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EFHW that weighs 2.7oz for QRP SOTA Activations

Discussion in 'Amateur Radio News' started by KK6USY, Mar 25, 2021.

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  1. NT9W

    NT9W Premium Subscriber QRZ Page

    KK6USY and KM1NDY like this.
  2. KF5FEI

    KF5FEI Ham Member QRZ Page

    Made by N5JLD using a binocular core for the balun, IIRC. Likely even smaller now as he has made some tiny 3D printed insulators.

    KI5HNX, KK6USY and KM1NDY like this.
  3. N5HXR

    N5HXR Ham Member QRZ Page

    The only simple article I've found on this that makes sense and appears to present a real methodology for calculating it is here:

    I'd be interested in materials with more discussion on it, though.
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  4. KM1NDY

    KM1NDY Ham Member QRZ Page

    N5HXR & NT9W:

    Thanks for those references. I am at the "How the heck does a transformer even work?" stage of learning, being of a non-electrical background.

    But, one might say it's a heavy area of interest! I might even take on an transformer reference as a project myself... I can't believe the dearth of digestible info on the topic (and am still more apt to believe I haven't looked in the right areas).

    Fyi, .pdf's of some of Sevick's earlier editions are posted online too.


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  5. KM1NDY

    KM1NDY Ham Member QRZ Page

  6. WA2LXB

    WA2LXB XML Subscriber QRZ Page

    Excellent thread...the antenna, balun and feed line often end up weighing nearly as much as the battery.

    Chuck, what are you using as a feed line to stay as light as possible? LMR240 superflex is my go-to for jumpers and light weight, low loss feed lines, but it's heavier than desired for POTA and SOTA.
    KK6USY likes this.
  7. VE3VXO

    VE3VXO Ham Member QRZ Page

    Hi Mindy

    You have asked a good question and I think very few QRP'ers ask and do the homework on this but of couse they can get away with things and not even know how much signal is being lost but if you try that approach at QRO you might have a fire on your hands! First you have to choose a core made from appropriate material for the frequency and type of application. Ferrite materials have published curves showing the complex permeability Ui and Uii and for a transformer you want a material where it is operating in a part of that chart where Ui is high and Uii is low. For power handling you have to look at core flux density for one thing which depends on several factors, mainly the peak driving voltage, the frequency and number of turns on the driving winding (usually the primary) and then the density of that flux being inversely proportional to the cross sectional area of the core. There are limits to the amount of flux the core can handle without getting hot. In some cases the windings themselves can be the area where things break down even when the core itself is in a reasonably safe area. As a rule of thumb a small core with few driving windings (like the common 1:49 autotransformer) at lower frequency end is asking for trouble.

    I found the best sources of info for me in understanding all this was in reading the technical info published by Amidon corp on their website. I will attach some of the best references here. The first one explains better what I wrote above and gives maximum flux density guidelines at the bottom. The second one goes through an example of the calculation. I also include a graph I made to visualize the max flux vs frequency according to the guidelines. Then there's a chart showing physical dimensions for common cores which you need for the cross-sectional area in the flux density calcs above. If you follow these references you can easily scale your designs for power and frequency and know exactly what you will get. You might even twist my arm to get you started on an example.

    Sevick's books are great for example transformer designs mostly QRO stuff but some of the material on baluns is quesionable at best and just plain wrong at worst.
    Don't believe anything you see about a 4:1 balun on a single core for example.

    Ham on....Joe

    Attached Files:

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  8. KK6USY

    KK6USY Ham Member QRZ Page

    That's great , I too just did a 9:1 and it works good . I do like the 49:1 antenna better though. Thanks for sharing!
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  9. KK6USY

    KK6USY Ham Member QRZ Page

    These are some great Ideas thankyou for sharing, everything goes in the memory bank lol if I can juse remeber it lol!
  10. KK6USY

    KK6USY Ham Member QRZ Page

    I don't use coax with the antenna unless I need some separation for setting up the radio. But I do carry a 1' piece of rg 316 if I need it. I have tried a counter poise also and it didn't seem to help or change anything.

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