Coax Loss Calculator

RF Feedline (Coax and Ladder-Line) Loss and ERP Calculators


This page is Copyright © 2015 by KV5R. The Javascript that runs the loss calculator was written in 2001 by Dan Maguire, AC6LA, based on ARRL's "Additional Loss Due to SWR" formula. He later withdrew the code in favor of more accurate formulae. For more info, please see this AC6LA page.

This program is provided “AS-IS.” It is the responsibility of the user to verify the accuracy of the calculations when using this program. Always look up the cable manufacturers’ specifications and installation guides for any RF cable prior to purchase and use.

Please don’t ask me to add another coax! I don’t know where or how the K1 and K2 constants used in the arrays were obtained or derived.

Coax and Ladder-Line Info

Basic Feedline Facts for Hams

  • RG-174 (50 ohms) is very small (~0.11") and lossy. Suitable only for short pigtails and jumpers at very low power, as in receivers, scanners, etc.
  • RG-58 (50 ohm) is about 0.195", quite lossy, suitable only for mobile installations (typically < 20 feet, < 150 watts).
  • RG-6 (75 ohms) is about 0.332", typically used for cable/satellite TV.
  • RG-8X (50 ohm); and and RG-59 (75 ohm) are about 0.24 inch. Suitable for medium power (~350 watts), HF and Lo-VHF.
  • RG-8 and RG-213 (50 ohm); and RG-11 (75 ohm) are about 0.405 inch. Suitable for higher power (~1800 watts) at HF.
  • 9913 and LMR-400 are popular “low-loss” RG-8 types. Suitable for VHF (~1.5dB loss per 100 feet at 146 MHz). The flexible types (9913F and LMR-400UF) are preferred, particularly for rotatable antennas. However, stranded center conductors have a little higher loss than solid ones.
  • There are at least four commonly-used types of coax dielectric: (1) semi-solid polyethylene (PE) for temps up to 80°C (Vp 66%); (2) gas-injected PE (foam PE) for temps up to 100°C (Vp ~85%); (3) air-core: a thinner PE with a PE strand spiraled around the inner conductor (Vp ~85%); and (4) PTFE (Teflon) for temps up to 250°C.
  • All foam-type coax cables should be taped—not attached with plastic cable ties or wire—to tower legs, etc., to prevent deforming of the foam dielectric.
  • Windowed ladder-line (WLL) should be (1) kept at least a few inches from metal (to prevent magnetic induction losses); (2) twisted 1/2 turn per foot (to prevent wind flap); and (3) turned at a large radius (12 inches or more). It has a VF of 91-95%. See also KV5R’s Ladder-Line pages.
  • Though frequently ignored, all RF transmission lines have maximum RMS voltage, and maximum power (wattage), which varies with type, brand, and frequency.
  • How much attenuation is acceptable? Well, a mere 3dB will waste 50% of your wattage!

Coax and Ladder-Line Data

Click the links below for manufacturers’ data-sheets.
Note: the RG numbers given are not actual labels, but are given merely for familiarity.

8215 RG-6A
8237 RG-8A
9913 RG-8A (low-loss)
9258 RG-8X
8213 RG-11
8261 RG-11A
8240 RG-58
9201 RG-58
8219 RG-58A
8259 RG-58C
8212 RG-59
8263 RG-59B
9269 RG-62A
8216 RG-174
8267 RG-213
Davis RF:
Times Microwave (all LMR PDFs):
LMR-100A RG-174
LMR-200 RG-58
LMR-240 RG-8X
LMR-400 RG-8
LMR-600 0.59"
LMR-900 0.87"
Wireman (coax data table):
CQ102 RG-8
CQ106 RG-8
CQ125 RG-58
CQ127 RG-58C
CQ110 RG-213
CommScope (Andrew):
Heliax LDF4-50A 1/2 in.
Heliax LDF5-50A 7/8 in.(obsolete)
Heliax LDF6-50 1-1/4 in. (obsolete)
Wireman (1" WLL):
551 (#18 solid)
552 (#16 stranded)
553 (#18 stranded)
554 (#14 stranded)
551 (wet*)
552 (wet*)
553 (wet*)
554 (wet*)
Generic 300 ohm Tubular
Generic 450 ohm Window
Generic 600 ohm Open
Ideal (lossless) 50 ohm
Ideal (lossless) 75 ohm
*"Wet" numbers are worst case
for lines with ice or snow.

Line Loss Calculator:

Note: Set Line Length 100 here to use the ERP Calc. Put actual line length in the ERP Calc.

  Parameters:   Results:
Line Type: Matched Loss: dB
Line Length: Feet Meters SWR Loss: dB
Frequency: MHz Total Loss: dB
Load SWR: :1 Power Out: Watts
Power In: Watts Power Loss: %
  before using ERP Calc.    

Effective Radiated Power Calculator:

  Parameters:   Results:
Loss Per 100' at Op Freq: dB Calculated Loss: dB
Actual Line Length: Feet Power Out: Watts
Power In: Watts Power Loss: %
Antenna Gain: dBd (λ/2 dipole=0dB) ERP: Watts

External Links

Transmission Line Loss technical tutorial from

Online Calculator from Times Microwave

—73, KV5R

30 thoughts on “Coax Loss Calculator
  1. Hello Everyone, my name is Christian Nartatez, I am really grateful for this site, since I have a better understanding about FM Radio Transmission, thank you all for your comments, I am a pastor of a Baptist church in South America, I have been working with a 150W supposed transmitter, but the thing is that it never gets more than 60Watts of output when I measure with the Wattmeter, and SWR is below 1.5, also it is a strange thing that when setting up bellow 99Mhz dial the power drops to 30Watts, but when moving dial to 106Mhz or above the 105Mhz it goes up to 60Watts, I have tried: Dipole Standard antenna and also Dipole with Gamma Match and in both scenarios I am only registering that 60Watts and no more than that, and the cables I have tried are: RG8X and Times LMR400 and with both even 100Feet and 50Feet, measurements in watts are the same, it doesn’t change, is this a problem of the transmitter? If you can help me out I will appreciate, we are preaching the gospel, God bless.

    • IMO you should have an RF choke close to the antenna feed point to prevent RF travelling down the outside of your coax and interferring with your wattmeter readings. You can make an RF choke by wrapping 4-5 turns of the coax around a 4 inch diameter non-conductive object or just open air with plastic tie-wraps. There are lots of articles on the internet about 1:1 baluns or rf chokes (even the ugly balun which has many more turns on it than you need for vhf).

  2. Harold Melton (kv5r) I have a fan dipole up for 160, 75, 40 meters on 160 swr is fine it is 100% but on 75 I cant get the swr down lower than 1.7 and on 40 meter it`s about 3 swr I tried to make the legs shorter but it would make them higher swr`s even if I make them longer thy would be higher too it works okay with my antenna tuner but on 160 I don`t need a tuner I can feed it directly to get a 1.1 swr you can tell me what to try or maybe I should get rid of the fan dipole and put up a multi band dipole fed with ladder line if you want I can send you pictures of what I have now give me your message address and I could sent it to you give me some help Harold Melton (KV5R) if I get rid of the fan dipole I then would only have one leg I don`t mind using a tuner but if I have to use a tuner I might put up a multi band dipole it would be okay let me know on what to do.Paul Wozniak…KB9VWD

    • Hello Paul,
      Adjusting the length only changes the reactance, not the resistance, at the feed-point of a dipole. It’s likely that even at resonance the FP resistance is low (way under 50 ohms) on your shorter dipoles due to interaction with the longer ones. I’d suggest spreading the ends of your fan way out, even to 60 degrees between each leg (6 end-points), to reduce interaction. If you need to stay with only 2 end-points, at least slacken the shorter dipoles so they droop several FEET below the longest one.
      But I’m no expert on fan dipoles, so read elsewhere to find people with more experience with them.
      Yes, you could also switch to a ladder-line fed doublet + tuner, they do work well, though have wacky patterns with multiple nulls on the higher bands. Please see the Ladder Line pages, and the 80-meter Doublet article, on this site.
      73, –KV5R

  3. Good resource. I have it bookmarked to find it easily when i need it. I like being able to input the variables to see what kind of loss to expect on the different types and lengths of coax i might need. Nice to be able to do a comparison and see the differences.

  4. Hi. Tnx for the calculator. I just built a 40 meter dipole fed with MFJ purchased 300 ohm twin lead. It is the 300 ohm foam insulated twin lead.
    Would I have significant improvement if I switch to 450 ladder line ( wireman 553)?
    My twin lead is 50 ft and I had a problem tuning 20 meters. The antenna is set up for 40 meters with each side measuring 33 ft. I assume I need to trim the twin lead to get better matching?
    Is there a difference in loss when one compares the old 300 ohm twin lead compared to the new foam insulated 300 ohm twin lead?
    Any comments on a recent QST article regarding 450 ladder line in 3/4 inch foam pipe insulation in order to run on the ground?

    • I have no experience with foam insulated 300 ohm twin-lead, but with continuous dielectric it will be lossy when operated at high SWR. Yes, window line will be better, and open-wire ladder line will be best. It’s all about getting lossy dielectric out of the line’s electromagnetic field.
      Running your twin-lead in poly-foam pipe insulation will not provide sufficient spacing between the line and ground. You need at least 3-4 times the line width of spacing, so for half-inch line you’d need insulation 3-4 inches in diameter (0.5 x 4 x 2). And the insulation would add additional dielectric loss.
      On your tuning problem, running a dipole on its second harmonic (i.e., running an 80 on 40, or a 40 on 20), you are feeding two have-wave end-fed elements their voltage anti-nodes, which is a very high impedance point. An odd-quarter-wave feed-line will transform that to a low impedance, but then it won’t work on its fundamental. The solution is to pick a line length in-between, so you get a medium impedance transformation on both bands. So, somewhere around an odd-eighth-wave multiple of the dipole’s fundamental frequency.
      For more info, please see Ladder Line Page 3 on this site.
      73, —kv5r

  5. Hello,

    I have a vertical dipole very much like the one in your Big Vertical Antenna Project blog. I used 450 ohm LL and trimmed the feed line length to 145ft to get an SWR on 40M of 2.2 without a tuner. However, my SWR for 20M is a whopping 16.4. Fortunately, I have a Palstar AT2KD tuner (awesome piece of gear) and I can tune 20M down to 1.7 SWR. Here is my question… does the 16.4 SWR still exist over the 145ft of my feed line and my tuner is just masking it to my radio or did my tuner really “fix” the mis-match and an SWR of 1.7 truly exists now between radio and antenna? Thanks!

    • The 16.4 exists on the line at the point where it attached to the tuner. Any transmission line that is not load terminated into its characteristic impedance has a variable impedance all along the line. And in the case of ladder line, SWR on the line doesn’t hurt a thing.
      A tuner doesn’t “fix” anything on the line, it just transforms the impedance that exists at the point where it’s connected, to 50 ohms on the radio side.
      Also, keep in mind that your SWR reading is referenced to 50 ohms, not 450, so the actual SWR on the line, referenced to 450, is 9 times less than indicated by a 50-ohm SWR meter. But all that matters is what the radio sees, so SWR meters are calibrated 1:1 at 50 ohms.
      The bottom line is that SWR on ladder line just doesn’t matter, as long as it’s within the range of the tuner to match it. Coax, oh the other hand, is very lossy at high SWR.
      73, –kv5r

  6. Great tool, but please don’t cheat yourself. This calculator will just show you a part of your loss – the cable. If your antenna is not an exact load of 50 ohms (do you know such an antenna?), the world looks completely different.


    73’s de OE1MWW

    • Antenna efficiency is not related to 50 ohms, or resonance. 50 ohms is just a design point to match to 50-ohm coax. If the antenna is not 50 ohms, the coax gets lossier, but the antenna does not. For example, a folded dipole is 300 ohms at resonance, and it’s just as efficient as a 50-75 ohm dipole. Ladder-line-fed non-resonant dipoles may, at various frequencies, have a feed-point impedance anywhere between 50 and 5,000 ohms, with the same efficiency.

      Regardless of feed-point impedance, an antenna’s efficiency is mostly determined by its aperture (physical size), and losses in the near-field (mostly ground). Antenna losses are resistive and ground heating. Feed-line losses are resistive and dielectric heating.

      Software like TLD usually model everything on 50 ohms, but you can model on any other impedance just as well.

      The tool on this page calculates feed-line losses (dielectric heating) as a result of voltage standing waves, those high-voltage anti-nodes that increase dielectric stress and therefore, heating. Antenna efficiency is a separate matter.

      73, –KV5R

  7. This is the best way to figure out what kind of performance I wanted from my installation. I’m glad I saw this first, what I had in mind would have been a waste of time had I not seen this.
    I decided on LMR600 (30ft) and a Tram 1481 dual band antenna with a 8.3/11.7 dBd gain which almost doubled the budget but it was well worth it, and the setup even exceeds the predictions my RF mapping software program showed my coverage would be. You saved me a ton of disappointment and trouble. THANK YOU!

  8. Very informative usefull tool when setting up a gateway calculating losses to get NOV will recomend to anyone doing same.

  9. very informative and useful site all the information you need on one webpage the calculator is brilliant .

    I teach the uk amateur radio exam courses and this will be a great asset.

    many thanks


  10. This was a Godsend. I have a inverted L 117ft beta matched on 1905KHZ.
    I am feeding it with rg-6 tv foam coax. I was thinking of using Rg-11
    foam. This antenna is 140 ohms or so both on 14mhz and 18 mhz. The
    75 ohm coax is 216 ft long, VF .82 guesstimated and is a perfect
    1/4 multible for 17 and 20 meters. The feed Z xmitter end is 40-45
    ohms j-0. The calculator says I would gain 1db or less by changing
    coax. My next change would to be to add short radials. I have 30
    100 footers now. I have put down a 1/4 wave radial for 20 meters
    and not enough current to light a flashlight bulb connected to
    main ground at 100w. Thanks Tim

    • The width of the line needs to be <1% of the wavelength. For 1" line, 2-meters is about the limit.
      Parallel line for 23cm would need to be under 2.3mm wide.
      But Wait! You can use G-line for 23cm; very low loss, but straight runs only. G-line is a single conductor with cones on each end; look it up.
      73, –kv5r

  11. This calculator helped me make my mind up to build a balanced feed line. Thanks for putting all of this together in one convenient place.

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