Coax loss: where your lost power goes

Lost power turns into heat

Picture your signal as a hundred sparks racing up the coax to your antenna. Along the way, some of them don’t make it. The center wire and the shield have resistance, and the insulation between them soaks up a little too.

The power that’s lost doesn’t raise your SWR, and it doesn’t come out as harmonics. It turns into heat: the cable gets warm. Through 100 feet of thin coax on 2 meters, only about a quarter of your power reaches the antenna.

Higher frequency, more loss

The higher the frequency, the more a coax loses. The same thin cable that gets about half your power to the antenna on 10 meters gets about a quarter there on 2 meters, and about one tenth on 70 centimeters.

Why almost everyone uses coax

Because it’s easy. The signal stays inside the shield, so you can run coax along a wall, through a window or right next to metal with few special precautions. That’s the exam’s answer: it’s easy to use and needs few special installation considerations.

Most coax in ham radio is 50 ohms, to match the radio and the antenna.

• Coax isn’t the lowest-loss feed line, though. That’s hardline, below.

Which cable loses least

Thicker cable loses less. RG-213, about 0.4 inches across, has less loss at a given frequency than thin RG-58, about 0.2 inches.

What’s inside matters too. Foam insulation loses less per foot than solid. And air-insulated hardline, a stiff metal pipe with the center wire held in air, has the lowest loss of all.

Keep the water out

Moisture is what kills coax. Water soaks into the braid of the shield, and the loss climbs.

Sunlight is one way it gets in. Ultraviolet light cracks a jacket that isn’t made for it and lets water into the cable, so use coax with a UV-resistant jacket outdoors.

Connectors are the weak spots. Outdoors, tape and seal every one, whether it’s a PL-259, a BNC or a type N.

Which connector

The PL-259 is the everyday connector, common on HF and VHF. It isn’t watertight. Above 400 MHz, use a type N.

Every connector costs a little power. So does water in a connector, and so does a high SWR: all three are sources of loss in coax.

Three things to remember

1. Lost power turns into heat.
2. Higher frequency, more loss.
3. Thicker, foam or hardline: less loss.

Every question this covers

13 questions from the 2026–2030 Technician pool, word for word. On the exam the choices may come in a different order.

1. T7C07 What happens to power lost in a feed line?

   1. A It increases the SWR
   2. B It is radiated as harmonics
   3. C It is converted into heat Answer
   4. D It distorts the signal

2. T7C09 Which of the following causes failure of coaxial cables?

   1. A Moisture contamination Answer
   2. B Solder flux contamination
   3. C Rapid fluctuation in transmitter output power
   4. D Operation at 100% duty cycle for an extended period

3. T7C10 Why should the outer jacket of coaxial cable be resistant to ultraviolet light?

   1. A Ultraviolet light can increase the resistance of the conductors
   2. B Ultraviolet light can increase losses in the cable’s jacket
   3. C Ultraviolet and RF signals can mix, causing interference
   4. D Ultraviolet light can damage the jacket and allow water to enter the cable Answer

4. T7C11 What is an advantage of foam-dielectric versus solid-dielectric coaxial cable?

   1. A It is more resistant to moisture contamination
   2. B It has higher voltage breakdown
   3. C It has less loss per foot Answer
   4. D It has a better impedance match to 50 ohms

5. T9B01 Which of the following connectors should be carefully taped for weather protection when used outdoors?

   1. A PL259
   2. B BNC
   3. C Type N
   4. D All these choices are correct Answer

6. T9B02 What is the most common impedance of coaxial cables used in amateur radio?

   1. A 8 ohms
   2. B 50 ohms Answer
   3. C 600 ohms
   4. D 12 ohms

7. T9B03 Why is coaxial cable the most common feed line for amateur radio antenna systems?

   1. A It is easy to use and requires few special installation considerations Answer
   2. B It has less loss than any other type of feed line
   3. C It can handle more power than any other type of feed line
   4. D It is less expensive than any other type of feed line

8. T9B05 What happens as the frequency of a signal in coaxial cable is increased?

   1. A The characteristic impedance decreases
   2. B The loss decreases
   3. C The characteristic impedance increases
   4. D The loss increases Answer

9. T9B06 Which of the following connector types is most suitable as an RF connector for frequencies above 400 MHz?

   1. A PL-259
   2. B Type N Answer
   3. C RS-213
   4. D DB-25

10. T9B07 Which of the following is true of PL-259 type coax connectors?

    1. A They are preferred for microwave operation
    2. B They are watertight
    3. C They are commonly used at HF and VHF frequencies Answer
    4. D They are a bayonet-type connector

11. T9B08 Which of the following is a source of loss in coaxial feed line?

    1. A Water intrusion into coaxial connectors
    2. B High SWR
    3. C Multiple connectors in the line
    4. D All these choices are correct Answer

12. T9B10 What is the electrical difference between RG-58 and RG-213 coaxial cable?

    1. A There is no significant difference between the two types
    2. B RG-58 cable has two shields
    3. C RG-213 cable has less loss at a given frequency Answer
    4. D RG-58 cable can handle higher power levels

13. T9B11 Which of the following types of feed line has the lowest loss?

    1. A 50-ohm flexible coax
    2. B Multi-conductor unbalanced cable
    3. C Air-insulated hardline Answer
    4. D 75-ohm flexible coax

Keep going

• Practice every question on HamStudy
• How to get your license, step by step
• Am I reaching the repeater?
• 2 meters vs 70 centimeters

Questions from the NCVEC 2026–2030 Technician question pool, with the errata of February 19, 2026.