Let's start with a simple definition of coaxial cable - A feedline that transports radio frequency energy from your radio to your antenna and from your antenna back to your radio.
A simple truth: The coaxial feedline is one of the most critical components of your radio station and, as such, deserves careful consideration.
Factors to Consider and Other Considerations
Length - How much cable do you need? Or, stated another way, how long does your feedline need to be? Why do we need to consider the length of cable? All feedlines have loss and the longer the feedline, the higher the loss.
Frequency - What frequencies do you intend to use? Loss also increases with increases in frequency. Loss in cable is rated in decibels usually expressed as Db loss per 100 ft of cable. You should be looking for lowest loss in Db per 100 ft for the highest frequency you plan to operate.
Let's stop here for a real world example using RG58 Coax.
At 75-80 meters, a 100 ft run of RG58 coax is going to exhibit a loss of about 1/2 dB. We are also going to assume that VSWR is low and by that I mean 2:1 or less. That equals about 14 watts of loss from a 100 watt transmitter. The received energy is diminished slightly but modern radios more than make up for that loss.That all sounds very much acceptable to me.
Lets run the same length of RG58 cable to an antenna at 70 cm (440 MHz). This is a huge increase in frequency and the loss now reaches over 9 dB, and this result is with a good VSWR match. That now translates to a loss of 80% of your transmitted power! That is terrible and should not be acceptable to any sane operator!
The solution is to use a cable type that has much less loss for that operating frequency.
Quick snapshot:
LMR400 Coax by Times Microwave - 3 dB loss at 100 ft, that's still 50% of power loss!
LMR600 Coax - 1.7 dB loss at 100 ft, much better but keep an eye on the cost. It can add up very quickly.
Note: if your cable length need is very short, such as a mobile setup or maybe even a portable setup where all you need is 10 or 15 feet, higher loss coax (less expensive) may be used because the resulting loss at short distances is much less than at 100 ft.
Another Note: Think about the future. You need to consider the frequencies that you may want to operate in the future when choosing a cable or multiple cable runs.
This is a great time to review your understanding of VSWR and antenna tuners.
I recommend these articles/videos:
Videos:
SWR - W6LG - https://youtu.be/VcC7j55_xqc
Using an Antenna Analyzer - K7AGE - https://youtu.be/jOCG6bFGfuE
Tuners - HamRadioConcepts - https://youtu.be/pdOEiML2XGk
Audio:
Selecting Coaxial Cable - ARRL - http://www.arrl.org/files/file/DoctorPodcast/2019/July%204%20-%20Selecting%20Coaxial%20Cable.mp3
SWR Simplified - ARRL - http://www.arrl.org/files/file/DoctorPodcast/2019/March%2014%20-%20SWR%20Simplified.mp3
Antenna Analyzers - ARRL - http://www.arrl.org/files/file/DoctorPodcast/August%2031%202017%20-%20Antenna%20Analyzers.mp3
Back to factors and other considerations.
So, if the solution is to use a cable type that has much less loss for that operating frequency, how do we do that?
RESEARCH!! ( A good ham uses all the tools at his/her disposal to research solutions. An unwitting ham asks their question on social media and gets 100 different opinions from 100 people. The good answers are not easily weeded about by a new ham. Join a club, find an Elmer there or through the ARRL. But I digress.)
To assist you in your selection of the proper coax, use Google search to look up cable losses and cable loss calculators: Here is a typical cable loss table from W4RP's website:
Results are shown in dB per 100 foot and (dB per 100 meters).
A very good cable loss calculator can be found here at KV5R's website: https://kv5r.com/ham-radio/coax-loss-calculator/
This calculator is very comprehensive. The drop down menu for Line Type has all the cable types you will ever need to consider. The above link is worthy of "Bookmark" status.
Your last consideration is one of economics. Only you can determine the best bang-for-the-buck for your particular situation.
In Summary:
1. Measure the distance between your radio and your intended antenna feedpoint.
2. Determine the highest frequency you will operate now and in the future.
3. Use Google to find an online coax loss calculator. I like https://kv5r.com/ham-radio/coax-loss-calculator/ .
4. Use the calculator to determine the cable that gives you the lowest loss for the frequency and length you will operate. PICK A FEW OPTIONS!. Cost of cable will vary a great deal so prepare a few options to review against your budget constraints. This is a hobby, folks. Feeding your family is a higher priority than feeding your antenna. :-) Be Practical!
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I hope you or someone you know can benefit from this article.
As always, please provide any feedback you wish.
Best regards, Joe, N2QOJ
Joe Sammartino
24/7: 480.270.4563, Mobile: 480.381.2993
jsammartino@gmail.com
