Coax

w hole

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#1
Found this article on an American WebSite it is mainly text so I thought I would post the whole thing rather than a link.

It is actually about 'scanners' but I'm sure the advice stands true for most types of kit.

I haven't explored the other links yet, I see that they haven't come across so here is the original site address if anyone is interested.

http://strongsignals.net/access/content/co-ax.html

Kind regards

W Hole

"Introduction
Co-ax, short for coaxial, and not meant to be confused with the verb coax, is the means by which radio transmissions are conducted from an external antenna to your scanner/receiver.
As such, it represents one of THE most vital links in your monitoring station.

People go out of their way to spend good money on a scanner/receiver and antenna but hamstring their investment by using inadequate (read cheap) cabling!

Most times this results from simple ignorance in thinking all co-ax cables are created equal. Far from it!

Hopefully this page can remedy such problems and help people "tune-up" their systems to get better performance!

Cable Tips
For the most part try and remember these simple rules : thicker, more rigid cables give better results.

If it's about a half-inch thick (take a look at your pinky finger) and likes to hold a bent shape, you've probably got a winner!

If it's only about a quarter-inch thick or less and flails about easily, you could be doing better!

You get what you pay for. Usually priced in cents per foot, the cheaper cables will run you 20 cents/foot while the more expensive ones run 70 cents/foot and more. And of course there's savings advantages in buying longer lengths or pre-cut lengths.

The longer the cable, the more it attenuates (weakens) the signals as they travel from the antenna to the receiver. For lengths of 25 feet and less, it's not a BIG concern as to which cable type you use. Beyond 25 feet, you should be paying more attention. If you want the best rececption, you'll use the best cable you can regardless of length!

The higher the frequency, the more a cable will attenuate it. HF frequencies (below 30MHz) are not affected very much so almost any cable will do. If you are listening to VHF frequencies (30 - 300MHz) then it becomes much more of a concern. For UHF (300MHz and up) you had better make cabling a big priority! Since most scanners/receivers go over 1GHz these days, you can see the importance of choosing the right cable.

Cable Types
The basic co-ax types you run across in our hobby are RG-58, RG-59, RG-174, RG-8, RG-6, RG-11 and 9913 (which is really a form of RG-8).

These types basically differ in diameter and stiffness which is due to differences in types of outer jacket material, braiding, center conductor type (solid or stranded), shielding (used or not), dielectric type (air spacing and gas injections) and materials used (copper, aluminum, etc.).

These differences in properties determine the capicitance and velocity factor which are terms used to indicate how efficiently various signals are conducted through the cable.

For our purposes, we want efficient cable so that the signals received at the antenna make it down to the receiver with as much strength as possible.

If a really weak signal is received at the antenna but a poor choice of cable is used, it's possible that it may attenuate the signal such that the receiver is unable to detect it! So if you use good cable, you WILL hear more signals.

Be aware that you will see some funky notations for cables at times. Usually these take the form of suffixes like '/U', 'A/U', '/CU' and '/X'. These represent slightly different constructions of the same cable type which means they'll have different efficiencies. So take heed! A /U cable is usually a much better choice than the same cable in its /X form.

You are also going to find that cables come in two different types of resistances : 50- and 75-ohm. RG-8/58/174 are all 50-ohm while RG-6/11/59 are 75-ohm.

While most scanners/receivers specify a 50-ohm load at their antenna jack, either type cable can be used. Why? Because the impedance of the entire scanner/cable/antenna system as a whole changes radically over the wide range of frequencies that such radios cover.

If we were just listening to one narrow range of frequencies then we would worry about matching the cable/antenna impedance to the receiver impedance to get the most efficient signal transfer. Since we listen to signals from 25MHz to nearly 1GHz it's just not practical so pay it no mind.

Cable Choices
Most cabling used in our hobby and supplied by manufacturers is the cheaper and less efficient RG-58 and RG-174. These are OK for very short cable runs and HF/VHF-lo frequencies but you can easily get more performance by replacing them if possible.

To compare the different types, we'll need to use a reference of some sort. For our purposes, let's use figures that tell us how much of a 100MHz signal is lost in 100 feet of cable. These are usually specified in dB (decibels). Keep in mind that a 3dB loss represents 1/2 the original signal strength (50% transmission) while a 6dB loss represents 1/4 the original signal (25% transmission).

Taking a look at some representative numbers for our 100MHz/100ft cable, here are the figures for cables that you're likely to come across :

Cable dB loss
RG-174/U 8.4
RG-58 4.2
RG-58/U 3.8 - 4.5
RG-8/X 3.4 - 3.7
RG-59 2.9
RG-8A/U 2.8
RG-59/U 2.5 - 4.0
RG-6 2.3
RG-213/U 2.1
RG-6/U 2.0 - 2.1
RG-11 1.4
RG-8/U 1.3 - 1.9
RG-11/U 1.2 - 2.0
9913 1.3
LMR-400 1.2
LMR-600 0.9
LMR-900 0.6
LMR-1200 0.45

Looking at the dB losses, you can see I've ranked the table from worst down to best. Makes it pretty clear why you should try and avoid RG-58 & RG-174 type cables for most situations!

For most purposes, the large LMR co-ax is not suitable. While the LMR-400 & 600 have 0.5 inch diameters, the 900 is 0.9 inches and the LMR-1200 is 1.2 inches!

Pricing on the LMR series is high as well. 400 is $1.50/foot, 600 is $3/foot, 900 is $8.50/foot and 1200 is $11/foot.

And just for another emperical reference point, if we looked at the dB losses at 400MHz instead of 100MHz, the values in the above table can be roughly DOUBLED (remember that doubling a dB value will scale it logarithmically; doesn't take long to kiss most of your signal good-bye!) I think you can project what will happen at 800MHz and therefore the importance cable length and type plays on how well you receive the higher frequencies!

Connectors
Along with cables come the connectors needed to hook everything together.

The connectors used in our hobby basically consist of BNC, PL-259, F-type, Motorola and in some cases RCA. If you see SO-239, it's a chasis-mount version of the PL-259 female connector.

BNC is the connector seen on most scanners and lab test equipment. PL-259 (or SO-239) is usually found on larger antennas and commercial equipment. F-type are used for most cable TV fittings. Motorola is used on some mobile antennas and commercial equipment. RCA types are usually used for TV and stereo equipment connections but can be found on some scanner gear as well.

Most of your smaller width cables (RG-6/58/59/174) will use BNC, F-type, Motorola and RCA. The thicker cables (RG-8 & 9913) usually use PL-259.

That's why it's always a good idea to have several adapters handy so you can cross-connect various equipment, cables and antennas to experiment with. Radio Shack and radio dealers carry a wide variety of such connectors.

When ordering cable from a radio dealer, they can often put different type connectors at each end alleviating the need for additional connectors to match the cable to your antenna and scanner/receiver. So be sure to know which type and gender you'll need on both ends.

With a little experimentation, you can buy your own cable and connectors to build your own. With a soldering iron or crimp tool, you'll be able to make a cable at a moment's notice for that emergency situation. Just the length you need with the proper connectors!

The golden rule is to use as few connectors as possible. Most good quality connectors attenuate the signal very little (0.3 d:cool: but some can be quite poor (1-3 db)! So if you use more than one, it could be as bad as selecting a poor type of co-ax! Or in other words, you could loose half your signal, or more, simply by using bad or multiple connectors.

As connectors age, are plugged and unplugged repetitively and exposed to the elements, they will attenuate more and more. Be sure to seal those connectors which are outside and give everything a good check annually to assure proper operation.

Web Links

AES cable
Belden Attenuation vs. Frequency table
Durham Radio cable
Grove Enterprises cable and accessories
HRO cable
Radio Shack RG-6
Radioware co-ax cable
Radioware co-ax connectors
Radioware attenuation table
R.F. Connection co-ax cable
R.F. Connection attenuation table
SSB Electronic cable
Texas Towers LMR page
Universal Radio Co-ax "
 

rolfw

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#2
Yep, the guy certainly knows his coax, I think the 1/2 inch coax for domestic installation is probably a bit of overkill.

I think the main guide in the UK is that if they sell it in B&Q it's crap :)

Most of the CT100 or equivalent coax cables are more than adequate for the average installation, being double screened with a loss of around 25 DB per 100 metres at 2Ghz. I personally use a foam cable with double screen, the outer being copper, this I find suitable for all but the longest runs, it is quite rigid and great for poking through cavity walls :)

Rolf
 

rolfw

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My Satellite Setup
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#4
They do?

Oh well that blew that theory out of the water :) :)

Rolf
 

rolfw

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My Satellite Setup
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#5
How much per metre as a matter of interest?

Rolf
 

ckrys1

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#6
I bought some a few weeks ago, an to be honest, can`t remember! Might have been 49p/m-does that sound right?

Ckrys1
 

rolfw

Believe it when I see it Admin.
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My Satellite Setup
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#7
Yes,probably about right for retail, they've obviously stepped up they're game due to so many people wanting to do things with their Digiboxes.


Rolf
 
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