Will I Blow Up my Unun? (#689)

Todd, N2MUZ, wants to add a 500 watt amplifier but is worried that it might be too much for his unun to handle. Is it going to hurt the unun and if it does what should he expect?



Edited by Callum Jakeman.


Theme music is "Sour Tennessee Red," by John Deley and the 41 Players, courtesy YouTube Music Library.



Twitter: @dcasler

[westchesterbeemers]

Dave, Thanks for your response ( and those of the commenters ). My UNUN is very heavily constructed with other users saying they run a KW, so it should be alright for 500W on the EFHW frequencies where SWR is low. However, even though my 1.5KW tuner will easily adjust the SWR down to 1:1 on non-resonant frequencies 17 or 12 (where SWR is 3:1 or more), I won't risk using 500W amp there, for the reasons you state.
And agreed about most often not needing an amp, but it sure does help, when all you have is a wire antenna and need to "compete" with stations running directional antennas at height! 73, Todd N2MUZ

[ryanmall]

I'm working on trying to pass my extra test I'm a visual learner in your videos really helps to explain stuff for me thank you

[kevinkconohelijeepworld]

Bob at Balun Designs said never run a >3:1 or the chances of the coax shorting internally are greater.
I prefer a balanced feed and antenna ☺️

[Cowboy_Bob]

Dave, word to the wise: If you’re using FT-8, then multiply the unun’s PEP power rating by 0.3 … so a balun (or unun) rated for 500 watts PEP can only safely handle 150 watts of FT-8 (e.g., 500 multiplied by 0.3). I myself have pushed an unun’s toroid beyond its temperature threshold using FT-8 and have then witnessed unexpectedly high SWR. (The “0.3 multiplier” guideline is recommended by Bob Brehm of Palomar Engineering.)

[twohandsandaradio]

< is a less than sign. > is greater than.

[billryland]

An EFHW antenna is only resonant as a half wave or multiple of that frequency so the high impedance point is at the end of the antenna. A 49:1 transformer is looking for about 2450 ohms. The coax should always see a 50 ohm load. The wrong core can overheat if not large enough or the wrong material. I use type 43 cores.

[rogerlafrance]

Ratings for coax and components are valid for less than 2:1 VSWR. Not a problem at low power, but at higher powers, the voltages and currents that a mismatch can generate can be. Running a tuner into long coax and transformer can melt or arc thru coax well below its ratings.

[donaldsmith]

My EFHW has a rating that is high but said that on one or two bands said to do lower power! If he has good SWR on most bands but High SWR on one or two bands I would do lower power on the bands with the higher SWR! Some think that you can put up an antenna that covers all the bands good. Most multi band antennas are not really good for all the bands. They can work them but not all of them really good!

[GVFY]

1. An end fed half wave for 20m, 40m or 80 will always have a high SWR because it is not a multiple of 10m, 15m, 20m, 40m or 80m
2. Why do you need a linear anyway? it will you give you a few extra S-points.
3. Most devices like UnUn's have VERY misleading optimistic power ratings, and usually this is 500w PEAK and not nominal. Also data modes have much higher duty cycles. This means an UnUn rated for 500watts peak on SSB will only manage 125watt to 250watts on RTTY or FT8 etc. If you really plan to run 500watts in data modes then you need a 1000w UnUn to be safe!

4. If you over drive any kind of balun or UnUn you risk the core getting HOT and then it can suddenly stop and your radio's PA or linear's PA stage can be damaged and they are VERY expensive to replace.
5. You can buy linear amplifiers with internal antenna matchers. These are the modern ones.
6. You are better off learning how antennas work and improve your antenna system than splashing out $2500 ++on a quality linear amplifier.

G7VFY

[ericdee]

I love that Yeasu FT-101 right behind you Dave, is it operable?

[JT-pylv]

Blowing up stuff ? Every 14 y/o boys dream !