Coax Cables and Associated Losses

As a newly licensed amateur, one of the points I found hard to grasp was feeder losses. While it was explained to me that feeder cable had losses, it was sometimes hard to visualise what this meant in the real world.

The maths can be a little scary, and so in this article, I have looked at some real-world examples.

So we all appreciate that coax has losses, and we understand these increase with frequency and have a direct relationship to the length of the coax. Coax of course also has a maximum power handling rating, although this isn’t applicable to foundation licence candidates it’s worth considering this as you work towards a full licence and the ability to run 400w.

The amount of RF power (and received signal) will always be less than we put in, it’s worth taking a moment to consider that this applies to both transmit and receive signals as losses work both ways. The addition of a Low Noise Amplifier at the masthead will help to offset the receive losses, but it will do little for your transmit power. Often we say that’s fine, I will just use more transmit power, but that’s not always possible unless you have linear amplifiers to hand.

At HF (below 30 MHz) the losses are not significant, and we can usually opt for RG58 or RG213 coax, but as we move up the spectrum 144 and 430 MHz present a different challenge. The quality of your coax now becomes a significant factor in how your stations perform, and investing in good quality coax will often provide a better return than spending more on your antenna.

Let’s assume we have a typical coax run of 30m (100ft in old money) and we wanted to review the actual power presented to the antenna for various coax choices.

In this example, we have a radio with 10w (Foundation Licence) out and will be using 30m of coax. So for each frequency, we can now read off how much power will actually reach the base of the antenna.

Power 10w 30 MHz 50 MHz 146 MHz 440 MHz
RG-58A/U 5.6w 3.9w 2.46w 0.9w
RG8(mini8) 6.3w 6.16w 3.54w 1.54w
RG-213 8.7w 7.4w 5.24w 3.1w
LMR-400 8.5w 8.12w 7.08w 5.36w

The same figures have been calculated with 50w (Intermediate Licence)

Power 50w 30 MHz 50 MHz 146 MHz 440 MHz
RG-58A/U 28w 19.5w 12.3w 4.5w
RG8(mini8) 31.5w 30.8w 17.7w 7.7w
RG-213 43.5w 37w 26.2w 15.5w
LMR-400 42.5w 40.6w 35.4w 26.8w

So for every pound invested in your coax, performance will be increased on both your transmit and receive path. The actual coax you choose may well be influenced by the route and the bend radius, large diameter coax doesn’t bend very well.

Just to complete the picture, the last example is using 400w (Full Licence)

Power 400w 30 MHz 50 MHz 146 MHz 440 MHz
RG-58A/U 224w 156w 98.4w 36w
RG8(mini8) 252w 246.4w 141.6w 61.6w
RG-213 348w 296w 209.6w 124w
LMR-400 340w 324.8w 283.2w 214.4w

The figures have been calculated by looking at the feeder loss in dB/100m. These figures are a guide and provide the typical power (loss) seen for these coax types. The actual figure may vary, I have not considered coax connectors and adaptors as the quality and losses vary, so your measured RF power is likely to be even lower as the losses increase.

In selecting your coax you may want to consider the dB loss figures quoted by the manufacturer, the lower the dB the better in this example.

loss/100m (dB) 30 MHz 50 MHz 146 MHz 440 MHz
RG-174 16.5 19.8 39 75
LMR-100A 11.7 15.3 26.4 46.8
RG-58A/U 7.5 12.3 18.3 31.2
LMR-200® 5.4 6.9 11.7 20.7
RG-59 x 7.2 x 22.8
RG-8X 6 6.3 13.5 24.3
LMR-240 3.9 5.1 9 15.6
LMR-240 Ultra 3.9 5.1 9 15.6
RG-8/U FOAM x 3.6 x x
RG-213 1.8 4.5 8.4 15.3
RG-214 3.6 4.8 8.4 15.3
LMR-400 2.1 2.7 4.5 8.1
LMR-400 Ultra 2.1 2.7 4.5 8.1
DRF-400 2.1 2.7 4.5 x
Bury-FLEX x 3.3 x x
9086 x x x 8.4
9913 2.4 x 4.5 8.4

And lastly maximum power rating for typical coax types.

Max Power Handling 30 MHz 50 MHz 150 MHz 450 MHz
LMR-100A 230 180 100 60
RG-58U 400 300 160 80
LMR-200 1020 790 450 260
RG-59 500 400 250  x
RG-8X 350 280 150 80
LMR-240 1490 1150 660 380
RG-213 1800 1200 620 300
RG-214 1800 1200 620 300
LMR-400 2100 1700 1000 550
DRF-400 3300 2570 1470 830
9913 2200 1700 900 450

 

The following online resources will help you calculate losses.

dB calculator

Coax attenuation chart

Coax type v loss

 

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2 Comments

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  1. Excellent article Dave. Thanks for doing the sums!

    Can I borrow it for the LEFARS newsletter? Full credit of course.

    Schedule 1 Notes (B) of the UK Amateur Radio Licence notes [Section 2, page 17 of 20] that the Maximum Peak Envelope Power level applied, as appropriate, is measured at the base of the antenna. As an example, a Foundation licence holder could run 40W on 2m if using a 30m length of RG-58U (18.3db/m loss per 100m) and just be within limit (10W). Notwithstanding ERP absolute ERP limits on 430-432MHz 472kHz and 135kHz.

    1. Yes no problem, that’s cool.

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