ei=cuKXTcL0CKu-0QGK5fHwCw&ved=0CFkQ8wIwAg#). At the higher frequencies everything needs to perform to a higher standard as losses are much higher per foot (7.3db/100ft) and per connector.

We also had special coax-cutting tools to ensure clean cuts of the core, and special quick stripper tools to strip the insulation and dielectric to the exact length every time.

With this mix of tools, we were able to strip a cable and install a connector in about 5 minutes or less, and they were reliable. I had over 1000 wireless end points on my network, and about 50% of them were connected from the transmitter to the antenna with a 3-4' length of LMR-600 and crimp connectors. They needed weatherproofing when using a crimped connector, but performed flawlessly.

Forrest15112's description of weatherproofing with vinyl (electricial) tape, then butal (aka mastic) tape, and another layer of vinyl tape is dead accurate.