Why coax loss matters
Every metre of coax turns a little of your transmitter power into heat, and it does the same to weak signals on the way in. The loss climbs with frequency, so a cable that's fine on 40m can waste most of your power on 70cm. On receive there's no amplifier that gets that signal back, because the cable's noise comes along with it.
A quick way to read the numbers: 3 dB is half your power. 1 dB loses about a fifth, 6 dB loses three quarters, and 10 dB leaves you with a tenth. A 100 W radio feeding 30 m of RG-58 on 2m delivers only about 25 W to the antenna.
How the calculation works
Coax loss comes from two things: resistance in the copper, which rises with the square root of frequency because of skin effect, and the plastic insulation, which absorbs energy in direct proportion to frequency. Manufacturers measure the loss at a range of frequencies and publish it per 100 m or per 100 ft.
The calculator uses those published figures for each cable, and works out values between them on a logarithmic scale, which follows the real curve closely. Below the lowest published frequency it scales with the square root of frequency, and above the highest it follows the trend of the last two figures. It then scales the result to your length.
When the SWR is above 1:1, some power is reflected back and forth along the cable and loses more on each trip. The extra loss uses the standard ARRL formula, where a is the matched loss as a power ratio and ρ is the reflection coefficient:
It also shows the SWR at the radio end. A lossy cable hides a poor match, because the reflected power is weakened before it gets back to you. If your meter reads 1.5:1 through a long run of thin coax, the antenna itself may be much worse.
Choosing a cable
- RG-58 is light and cheap. It's fine for short HF runs and patch leads, but avoid long runs above 30 MHz.
- RG-8X (mini-8) is a good compromise for portable HF and short VHF runs.
- RG-213 is the traditional base station cable for HF. It's stiff and heavy, and only average at VHF.
- 10 mm low-loss cables such as M&P Hyperflex 10 and Ultraflex 10, Ecoflex 10, LMR-400 and Westflex 103 are the usual choice for VHF and UHF base stations, with roughly half the loss of RG-213 on 2m. Stranded centre conductors (Hyperflex, Ultraflex, Ecoflex) are more flexible and easier to route than solid ones.
- Lightweight cables such as M&P Airborne and Aircell, and the 7 mm types like Ultraflex 7, suit portable operating and SOTA or POTA, where weight matters.
- 13 to 15 mm cables such as Hyperflex 13, Ecoflex 15 and LMR-600, or hardline like LDF4-50A, are worth it for long runs at UHF and above, if you can handle the stiffness.
Connectors add loss too: allow about 0.1 dB per good-quality PL-259 at VHF, more for adapters. Water getting into the cable can increase its loss dramatically, so seal outdoor joints with self-amalgamating tape.
Figures come from manufacturers' datasheets where available: Messi & Paoloni, SSB Electronic and Times Microwave. Generic types such as RG-58 and RG-213 use typical published figures, and different makes can vary noticeably, so check the datasheet for the exact cable you buy.
Questions
How much coax loss is acceptable?
Under 1 dB is a good target for most stations, and under 2 dB is acceptable. Above 3 dB you're losing half your power and a similar share of weak received signals, so a better cable is usually cheaper than a bigger amplifier.
Does a high SWR damage the coax?
Not at normal amateur power levels, but it increases the loss and the voltage on the cable. The bigger concern is your radio, which will usually reduce power to protect itself.
Is RG-58 OK for 2 metres?
For a few metres to a mobile antenna, yes. For a 20 m run to a roof antenna, RG-58 loses well over half your power on 2m, so LMR-400 or Ecoflex 10 is a much better choice.
Which is better, Hyperflex 10 or Ultraflex 10?
They're very close. On the datasheets Hyperflex 10 loses a little less above about 400 MHz, for example 15.4 against 16.4 dB per 100 m at 1296 MHz, and has a more flexible 19-strand centre and a higher velocity factor. On HF and 2m the difference is negligible.
Is Airborne 5 better than RG-58?
Yes, by a big margin. On 2m, Airborne 5 loses about 11 dB per 100 m against roughly 20 for typical RG-58, and it's lighter, which is why it's popular for portable antennas.
Why does my SWR look better with a long cable?
Cable loss weakens the reflected power before it gets back to your meter, so the radio sees a lower SWR than the antenna really has. The calculator shows both values.