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Author Topic: Is a full strap 49:1 transformer for a 40 m half-wave end fed antenna possible?  (Read 2482 times)
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WO4K
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« on: October 02, 2019, 07:08:53 PM »

Has anyone here constructed a 49:1 transformer for a 40 meter half-wave antenna that can withstand full strap AM duty cycle? If so, I'd like to get the design info. I'm not sure it could be done in a way the torroids could withstand the heat produced without reaching the Curie point (without some kind of cooling mechanism). Has anyone successfully built one? Or tried and failed?
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PA0NVD
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Nico and Chappie (Chappie is the dog...)


« Reply #1 on: October 02, 2019, 07:37:40 PM »

If it runs hot, it has losses. That's not where you want your power to go. I used a lot of end-fed antennas, mainly 1/2 wave verticals, and always use an LC circuit with coupling loop. It runs 1 kW CW and stays cool, no losses. Many high ratio transformers match well due to the high losses.
The end fed is approx 3200 Ohm when tuned well, so I normally design for a Q of 10. That means that both the L and C have to have an impedance of 3200/10 + 320 Ohms  This is off course not a critical value. I couple wit a two turn coupling loop tight over the cold end of the coil. but calculating the coupling loop is easy. Just the number of turns of the coil divided by the sq. root of the impedance transformation, 3200 / 50 . Round off high because the coupling coefficient is not 100 %. With the coil I use I should have 1.8 turns, so I took 2 and it matches really very well.
If you want to calculate the max voltage, just use the peak power over 3200 Ohms.
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W1ITT
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« Reply #2 on: October 02, 2019, 09:11:31 PM »

Check out the feed method in the antenna books that is commonly used with  half square antennas.  It uses a parallel LC network with the antenna lead coming off the top and the 50 ohm line tapped up a bit from ground.    It'll work with an end feed half wave just as well.  I think most of the magic end fed wires sold in a plastic bag are essentially done the same, but with small components that won't handle legal limit AM for long.  Use a vacuum variable capacitor or wide spaced air variable and a stout coil for reliability.
A high ratio transformation is generally done up as a series of smaller steps, both for reasons of efficiency and for bandwidth.  And 49 to one is a big hop for a single stage.
 
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W4DNR
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« Reply #3 on: October 02, 2019, 11:30:38 PM »


A high ratio transformation is generally done up as a series of smaller steps, both for reasons of efficiency and for bandwidth.  And 49 to one is a big hop for a single stage.
 

There is a Facebook 49:1  group selling what I call, "Snake Oil"....   The Owner may have found the magic combination on his EFHW , but there have been hundreds of toroids sold and cracked due to heat. 

   Palomar stock is surging while toroids are burning.

   A lot of unhappy new hams looking for a "magic antenna matcher". They want all bands to match below 1.5:1 .

  Admittedly, my only foray into efhw used a 1/4 wave matching section of home-made ladder line and had a 2:1 bandwidth of 
 around 50 kc on 75 meters .... totally off scale every where else.    But the matching section was a direct ground at the cold
 end for lightning and static dissipation.  I have dreams of a tower mounted air coil with a motorized vacuum variable in a weatherproof box.   I also dream of those years when climbing the tower didn't take three or four rest stops... LOL !

Don W4DNR

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WA1LGQ
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« Reply #4 on: October 03, 2019, 09:09:48 AM »

End fed 1/2 wave? Do it like a J-pole, unless it has to be a transformer or you have to feed it right at the very end.
Just an  idea. J-poles don't have to be vertical.
Larry
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PA0NVD
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« Reply #5 on: October 03, 2019, 09:23:04 PM »

Hi Don
The small bandwidth is mainly because the loaded Q is too high. With a 1/4 wave matching, the BW is in the order of the transformation ratio, that is 3200/50=60. So quite narrow band and indeed in the order of 50 kHz for the 80 MHz band. This is NOT the case for a transformer or a resonating transformer as a LC parallel circuit with coupling loop or tap. With the parallel circuit it is the same if you take too low impedance for the coil / capacitor. It simply is a matter of resonance circuit impedance vs load. The load is 3200 Ohms, so when the coil and cap have an impedance of 320 Ohms, you will have a Q of 10 with the appropriate bandwidth. If you need more bandwidth, yo need to increase the impedance of the coil and cap (bigger coil, smaller cap) to get a lower LOADED Q. But there is a limit, mainly a practical one, big coils and some decrease in coil Q so somewhat more losses. With a Q of 10 you get a big part of the 80 meter band.
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