Coaxial Length Calculator Version 5.1 Free Download (uses Microsoft Excel) Updated 15th July 2015. What is the correct length of coax to use between your transmitter and aerial, or should there be a correct length? Some say it does not matter and any random length will do as long as it reaches where it needs to go. The next question is. What coax should I use? Some say it does not matter as long as it is of the correct impedance. Now we ask the question. How much power is being radiated by the aerial? Many believe that if your transceiver has an output of 100w then that is what the aerial is radiating .... But is it? At this point, let it be said that a random length of coax will work at the correct impedance but the question remains as to the efficiency of the coax. The aerial is a tuned resonant device with a nominal impedance of 50ohms set for maximum gain at the chosen frequency. As we know the transceiver is a variable tuned resonant device capable of receiving and transmitting at the chosen frequency with an impedance of 50ohms. It makes common sense to make the transmission line match the aerial and transceiver for maximum efficiency from the system. Therefore the answer is a tuned transmission line. In the case of low HF frequencies this may not be practical due to the long lengths that would be required. All coax cables have a velocity factor and will have some loss along the length. The losses are more noticeable in the higher frequencies. So to determine which coax to use, the questions to ask yourself would be. Which Frequency is the coax to be used for? What length do I need and importantly how much of my transceivers output power do I want at the aerial? At VHF/UHF the choice of coax is important to minimise loss on both transmit and receive. Below is a screenshot of a Microsoft Excel based calculator which is a valuable tool to simply and quickly calculate coax lengths and losses. You can download Coaxial Length Calculator for free but you will need Microsoft Excel to use it or another spreadsheet program capable of using Excel. You can see from the screenshot a league table of coaxial cables. Which one are you using? In the example below, the frequency given is 50.150 and the chosen coax velocity factor is 0.87. You can see the calculator has worked out the electrical lengths which should be used. A quick look at the table on the right displays the multiples of half wavelengths. Your coaxial system including patch leads should be made up of multiple half wavelengths with an odd quarter wave which will act as an impedance transformer to achieve resonance. If you do this, you will get the maximum performance from your chosen coax. Take a look at the Coaxial Loss Calculator and the Power Compensatoras the two are linked and work in conjunction with each other. Here you can see that the total length of cable being used is 19.5m which is 7 half wavelengths taken from the Multiple 1/2 Wavelengthstable with a single 1/4 wavelength added as shown in the Coaxial Length Calculator. Using the league table, you can see that the dB loss for 100m of LMR-600 coax at 50.150MHz is 1.8dB. The calculator now shows that for the length of coax the loss is 0.35dB. This may not seem much of a loss but look at the Power Compensatoragain and you will see that 100w from your transmitter would have to be increased to 108.4w to achieve 100w at the aerial. Simply put, the coax is losing 8.4w. This example is for LMR-600 which is very low loss coax. Imagine how much transmit or receive power you would lose if the coax was RG-58!!! Download Coaxial Length Calculator and you will be able to calculate and see your coax losses, You can use the Power Compensatorto calculate dBW. Just enter any power in watts and the calculator will convert it to dBW. You can use the Coaxial Length Calculator for any cable that is not shown in the table providing you know the velocity factor and loss per 100m at any frequency you wish to use.Now we have dealt with coaxial cables, we should turn our attention to the various connectors and their losses. John Huggins KX4O has provided detailed analysis on his website http://www.hamradio.me/connectors/uhf-connector-test-results.htmland makes for very interesting reading.
It does work with Open Office but may look a little untidy. Other programs may work, but I have not yet tested any.
I tune all my antennas as close to 50 ohms +/- J0 as can and use good quality coax cable, LDF4-50. I have never in 36 years needed to calculate a special length of cable using anything other than a tape measure to make sure it was long enough and most of the time not even that. I started at one end and unspooled the cable until I had enough. Losses were/are always low and the match is good. Making up phasing lines however is a different story and I use my MFJ-269 for that usually.
See his posting on the eHam QRP group. The theory is incorrect. The correct answer is the shortest length that reaches from the antenna to the rig. This multiple half wave stuff started back in the CB days as they thought the SWR repeated each half wave. What does repeat is the impedance- the SWR (neglecting small feedline losses) is constant along the transmission line. Dale W4OP
It looks as if the OP was educated at the school of the CB Modge. The company I worked for used miles if not tens of miles of the stuff, and we never measured any lengths... It all worked beautifully, before Fibre Optic Cable and Satelites took over.... Hi.
Absolutely ! The only time the length of coax is important is when you actually have mismatch, like on a G5RV, and you are using the coax part of the feeder to try and fool the rig into making the impedance look OK at the rig end. With a resonant aerial and even half-decent coax on the lower bands, feeder losses are totally negligible! Something I AM interested in (but never seen any figures for) is what kind of feeder losses you get in practice when the feed impedance ISN'T correct, like using a dipole on the wrong band. You would imagine they would be terrible . . . but in practice they don't seem to be that bad. Like if the antenna was about 300 ohms at the feed point . . . but you're just using 50 ohm coax . . . I wonder what the feeder loss would be on a 60ft run? (not sure why this thread is in this section . . . maybe somebody will move it?)
http://www.ac6la.com/tldetails.html This free program gives you all sorts of information about where your power goes when you have a mismatch. Zack W1VT
I found with LINUX I use Libre Office, and there seems to be that .docx files ,etc work ok, I have used MS Office, and you should be able to do file conversions without any issues, and check their web site or search for open office forums.
Ditto! Something about tilting at windmills & grasping at straws................. I have NEVER, EVER worried about a specific length of coax. Listen: The screwdriver antenna is a perfect example since it operates on multiple frequencies. If that coax length bull**** was true, you'd have to have 8-10 coaxes, one per band, each cut to this "magic" length, in order for it to work! Yet many hams, military and commercial ops run these multiple bands with great success, yours truly included, with "them swr'zzzzzzzzz" at 1.5 or less (mostly less). Now WHERE would you store/run all those coaxes in a small S-10 pickup if you had to keep up with all that jazz? If you want to give them f'ar in th' w'ar CBers fits, explain the screwdriver antenna to them, the multiple bands, and ONE coax!!! It will drive them crazy trying to rationalize/explain their "coax length" theory. I have presented this problem to some of them in the past, and not ONE has ever been able to explain how I can operate 10 bands (amateur, CAP, FEMA) with ONE coax and a 1.2 on ALL bands--Plus a very respectable signal, too! Its hilarious to see! Bottom line: Tune the antenna, NOT the coax.
