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|
The same figures have been calculated with 50w (Intermediate Licence)
|Power 50w||30 MHz||50 MHz||146 MHz||440 MHz|
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|
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|
And lastly maximum power rating for typical coax types.
|Max Power Handling||30 MHz||50 MHz||150 MHz||450 MHz|
The following online resources will help you calculate losses.